Cooper Basin HDR Seismic Hazard
Evaluation: Predictive modelling of local
stress changes due to HFR geothermal
energy operations in South Australia
Dr Suzanne P Hunt and Mr Cameron P Morelli
The University of Adelaide
October 2006
Report Book 2006/16
Petroleum and Geothermal Group
Division of Minerals and Energy Resources
Primary Industries and Resources South Australia
Level 6, 101 Grenfell Street
GPO Box 1671
ADELAIDE SA 5001
Phone
+61 8 8463 3204
Fax
+61 8 8463 3229
Email
pirsa.petroleum@saugov.sa.gov.au
Website www.petroleum.pir.sa.gov.au/
© Government of South Australia 2006
This work is copyright. Apart from any use as permitted under the Copyright Act 1968 (Cwlth), no part may be reproduced by any
process without prior written permission from Primary Industries and Resources South Australia. Requests and inquiries concerning
reproduction and rights should be addressed to the Director, Petroleum and Geothermal Group, PIRSA, GPO Box 1671, Adelaide SA
5001.
Disclaimer
Primary Industries and Resources South Australia has tried to make the information in this publication as accurate as possible, however,
it is intended as a guide only. The agency will not accept any liability in any way arising from information or advice that is contained in
this publication.
Preferred way to cite this publication
Hunt, S.P. & Morelli, C.P., 2006. Cooper Basin HDR hazard evaluation: Predictive modelling of local stress changes due to HFR
geothermal energy operations in South Australia. South Australia. Department of Primary Industries and Resources. Report Book
2006/16.
CONTENTS
Executive Summary..................................................................................................................................................................4
Future Hazard Assessment Policy Recommendations..............................................................................................................5
1.0 Introduction ........................................................................................................................................................................6
1.1 General discussion of potential seismic hazards.............................................................................................................6
1.2 Earthquake hazard for natural seismic risk assessment ..................................................................................................7
1.3 Earthquake hazard for induced seismic risk assessment.................................................................................................7
2.0 Module A Literature review of seismic hazard assessment for other international hot fractured rock geothermal energy
sites ....................................................................................................................................................................................9
2.1 Previous Work ................................................................................................................................................................9
2.2 General review of seismic events caused by stress triggering ........................................................................................9
3.0 Module B Data collection for basement granite structural data in Cooper Basin and assessment of critically stressed
basement-offsetting faults in these areas (from basement maps) .....................................................................................11
3.1 Introduction – slip tendency analysis for faults in the Cooper Basin region basement rocks.......................................11
3.2 Natural tectonic seismicity in the Cooper Basin region................................................................................................11
3.3 Basement fault interpretation........................................................................................................................................12
3.4 Stress data used for fault mechanical modelling...........................................................................................................13
3.5 Analytical modelling ....................................................................................................................................................13
3.6 Numerical modelling ....................................................................................................................................................13
3.7 Results ..........................................................................................................................................................................14
3.8 Discussion ....................................................................................................................................................................14
3.9 Conclusions ..................................................................................................................................................................15
4.0 Module C HFR site induced seismicity and hazard to local geological and built structures ............................................29
4.1 Problem statement ........................................................................................................................................................29
4.2 Introduction ..................................................................................................................................................................29
4.3 Effect of geothermal activity on the probabilistic earthquake risk maps for the Cooper Basin region.........................29
4.4 Background seismicity for the Cooper Basin GEL sites and recurrence relationships at the site of interest................29
4.4 Geodynamics Ltd event description (GEL98) ..............................................................................................................30
4.5 Results for seismic risk determination..........................................................................................................................30
4.6 Attenuation effects........................................................................................................................................................30
4.7 Conclusions ..................................................................................................................................................................30
5.0 Module D Development of numerical models to assess permanent impact of the developed reservoir structure on the
local in-situ stress field.....................................................................................................................................................40
5.1 Problem statement ........................................................................................................................................................40
5.2 Introduction ..................................................................................................................................................................40
5.3 Static stress changes .....................................................................................................................................................40
5.4 Model development ......................................................................................................................................................40
5.5 Conclusions ..................................................................................................................................................................41
6.0 Proposed Future Hazard Management..............................................................................................................................46
6.1 Hazard Management Approach ....................................................................................................................................46
6.2 Proposed Future Monitoring Arrangements ...............................................................................................................468
6.3 Uncertainties and Proposed Future Research .............................................................................................................469
APPENDIX A ........................................................................................................................................................................49
Useful terminology and abbreviations................................................................................................................................49
APPENDIX B.........................................................................................................................................................................51
HDR Rosemanowes Data ...................................................................................................................................................51
REFERENCES.......................................................................................................................................................................52
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EXECUTIVE SUMMARY
The aim of this study was to evaluate hazard due to seismicity generated by the development of hot fractured rock
operations in the Cooper Basin region of South Australia. This has been performed (divided into four modules) and the
conclusions of this study are given in summary below.
• Module A A primary focus of the work was to bring together all information on this topic both from an international
and national review.
• A product of this work has been the development of links with international and national groups working on this
topic. The participants have included:
The International Energy Agency (IEA) and the Geothermal Resources Council (GRC), who have developed a
working group and collaborated in the production of a white paper.
The Australian Earthquake Engineering Society (AEES)
Geoscience Australia (GA)
• Papers from the work presented herein have been accepted for publication and will be presented at both the GRC
Annual Meeting and at the AEES Annual Conference. This is an excellent opportunity for international peer
review.
• A flowchart describing earthquake risk assessment process usual for natural seismicity and that proposed for
induced seismicity is given in Module A. The newly proposed workflow encourages the study of the effects of
natural seismicity, not only on man-made built structures, but also natural large scale structural features such as
nearby basement faults, which may be prone to damage and reactivation under certain levels of enhanced
seismicity.
• Module B Data was collected for basement granite structures in the Cooper Basin and assessment of critically stressed
basement-offsetting faults in these areas. Analytical and numerical techniques were used to assess whether these
basement faults are critically stressed under the current tectonic stress regime.
• The region has a history of low level of seismic activity and is classified as a background area in terms of
earthquake risk. Damaging earthquakes are known to predominantly occur at depths of 5–10 km in seismically
active regions. The region has nationally low levels of seismic activity. Therefore the region is ideally suited to
HFR activities in terms of natural background seismic hazard.
• 107 faults were modelled analytically to distinguish any faults at risk of reactivation under the current in-situ
stress regime. It was found that assuming a Mohr-Coulomb strength relationship for the faults and a base friction
angle of 37°, that all faults have a mechanical safety factor F>1, tested under a series of extreme case scenarios.
The implication is that reactivation on any of the basement faults in this region is unlikely.
• 107 faults were modelled numerically and no slip occurred when applying the predicted base friction angle of 37°
for these faults with zero fault cohesion. This again suggests that reactivation of basement faults in the region is
unlikely. Unless the faults experience a direct static stress state change.
• There is concern about part of one fault, which could potentially experience a direct static stress state change. This
is part of the northerly Big Lake fault, which under the worst case scenario, Case 3 strike-slip stress regime, might
be close to limiting friction. This should be considered further in terms of possible developments in GEL99.
• Module C Assessment was made of the seismicity generated at the Geodynamics site at Innamincka to study the likely
level of seismicity and dynamic stress wave attenuation leading to ground vibration hazard.
• The events generated at a current test site, Geodynamics Limited Innamincka, fall below the background
coefficient of ground acceleration which is 0.05 g, thus not exceeding the government current building
design standards for peak ground accelerations. To summarise findings, the regional earthquake return period
is shortened by the HFR operations but there is no change in the risk to surface structures due to the low
magnitude and resultant ground acceleration of each event. Future and current EGS companies should be required
to report the Gutenburg-Richter relationship ie recurrence relationships throughout development and production of
the reservoir.
• Evidence of fracture growth during reservoir development to date in this region suggests that larger unexpected
earthquake magnitudes are unlikely due to the direct relationships observed between fluid injection rates and
seismicity.
• Toro’s intraplate formula (Toro et al. 1997) was used to calculate attenuation distance and it was shown that the
attenuation distance would not extend to natural structural features (i.e. faults) for either GEL97 or 98.
However, for the south eastern corner of GEL99 there is the Big Lake fault, which might overlap the
attenuation distance effect and could subsequently be influenced by associated stress changes.
• HFR events are generally of short duration and high frequency and this is the case for other HFR geothermal
fields, e.g. Rosemanowes UK where events are in the range 100–500 Hz. Gibson and McCue (2001) state that no
properly engineered structure should be affected by this type of event, also Figure 4.1 shows event attenuation
increases for these short high frequency events.
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•
Module D Assessment of static stress changes in the rock mass surrounding a HFR reservoir and the impact this
may have on local structures.
• A numerical model was built of the Geodynamics reservoir as an example scenario. A worst case static stress
scenario (most conservative) was assessed whereby the whole reservoir was treated as a failed rock mass with no
residual strength, i.e. a void. It was found that the extent of stress perturbation is localised to less than the width of
the reservoir itself extending to around 200 m beyond the edge of the reservoir. Therefore this static stress
damage zone would not be expected to have any impact on identified local structural features.
• It is planned to follow-up this study with some further modelling work to examine the effect of the differences in
the reservoir structure as there is some uncertainty relating to the reservoirs internal structure.
FUTURE HAZARD ASSESSMENT POLICY RECOMMENDATIONS
• Keep abreast of national and international efforts in this area through regular review of literature and attendance at
international meetings. In particular the annual international Geothermal Resource Council Meetings (GRC).
Introduce a “traffic light” seismic hazard control system for all current and future geothermal reservoir development
operations. Particularly during initial fluid injection phases. The idea of this system is that injection volumes should be
reduced if ground motion levels and events magnitudes are raised above a predetermined level. Incorporating observations
recently made at Basel in Switzerland as an add-on to this; if magnitudes remain high or even keep increasing at reduced
injection rates (“amber”) then the well should be allowed to flow rather than just shutting the well in, this will allow a
reduction in reservoir pressure and subsequent hydraulic energy (Baisch 2007). Also the seismic monitoring should start
well before the start of stimulation operations to get baseline information.
In the case of the Cooper Basin region where seismic recurrence relationships are at nationally low levels, a straight
forward strategy would be to use regional design factors (i.e. ground motion vibration design levels) as described in
Australian/New Zealand standard AS/NZS 4360: 2004 (Standards Australia/Standards New Zealand 2004) and maintain
these levels during all operations so that background values are not exceeded.
The traffic light system should be based on these background levels.
• It is usual for geothermal energy (either HFR or HDR) development that a seismic network is developed and runs only
during stimulation operations, whereby the system monitors seismicity generated during fracturing activity only. It is
advised that this system be maintained and run during the lifetime of the reservoir, i.e. during all production operations.
There is evidence from other HFR fields that seismicity can continue after shut-in following pumping operations.
• Furthermore this monitoring system should incorporate strong motion accelerometers placed at the HFR site, which
would be able to directly measure ground motion. The current work used previously derived attenuation relationships to
estimate the ground motion at the Geodynamics site. It would be preferable to have direct measurements of this activity.
The monitoring suggestions made above will not only allow reduction in the hazard associated with HFR operations, but
will provide data of use to the scientific community and the opportunity to enhance our understanding of rock/fluid
interactions and general mechanisms at work within the reservoir, which may benefit future operations.
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1.0 INTRODUCTION
The development of hot rock projects in South Australia has been in significant growth mode since 2003. There are
currently 12 companies holding geothermal energy (GE) leases, and 98 geothermal lease areas covering over 46 000 km2.
The current 5 year work programs correspond to A$515 million and this excludes demonstration and upscaling projects on
the road to commercialisation. The Commonwealth Government of Australia provides renewable energy initiatives as an
incentive to develop this emission-free energy source.
In light of the growth in this area, concerns have arisen relating to safety aspects of project development. This has lead to
an investigation of seismic hazard associated with geothermal energy production for the regions of interest. The purpose of
this project is to study the likelihood of enhanced seismic hazard in Australia’s Cooper Basin petroleum province as a result
of geothermal energy production in the region. The work is pursued in collaboration with the International Energy Agency
(IEA) who have officially stated, as part of a cooperative effort in addressing induced seismicity associated with enhancing
output of geothermal systems, that;
“Participants will pursue a collaborative effort to address an issue of significant concern to the acceptance of
geothermal energy in general but Enhanced Geothermal Systems (EGS) in particular. The issue is the occurrence
of significant seismic events in conjunction with EGS reservoir development or subsequent heat extraction.”
