Demonstrating a Microearthquake- and Magnetotelluric-based
Production and Injection Well-Targeting Technology
Developed under CEC Support
Case Study of the Rhyolite Plateau Exploration Area
Long Valley Caldera, California
Contract Project Manager:
Robert Sullivan
Commission Project Manager:
Gail Wiggett
Duke University Collaborators:
Peter Malin,
Eylon Shalev
Stephen Onacha
February 28, 2006
Department of Earth and Ocean Sciences
Duke University
Table of Contents
Table of Contents
List of Figures
Executive Summary
1.0
Project Objective
1.1
Introduction
1.2
Microearthquake Survey
1.3
MT and TEM
1.4
Purpose
1.5
The Overall Project Objectives
2.0
Project Outcomes
2.1.
Data acquisition and analysis
2.1.1
Mobilization and data acquisition and demobilization
2.1.1.1 Arrange for cost-shared seismic Instruments
2.1.1.2 Identify Required Permits
2.1.1.3 Set up seismic array at the target area and demobilization
2.1.1.3.1 Instrument deployment
2.1.1.3.2 Test of array.
2.1.1.4 Array maintenance
2.1.1.4.1 Visit and maintain stations
2.1.1.4.2 Data downloading
2.1.1.4.3 Data archiving
2.1.1.5 Demobilizing
2.1.2 Analyzing microearthquake (MEQ) and magnetotelluric (MT) data
2.1.2.1 Purchase mass storage unit
2.1.2.2 Read data tapes
2.1.2.3 Locate seismic events.
2.1.2.4 Identify shear-wave splitting.
2.1.2.5 Velocity and crack-density inversion.
2.1.3 Initial and final scientific interpretations
2.1.3.1 Create 3-D geophysical map of the target area
2.2. Combine new data with existing geophysical data to create a well-targeting map
2.2.1 Pre-drilling geological and geophysical interpretation.
2.2.2 AGU presentation.
2.3. Site and drill a test well to ground-truth the value of well-targeting
2.3. Hold a Critical Pre-drilling Project Review Workshop
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2.4. Evaluate success of the test based on comparison of Pre-drilling/Post-drilling. 22
2.6.
Reporting
23
3.0
Results
24
3.1
Geological Setting and Tectonics
24
3.2
Seismic Activity in the Long Valley Caldera
28
3.3
Hydrothermal activity and exploration history of LVC
29
3.4
Rhyolite Plateau geophysical and geological related geothermal exploration.
34
3.4.1
Ground Magnetic, Self Potential (SP) and Fracture Density
36
3.4.2 Fracture Density
36
3.5
2004 Electrical Resistivity and Microearthquake studies
39
3.5.1
Microearthquake Studies (MEQ)
39
3.5.1.1 Shear-wave splitting.
Error! Bookmark not defined.
3.5.1.2Definition of crack density
Error! Bookmark not defined.
3.5.13 Velocity and crack-density inversion
Error! Bookmark not defined.
3.5.2
Magnetotelluric Studies (MT)
Error! Bookmark not defined.
3.5.2.1 Introduction
Error! Bookmark not defined.
3.5.2.2 MT Data Acquisition
Error! Bookmark not defined.
3.5.2.3 MT Data Processing
Error! Bookmark not defined.
3.5.2.5 Inversion for Subsurface Resistivity
Error! Bookmark not defined.
3.5.2.5.1 1-D Resistivity Maps and sections
Error! Bookmark not defined.
3.5.2.5.1.1 Analysis of 1-D Resistivity sectionsError! Bookmark not defined.
3.5.2.5.1.2 Analysis of 1-D Resistivity maps Error! Bookmark not defined.
3.5.2.5.2 2-D Inversion
Error! Bookmark not defined.
3.5.2.5.2 2-D Inversion
Error! Bookmark not defined.
3.5.2.5.2 Analysis of 2-D resistivity sections
Error! Bookmark not defined.
3.5.2.6
Comparison of shear wave splitting and MT polarization
Error! Bookmark not defined.
3.5.2.7 Data Interpretation
Error! Bookmark not defined.
3.5.2.8 Conceptual Model
Error! Bookmark not defined.
3,5,2,9 Discussion of recommended sites for drilling
75
4.0
Conclusions
Error! Bookmark not defined.
5.0
Recommendations
Error! Bookmark not defined.
6.0
References
Error! Bookmark not defined.
APPENDIX A: AGU Poster. Deep Borehole Reciever Functions in Long Valley Caldera, CA,
Chavarriia et al.