Risk assessment and its associated management is the most effective approach to addressing the impact of natural hazards
on a region. It combines the hazard level with the vulnerability of the local infrastructure to give a picture of the aggregated
financial consequences of a region’s hazard and how this can be reduced through mitigation strategies (Sinadinovski et al.
2005). This statement is equally applicable to man-made induced hazards such as those associated with the creation of a
geothermal reservoir or any induced fracturing activity in a rock mass.
It is usual for natural earthquake risk assessment to use the Australian/New Zealand standard, AS/NZS 4360: 2004
(Standards Australia/Standards New Zealand 2004). It can be expressed conceptually as a convolution of the following:
Risk = Hazard x Elements at Risk x Vulnerability (earthquake resistant design) of the elements at Risk
A typical risk assessment process for a city such as Perth would be as shown in Figure 1.1, where the column to the right
shows a typical risk assessment, that is, a workflow for naturally generated seismicity.
To investigate seismic hazard associated with the geothermal energy leases in South Australia, we first need to assess
potential seismic hazards and they can be classified as follows:
1.1 GENERAL DISCUSSION OF POTENTIAL SEISMIC HAZARDS
Seismic hazards having any reasonable likelihood of affecting the area of interest are as follows:
1) Surface rupture potential
Surface rupture can occur where earthquake magnitudes are large or where epicentres of fault failure are shallow. Surface
rupture is more likely to occur on ‘active’ faults which are defined as faults offsetting material less than 11 000 years old or
exhibiting significant seismic activity (Hart & Bryant 1997). In the Moomba/Big Lake area and other parts of the Cooper
Basin, faults do not extend to surface and cannot be classified as exhibiting significant seismicity, they therefore would not
be classified as active.
2) Ground shaking potential
Ground shaking in an earthquake depends on magnitude, distance from the fault and local geological conditions. The level
of shaking is controlled by the local geology and characterisation of this should be available from seismic data for the
region. The local geology of the Cooper Basin area is well understood as the region has been an oil and gas producing
region for over 40 years. Two important characteristics are ground softness at a site and total sediment thickness. Seismic
waves travel faster through hard rocks, and as waves pass from harder to softer rocks, the energy transfer can be from faster
to greater amplitude. Thus, strength of shaking is often greater in softer surface layers (Field et al. 2000) and by calculating
the energy content of the induced event and thus predicting the resulting ground displacement based on event propagation
through the basement and overlying sedimentation it is possible to quantify this assessment. Ground shaking can be
expressed quantitatively using peak horizontal ground acceleration and calculated from the seismic event most likely to
occur on a fault. Determining the likely displacement generated on a fault plane of interest can assess this.
3) Liquefaction potential
Loss of grain to grain contact can occur in a granular material as a result of increased pore pressure, which can lead to
sediment liquefaction. Seismic ground shaking is also capable of providing a mechanism for liquefaction, which can lead to
settlement of surface structures. Groundwater assessment and near surface sediment classification is important in
determining this risk potential.
4) Subsidence potential
Subsidence is the phenomenon where soils and other earth materials underlying a site settle or compress resulting in lower
ground surface elevation. A net decrease in the pore pressure of a formation will give rise to compaction of particles in a
granular material, but subsidence can be arrested by water injection into a reservoir.
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1.2 EARTHQUAKE HAZARD FOR NATURAL SEISMIC RISK ASSESSMENT
The following section is extracted from Sinadinovski et al. (2005). The earthquake hazard in a region can be described in
terms of the level of ground shaking that has a certain chance of being exceeded in a given period of time. It is common to
describe earthquake hazard in terms of the level of ground shaking that has a 10% chance of being exceeded in 50 years. In
order to calculate the earthquake hazard, three key models are needed, specifically;
1. a regional seismicity model, which describes the chance of an earthquake of a given magnitude occurring in a
year in various parts of the region;
2. an attenuation model, which describes generally how earthquake ground shaking (acceleration coefficient) or
intensity decreases with distance away from the earthquake source, and;
3. a site response model, which describes how the local regolith (soils sediments and weathered rock) will affect the
ground shaking experienced during an earthquake.
Building design standards (Standards Australia 1993) use earthquake design categories for all buildings and the earthquake
design depends on:
(i)
the structure classification
(ii)
the acceleration coefficient (obtained from maps based on points 1 and 2 above)
(iii)
the site factor (based on soil types and thickness).
1.3 EARTHQUAKE HAZARD FOR INDUCED SEISMIC RISK ASSESSMENT
In this instance, when an event is generated at a site some enhancements are required to the normal hazard assessment
procedure described above. The regional seismicity model should be adapted to account for the additional seismic hazard
and this can be done by adding the additional events to the regional seismicity model. Furthermore, work must be carried
out to assess the likelihood of induced events triggering further events due to local dynamic and static stress changes in the
local geology. Site locations for creating induced seismicity can then be selected to avoid these additional risk elements see
Figure 1.1. In other words there is an option to manage the risk through control of the hazard possibly without intervention
in any aspect of the building vulnerability (vulnerability is an assessment of the resistance of a building design to
earthquake damage); this has been discussed previously by Bommer et al. (2006).
To investigate the resultant seismic hazard potentially generated at the geothermal energy leases in this region, the project
research activities were divided into five categories/modules as follows:
Module A
Literature review: 1) Seismic hazard assessment at other HFR sites internationally and 2) general review of seismic events
caused by stress triggering.
Module B
The collection of basement granite structural data in Cooper Basin and assessment of critically stressed basement-offsetting
faults in these areas (from basement maps). The development of analytical and numerical techniques, to assess whether
local basement-cutting faults are critically stressed under the current tectonic stress regime.
Module C
The assessment of likelihood of microseismic activity, caused by the HFR fracturing of basement, which may trigger
seismic activity on local faults. This can be achieved by studying the likely magnitude of HFR generated events. This is
followed by analytical work to assess the effect of a seismic wave propagating within the local structural environment,
which can be achieved through evaluation of risk maps.
Module D
The development of numerical models to assess permanent impact of the developed reservoir structure on the local in-situ
stress field.
Module E
The development of finite difference models of well-bore and completion designs to assess the likelihood of damage
caused by a seismic wave hitting a well-bore at various depths. Module E is currently on hold until suitable data becomes
available.
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Hazard
Natural Seismicity Workflow
Induced Seismicity Workflow
Regional Seismicity Model
Induced Enhanced Seismicity Model
Attenuation Model
Enhanced Seismicity model cf Regional
Seismicity Model
Regolith Site Response Model
Elements
at Risk
Vulnerability
of the
elements at
risk
Risk
Identification of Natural Structures i.e.
faults at risk under dynamic and static
local stress changes
Building Inventory
Building Vulnerability Model
Assess Vulnerability of Faults (segments)
at Risk. Natural Vulnerability
Economic Loss Model
Earthquake Risk Assessment
Figure 1.1 Flowchart describing earthquake risk assessment process usual for natural seismicity and that proposed for induced
seismicity (after Sinadinovski et al. 2005).
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2.0 MODULE A
LITERATURE REVIEW OF SEISMIC HAZARD ASSESSMENT FOR OTHER
INTERNATIONAL HOT FRACTURED ROCK GEOTHERMAL ENERGY
SITES
2.1 PREVIOUS WORK
After a review of the international literature it was found that there are no publicly available reports on the hazard
assessment of induced seismicity for enhanced geothermal systems EGS systems or guidelines on how to assess and
maintain acceptable levels before 2005.
In 2005, Majer et al. described a collaborative international effort currently underway to understand and mitigate any
potential seismicity that may limit the advancement of the use of geothermal energy systems. As a result of this work Majer
(2005) published a white paper and his conclusions clearly state that ‘the majority of negative aspects seem to be associated
with the impact of seismicity on the surrounding community.’ This is important in the Cooper Basin where geothermal
exploration licences (GELs) and petroleum exploration licences (PELs) are co-located, i.e. overlap. Majer et al. (2005) go
on to state that ‘Other effects such as well failure due to subsidence, wellbore damage and damage of surface facilities are
minimal or has not significantly impacted upon the cost benefit ratio of the geothermal operations. In a number of
geothermal fields and potential geothermal fields in the U.S. the induced seismic activity or potential for seismicity seems
to be below the significant damage potential (less than magnitude 5.0)’. In the Cooper Basin subsurface and surface
facilities are close and this has been a point of concern for stake-holders. This cause for concern is assessed in the following
work. Majer (2005) continues, ‘This is for several fundamental reasons, 1. there are no faults close enough to create a large
damaging event, 2. if there are large faults nearby, then it is usually the case that large events are initiated at depth (5 to 10
kilometres), and most geothermal production and injection activities are shallower than 5 kilometres, thus making it
difficult to trigger a large event, 3. in many cases there may be no negative effects if the seismicity is small or that the
geothermal area is in a remote location’.
Bommer et al. (2006) have provided suggested means of controlling hazard due to seismicity induced by a hot fractured
rock geothermal project. They first identify the fact that in earthquake engineering the premise is that the hazard due to
earthquakes cannot be altered, this involves reducing exposure (through relocating) or the vulnerability (through earthquake
resistant design). This is not the case for induced seismicity where the option exists to manage the risk. Bommer et al.
(2006) have proposed an approach, which they describe as a traffic light system that enables real-time monitoring and
management of the induced seismic vibrations. They also state that in an area of high natural seismicity it might be unclear
whether a natural event or an induced event caused the damage. In a discussion of liability under US law for the effects of
induced seismicity, the following was stated, ‘seismicity induced by one source might accelerate failure of support
originating from another source, leaving both of the parties at fault proportionally liable to the injured party’.
Bommer et al. (2006) suggest that for real-time risk management of induced vibrations, the induced vibrations should be
analysed with as little delay as possible so that they can be compared with pre-established thresholds and used to guide
decision making. The second feature is that it must be possible to verify unambiguously the induced levels of motion. The
method for this is to use instrumental measures of ground motion with complimentary instrumental arrays. A seismic
monitoring system is suggested that calculates almost real-time hypocentres (event source location) and magnitudes from
which median estimates of peak ground velocity (PGV) at the surface can be obtained. A small network of accelerographs
was also installed at key locations in order to provide instrumental verification of the estimated PGV levels. The traffic
light system can be operated allowing those on site to determine simply and rapidly whether it is possible to proceed
(green), to invoke caution (amber), which may mean adjusting levels of operation (the hydraulic injection rate) or simply to
stop (red).
Although Bommer et al. (2006) evaluate in detail the possibility of risk management through the ‘traffic light system’.
They do not discuss the impact of site selection and possible induced stress changes giving rise to static or directly dynamic
induced stress changes on local geological features, which may themselves reactivate creating a larger induced event—this
effect is generally known as event ‘stress triggering’. This has been the case for many mining induced seismic events. In
this report considerable effort has been made to consider ‘stress triggering’ which may itself induce a damaging event. In
order to manage this risk it may be necessary to move the geothermal energy site away from any vulnerable geological
structures.
2.2 GENERAL REVIEW OF SEISMIC EVENTS CAUSED BY STRESS TRIGGERING
This section presents a methodology for predicting local rock-mass stress changes and evaluating risk associated with
developing a reservoir.
For many years seismologists have been attempting to predict earthquake aftershock locations. Recently they have used a
method described as prediction of the ‘Coulomb Static Stress Change’ in the earths crust resulting from permanent
displacement on a ‘natural’ earthquake fault plane (King et al. 1994; Toda et al. 1998). The Coulomb stress change
methodology is equally applicable to the earth stress changes generated by ‘induced’ seismic activity, such as those
produced during the fracturing process during development of the geothermal energy fractured reservoir.
This methodology will allow one aspect of the risk evaluation to be undertaken through assessing the vulnerability of local
natural structures to this stress change in the case of geothermal energy production to be assessed prior to reservoir
development. It may be used to define the spatial limit of the reservoir which can be assessed for planning purposes.
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The economic benefit of this proposed method is that risk associated with geothermal energy reservoir development can be
assessed. Currently fields are located far from population but this may not always be the case, as geothermal energy
becomes an, increasingly viable, ‘green’ energy source.
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3.0 MODULE B
DATA COLLECTION FOR BASEMENT GRANITE STRUCTURAL DATA IN
COOPER BASIN AND ASSESSMENT OF CRITICALLY STRESSED
BASEMENT-OFFSETTING FAULTS IN THESE AREAS (FROM BASEMENT
MAPS)
Use of analytical and numerical techniques to determine whether interpreted faults are critically stressed under present-day
stress conditions
Problem Statement
Assessment of seismic hazard associated with the co-location of the Moomba/Big Lake fields and an overlying GEL
(Figure 3.1) and an assessment of all GELs in the Cooper Basin. A general analysis of significant basement faults in the
Cooper Basin province has been undertaken in this section to determine likelihood of fault reactivation under stress
changes associated with geothermal operations.
3.1 INTRODUCTION – SLIP TENDENCY ANALYSIS FOR FAULTS IN THE COOPER
BASIN REGION BASEMENT ROCKS
Majer (2005) reviews aspects of seismicity and risk associated with geothermal energy production and relevant extracts are
taken from his report. Seismicity occurs at many different times and spatial scales. Creep on a fault (i.e. growth faults)
generates seismicity just as much as a sudden loss of cohesion. In this section we will discuss events that are sudden and
can generate ‘earthquakes’. The size of an earthquake (or how much energy is released) depends upon:
• how much stress there is on the fault before slipping
• how fast it fails
• and over how large an area it occurs.
Damaging earthquakes (usually greater than magnitude 4 or 5) require the surfaces to slip over relatively large areas
(kilometres).
Over millions of years of movement, the surface of the earth has deformed and become fractured and faulted. In some areas
where there has been consistent movement, large fault systems have formed. If the forces are still present then there is a
potential for earthquakes to occur. The slip along a fault does not have to occur in discrete or sudden jumps. Along some
parts of the San Andreas Fault it is creeping rather than jumping in a ‘stick-slip’ type of movement. This concept accounts
for the high level of seismicity in some areas and lower levels in others. Large or damaging earthquakes tend to occur on
well developed or active fault systems and rarely occur where there is not a fault large enough or long enough to release
enough energy. It is also difficult to create a large new fault, because there is usually a pre-existing fault that will slip first,
rather than a new fault created. It has been shown, that in almost all cases, large earthquakes start at great depth. It is only at
depth (5–10 km) where there can be enough stored energy to provide the adequate amount of force to move the large
volumes of rock.
A fault will slip (an earthquake) when the forces acting to cause slip are greater than the forces keeping the fault together.
The forces keeping it together are friction, the inherent strength of the rock and the pressure (from the surrounding rock). If
there are forces acting in a suitable direction to cause an earthquake, pore pressure will reduce the forces holding the fault
together and the surfaces will then slip, caused by the deviatoric stress field in the subsurface i.e. how much excess stress
there is available to cause an earthquake. An indication of how much excess force is available in the subsurface is the
amount of historical seismicity there has been in the area. In almost all cases of induced seismicity, there has not been any
induced seismicity (greater than magnitude 4.0) where there has not been historical seismicity of the same size or larger. In
other words it is difficult to induce seismicity of a large magnitude where the faults are small, the rocks are weak (large
amounts of energy can not be built up and energy is dissipated during minor slip events), and/or where there is not
sufficient energy (stress build up) to cause an earthquake.
In conclusion, several conditions must be met for a significant earthquake to occur (damaging events). There must be a
large enough fault system so that there is fault slip, there must be forces present to cause slip along the fault (as opposed to
some other direction) and these forces must be greater than the forces holding the fault together (the sum of the forces
perpendicular to the fault plus the strength of the material in the fault).
In order to assess the characteristics of the faults and forces in the region of interest, the mechanical behaviour of faults in
the region of interest was assessed as a part of the hazard assessment study.
3.2 NATURAL TECTONIC SEISMICITY IN THE COOPER BASIN REGION
A seismicity map of south eastern Australia is shown in Figures 3.2 and 3.3, but there are a number of methods for zoning
Australia in terms of seismic activity. These zoning methods are described by Gibson and McCue (2001). The most recent
method to be used is that described by Gaull et al. (1990) and this zoning method is used for the Australian standard AS
1170.4: 1993 (Standards Australia 1993). The zones are shown in Figure 3.4. Details relating to these zones are given in
Gaull et al. (1990) but generally they were chosen by the areal distribution of epicentres and by using relevant geological
and tectonic factors. Regions outside the source zones in Figure 3.4 exhibit sparse or little known seismic activity. This
region is treated as having ‘background seismicity’ and the level for each region is individually determined and normalized
to 10 000 km2. Regions ‘West’ and ‘East’ represent the regions west and east of 129° E longitude. Hence our region (the
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Cooper Basin region) falls into the background ‘East’ zone. This eastern zone is characterised by the parameters given in
Table 3.1.
Table 3.1 Characteristics of earthquake parameters used in risk calculations and sensitivity tests for Gaull et al. (1990) zone
‘east’.
Area (km2)
h (km)
MLmin
MLmax
b
Amin
σ
σ
East
10000
10
2.0
5.5
0.83
0.1*
0.14
0.2*
*Estimates only. Values in last column are the standard deviation of logAmin.
h=average focal depth; MLmin=lower bound Richter magnitude from completeness tests; MLmax=upper bound Richter magnitude (largest known ML for
source zone +0.5); b=constant (termed b-value), slope of Gutenburg and Richter (1954) magnitude-frequency relationship (logN=A-bML); Amin=number
of earthquakes per year with ML≥MLmin; intercept of equation at ML=MLmin σ=standard deviation of corresponding parameter in zone.
Zone
Figure 3.5 shows the map of the acceleration coefficients for South Australia and a description of how these are used is
given in Module C. The map shows that for the region of interest the acceleration coefficient ranges between 0.04–0.06 g.
Although natural seismicity in the Cooper Basin region is low (events are plotted on Figures 3.2, 3.3 and 3.6 and are given
in Table 3.2), it was felt necessary to perform an analysis of faults in the area to assess their likelihood of stress reactivation
given a possibly changed static or dynamic stress state incurred by basement fracturing activity. An analysis was made of
fault slip tendency (factor of safety) under the currently acting tectonic stress. Slip tendency calculations were undertaken
for the major faults identifiable from seismic data for the Cooper Basin for depths between 2.5 and 4 km. Two methods
were used, distinct element numerical modelling and an analytical procedure. The main difference between these two
methods is that the analytical method cannot take into account fault interaction and the faults are therefore treated
individually, whereas the numerical procedure allows the interaction of nearby faults to be taken into account.
Table 3.2 Seismic events historical record from the Earthquake database (2006), between latitude 26.5° S to 29.0° S and longitude
139° E to 142.5° E. Events marked in grey are outside the area of the regional map (Figure 3.6), the others (highlighted in blue)
are plotted, that is except for the Innamincka events (white) these are discussed in more detail in Module C.
DATE
28/12/1961
30/03/1963
31/03/1963
10/06/1979
30/01/1985
22/06/1988
08/08/1989
05/02/1997
06/03/1999
03/06/1999
27/02/2001
TIME (UTC)
07:34:35.6
12:40:27
00:25:45
00:33:23.9
20:41:29.6
06:31:33
06:54:23.4
00:15:14.6
23:54:40.9
15:53:43
01:02:06.4
LATITUDE (°)
-28.12
-27.2
-27.2
-28.027
-26.58
-26.706
-27.63
-28.803
-28.506
-27.128
-28.67
LONGITUDE (°)
141.57
140.9
140.9
140.336
140.94
140.28
141.52
139.117
139.072
140.8
142.082
DEPTH (km)
10
10
10
15
0
7
10
0
5
0
0
MAGNITUDE
4.7
3.8
4
2.9
3.1
3.5
3.3
4.3
4.1
2.6
3.3
COMMENTS
09/03/2001
16:10:01.7
-28.604
141.995
8
3.2
Cameron Corner area Qld
12/12/2001
11/10/2002
13/11/2003
02/12/2003
04/12/2003
05/12/2003
07/12/2003
07/12/2003
07/12/2003
08/12/2003
08/12/2003
08/12/2003
09/12/2003
21/12/2003
12/09/2005
12/09/2005
13/09/2005
00:43:25.97
07:09:33.13
14:03:27.7
14:00:24.2
01:55:45.32
17:45:38
01:21:42.14
08:03:03.06
18:31:42.3
07:09:32.06
12:42:12
12:50:07.9
06:58:41.37
21:15:11.7
16:04:04.5
16:36:58
03:21:04.6
-28.63
-28.007
-27.884
-27.846
-27.858
-27.776
-27.73
-27.741
-27.757
-27.892
-27.859
-27.883
-27.757
-27.862
-27.737
-27.744
-27.853
139.121
140.698
140.744
140.711
140.65
140.632
140.523
140.548
140.52
140.735
140.683
140.712
140.593
140.804
140.558
140.564
140.76
10
9.9
0
1
0
6.7
5
10
5
0
5.1
5
5
6.2
2
2
2.5
3.1
2.8
3
3.3
3.6
3.7
3.3
2.7
3
2.6
2.7
2.5
2.7
2.9
3
2.5
2.9
Etadunna Area SA
Innamincka SA.
Innamincka SA.
Innamincka SA. Felt.
Innamincka SA.
Innamincka SA. Felt.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
Innamincka SA.
SW Qld
BIRDSVILLE
Mid North SA
Near Mungeranie Homestead SA
Near Haddon Corner SA
Grey Mare Range near Cameron Corner Qld
3.3 BASEMENT FAULT INTERPRETATION
In order to evaluate the mechanical behaviour of large faults in the region, it was first necessary to obtain a reliable
basement fault map. Initially a contour depth to basement map was obtained (P&G, PIRSA 2006), which contained a series
of faults interpreted and overlain on the basements structure (Figure 3.6). Initially this was analysed and it became obvious
that a number of faults might be missing from this map, particularly in the Gidgealpa Ridge region. Further investigation
into previous reports provided a figure of the Pre-Permian structure of the Gidgealpa Ridge region from the South
Australian Oil and Gas Corporation (SAOGC) (cited in Boult 1996). This contained a series of additional faults which were
then also incorporated in the map (Figure 3.7). Figure 3.7 shows the additional Pre-Permian structure overlain on the
original basement map and the additional faults were then incorporated in the new model.
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Furthermore, additional basement fault data was located in the Warburton Basin GIS data atlas (2002), where a series of
basement images were evaluated. Deep tectonic faults were identified from this GIS data set and, as shown in Figures 3.9
and 3.10, from basement rocks modelled using field potential data, i.e. gravity and magnetic data (Figures 3.11a and 3.11b).
These additional faults were studied in some detail and as a result of closer inspection two additional faults were added to
the west of the Pondrinnie well site. All faults were assumed to be vertical. The final fault map used for the analysis is
shown in Figure 3.10.
3.4 STRESS DATA USED FOR FAULT MECHANICAL MODELLING
The regional stress state has been analysed previously by Reynolds et al. (2006) and this data is presented in Figure 3.12.
Overlain on this data set is an estimate of the stress state obtained from Geodynamics Limited (2006). Evidence at the
Geodynamics (Innamincka) site suggests the active stress regime is reverse-thrust in that region, but in other local areas
evidence suggests a strike-slip regime is present. In fact the stress regime within the Cooper-Eromanga Basin is unknown
to range from strike slip to reverse, hence upper and lower limit bounds are given on Figure 3.12. Of interest is evidence of
tectonic reverse faulting in the basement structure tectonic map, Figures 3.11a and 3.11b. A mean maximum principal
stress orientation (of 101° N) was applied to the model boundaries.
For the modelling work, three scenarios (Case 1, 2 and 3) are chosen for a mean depth of 4 km.
Table 3.3 Three principal stress regime cases used for the mechanical modelling analysis.