APPENDIX B: EVENT CATALOGUE
3
List of Figures
Figure 1 Tectonic setting of the Long Valley Caldera (LVC) showing major faults and
earthquakes of M5 or higher. Eathquakes occur mainly to the south of the LVC and along active
faults. Mono Lake is to the north of LVC, Owens Valley to the south and Glass Mountain
(modified from Hill et al., 1989). The location of slip of the 1872 earthquake in the Owens
Valley is shown.
26
Figure 2 Simplified Geological Map and history of the Long Valley Caldera and Mono-Inyo
Craters (Modified from Bailey USGS). The history covers the period between 2.2 million and
300 years ago. The location of earthquakes is shown as swarms of dark circles.
27
Figure 3 Location of wells, hot springs, streams fumaroles and precipitation recording stations in
LVC for studying the natural hydrological variations and the response of the hydrothermal
system to volcanic and tectonic processes (http://lvo.wr.usgs.gov/HydroStudies.html)
29
Figure 4 Map of LVC showing major faults, postulated ring fracture (RFC), the Resurgent
Dome, Rhyolite Plateau (RP) and the Temperature exploration wells. Rhyolite Plateau is in the
Discovery Fault Zone (DFZ) with a NE-SW trend. HSF is the Hartley Springs Fault, LCF is
Laurel-Convict Fault, FLK is Fern Lake Fault and HCF is Hilton Creek Fault (From Sorey et al.,
1991)
33
Figure 5 Rhyolite Plateau Exploration Area showing the location exploration wells and faults.
The faults have orientations in the NW-SE, EW and NE-SW
32
Figure 6 NW-SE geological section through the exploration wells in the western part of Long
Valley Caldera showing stratigraphy and faults. Maximum measured temperatures measured in
the wells are shown. The highest temperatures occur within the Rhyolite Plateau (Modified from
Sorey et al., 1991)
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Figure 7 Location of MEQ and MT stations in the Rhyolite Plateau area. Geothermal exploration
wells are shown in purple. MEQ stations from the 2003 deployment are as solid black squares
while the stations from the 2004 deployment are shown as labeled red squares. The MT stations
by CGG, Wannamaker and Duke are also shown in triangles of different colors
40
Figure 8 Components of the new GS-1 seismometer. The seismometer and the data recorder
couple together into one Unit which is anchored securely to the ground by the ground auger41
Figure 1 Mapped locations for all events recorded in 2003 and 2004. Colored dots represent
earthquakes with specified depths. Dots and triangles represent recording stations deployed in
2003 and 2004,
40
Figure 2 Polarization direction observed at the recording stations, Mammoth, 2004
42
Figure 11 Ray paths from some events from the MEQ data acquisition by Duke University in
2004, including those that occurred outside of the figures. The distribution is representative of
events shown in Figure 9
Error! Bookmark not defined.
Figure 12 Location of MT soundings in the Rhyolite Plateau Exploration area. The MT stations
acquired by Duke University in 2004 are shown in blue triangles, CGG data is shown in dark
light blue triangles while that by Wannamaker is shown in light green triangles
Error!
Bookmark not defined.
4
Figure 13 Location of MT profiles used in the data interpretation. Profile NW4 coincides with
geological profile (see Figure 6) through the existing wells and the caldera rim.
54
Figure 14 Resistivity across Profile NE-1. The station spacing in the western part of the profile is
poorly resolved
Error! Bookmark not defined.
Figure 15 Data from MT site D33 which shows low resistivity at long periods corresponding to
deeper resistivity structure. The apparent resistivity is very low and the phase is very consistent
Error! Bookmark not defined.
Figure 16 1-D resistivity cross section across profile NW1, the NW direction is to the left while
the SE direction is to the right.
Error! Bookmark not defined.
Figure 17 The high quality data along profile NW1. The data indicates a low resistivity at long
periods
Error! Bookmark not defined.
Figure 18 Stitched 1-D profile in the NW-SE direction along profile NW1. The Rhyolite Plateau
area is between stations D04 and D32
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Figure 19 Stitched 1-D NW-SE resistivity section across the Rhyolite Plateau and Shady rest
Area
Error! Bookmark not defined.
Figure 20 Resistivity at 2000 meters above sea level. Low resistivity areas are within areas of
less than 20Ωm. Locations of MT soundings are shown as filled triangles while the locations of
the existing exploration wells are labeled in pink
Error! Bookmark not defined.
Figure 21 Resistivity at 1700 meters above sea level. Low resistivity areas are within areas of
less than 20Ωm. Locations of MT soundings are shown as filled triangles while the locations of
the existing exploration wells are labeled in pink
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Figure 22 2-D resistivity along profile NW-4 through the existing exploration. The area between
wells 44-16 and PLV-1 shows a higher intensity of alteration close to the surface above
1500masl. The hydrothermal system is interpreted to be below the area on near surface low
resistivity. The deep low resistivity is interpreted as the outflow which dips to the west and to the
east
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Figure 23 MT Polarization in the study area based on average direction at each site over the
frequency window of 317-126 Hz. The locations of the MT stations are shown in triangles. The
location of MEQ stations is shown in red squares while earthquakes are shown as brown solid
circles. The faults are shown as yellow traces.