Case 1 Principal Stresses –strike-slip/reverse
[σ123]
σH (101°N)
σh
σ vert
MPa
MPa
MPa
128.0
60.0
60.0
Case 2 Principal Stresses-reverse
[σ123]
σH(101°N)
σh
σ vert
MPa
MPa
MPa
93.5
82.9
166.0
Case 3 Principal Stresses - extreme case strike-slip
[σ123]
σH(101°N)
σh
σ vert
MPa
MPa
MPa
166.0
62.0
93.61
3.5 ANALYTICAL MODELLING
Analytical modelling was used to determine the effective friction angle (or slip tendency) of fault segments. This was
achieved by determining the stress-shear relationships applied to the individual fault segments via matrix manipulation of
the principal stresses as shown in Angelier (1994). The slip tendency angles (Morris et al. 1996) were then calculated using
Eq. (3.1), where φe is the ‘equivalent friction angle’ (slip tendency) and τ and ν are the shear and normal stress vectors
acting on the fault plane respectively.
r
⎛τ
ϕe = tan ⎜⎜ r
⎝ν
−1
⎞
⎟
⎟
⎠
(3.1)
This allowed each fault segment to be ranked from low to high in terms of slip tendency or risk of reactivation, under the
current tectonic stress regime, acting at a mean depth of 4 km.
3.6 NUMERICAL MODELLING
The distinct element model (DEM) simulates fault-related deformation using the Coulomb slip model (UDEC Itasca) and in
this model, only elastic deformation (strain) can occur. At a constant magnitude of applied differential stress the resultant
elastic deformation (strain) is constant and governed by Young’s modulus. Elastic deformation is independent of the
Coulomb parameters below the frictional limit. Once the fault reaches its frictional limit/shear stress limit, as defined by the
strength envelope, shear displacement occurs along the length of the fault. In the model created for this study, a low friction
angle was applied to the faults to produce significant displacement, an elastic rock mass was applied which restricts any
fault propagation effects, but stress changes are imparted to the surrounding rock mass.
Previously in order to better understand the parameter sensitivity of the basin-wide DEM model, stress perturbations
around a single fault were modelled (Hunt & Boult 2005). In summary, it was observed during the calibration studies that
the output model result is highly dependant on the following parameters:
• the ratio k = σ1 / σ3,
• θ, the angle between the maximum principal stress σ1 and the fault strike
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• the fault friction angle
The properties required for description of the intact rock’s mechanical behaviour were: bulk modulus (K), shear modulus
(G) and density (ρ). The bulk modulus and shear modulus being obtained from the Young’s modulus and Poisson’s ratio.
The values used for this 2D model of the Cooper Basin, are given in Table 3.4. The current in-situ tectonic stress
orientation and magnitude (table 3.3) were applied to the boundaries of the model.
Table 3.4 Mechanical properties applied to the model.
Block Density, ρ (kg.m-3)
Bulk Modulus, K (GPa)
Block Shear Modulus, G (GPa)
2600
1.5
0.6
Rock Mass
Shear stress and displacement were calculated on each modelled fault. In total 107 basement faults were included in the
model. The faults were assumed to be vertical, (this is likely from basement seismic interpretation work) hence a plane
strain model was applicable.
3.7 RESULTS
3.7.1 Analytical modelling
The results of this analysis are shown in Figures 3.13, 3.14 and 3.15 for Case1, 2 and 3 respectively and none of the faults
analysed reached a slip tendency angle greater than approximately 28° degrees. This is shown clearly from Figures 3.15
and 16, where the slip tendency angle is produced as a frequency distribution for the fault segments analysed. Figure 3.16
shows the slip tendency value for varying fault orientations and these data suggest the worst fault orientation is between 62
and 75°N (26-39° from SigHmax) (cf 15 and 45°)(Pine 1986). If these faults are assessed in terms of factor of safety where
Capacity/Demand = ‘Factor of Safety’, F. If F<1 failure occurs. We can use relative friction angles to replace stress
magnitudes acting on the fault planes as τ/ ν=tan-1φ, then if slip-tendency equates to Demand. The Capacity is the actual
fault friction angle [Byerlee (1978) quotes between 30° to 45° for fault friction angle, and basic friction angles for joints in
granite have previously been measured at 37° (Pine 1986) which falls within the range of Byerlee (1978) suggested fault
friction angles]. We find that after calculating the slip tendency angle for all faults, a factor of safety greater than 1 is
found. This is hence a relative comparison of the likelihood of slip along each fault segment – we are clearly making an
estimate of the likely fault friction angle, here there is a level of uncertainty that can be reduced by sampling the fault plane
and performing laboratory based rock mechanics test, at present no data is available to do this.
A summary of data obtained from this analysis is given in Figures 3.16, and 3. 17 for stress regime Cases 1, 2 and 3.
3.7.2 Numerical modelling
A numerical model was run for the 107 faults interpreted in the region. The cases studies were both strike-slip or worst case
and the expected case. The reason that reverse was not included was because a reverse stress application is not possible for
a 2D plane strain model. The faults in the model were initially all given a low friction angle in order to assess the relative
slip behaviour of all faults, the numerical and analytical models match well, although there are some instances of variation
for the case scenarios analysed. The same faults appear to be higher risk for reactivation in both cases. Both models were
then run, for Case 1 and Case 2 scenarios. It was found that for a predicted fault friction angle of 37°N there is no fault slip
on any of the faults mapped, agreeing with the factor of safety predictions from the analytical models (see figures 3.18 and
3.19).
3.8 DISCUSSION
The GELs located in the central region of the basin adjacent to Innamincka and GELs 97 and 98 are not located near any
large basement structural faults (Figure 3.10). But there are two fault segments north of Napacoongee, that if low friction
angles did occur, slip may be possible. But neither of these segments is at the fault tip, where fracture propagation is likely
to occur and they are of limited length suggesting that if slip did occur along these segments the energy release would be
relatively low.
GEL99, which is co-located with the Moomba and Big Lake fields, overlaps a portion of the Big Lake fault (Figure 3.1)
and sensitivity analysis on this fault shows that if it had a low fault friction angle, then the fault may be at a higher risk of
slip. Previous modelling work at Big Lake, where the ISIP data was compared with the Shmin magnitude (Hunt 2000),
suggested that the Big Lake fault had a friction angle of 25° and for the expected strike-slip Case 1, this is acceptable. For
the worst case scenario this friction angle is below the slip tendency value, i.e. giving a factor of safety less than 1. This
should be considered carefully in terms of the location of GEL99 and a considered effort made in understanding the
mechanical behaviour of the Big Lake faults. If possible core through these faults should be analysed to determine actual
base fault friction angles. It is known (Dieterich et al. 1972) that the frictional strength of a fault can vary throughout a fault
area, leading to shear stress concentrations, which can become relieved by local friction when the local static friction is
overcome i.e. there are local variations in frictional strength and stress due to fault surface irregularities. This would have to
be assessed at a smaller scale i.e. through detailed 3D seismic analysis for each fault plane.
Further evidence of friction angle parameters for these basement faults would be useful as it is possible that lower friction
angles exist here. However even without this evidence, Figures 3.12, 3.13 and 3.15 clearly show the relative susceptibility
of faults and fault segments to reactivation. Thus the extent of any stress changes associated with geothermal activities can
be made relative to these locations. It is important to remember that the region has been operating as a significant
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petroleum province for some time as shown by the wellbores indicated on Figure 3.1 and little seismicity has been
generated from production of hydrocarbons in the region.
There is also a possibility that additional unidentified faults occur in the basement within the study area and have not been
included in the modelling work. Faults of significant interest would be those in the basement of the Moomba region and
those associated with the Geodynamics project location.
3.9 CONCLUSIONS
• The region has a history of low level of seismic activity and is classified as a background area in terms of earthquake
risk. Damaging earthquakes are known to predominantly occur at depths of 5–10 km in seismically active regions.
• 107 faults were analytically modelled to distinguish any faults at risk of reactivation under the current in-situ stress
regime. It was found, when assuming a Mohr-Coulomb strength relationship for the faults and a base friction angle of
37°, that all faults have a mechanical safety factor F>1, implying that reactivation on any of the basement faults in this
region is very unlikely.
• 107 faults were numerically modelled and no slip occurred when applying the predicted base friction angle for these
faults of 37° with zero fault cohesion. This again suggests that reactivation of basement faults in the region is very
unlikely. Unless the faults experience an anthropomorphic direct static stress state change.
• One fault of some concern which could potentially experience a direct static stress state change is the northerly Big
Lake fault, which under the worst case scenario of Case 3 the strike-slip stress regime, portions of this fault might be
close to limiting friction. This should be considered further in terms of possible developments in GEL99.
Big Lake Field
Big Lake Fault
Figure 3.1 Co-location of Moomba Field and Geothermal Energy Lease 99.
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Figure 3.2 Map of earthquakes locations and magnitudes in South Eastern region of Australia (Earthquake database 2006);
colour scale given below.
Figure 3.3 Seismicity for north eastern South Australia, note the low level of activity in between latitude 139–142.5 and
longitude 26.5–28.5, the Cooper Basin province (Earthquake database 2006).
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Figure 3.4 Gaull, Micheal-Leiba and Rynn seismic risk zones, the numbers represent the
identification system for each zone (Gaull et al. 1990).
Figure 3.5 Acceleration Coefficient map of South Australia, gives the acceleration
coefficient with a 10% chance of being exceeded in 50 years (Standards Australia 1993).
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Figure 3.6 Natural seismic events recorded in Cooper Basin region (after P&G, PIRSA 2006).
Figure 3.7 Fault map for basement in the Cooper Basin (P&G, PIRSA 2006), the insert is from SAOGC (cited in
Boult 1996) where additional fault data is interpreted for the Gidgealpa region.
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Figure 3.8 GIS Map through region from the Warburton Basin GIS data atlas (2002).
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Figure 3.9 Composite GIS map showing depth to basement and interpreted tectonic faults from magnetic and gravity interpretations (Warburton Basin GIS data atlas 2002)
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Figure 3.10 Final fault map used for both analytical and numerical models (after P&G, PIRSA 2006).
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Figure 3.11a and 3.11b Basement structure map (Warburton Basin GIS data atlas 2002).
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Stress Magntudes with Depth - Cooper Eromanga Basin
Stress (M Pa)
0
50
100
Shmax (UB)
150
200
Sigv (LB)
0
Shmin (UB)
Shmin (LB)
1
Shmax (LB)
y = 0.0427x
Sigv (UB)
Doone Shmin
2
Depth (km)
Donne Sv
Doone SHmax
3
Model Smax
Model Sh
Model Sv
4
Linear (Sigv
(UB))
Linear (Sigv
(LB))
5
y = 0.0481x
6
Figure 3.12 Mean stress magntiude with depth plot as for the Cooper–Eromanga Basin (Reynolds et al. 2006; Geodynamics
Limited 2006).
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Figure 3.13 Slip tendency values for faults in the Cooper–Eromanga Basin basement faults. Stress regime expected strike-slip; Case 1 (after P&G, PIRSA 2006).
Fault segment
slip-tendency
angle
(degrees)
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Figure 3.14 Slip tendency values for faults in the Cooper–Eromanga Basin basement faults. Stress regime reverse; Case 2 (after P&G, PIRSA 2006).
Fault segment
slip-tendency
angle
(degrees)
Figure 3.15 Slip tendency values for faults in the Cooper–Eromanga Basin basement faults. Stress regime greatest deviatoric i.e. worst case strike-slip; Case 3 (after P&G, PIRSA 2006).
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Frequensy Distribution of Slip Tendency For Interpreted Faults in the Region
160.000
140.000
Nu m b er o f fau lts
120.000
100.000
Case 1(Strike-slip)
80.000
Case 2 (Reverse Regime)
Case 3 (Worst cse strike-slip)
60.000
40.000
20.000
0.000
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Slip Tendency Angle
Figure 3.16 Frequency distribution for slip tendency angle, applying three stress regimes. Case 1 Strike-slip (expected), Case 2
(Reverse), Case 3 Strike-slip (worst case scenario).
Slip Tendency Angle Against Fault Orientation
30
S lip Tendency Angle (deg)
25
20
Case 1
Case 2 Reverse
15
Case 3
10
5
0
0
20
40
60
80
100
120
140
160
180
200
Orientation from North for all interpreted faults in the region (deg)
Figure 3.17 Slip tendency angle against fault segment orientation from north, suggesting most likely slip on fault segments
oriented between approximately 60–80°N.
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Figure 3.18 Case 1 strike-slip stress regime. Expected stress state applied as boundary conditions to the model. Thickness of blue line is 145.7 m. Low friction angle 5° (after P&G, PIRSA 2006).