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Figure 24 Map showing both MT (black lines) and shear wave (red lines) polarization directions
in the Rhyolite Plateau area. The shear wave polarization length and thickness are propagation to
the number of events and amount time delay time between the fast and slow S-waves. Fault
traces are shown in yellow while the locations of earthquakes are shown in brown.
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Figure 25 Map of resistivity at a constant depth of 500 meters showing the areas for exploration
drilling in Rhyolite Plateau and Shady Rest. Low resistivity areas are indicated bright yellow –
red colors while the high resistivity areas are indicated by blue colors. Low resistivity areas are
controlled by NW-SE and NE-SW trending faults (yellow traces). Location of MT data is shown
in triangles.
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5
Figure 26 Map of LVC showing location of area of study relative to location of hot springs and
fumaroles (from USGS). Map of the known faults has been superimposed to show structural
control of the hydrothermal manifestations. The hot springs are marked as orange squares,
fumaroles as green filled ellipses and water wells as blue circles. The red arrows show the
postulated flow of hydrothermal fluids while the blue arrows show the flow of meteoric waters
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Figure 27 Postulated hydrogeological model at 1900masl based on resistivity data and faults. The
orange arrows indicated the postulated flow of hydrothermal fluids while the blue arrows
indicate the possible flow paths for meteoric water mainly from the north.
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Figure 28 Recommended target depths for drilling exploration wells along the 2-D resistivity
profile NW-4 through the existing exploration. The area between wells 44-16 and PLV-1 shows
a higher intensity of alteration close to the surface above 1500masl. The postulated hydrothermal
system is shown below the area on near surface low resistivity. The deep low resistivity is
interpreted as the outflow which dips to the west and to the east. The priority target area between
44-16 and PLV-1
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Figure 29 Location of proposed drilling sites in the Rhyolite Plateau Area and Shady Rest. The
proposed sites are shown in red. Rhyolite Plateau sites are labeled by RW while those in Shady
Rest are labeled as SW. The fault traces are shown in yellow.
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APPENDIX A. AGI Poster. Deep Borehole Reciever Functions in Long Valley Caldera, CA,
Chavarriia et al.
APPENDIX B. Microearthquake event catalogue
6
Executive Summary
The purpose of this project was to conduct a “ground-truth” test of well-targeting
technology developed by Duke University under CEC support in 1996 to 1998.
Microearthquake and magnetotelluric field surveys were conducted in 2003 and 2004 and the
data from these surveys were analyzed to provide the basis for the well targeting test. The
major finding of this current study is that microearthquake shear wave slitting directions for
events occurring outside the study area are consistent with maximum and minimum conduction
directions obtained from magnetotelluric data and known faults. While the microearthquakes
within the study area proved to be too few to allow creation of a fracture density map for
production well targeting, the magnetotelluric data are sufficient to specify potential exploration
well sites. These sites as discussed in this report.
During the 1996 to 1998 support period more than 10,000 microearthquakes (MEQ) were
recorded in the Casa Diablo Geothermal area of the Long Valley Caldera (LVC). The proposed
well-targeting technology for the 2003-2004 studies was based on use of microearthquake
locations and wave propagation effects to find shallow, high-production geothermal zones. The
well-targeting test, the data for which was to be gathered only in 2003, was designed to combine
this exploration method with existing geophysical measurements to guide drilling of a 600 to
1000m well in the Rhyolite Plateau and Dry Creek Basin of Long Valley Caldera. Making use
of a new cost share opportunity that arose shortly after the start of this test project, a second field
effort was made in 2004.
In this latter effort, the 2003 field survey was augmented at no cost to CEC with the
addition of magnetotelluric (MT) data acquisition and by a new seismograph array. The MT
technique is routinely used in geothermal exploration and its addition to the CEC supported
survey at no cost was an important benefit. It provided the basis for proposing the exploration
7
wells. The new seismograph array included a highly sensitive borehole seismometer in the 4416 well in the center of the targeting area. This special station was used to detect MEQ’s at
magnitudes as low as one unit below those observable at the surface. The data from this station
was also used to complete a no-cost receiver function study of the deeper velocity structure
below the target area (see Appendix; Chavarria et al., 2004.
Duke University deployed the MEQ equipment as planned in the proposal and made all
arrangements to collect high quality data. Previous work in Basalt Canyon proven that MEQ
locations and wave propagation effects can be used to find the shallow, high-production zones
needed to reduce the number of wells drilled (ORMAT, internal data; Shalev; personal
communication). The exploration value of this technology was therefore to be tested by
targeting and drilling a demonstration well. In contrast to results of the 96-98 study and
extrapolations of seismicity rate to lower magnitudes, very few earthquakes were recorded
beneath the Rhyolite and Dry Creek target areas during the 2003 survey. This was despite the
fact that Duke University had successfully increased the sensitivity of the monitoring array to
lower magnitude microearthquakes than in the 1996 to 1998 study.
During the 2004 survey, Duke University deployed new seismographic equipment with
capability of continuous recording and sampling rate of 250 samples per second. The 2003 MEQ
survey was repeated in 2004, with the hope that by changing the configuration to record lower
magnitude earthquakes, sufficient data would be acquired for shear wave and shear-wave
splitting tomography. While this second MEQ survey, which included the borehole sensor, also
failed to detect a sufficient number of microearthquakes within the area of the seismic network
suitable for carrying out the shear wave splitting tomography for well targeting, the cost shared
MT data set did provide suitable well-target testing information.
8
Thus a major finding of this study is that successful use of MEQ shear wave and splitting
tomography for geothermal exploration can be unreliable if the target area is not in a seismically
active phase. This hold true even if the array is designed to detect very low magnitude
microearthquakes. On the other hand, we have found that shear wave polarization results based
on MEQ locations occurring outside the study area are consistent with those obtained from MT
data and known faults.
In particular, the 2003-2004 MEQ data shows spatial variability in shear wave time
delays which we attribute to variability in fluid filled fracture density. The spatial variability in
the magnitude and direction of shear wave splitting in the Rhyolite Plateau area is interpreted to
indicate the variability in the direction of fluid filed cracks. While useful in determining
directions, the MEQ data were not sufficiently numerous for shear wave tomography to image a
well target, as had been successfully performed in Basalt Canyon.
In contrast to the MEQ data, the interpretation of the 2004 MT data has provided a more
definitive delineation of the geothermal prospective area. It has also provided a conceptual
model of the Rhyolite Plateau useful in targeting exploration wells there. This aspect of our
project has resulted in general recommendations for geothermal exploration well locations. The
MT data has indicated the possibility of a deeper low resistivity zone at a depth of 7 km, which
might be interpreted as a heat source for the geothermal system.
This report includes both an American Geophysical Union meeting poster on the crustal
receiver functions calculated from the special seismic station installed in the 44-16 well and the
catalogue of microearthquakes recorded during the 2 field seasons completed during this study.
The research conducted here is part of a PhD Thesis in geothermal exploration methods by
Stephen A. Onacha
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1.0
Project Objectives
1.1
Introduction
The general purpose of this project was to conduct a test of a well-targeting technology
developed by Duke University under CEC support in 1996 to 1998. The technology is based on
the use of microearthquake locations and wave propagation effects to find high-productivity
geothermal zones, as demonstrated by Duke University in previous work (Malin and Shalev
1999). The immediate goal was to combine this exploration method with existing geophysical
measurements to guide the drilling of a 600-1000m geothermal exploration well in the Rhyolite
Plateau Area of Long Valley Caldera (LVC). The initial field study was completed as planned in
2003; however, because of a lack of earthquake data, no results could be reported. In the
summer of 2004, Duke University deployed another MEQ array, as well as 3 magnetotelluric
(MT) systems to record 33 MT stations. The MT data was to be used to supplement the MEQ
data, and assist in delineating the best targets for drilling exploration wells.
The Technical Objective of this project was to demonstrate that microearthquake location
and waveform studies can be used in combination with other geophysical measurements to target
high-productivity geothermal wells. The Economic Performance Objective of this project was
to lower the cost of geothermal development by reducing the costs of drilling multiple
geothermal production wells. Furthermore, the Economic Performance Objective consisted of
reducing the risk of target production wells and thereby reducing the capital and environmental
costs that are the principal barriers to rapid geothermal power development.
The value of this
technology was to be confirmed by drilling an exploration well in the Rhyolite Plateau and Dry
Creek Basin.
10
In the summer of 2004, Duke University in collaboration with the California Energy
Commission (CEC), Mammoth Pacific, and Kenya Electricity Generating Company (KenGen),
installed 20 short period portable seismographs to record microearthquake (MEQ) and 3
magnetotelluric (MT) systems to record 33 MT stations. All of the MEQ stations were co-located
at the same locations as MT stations. This project was carried out to acquire data that could be
used in an integrated interpretation scheme used to guide the targeting of geothermal exploration
wells in the Rhyolite Plateau Lease Area of Mammoth-Pacific in the west moat of the Long
Valley Caldera (LVC). The main purpose of this study was to identify and propose areas for
targeting shallow (less than 600 m deep) wells for harnessing steam at a temperature of more
than 170˚C. The MT and MEQ data acquisition and objectives are discussed below.