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Figure 3.19 Case 1 strike-slip stress regime. Expected stress state applied as boundary conditions to the model. Thickness of blue line is 256.1 m. Low friction angle used 5° (after P&G, PIRSA
2006).
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4.0 MODULE C
HFR SITE INDUCED SEISMICITY AND HAZARD TO LOCAL
GEOLOGICAL AND BUILT STRUCTURES
This module can be achieved by studying the likely magnitude of HFR generated events, followed by analytical work to
assess the effect of a generated seismic wave propagating within the local structural environment.
4.1 PROBLEM STATEMENT
In this module the effect of site induced seismicity is discussed in terms of hazard and possible impact on nearby structures,
i.e. elements at risk. This is performed through an evaluation of dynamic stress changes, i.e. seismic event propagation and
attenuation during reservoir development.
4.2 INTRODUCTION
Gibson and McCue (2001) discuss earthquake hazard. They state ground vibration hazard depends on amplitude, frequency
content and duration:
• The amplitude is affected by magnitude and distance, represented by an attenuation function. The amplitude reduces
with distance by geometric spreading, by absorption of energy, and by scattering.
• The frequency content depends firstly on initial frequency magnitude and the fact that the high frequency motion is
attenuated more quickly with distance than low frequency motion.
• The duration depends mainly on the magnitude, with the strong motion from earthquakes less than magnitude 5 lasting
less than a second.
Ground vibration can be represented:
•
In the time domain by acceleration (m/s2 or g), velocity (mm/s), displacement (m) or by their peak values PGA, PGV
and PGD.
•
In the frequency domain by a Fourier spectrum or response spectrum.
•
As a simple number of intensity determined empirically (Modified Mercalli) or computed from the time series and/or
spectrum of the motion, such as the Arias intensity.
The time domain series is used for ground vibration predictions in this study. Figures 4.1 and 4.2 give an example of how
these measurements can be used. Figure 4.1 shows the effect of the event frequency on acceleration (g) for different
magnitude events measured at 20 km distance. Also Figure 4.2 shows how the frequency can affect human perceptibility of
a seismic event. In Figure 4.3 the influence of PGV (peak particle velocity) on building damage is shown as a series of
fragility curves (damage curves) for sun-dried earthen bricks, D1, D2 etc represent the damage level, D1 being the lowest
(i.e. no damage) and 5 the highest (i.e. complete damage). Table 4.1 shows the proposed ‘traffic light’ thresholds based on
PGV used by Bommer et al. (2006) at the geothermal field in eastern El Salvador, Central America.
4.3 EFFECT OF GEOTHERMAL ACTIVITY ON THE PROBABILISTIC EARTHQUAKE
RISK MAPS FOR THE COOPER BASIN REGION
Earthquake risk maps for Australia have been developed previously by seismologists at Geoscience Australia in Canberra.
As a result, Australian standards have been produced for minimum design loads on structures for earthquake loads
(Standards Australia 1993). The earthquake design category depends on (i) the structure classification (ii) the acceleration
coefficient for the site location and (iii) the site factor. The acceleration coefficient is obtained from the earthquake hazard
maps which give acceleration coefficient (a) for a 10% chance of exceeding this once in 50 years.
The region of interest shown in Figure 4.1 has a low acceleration coefficient of 0.05 g. If we assume this is natural
background seismicity, then an analysis of seismic hazard post-geothermal reservoir development would give an indication
of the additional seismic hazard generated by this induced activity.
4.4 BACKGROUND SEISMICITY FOR THE COOPER BASIN GEL SITES AND
RECURRENCE RELATIONSHIPS AT THE SITE OF INTEREST
Background seismicity for the Cooper Basin is discussed in section 3.2. Figure 3.5 shows that for the region of interest the
acceleration coefficient ranges between 0.04–0.06 g, which equates to 0.4–0.6 m/s2 or in PGV a 20–40 mm/s which is 2–4
cm/s or on the MM intensity scale a value of 5. A Gutenburg-Richter relationship is also shown in Figure 4.5, this is the
frequency magnitude relationship for the background east region of Australia as defined by Gaull et al. (1990).
This relationship defines the level of earthquake activity in a region i.e. less than 1 magnitude 2 ML event occurring every
year in this region.
In order to assess the impact of reservoir development the induced activity should be compared against these background
levels. All building codes in this region, which are based on these, are above design specification for background levels and
so it is wise to assess whether the events generated at the sites of interest are significant. To date the only site where
reservoir development has occurred is the Geodynamic Limited site within GEL98.
After some discussion with staff from Geoscience Australia (pers comm. C. Sinadinvski, P Cummins and J. Schneider), a
decision was made as to the best way forward in terms of evaluating induced HFR seismic hazard. Figure 4.4 shows the
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initial proposed workflow for assessing events generated from the Geodynamics site. Referring to Figure 4.4 the initial
proposed workflow was to assess all events of magnitude greater than a magnitude of 2.5, and then discuss how this would
change the risk map for the background ‘East’ region. A later study, which was proposed, was to look at all events
incorporating those of less than magnitude of 2.5 and produce a Gutenburg – Richter relationship for the HFR site that
could be compared directly with the background relationship, in general performing a more detailed analysis of all
generated seismicity.
4.4 GEODYNAMICS LTD EVENT DESCRIPTION (GEL98)
A comparison was made of those events generated at the nearest PIRSA seismological station and those recorded by
Geoscience Australia as well as those given to us as records from the Geodynamics Limited seismic array (Wyborn et al.,
2004). The magnitude of events that the recording system detected range to below a value of -2. Several events were
around +3, but 98% were in the range -2 to 1.
4.4.1 Geoscience Australia, PIRSA event catalogues and Geodynamics event catalogue data
collection and reporting
A record of events greater than a magnitude of 2.5 is shown in Table 4.2. Event locations in plan are shown in Figure 4.6,
and the frequency and magnitude relationships are also given in Figure 4.6 extracted from Asanuma et al. (2005) during
these LFT- long term flow tests a total of 20 000 m3 of water was injected with the highest pump rate of 48 l/s. Figure 4.6b
shows a more recent interpretation of event locations (Baisch et al 2006) using a different velocity model. The
accumulative histogram represents the Gutenburg-Richter relationship i.e. number of events (on a log-scale) versus
magnitude, a straight line is fitted to the data Asanuma et al. (2005) identified the inflection point in this data at low
magnitudes.The event magnitudes larger than 1.7 are missing from this figure as they could not be recorded by the local
recording system, but the higher magnitude events are given in Table 4.2. These two datasets should be combined for
generic studies and an accurate relationship obtained but for the purposes of this study it is clear that the recurrence
relationship is well above background, as background rates is so small.
4.5 RESULTS FOR SEISMIC RISK DETERMINATION
This was performed for seismic event data generated by the development of the reservoir in GEL98 in the Innamincka
region by Geodynamics Limited from November 2003 to September 2005. The events were analysed and the following
findings were made — the region of Innamincka does not fall into any of the Gaull et al. (1990) zones, so it is treated as
part of a large background zone with pre-defined seismicity in which the biggest seismic event of magnitude between 3.5
and 4.0, normalised to 1x1 degree area, has probability of occurrence of once every 100 to 167 years. The region of
Innamincka falls into the Eromanga zone according to Brown and Gibson (2000) model AUS5 in which the biggest
seismic event of magnitude between 3.5 and 4.0, normalised to 1x1 degree area, has probability of occurrence of once
every 50 years. According to the Earthquake database (2006), the latest event occurring in the study area prior to 2003–05
sequence was in 1979 with magnitude 2.9. Thus as stated in the current Australian standard AS 1170.4: 1993 this region
has a 10% chance in 50 years of experiencing a peak ground acceleration of 0.05 g (Standards Australia 1993). If Toro's
intra-plate formula (Toro et al. 1997) is used, values of 0.041 g are calculated for ground acceleration at the epicentre of the
expected largest induced Innamincka event. These do not exceed the current design standard for peak ground accelerations.
The risk analysis process for earthquake recurrence relies on the assumption that earthquake occurrences are independent
and that the earthquake process is stationary in time. Therefore because aftershocks and foreshocks are not independent
events these are removed from the dataset. This concept has been applied to the Geodynamics data and the activity treated
as a single event.
4.6 ATTENUATION EFFECTS
Toro’s intraplate formula (Toro et al. 1997) was used to calculate attenuation distances. This attenuation model was
developed for central and eastern North America, a region of the world thought to have similar attenuation characteristics
as proposed in AS 1170.4: 1993 (Standards Australia 1993). The Toro intraplate formula (Toro et al. 1997) was used to
produce Figure 4.8 which shows the calculation of attenuation distance for peak ground acceleration (in units of g)
generated at Geodynamics site. The map in Figure 4.9 shows the overlap of this attenuation effect with nearby structures
such as the abandoned wells. The extent of the attenuation effect using this measure does not extend to reach any natural
structures.
It is noted that acceleration attenuation is 10 to 100 times greater in soft rocks and soils than in crystalline rocks. So event
propagation through the basement will not attenuate as fast, but counteracting this are the nature of the HFR events
generally high frequency which will have a greater attenuation.
4.7 CONCLUSIONS
• When comparing the frequency and magnitude event relationships generated at the Geodynamics site against
background seismicity for the ‘Eastern’ Gaull et al. (1990) background zone, the geothermal energy site is shown to be
above that observed for the background where only 0.14 earthquakes occur per year with ML>2.0 (Figure 4.5). At the
Geodynamics site events occurred in rapid succession during reservoir development and have reached magnitudes of up
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•
•
•
•
•
to 3.7 (GA magnitude). So certainly events generated at the site are above the frequency magnitude relationship for the
‘East’ background region.
When assessing likely damage in the region the events generated at the Geodynamics site fall below the background
coefficient of acceleration of 0.05 g specified by the Australian standard, AS 1170.4: 1993 (Standards Australia 1993),
thus not exceeding current design standard for peak ground accelerations.
Thus to summarise the above two points; the earthquake return period is shortened by the events occurring at the
Geodynamics site (i.e. earthquakes are occurring more frequently) but there is no change in the risk to surface
structures.
In terms of the rock mechanics at the site and the likelihood of further large earthquakes, there is no evidence of
continued event propagation and uncontrolled fracture growth post injection. Therefore earthquake magnitudes are
unlikely to rise. Growth of the reservoir is in an approximately horizontal direction with propagation occurring around
the reservoir perimeter during injection and fingering is followed by infilling, which suggests a controlled growth
pattern. Thus the evidence so far suggests that larger earthquake magnitudes are unlikely.
Toro’s intraplate formula (Toro et al. 1997) was used to calculate attenuation distances and it was shown that these
would not extend to natural structural features i.e. faults for either GELs 97 or 98. However, in GEL99 the Big Lake
fault might overlap the attenuation distance effect and could be influenced by associated stress changes.
The Geodynamics events are of short duration and high frequency and this is the case in general for HFR geothermal
fields e.g. Rosemanowes UK where events have a frequency of 100–500 Hz (Appendix B). Gibson and McCue (2001)
state that no properly engineered structure should be affected by this type of event, also Figure 4.1 shows an increase in
attenuation for the higher frequency events. Frequency is accounted for in the Toro relationship (Toro et al. 1997) and
reached a value of 35 Hz. The geothermal events at Habanero varied in range from 1-100Hz as given by the event
spectral analysis, attenuation was obtained from the Toro relationship based on this frequency range.
Figure 4.1 Effect of frequency on acceleration (g) for different
magnitude events measured at 20 km distance. (Sadigh et al. 1997).
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Figure 4.2 Recommended levels of human sensitivity
to vibration due to blasting (USACE 1972).
Report Book 2006/16
Figure 4.3 Fragility curves for adobe (sun-dried earthen bricks) dwellings (Bommer et al. 2006)
D1=damage level 1 i.e. no damage.
Preliminary proposal for ‘traffic light’ thresholds
PGV (cm s-1)
Description of response
0.10
0.65
1.30
3.00
6.00
12.00
Just perceptible (weak shaking, no damage)
Clearly perceptible (light shaking, no damage)
Disturbing (moderate shaking, very light damage)
Frightening (strong shaking, light damage)
Alarming (strong shaking, damage in weak structures)
Damaging (severe shaking)
Table 4.1 Proposal from Bommer et al. (2006) for ‘traffic light’ thresholds. These were later applied to PGV equivalent
magnitude recurrence relationships to produce traffic light boundaries (PGV 0.1–2.0 green; 2.0–3.0 amber; 3.0–12.0 red).