1.2
Microearthquake Survey
The MEQ survey in the Rhyolite Plateau Area was motivated by the success of acquiring
high quality data during CEC supported field campaigns in 1997, and good lateral correlation of
those results with the existing Casa Diablo wells. During that field campaign the south moat of
the LVC experienced several earthquake swarms, which provided suitable sources to image the
area where the stations were deployed. The data successfully defined shear wave splitting and
fracture density in Basalt Canyon, which was consistent with the regional and local structures. In
addition, the MEQ data from 1997 was used to identify a possible heat source below 6 km from
observations of shear wave attenuation.
Despite numerous studies (USGS), there are still major unanswered questions about the
mechanism of the seismicity and whether, in the long term, it is likely to decrease, remain in a
steady state, or increase. It is also unclear whether active dike intrusion is occurring along the
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Mono-Inyo series of craters. Understanding the intrusion mechanisms is important to
understanding the genesis of the hydrothermal systems in the LVC. More importantly for this
particular survey, it required that Duke University’s MEQ survey rely on an expectation that
lower magnitude seismicity could be recoded by using new 1 Hz seismometers set to record
continuously at 250 samples per second, rather than on trigger mode. Duke University had also
relied on previous studies that had shown the presence of events with different spectral
signatures, corresponding to either microearthquakes or events that were interpreted as
interactions between fluid movement and seismicity (Cramer and McNutt, 1997). Analysis of
near source characteristics of microearthquakes recorded from the Long Valley Exploratory Well
at a depth of 2.5 km, indicates that both statistics and near source spectra show significant
departures from standard earthquake models. These departures can be due to spurious downhole
seismometer sonde responses, propagation effects like high frequency scattering, and complex
source processes. Moreover, the statistics show that the scaling of small events changes at low
magnitudes. Duke University had therefore relied on the premise that by recording events with
small magnitudes and high frequency, this project would acquire enough information for shear
wave splitting tomography.
The microearthquake imaging included data acquired by Duke University in the CEC
supported MEQ data acquisition campaigns in 2003 and 2004. Only the 2004 data set yielded
useful shear wave splitting results. Although these were similar to that observed in the 1997 CEC
supported study and subsequent publications (Stroujkova and Malin, 2000; Stroujkova and
Malin, 2001; Stroujkova and Malin, 2002), there was insufficient data to support the shear wave
tomography for well targeting. The events included in the catalogue (Appendix B) of orientations
and time differences of S-wave arrivals were only from the 2004 survey. In addition, we used
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the clearer splitting in these events to develop more advanced software for identifying and
measuring orientations and fast and slow time differences. All of the related tasks enumerated in
the CEC agreement have been completed.
In addition to the surface networks deployed in 2003 and 2004, a deep borehole
seismograph was placed into the 44-16 well. This station was close to the center of the study
area and provided a high sensitivity station for detecting the times of even very small
earthquakes. The recorder for this station was run continuously and careful examination of these
data demonstrated a very low level of seismicity in the study area. The station did pick up many
significant but more distant earthquakes that were used to calculated the velocity oy structure and
layering of the caldera beneath it. Please see Appendix A for further detail on this study.
A catalogue of the microearthquakes recorded in 2003 and 2004 is given in Appendix B.
1.3
MT and TEM
MT and TEM data are routinely used in geothermal exploration to guide the targeting of
exploration and production wells. The resistivity data in most cases is used to map the clay cap,
indicative of the alteration rank that can be used as an indicator of reservoir temperatures. The
clay cap usually coincides with smectite alteration, which forms at slightly elevated temperatures
but disappears as temperatures exceed 150-200˚C. Since the target temperature for MammothPacific in the low temperature part of the field is 170˚C, the best targets for drilling exploration
wells in that area would be where the base of the clay cap is shallower than depths of 1000 m.
The resistivity data can be tied to temperature, permeability, and alteration data from existing
wells. The data is routinely used to define the likely geometry of the conceptual targets for
drilling geothermal exploration wells and characterize their relative risk.
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While, this was not part of the original proposal, it was added when MT equipment
became available through KenGen and Mammoth-Pacific provided funds to support equipment
shipment from Kenya and the MT acquisition. This survey also increased the chance of a
technical success because it had a lower component of risk due to new technology development.
Duke University also digitized paper resistivity data acquired by CGG and Woodward-Clyde in
1980 and 1982. These data sets have been incorporated in the new data sets acquired by Duke
University in 2004. Previous data acquired by EarthTech 1985 TDEM, UnoCal 1983 and
Wannamaker in 1991, have been included in the interpretation.
The Duke University MT project using three (3) systems manufactured by Phoenix
Geophysics of Canada was carried out to address the following concerns in these existing data
sets:
1
Fill in gaps in existing electrical resistivity data sets.
2
Acquire 5 channel MT data close to existing TDEM data to allow for static shift
effects common in MT data and the limited depth of penetration of TDEM data.
3
Re-interpretation of poorly resolved resistivity features and use of consistent
conceptual models to correlate resistivity with likely geology.
4
Local power line distortions of the electrical resistivity data.
5
Resistivity geometry, which is not consistent with 2 dimensional assumptions
used in early scalar AMT data acquisition.
6
Focus on evaluating the resolution and uncertainty in resistivity images so as to
establish realistic models that can be used for targeting wells.
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1.4
Purpose
The purpose of this project was to conduct a test of well-targeting technology based on use of
microearthquake locations and wave propagation effects to find shallow, high-production
geothermal zones. This test was designed to combine this exploration method with existing
geophysical measurements to guide drilling of a 600 to 1000m well in the Rhyolite Plateau and
Dry Creek Basin of Long Valley Caldera. The originally planned survey was supplemented by a
no project cost magnetotelluric (MT) data acquisition, routinely used in geothermal exploration.
The Economic Performance Objective of this project was to lower the cost of geothermal
development by reducing the costs of drilling multiple geothermal production wells.
Furthermore, the Economic Performance Objective consisted of reducing the risk of target
production wells and thereby reducing the capital and environmental costs that are the principal
barriers to rapid geothermal power development.
The value of this technology was to be
confirmed by drilling an exploration well in the Rhyolite Plateau and Dry Creek Basin.
1.5
The Overall Project Objectives
1. Collect and analyze microearthquake (MEQ) and magnetotelluric (MT) measurements
from Rhyolite Plateau.
2. Combine new data with existing geophysical data to create a well-targeting map.
3. Site and drill a cost-shared test hole to ground-truth the value of well-targeting map
4. Hold a Critical Pre-drilling Project Review Workshop to substantiate MEQ based targets.
5. Evaluate success of the test based on comparison of Pre-drilling/Post-drilling MEQ facts.
6. Reporting
15
2.0
2.1.
Project Outcomes
Data acquisition and analysis
In addressing Objective 1 the project accomplished the following:
2.1.1 Mobilization and data acquisition and demobilization
2.1.1.1 Arrange for cost-shared seismic Instruments
The Duke University obtained 24 L22 2Hz 3-component seismometers or equivalents of
GS-1 1-Hz sensors MEQ stations from the IRIS/PASSCAL Instrument Center which were
deployed in the field during the summer of 2003. In the summer of 2004, Duke University
obtained cost-shared 20 additional new cost shared MEQ instruments from KenGen
capable of continuous recording at 250 samples per second. A computer control system
for data management was also installed at Duke University. The time frame for the field
experiment was the summer of 2003 with a starting date of May 27, 2003. The new cost
shared seismographs from KenGen were deployed in the field in the summer of 2004 for a
period of two weeks
2.1.1.2 Identify Required Permits
The permits and written certification for the project were acquired by Robert Sullivan of
Mammoth-Pacific
2.1.1.3 Set up seismic array at the target area and demobilization
The seismic array of 30 L22 2-HZ seismometers were set up in the Rhyolite Plateau area
as scheduled from May 27, 2003; demobilization took place on August 24, 2003. Duke
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University rented a house in Mammoth for the field crews during that period. The follow-up
survey was carried between May and June 2004.
The Duke University and Mammoth Pacific LP team identified all suitable locations for
seismic station placement. The seismometers were deployed in the field with a distance of about
1 km between stations. Some instruments were outside the target area to assist in locating
earthquakes accurately. The 2003 and 2004 MEQ surveys were located in slightly different
locations.
2.1.1.3.1 Instrument deployment
The Duke team installed the seismic equipments at the chosen locations. During the
2003 survey, the seismographs were attached to outcrops with plaster to assure good
coupling with local geology; during the 2004 MEQ survey, the new seismometers were
coupled to the ground by a 70 cm long augur.
For the supplementary MT survey, 3 MT recorders were deployed in the field every day
and data acquired over night. A total of 33 stations were recorded. Some stations were
repeated so that the best data could be acquired while still in the field. Several data
acquisition quality challenges were encountered and addressed during the course of the
survey as discussed in the MT section of this report.
2.1.1.3.2 Test of array.
The MEQ data was collected from the field after the first three days and evaluated to
ensure that recording parameters were set correctly to acquire high quality data. Some of
the stations were moved after inspecting the data and adjusting the recording parameters.