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Assessment of the likelihood of microseismic activity, due to HDR fracturing of basement, setting
off earthquakes on faults in the study area. This can be achieved by studying the likely magnitude
of HDR generated events. This will be followed by analytical work to assess the effect of a seismic
wave propagating within the local structural environment?
Max mag G-R- attenuation function for zones
Stage 1 Recreate maps with current
GA recording for Innamincka.
Collect and check all Innamincka
seismic data
Risk Map 2003 – present
from GA website
New risk maps
incorporating
Geodynamics data,
>2.5 mag events
Action: PIRSA seismologist discuss with
GA seismologist
Part of geodynamics dataset obtained from
Geodynamics Limited (2006) >2.5 Mag events
PIRSA report and Geodynamics
Post PIRSA report
New map incorporating
complete Geodynamics
dataset
<2.5 mag events
Stage 2 Recreate maps with current
GA recording for Innamincka.
Collect and check all Innamincka
seismic data
Action: Obtain geodynamics dataset
International
Linkage Proposal
Figure 4.4 Initial proposed workflow for analysis of seismic activity at geothermal energy sites.
Richter Magnitude ML
2
2.5
3
3.5
4
4.5
5
5.5
Cumulative Number of Earthquakes per year
N/yr
1
0.1
0.01
0.001
Figure 4.5 Least squares fit given by average recurrence
relationship parameters for Gaull et al. (1990) background
region east.
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Figure 4.6a Plan view of all seismic events associated with the fracture initiation phase of the Geodynamics project in yellow,
main stimulation in red and second stimulation in blue. Habanero 1 is located at the centre at co-ordinates 0,0, scale in metres
(Wyborn et al. 2004).
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Figure 4.6b Absolute hypocenter locations in map view. Grading scales denotes origin time. Co-ordinates are given with
respect to the top of Hananero#1.( Baisch et al 2006).
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Figure 4.7 Histogram of the estimated moment magnitude (Mw) plot for the events at Geodynamics site (Asanuma et al. 2005).
LFT=long-term flow tests. The accumulative histogram represents the Gutenbeg-Richter relationship i.e. number of events (on a
log-scale) versus moment magnitude (Mw), a straight line is fitted to the data Asanuma et al. (2005) identified the inflection point
in this data at low magnitudes.
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20
15
10
5
0
-5
-10
-15
-20
-20
-15
-10
-5
0
5
10
15
20
Figure 4.8 Toro relationship (Toro et al. 1997) calculation of attenuation distance (km) of peak ground acceleration (g) generated
at Geodynamics site.
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Table 4.2 All events greater than a Richter magnitude generated at the Geodynamics site (Earthquake database 2006; Geodynamics Limited 2006).
SHunt comments
Ok
missing from GA and Geodynamics low mag but >2.5
DATE
TIME
UDC
UTC
GA
Magnitude
UT time
polarity
magnitude
3:11:14
0:03:31
13-Nov
14:03
0:04:42
13-Nov
14:04
2.9
3:11:14
0:06:33
13-Nov
14:06
2.5
3:12:01
19:29:44
1-Dec
9:29
3.3
13/11/2003
ML
ARK
3:11:14
Ok
3.0
Date
MD
ARK
2.8
1/12/2003
0930
down
2.6
2/12/2003
1400
down
3.0
3/12/2003
1835
2.2
3
missing from GA &Geodynamics - low mag ,2.3
3:12:04
4:35:00
3-Dec
18:35
earlier than GA event
3:12:04
11:55:43
4-Dec
1:55
3.6
4/12/2003
0155
up
3.0
3
Ok
3:12:06
3:45:42
5-Dec
17:45
3.7
5/12/2003
1745
down
3.0
3.1
down
3.0
Ok timing different
3:12:07
11:21:47
7-Dec
1:21
3.3
7/12/2003
0122
Ok
3:12:07
18:03:04
7-Dec
8:03
2.7
7/12/2003
0803
3.0
7/12/2003
1832
Ok
Ok
3:12:08
17:09:32
8-Dec
7:09
Ok
2.3
2.6
2.3
1.6
2.7
2.6
2.6
2.3
down
2.9
2.9
2.6
2.4
2.6
8/12/2003
0709
down
2.6
2.6
2.7
8/12/2003
1242
down
2.6
2.6
2.4
Ok
3:12:08
22:50:22
8-Dec
12:50
2.5
8/12/2003
1250
down
2.6
2.6
2.4
Ok
3:12:09
16:58:45
9-Dec
6:58
2.7
9/12/2003
0658
down
2.7
2.7
2.4
Ok GA only
3:12:22
7:15:15
21-Dec
21:15
2.9
21/12/2003
2115
TIME=Local time; UDC=Universal date; UTC=Universal time; GA magnitude=Geoscience Australia magnitude; ARK polarity Arkaroola polarity; PIRSA magnitude; ARK Richter magnitude; ARK duration
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20
15
10
5
0
-5
Habanero -1
Geodynamics well
-10
-15
-20
-2 0
-1 5
- 10
-5
0
5
10
15
20
20km
Figure 4.9 Map of basement in the Cooper Eromanga basin showing well locations. The inset bullet figure shows the attenuation radiation distance from
Habanero 1 at the centre of the Geodynamics site (after Warburton Basin GIS data atlas 2002).
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5.0 MODULE D
DEVELOPMENT OF NUMERICAL MODELS TO ASSESS PERMANENT
IMPACT OF THE DEVELOPED RESERVOIR STRUCTURE ON THE LOCAL
IN-SITU STRESS FIELD
5.1 PROBLEM STATEMENT
To assess the static stress change induced in the surrounding rock mass caused by the developed HFR reservoir.
5.2 INTRODUCTION
The distinct element method (DEM) was used to build a geomechanical model of the Geodynamics field in order to assess
changing field stress conditions generated by the reservoir.
The authors are current research leaders in the application of this code and have developed a workflow for petroleum seal
risking and hydrocarbon fluid migration potential. A model has been produced for the Big Lake region of the Cooper Basin
which demonstrates a stress low (high fluid migration potential region) which corresponds to the Big Lake sweet-spot and
instantaneous pressure test data (ISIP equates to σmin) for the field (Figure 5.1).
Figure 5.1 3D DEM model and minimum stress data obtained from stress model (Camac & Hunt 2004).
5.3 STATIC STRESS CHANGES
It is known that static stress changes which occur after a natural earthquake can give rise to local aftershocks in the
surrounding region and that the study of static stress change processes has lead to greater understanding of earthquake
trigger mechanisms (King et al. 1994; Toda et al. 1998). Earthquake triggering resulting from geothermal energy
production could be of concern to the community, and so the extent of the change in expected static stress should be
analysed. In order to do this a numerical approach has been used. It is also suggested that the method of analysing static
stress changes is equally applicable for induced activity around three dimensional structures such as a geothermal reservoir,
where damage is induced within the reservoir region. The example given below is for the reservoir generated by
Geodynamics Limited, Wyborn et al., (2004). Here the reservoir is defined with a numerical block volume and a distinct
element model is used (Itasca). The reservoir shape is defined initially in a rudimentary manner to make a first pass
assessment of the model applicability (Figure 5.2).
5.4 MODEL DEVELOPMENT
The model was constructed using a volume as shown in Figure 5.2. The structure for the first pass study was assumed to be
planar with an extent as given in Wyborn et al. (2004). In-situ stress conditions were set for the volume using results from
Figure 5.3, where the green trend line was used for the in-situ stress conditions i.e. a reverse-thrust stress regime, assuming
a linear gradient with depth. The rock mass surrounding the reservoir was treated as an elastic medium as this is a first pass
model and in a continued study the rock mass will be defined as a Mohr-Coulomb material as in the stress triggering work
described previously. Two models were developed using this set-up, the first model (A) used a void to represent the
reservoir and this was assumed to be a worst case scenario, whereby the rock mass defined by the assumed volume was no
longer capable of sustaining any load therefore modelled as a void. For the second model (B) the volume was defined with
a bilinear ubiquitous joint model, making both the intact rock mass and the contained jointing weak, i.e. low friction
parameters (Figure 5.4).
The stress perturbation effect is shown in Figures 5.5 and 5.6 for model A and model B respectively. Model A shows that a
rise in the in-situ stress state around the reservoir is limited to regions between the fingered extensions of the reservoir
volume, as stress perturbation is generated in this area. The extent of the stress perturbation is localised and it appears the
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static stress change extent is significantly less than the width of the reservoir itself (approximately 200 m) away from the
edge of the reservoir.
Model B shows little to no perturbation in the rock mass surrounding the reservoir. However, a significant stress change is
observed within the volume and the highs have a NE–SW trend reflecting the propagation direction of the reservoir as it
has continued to be pressurised.
Figures 5.7 and 5.8 are cross sections (E–W) through the reservoir. Here stress variation is plotted as a tensor, highlighted
for σ1 which is the maximum principal stress. For Model A, curvature is concave downwards where the stress field is
perturbed by the opening. Very little effect is evident for Model B.
Further work is continuing using this technique to assess static stress changes for a Mohr-Coloumb material in the
surrounding rock mass, varying the constitutive behaviour of the reservoir rock mass and investigating effects of
uncertainty on the applied stress state. This modelling work suggests the extent of static stress changes is minimal and
unless a fault is located close to the field stress triggering is unlikely.
5.5 CONCLUSIONS
The conclusions to date for this module are given as bullet points below:
• For the worst case static stress change scenario generated by the Geodynamics reservoir,. the extent of the stress
perturbation is localised to within 200 m beyond the edge of the reservoir, which is significantly less than the width of
the reservoir itself. Further work is being undertaken to assess this by reassessing the structure of the reservoir as there
is some controversy over the exact shape and structure of the field.
• It is planned to follow-up this study with further modelling work to examine the effect of the differences in the reservoir
structure and how these variations would effect static stress changes.
Figure 5.2 Dimensions of the modelled region, inset figure in blue shows final simplified model form.
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Shmax (UB)
Stress Magntudes with Depth - Cooper Eromanga Basin
Sigv (LB)
Stress (MPa)
0
50
100
150
Shmin (UB)
0
Shmin (LB)
1
Shmax (LB)
y = 0.0427x
2
D ep th (km )
200
Doone
Shmin
Donne Sv
3
4
5
6
Sigv (UB)
Doone
SHmax
Model Smax
y = 0.0481x
min vert
Model Sh
Vert, Sh, SH
Model Sv
SHmax orientation
Figure 5.3 Principal stress variation with depth for the Cooper Basin, values indicated are those used for numerical modelling
analysis.
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z
y
x
Figure 5.4 Modelled volume dimensions shown in 3DEC (volume x (E–W) = 6 km, y (N–S) = 8 km, z (depth) = 6 km).
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z
y
x
Figure 5.5 Static stress change associated with reservoir development Model A – reservoir defined as a void (volume
x (E–W) = 6 km, y (N–S) = 8 km, z (depth) = 6 km).
z
y
x
Figure 5.6 Mean stress plotted at reservoir depth using Model B – ubiquitous joint model (volume x (E–W) = 6 km, y
(N–S) = 8 km, z (depth) = 6 km).
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Figure 5.7 Stress tensors coloured for Sig 1 Model A – Void section E–W (width 6 km).
Figure 5.8 Stress tensors coloured for Sig 1 Model B – Ubiquitous joint model section E–W (width 6 km).
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6.0 PROPOSED FUTURE HAZARD MANAGEMENT
Generally conclusions are drawn at the end of each section, but a discussion ensues regarding possible hazard management
strategies.
6.1 HAZARD MANAGEMENT APPROACH
Bommer et al. (2006) summarise previous data which correlates seismic moment with injected fluid volume. They state
that McGarr (1976) has argued and demonstrated that total seismic moment release shows a direct proportionality to the
total volume injected during an experiment, similarly the total moment release is seen to vary approximately linearly with
volume of rock extracted in mining seismicity examples. Hydraulic injections at the Soultz Hot Dry Rock geothermal site
can be seen as an archetype for this kind of behaviour with very high detected event rates correlating precisely with the
onset of pumping into the deep granite (Weidler et al. 2002).