To ensure that triggering requirements in the 2003 MEQ survey were not the cause of the
17
low level of seismicity, the 2004 array was adjusted to record continuously, using new
equipment capable of sampling at 250 Hz.
2.1.1.4 Array maintenance
2.1.1.4.1 Visit and maintain stations
Each seismic station was serviced once a week. The data was downloaded and
transferred to a computer for additional analysis and storage. This was done routinely while
the MEQ equipment was in the field for both the 2003 and 2004 surveys.
2.1.1.4.2 Data downloading
The Data was collected everyday from the field and downloaded to the main computer
at project headquarters.
2.1.1.4.3 Data archiving
Two copies of the data was recorded on DVD and saved to a hard disk.
2.1.1.5 Demobilizing
All the seismographs and recorders used during the 2003 field deployment were shipped
back to IRIS PASCAL. The equipment supplied by KenGen for the 2004 survey was
shipped back. The equipment locations were restored to their original appearance by
removing all deployment materials, filling any holes, and replacing native ground cover.
2.1.2 Analyzing microearthquake (MEQ) and magnetotelluric (MT) data
2.1.2.1 Purchase mass storage unit
18
In anticipation of a large data set to be collected from the project (400-500GB), Duke
University purchased an 802F mass storage unit from NextSore to be used as online data
location.
2.1.2.2 Read data tapes
The data collected was transformed to both SEGY and SEGD format for further
processing
2.1.2.3 Locate seismic events.
Duke University staff picked all phase arrivals and located the MEQ events recorded
during the 2 seasons of data collection supported, in part, by the CEC in 2003 and 2004.
Initially, these events were located using the standard HYPOINVERSE algorithm. All data
was inspected by a technician and a seismologist for integrity. A total of 434 events were
located in the combined 2003 and 2004 operation periods of approximately three months.
However, most of the events located were outside the Rhyolite Plateau research area. This
seriously affected the goals of our research proposal to use the data for 3-D tomography.
2.1.2.4 Identify shear-wave splitting.
Because the MEQ data could not be used for the initially planned tomography, Duke
University researchers identified and picked orientations and travel time differences of
polarized shear waves (shear-wave splitting) identified in the 2004 events. Very few
events in the 2003 survey had significant splitting. While each event in the 2004 catalogue
was tested for splitting, only a subset of approximately 30 events showed measurable
directions and differences in fast and slow S-waves at the second deployment stations.
19
These have been used to identify shear wave polarization and quantify the delay times
between the fast and slow S-wave polarizations.
2.1.2.5 Velocity and crack-density inversion.
The data collected within the research area was not adequate for performing a 3dimensional inversion of P-velocity, S-velocity, Vp/Vs ratio, and crack-density structures.
This has implications for use of MEQ for exploration in fields without well established
patterns of microearthquake activity regardless of the magnitude sensitivity of the array.
The technology is still applicable in areas with a known and suitable pattern of
microearthquake activity, typically in developed fields where reservoir management or
extension are economically significant issues. Appropriate MEQ patterns are particularly
common where deep injection wells consistently induce seismicity like at The Geysers and
Coso Geothermal Fields.
2.1.3 Initial and final scientific interpretations
2.1.3.1 Create 3-D geophysical map of the target area
The data collected was not adequate for 3-D modeling of the MEQ data. Therefore,
Duke University carried out an interpretation emphasizing existing electrical data and
newly acquired MT data to develop a conceptual model used to target successful
exploration wells. The probable boundary of the resource area is well defined in the
integrated MT interpretation and several specific exploration drilling targets are identified.
20
2.2.
Combine new data with existing geophysical data to create a welltargeting map
The main focus of the research has been to carry out detailed review of relevant data and
information to enable Duke University to combine new data with existing geophysical data to
create a well-targeting map. In addressing Objective 2 the project accomplished the following:
2.2.1 Pre-drilling geological and geophysical interpretation.
Duke University carried out a detailed review and interpretation of the geological and
geophysical data which was used to develop a conceptual model of the Rhyolite Plateau
area. This model has been used to recommend geothermal exploration drilling sites. The
detailed review of the existing data and interpretation are given in sections 3 of this report.
2.2.2 AGU presentation.
The Duke team presented preliminary results of some of the data, including a
complementary set of down hole measurement of microearthquakes in well 44-16 and the
LVEW well, at the 2004 AGU meeting in San Francisco (Chavarria et al., 2004).
2.3. Site and drill a test well to ground-truth the value of well-targeting
In addressing Objective 3 the project accomplished the following:
The recommended well sites and locations are given in section 3 of this report. The sites
are based on the conceptual model developed from combining some old data with new
data. The sites have been located with emphasis in targeting faults and also intersections of
the NW-SE and NE-SW faults. The proposed sites will ground-truth the value of welltargeting map.
21
2.3.