At the Berlin field Bommer et al. (2006) after observing two distinct zones of seismicity. Data was analysed for cumulative
seismic moment release together with cumulative pumped volume for three periods of injection. The correlations clearly
showed a direct relationship between seismic activity and the fluid injected during the rock fracture stimulations (Figure
6.1).
Also a review was performed for data from the HDR site at Rosemanowes in Cornwall. Figure 6.2 shows once more a
direct relationship between cumulative seismic moment and net injected fluid volume.
Interestingly net volume injected varies between sites as follows, in the case of the Geodynamics site at Innamincka the site
is overpressured hence significantly less additional fluid is required to stimulate fracturing. Geodynamics site 20 000 cum
were pumped into the reservoir; at Rosemanowes 400 000 cum were pumped into RH12; at Berlin el Salvador 300 000 000
litres equivalent to 300 000 cum were pumped into their wells.
Figure 6.1 Comparison of cumulative volume of pumped liquid (litres) and induced seismicity (in terms of cumulative seismic
moment) (Berlin Field, El Salvador).
6.2 PROPOSED FUTURE MONITORING ARRANGEMENTS
Given these strong correlations between net injected fluid volumes and seismicity during HFR operations we have an
excellent opportunity to address seismic event generation.
It is strongly suggested a ‘traffic light system’ similar to that used at the Berlin field in El Salvador be implemented at all
future HFR operations, that is a seismic hazard control system for all current and future geothermal reservoir development
operations. This would be of particular importance during the initial fluid injection phases of any future project. The idea of
this system is that injection volumes should be reduced if ground motion levels and events magnitudes are raised below a
predetermined level.
In the case of the Cooper Basin region where seismic recurrence relationships are at nationally low levels, a straight
forward strategy would be to use regional design factors i.e. ground motion vibration design levels, as described in
Australian/New Zealand standard AS/NZS 4360: 2004 (Standards Australia/Standards New Zealand 2004) and maintain
these levels during all operations so that background values are not exceeded. The seismic monitoring should start well
before the start of stimulation operations to get baseline information.
The ‘traffic light system’ should be based on these background levels, and when a background level is approached (amber)
the injected fluid volume should be reduced and proceed at a reduced rate.
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Also it is proposed that as a further conservative measure the following take place:
• Usually for HFR development a seismic network is developed and runs only during stimulation operations whereby the
system monitors seismicity generated during fracturing activity only. It is advised that this system be maintained and
run during the productive lifetime of the reservoir. There is evidence from other HFR fields that seismicity can continue
after shut-in following pumping operations. Also relating to this observation Baisch (2007) suggests a number of cases
where the McGarr relationship does not hold this is discussed further in Section 6.3, but a proposal to account for this
uncertainty is given.
• The monitoring system should incorporate strong motion accelerometers also in place at the site which would be able to
directly measure ground motion. As well as the usual geophone array for event location and moment analysis (detector
redundancy is also advised). The work here used previously derived attenuation relationships from other sites to
estimate the ground motion at the site. It would be preferable to have direct measurements of this activity.
• The monitoring suggestions made above will not only allow reduction in the hazard associated with HFR operations but
will also provide data of use to the scientific community and the opportunity to enhance our understanding of rock/fluid
interactions and general mechanisms at work within the reservoir, which may be of benefit in future operations.
HDR Poject Rosemanowes UK, Relationship Between Net Injected
Volume (cum) and Cumulative Seismic Moment (Mnm)
Cumulative Seismic Moment (MNm)
2.5
y = 3.0995x + 0.0403
R2 = 0.9281
2
Cumulative
seismic
moment
(MNm)
1.5
1
Linear
(Cumulative
seismic
0.5
0
0
0.2
0.4
0.6
0.8
Net Injected Volume (cu.m) x105
Figure 6.2 Correlation at the UK HDR Geothermal Energy Project between net injected fluid volume and resultant cumulative
seismic moment.
6.3 UNCERTAINTIES AND PROPOSED FUTURE RESEARCH
Recent findings with regard to the “traffic light” system work only in a reactive way i.e. only after a seismic event with
significant magnitude (ML ~ 2.5) has already happened (Rybach L, 2007). At Basel Switzerland recently an M=3.4
earthquake was induced in a geothermal reservoir, the earthquake followed after project management had stopped the
injection because seismic event magnitudes became too large. Local authorities in Basel Switzerland now clearly state that
the “traffic light” system failed: it was unable in spite of immediate management actions to reduce/stop water injection for
stimulation to prevent the occurrence of later, larger events.
This case should be reviewed in detail and lessons learned incorporated in the protocol proposed. At Basel (Baisch, 2007)
the largest magnitude event occurred several hours after the injection rate had benn reduced to zero (ie shut-in). A decrease
in seismic activity occurred only after they started flowing back the well ie decreasing the reservoir pressure. Further
information relating to this topic and long term data is provided by Ake et al 2005. It has been suggested that correlations
between individual events and pumping rates should be made (Rybach, 2007), certainly all data relating to surface activities
and event timing and magnitude is of critical importance in fully reducing risk factors as proposed previously these data
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should be acquired conventionally. Once events of high magnitude occur and the warning stage is reached it is important
to reduce the pressure within the reservoir to prevent further large magnitude events. This, as suggested, would be
achievable by alleviating bottomhole pressure, this may not require significant pressure depletion but the safest option is to
flow the well at maximum rate to decrease reservoir pressure as much as possible, thus reducing effective reservoir stress.
The next phase of development would then be to allow the reservoir to reach stress equilibrium, ie wait until the reservoir is
seismically quiet, reviewing prior events and activities to understand in detail the results obtained. This will then require
extending the reservoir further under a different more controlled pumping regime.
Also Baisch (2007) suggests the “circulation” seismicity relationship and the “stimulation” relationships should be treated
separately, to evaluate the seismic hazard associated with the circulation also. A review of all these relationships published
to date should be performed to assess the influence of geological, tectonic and structural settings.
It is proposed that the fluid injected volume and hydraulic energy (simply a product of the injection volume and pressure)
versus seismic moment should be monitored continuously throughout development of the reservoir both within the
“stimulation” and “circulations” stages of the project. Also a review of all previous relationships should be made to assess
any geological relationships.
Generally the Gutenberg-Richter relationship has been discussed throughout the text in terms of the seismic event
recurrence relationship or the b-value; the b-value has not been represented completely for the Habanero Geodynamics
project. We are aware that the probability of event recurrence has increased in this region of Australia, this is discussed as
follows: the region is determined to experience a peak round of acceleration of 0.05g with a 10% chance of this being
exceeded in 50 years. Thus the presence of the geothermal reservoir has increased the “recurrence” component of the risk
for this region. Despite this the hazard map is unaffected because the Habanero events are not treated independently.
Usually when creating a hazard map earthquake aftershocks and foreshocks are not included in the calculations; only the
main event is assessed for the probability predictions. As part of this study the Habanero events are treated as one main
event as they are so closely spaced in time, this means the risk map will not change, this is a reasonable approach due to the
nature of the events in space and time. This has been assessed as a reasonable treatment by the seismological community,
but further work on the seismicity of each site should be undertaken to study “ recurrence” in more detail to justify this
assumption further . In particular with relation to the temporal spacing of events and the geometric
development/classification of the reservoir, whether as a single fracture zone or multiple fracture zones. This should be an
area for future research work.
In light of this work, seismic event recurrence relationships for any new projects should be published and made available
for discussion in terms of seismic hazard.
A further topic is the uncertainty associated with the fault friction angle chosen for the safety factor calculation in section
3.7.1. Despite this there is confidence in the relative comparison of the fault segments in terms of the likelihood of slip
along each fault segment – we are clearly making an estimate of the likely fault friction angle but this uncertainty that can
be reduced by sampling the fault plane and performing laboratory based rock mechanics test, at present no data is available
to do this. This is an opportunity for continued research.
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APPENDIX A
USEFUL TERMINOLOGY AND ABBREVIATIONS
Modified Mercalli Intensity Scale (after Wood & Neumann 1931)
I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an
earthquake is occurring.
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels
like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken.
Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open
containers.
VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture
moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural
damage.
VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from
buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.
VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as
towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe
damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change.
IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some
underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large
landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are
bent slightly.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are
destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts
of rock may move.
Aftershock An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger
earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with
time.
Amplitude The maximum height of a wave crest or depth of a trough.
Array An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.
Arrival The appearance of a seismic wave on the seismic record.
Arrival time The time at which a particular wave phase arrives at a detector.
Aseismic area An area that is almost free of earthquakes.
Body wave A seismic wave that travels through the interior of the earth and is not related to a boundary surface.
Crust The outer layer of the Earth's surface.
Earthquake Shaking of the earth caused by a sudden movement of rock beneath its surface.
Earthquake swarm A series of minor earthquakes, none of which may be identified as the main shock, occurring in a
limited area and time.
Elastic wave Rock is an elastic material that when strained by normal external forces can return to its original state. When
the strength of the rock is exceeded, the rock ruptures, generating elastic seismic or earthquake waves.
Epicentre That point on the Earth's surface directly above the hypocentre of an earthquake.
Fault A weak area in the Earth's crust where two sides of a fracture or fracture zone move relative to each other.
First arrival The first recorded signal on a seismogram is the direction of the first P-wave, where upward ground motion is
compressional and downward motion is dilatational.
Focus The point where earthquake rupture or fault movement originates.
Foreshock A small earthquake that may precede a larger earthquake or main shock and that originates at or near the focus
of the larger event.
Frequency The frequency of a wave (Hz) is the number of wave cycles per second.
Hypocentre The calculated location of the focus of an earthquake.
Induced seismicity Non-natural events induced by man's activity. These include mining induced events, events caused by
loading of dams or pumping of water in geothermal areas.
Intensity A measure of the effects of an earthquake at a particular place on humans and (or) structures. The intensity at a
point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to
the epicentre and the local geology at that point.
ISIP
Isoseismal line A line enclosing points on the Earth's surface at which earthquake intensity is the same. It is usually
elliptical in shape.
Love wave A major type of surface wave having a horizontal motion that is shear or transverse to the direction of
propagation. It is named after A.E.H. Love, the English mathematician who discovered it.
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Magnitude A measure of the strength of an earthquake. There are several scales depending on which part of the
seismogram is examined. These include Richter local magnitude (ML), Body wave magnitude (Mb) and surface wave
magnitude (Ms). Moment magnitude (Mw) is calculated from spectral analysis. Advised by Geoscience Australia
seismologist (Sinadinovski, 2006) that for the purpose of this study ML and Mw can be treated as equivalent.
Mantle The layer that lies between the crust and the core of the earth.
Microseism A motion in the Earth that is unrelated to an earthquake. It is caused by a variety of natural and artificial
agents, for example wave action, wind, traffic and industrial noise.
MM Modified Mercalli.
MSK MSK intensity is the intensity scale used in Europe before the introduction of the EMS scale. It is a 12-grade scale
ranging from not felt to complete devastation.
PGA Peak acceleration (m/s2 or g).
PGV Pak velocity (mm/s).
PGD Pak displacement (m).
P wave The first and faster of the body waves which moves by a series of compressions, similar to a sound wave. They can
travel through both solid and liquid.
Phase The onset of a displacement on a seismogram indicating the arrival of the different types of seismic wave.
Plate One of the segments which make up the Earth's crust. The plates are continuously moving relative to each other.
Plate boundary The place where two or more plates in the Earth's crust meet.
Prediction Predicting the time, place and magnitude of an earthquake.
Rayleigh wave A type of surface wave having a retrograde, elliptical motion at the free surface. It is named after Lord
Rayleigh, the English physicist who predicted its existence.
Reflected wave A wave that has turned back from a boundary or discontinuity in the earth's crust.
Refraction The change in direction of a wave on reaching a boundary of different density and velocity.
Richter scale A popular name for the local magnitude scale (See Magnitude).
S wave The second arrival on a seismogram, the S wave, is slower than the P-wave. It is a shear wave and cannot travel
through liquids.
Seismogram A record of an earthquake or ground vibration. The wave trace is made up of P-waves, S-waves and surface
waves, the pattern of onsets of the first two arrivals help to determine the location. The seismogram can be either a paper
record or a digital record that is analysed by computer.
Seismograph An instrument that registers the occurrence of an earthquake and the time it occurred as a written record.
Seismologist A scientist who studies earthquakes.
Seismometer An instrument that not only measures the time of the arrival of earthquake waves, but also allows the exact
motion of the ground to be computed from the record.
Seismoscope An instrument that registers the occurrence of an earthquake, but not the time.
Signal-to-noise ratio The comparison between the amplitude of the seismic signal and the amplitude of noise caused by
seismic unrest and (or) the seismic instruments.
Subduction zone An elongated region along which a crustal plate descends relative to another crustal block, for example,
the descent of the Pacific plate beneath the South American plate.
Surface waves Seismic waves with motion restricted to near the ground surface (Love and Rayleigh).
Teleseism An earthquake that is distant from the recording station.
Travel time The time required for a wave train to travel from its source to a point of observation.
Tsunamis A huge sea wave caused by earthquakes (referred to by many as a tidal wave).
Volcanic earthquake Earthquakes associated with volcanic activity.
Wavelength The distance between two successive crests or troughs of a wave.
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APPENDIX B
HDR ROSMEANOWES DATA
Extract 1: Rosemanowes, UK HDR Data
An assessment of the likely microseismic characteristics generated at an HDR project site in the basement granite colocated at the Moomba and Big Lake Fields, can be made by assuming the microseismic events generated will be of similar
magnitude as those generated in granites studied for other operating HDR project sites internationally (although this will
have to be upscaled for the reservoir size predicted). An example of this is the Camborne School Of Mines HDR site
created by means of high pressure injection into granite during the period 1981–1986. A direct relationship was observed
between cumulative seismic moment (magnitude proportional to log10 of moment) and net injected volume. Two seismic
monitoring systems were used to analyse the seismic events produced during hydraulic injection. There were two main
joint sets present in the granite both sub-vertical and orthogonal, with an average of 5 to 20 m spacing. This data and details
of the seismic events generated are given below and summarised from Pine (1989):
• Spatial distribution – event locations with a progressive downward trend, flattened ellipsoid height, plan length and
plan thickness of 2500, 1100 and 300 m respectively
• Measured signal characteristics (damage potential) – range from background noise of 15 μg up to 250 μg with an
average of about 125 μg, signal frequencies in range of 100–500 Hz peaks at 200–300 Hz
• Source inferences (highest average signal energies) – source radius 8–10 m stress drop 30–120 kPa, seismic moment
30–80 MNm
• Fault plane solutions – recorded motions compatible with sinistral strike-slip shearing on joints striking approximately
10° west of north or dextral strike-slip shearing on joints striking approximately 10° north of east. This was compatible
with a maximum horizontal stress striking at approximately 55° west of north (305°). Assuming that the joint
orientations in the reservoir are similar to those mapped at the ground surface, it is most likely that set 1 (striking
approximately 320–340°) is the source for most of the shearing
• Energy considerations – seismic moments recorded gave resulting seismic magnitudes of the order -2.5 ML, the
suggested upper extent from the second seismic network had events at -1.0 to +1.0 ML.
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REFERENCES
Ake, J. & Mahrer, K. & O’Connell, D. & Block, L., 2005. Deep Injection and Closely-Monitored, Induced Seismicity at
Paradox Valley, Colorado. Bull. Seism. Soc. Amer., 95, 664-683.).
Angelier, J 1994, ‘Fault slip analysis and palaeostress reconstruction’, in PL Hancock (ed), Continental Deformation,
Pergamon Press, Oxford, South East England, pp. 53–100.
Asanuma, H, Nozaki, H, Niitsuma, H & Wyborn, D 2005, ‘Interpretation of microseismic events with larger magnitudes collected at
Cooper Basin, Australia’, Geothermal Resources Council Transactions, vol. 29, pp. 87–91.
Baisch, S. Weidler, R. Voros, R., Wyborn D., and de Graaf L. (2006) Induced Seismicity during the stimulation of a Geothermal HFR
reservoir in the Cooper Basin, South Australia. Bulltein of the Seismological Society of America, Vol. 96. No.6 pp 2242-2256, Dec
2006.
Baisch, S. (2007) Peer review comments for this report.
Bommer, JJ, Oates, S, Cepeda, JM, Lindholm, C, Bird, J, Torres, R, Marroquin, G & Rivas, J 2006, ‘Control of hazard due to seismicity
induced by a hot fractured rock geothermal energy project’, Engineering Geology, vol. 83, pp. 287–306.
Boult, PJ 1996, ‘An investigation of reservoir/seal couplets in the Eromanga Basin; implications for petroleum entrapment and
production. Development of secondary migration and seal potential theory & investigation techniques’, Ph.D thesis, University of
South Australia, Adelaide, SA.
Brown, A & Gibson, G 2000, ‘Reassessment of earthquake hazard in Australia’, Proceedings of the 12th World Conference on
Earthquake Engineering, International Association for Earthquake Engineering, Auckland, New Zealand.
Byerlee, J 1978, ‘Friction of rocks’, Pure and Applied Geophysics, vol. 116, pp. 615–626.
Camac, BA & Hunt, SP 2004, ‘Applications of stress field modelling using the distinct element method for petroleum production’,
Proceedings of the 2004 SPE Asia Pacific Oil and Gas Conference and Exhibition, Society of Petroleum Engineers, Perth, WA.
Dieterich, JH, Raleigh, CB & Bredehoeft, JD 1972, ‘Earthquake triggering by fluid injection at Rangely, Colorado’, Proceedings of the
ISRM/IAEG Percolation Through Fissured Rock Symposium, International Society for Rock Mechanics/International Association for
Engineering Geology, Stuttgart, Baden-Württemberg.
Earthquake database 2006, Geoscience Australia, Canberra, ACT, <http://www.ga.gov.au/oracle/quake/quake_online.jsp>.
Field, EH & the SCEC Phase III Working Group 2000, ‘Accounting for site effects in probabilistic seismic hazard analyses of southern
California: overview of the SCEC phase III report’, Bulletin of the Seismological Society of America, vol. 90, pp. S1–S31.
Gaull, BA, Michael-Leiba, MO & Rynn, JMW 1990, ‘Probabilistic earthquake risk maps of Australia’, Australian Journal of Earth
Sciences, vol. 37, pp. 169–187.
Geodynamics Limited 2006, supply of data.
Gibson, G & McCue, K 2001, ‘Seismological contributions to earthquake loading codes’, Earthquake Codes in the Real World –
Proceedings of the 2001 Australian Earthquake Engineering Society Conference, Australian Earthquake Engineering Society,
Canberra, ACT.
Gutenberg, B & Richter, CF 1965 (1954), Seismicity of the earth and associated phenomena, 2nd edn, Hafner Publishing Company, New
York, NY.
Hart, EW & Bryant, WA 1997, Fault-Rupture Hazard Zones in California: Alquist-Priolo Earthquake Fault Zoning Act with Index to
Earthquake Fault Zones Maps, special publication 42, Division of Mines and Geology, California Department of Conservation,
Sacramento, CA.
Hunt, SP & Boult, PJ 2005, ‘Discrete element stress modelling in the Penola Trough, Otway Basin, South Australia’, in PJ Boult & J
Kaldi (eds), Evaluating fault and cap rock seals, AAPG Hedberg series, no. 2, American Association of Petroleum Geologists, Tulsa,
OK, pp.199–213.
King, GCP, Stein, RS & Lin, J 1994, ‘Static stress changes and the triggering of earthquakes’, Bulletin of the Seismological Society of
America, vol. 84, pp. 935–953.
Majer, E, Baria, R & Fehler, M 2005, ‘Cooperative research on induced seismicity associated with enhanced geothermal systems’,
Geothermal Resources Council Transactions, vol. 29, pp. 99–101.
Majer, EL 2005, White paper: Induced Seismicity and Enhanced Geothermal Systems, Centre for Computational Seismology – Lawrence
Berkeley National Laboratory, Berkeley, CA, <http://esd.lbl.gov/EGS/Workshop2/White_Paperfafsave.doc>.
McGarr, A 1976, ‘Seismic moments and volume changes’, Journal of Geophysical Research, vol. 81, pp. 1487–1494.
Morelli, C.P. and Hunt S.P. (2006) Calibration of Distinct Element Models for The Cooper-Eromanga Petroleum Province in Australia Towards Improved Hydrocarbon Production. American Rock Mechanics Association Annual Conference, Golden Rocks 2006, The 41st
U.S. Symposium on Rock Mechanics (USRMS): Rock Mechanics for Energy, Mineral and Infrastructure Development.
Morelli, C. And Hunt S.P. (2007) – ???
Morris, A, Ferrill, DA & Henderson, DB 1996, ‘Slip-tendency analysis and fault reactivation’, Geology, vol. 24, pp. 275–278.
Petroleum & Geothermal Group, Primary Industries and Resources SA (P&G, PIRSA) 2006, supply of data.
Pine, RJ 1986, ‘Rock joint and rock mass behaviour during pressurised hydraulic injections’ Ph.D thesis, Camborne School of Mines,
Penryn, South West England.
Pine, RJ 1989, ‘Seismic hazard assessment’, in RH Parker (ed), Hot Dry Rock Geothermal Energy: Phase 2B Final Report of the
Camborne School of Mines Project, vol. 2, Pergamon Press, Oxford, South East England, pp. 1345–1385.
Rybach L. (2007) Peer review comments for this report.
Reynolds, SD, Mildren, SD, Hillis, RR & Meyer, JJ 2006, ‘Constraining stress magnitudes using petroleum exploration data in the
Cooper–Eromanga Basins, Australia’, Tectonophysics, vol. 415, pp. 123–140.
Toda, S, Stein, RS, Reasenberg, PA, Dieterich, JH & Yoshida, A 1998, ‘Stress transferred by the 1995 Mw=6.9 Kobe, Japan, shock:
effect on aftershocks and future earthquake probabilities’, Journal of Geophysical Research, vol. 103, pp. 24,543–24,566.
Toro, GR, Abrahamson, NA & Schneider, JF 1997, ‘Model of strong ground motions from earthquakes in central and eastern North
America: best estimates and uncertainties’, Seismological Research Letters, vol. 68, pp. 41–57.
Sadigh, K, Chang, C-Y, Egan, JA, Makdisi, F & Youngs, RR 1997, ‘Attenuation relationships for shallow crustal earthquakes based on
California strong motion data’, Seismological Research Letters, vol. 68, pp. 180–189.
Sinadinovski, C, Edwards, M, Corby, N, Milne, M, Dale, K, Dhu, T, Jones, A, McPherson, A, Jones, T, Gray, D, Robinson, D & White,
J 2005, ‘Chapter 5: earthquake risk’, in T Jones, M Middelmann & N Corby (comps), Natural hazard risk in Perth, Western
Australia, Geoscience Australia, Canberra, ACT, <http://www.ga.gov.au/image_cache/GA6529.pdf>.
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Standards Australia 1993, Australian Standard: Minimum Design Loads on Structures – Part 4: Earthquake Loads (AS 1170.4: 1993),
Standards Australia, Sydney, NSW.
Standards Australia/Standards New Zealand 2004, Australian/New Zealand Standard: Risk Management (AS/NZS 4360: 2004),
Standards Australia (Sydney, NSW)/Standards New Zealand (Wellington).
U.S. Army Corps of Engineers (USACE) 1972, Systematic Drilling and Blasting for Surface Excavations, engineer manual 1110-2-3800,
USACE, Washington, DC.
Warburton Basin GIS data atlas 2002, CD-ROM, Primary Industries and Resources SA – Petroleum Group, Adelaide, SA.
Weidler, R, Oates, SJ, Dyer, B, Baumgartner, J & Gerard, A 2002 ‘Integrated microseismic and hydraulic monitoring of HDR reservoir
stimulation, Soultz 2000’, Proceedings of the 64th EAGE Annual Conference & Exhibition, European Association of Geoscientists &
Engineers, Florence, Tuscany.
Wood, HO & Neumann, F 1931, ‘Modified Mercalli intensity scale of 1931’, Bulletin of the Seismological Society of America, vol. 21,
pp. 277–283.
Wyborn, D, de Graaf, L, Davidson, S & Hann, S 2004, ‘Development of Australia’s first hot fractured rock (HFR) underground heat
exchanger, Cooper Basin, South Australia’, Proceedings of the 2nd Eastern Australasian Basins Symposium, Petroleum Exploration
Society of Australia, Adelaide, SA.
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