Hold a Critical Pre-drilling Project Review Workshop
Because the MEQ survey did not provide adequate data for shear wave tomography, the
Duke team has not formally presented and substantiated to MPLP and CEC all the evidence
obtained from the MEQ study for a shallow, 300 to 1000m deep drilling target in a Critical
Pre-drilling Project Review Workshop. Additionally, Duke has not prepared an informal
Workshop Monograph. Given the unexpected results of the MEQ experiment, it is
recommended that this workshop not be convened and that this report be accepted as the
final documentation on this project. However, the results of both MT and shear wave
splitting have been useful in locating drilling targets. We recommended that a well
targeting workshop be held.
The coordination for drilling Slim Holes and exploration wells will be coordinated by
Mammoth Pacific.
2.4.
Evaluate success of the test based on comparison of Pre-drilling/Postdrilling MEQ facts.
Although the well has not been drilled, the interaction of the MEQ part of the project
with the drilling is considered complete because no drilling recommendation could be
based on the MEQ data set only. The MT data set is relevant but was not considered as part
of the original proposal scope. The targets presented in this report are based on MT data
are, therefore, considered to be supplementary information, and can be reviewed in an
evaluation meeting. All of the digital MT data in standard EDI format have been provided
to MPLP for their independent evaluation of Duke’s conclusions.
22
2.6.
Reporting
Because the initial survey in 2003 did record enough microearthquakes for shear wave
tomography, not all the quarterly reports that Duke University expected to submit were
prepared in a timely fashion. Quarterly reports on the MEQ part of this project were
prepared for CEC. For the in quarters in 2004, no reports were submitted to MPLP because
no progress on the MEQ portion of the project was made due to the lack of data collected.
After, the data acquisition in 2004, a total of 10 quarterly reports were made.
This report completes this requirement for Duke University’s component of this project.
This specific review of the MEQ method required in the CEC agreement is addressed by
this report. The final Critical Post-drilling Project Review Workshop will be coordinated
by Mammoth Pacific.
The scientific contents of this report are part of a PhD thesis in geothermal exploration
methods research at Duke University by Stephen A. Onacha.
23
3.0
Results
3.1
Geological Setting and Tectonics
The Long Valley Caldera (LVC) is a topographic elliptical depression, approximately 15 by
30 km (9 by 18 miles) at the base of eastern escarpment of the Sierra Nevada range in California
(Figure 1). The LVC has been extensively studied for many years and the USGS has compiled an
extensive bibliographic list (Ewert et al., 2005). The LVC has been a persistent source of
volcanic activity throughout much of its geologic history (Bailey et al., 1976).
The most recent episode of volcanism began about 3 million years ago with widespread
eruptions of intermediate and basaltic lavas (Figure 2) accompanied with normal faulting. These
repeated episodes of volcanism and faulting might account for the variability in permeability and
fluid flow mechanism. Beginning about 2 million years ago, multiple rhyolitic eruptions from
vents along the northeast rim of the present day caldera formed the Glass Mountain complex.
LVC was formed about 760,000 years ago by the catastrophic eruption of more than 600 km3
(130 miles3) of rhyolitic lavas (the Bishop Tuff). This eruption was accompanied by subsidence
of crustal block 1 to 2 km (0.6 to 1.2 miles) into the partially evacuated magma chamber.
Subsequent eruptions of rhyolite lavas occurred around the margin of the resurgent dome
(Bailey, 1989, 1990; Bailey et al., 1976). Smaller eruptions from a residual magma chamber
accompanied by uplift of the central section of the caldera, is postulated to have formed the
resurgent dome. This hypothesis was the basis of drilling the LVEW, which was targeted with
expectations of finding a higher geothermal gradient. The test results show that this hypothesis
was not correct and therefore there is need for further research to understand the hydrothermal
system in LVC.
24
The most recent eruptions in the region occurred along the Mono-Inyo Craters volcanic
chain. Rhyolitic eruptions began along this chain about 40,000 years ago and have continued
through recent times with eruptions along the north end of the Mono Craters about 600 years ago
(Bursik and Sieh, 1989) and along the south end of the Inyo Craters about 550 years ago (Miller,
1985).
25
Figure 1 Tectonic setting of the Long Valley Caldera (LVC) showing major faults and
earthquakes of M5 or higher. Eathquakes occur mainly to the south of the LVC and along
active faults. Mono Lake is to the north of LVC, Owens Valley to the south and Glass
Mountain (modified from Hill et al., 1989). The location of slip of the 1872 earthquake in
the Owens Valley is shown.
26
Figure 2 Simplified Geological Map and history of the Long Valley Caldera and MonoInyo Craters (Modified from Bailey USGS). The history covers the period between 2.2
million and 300 years ago. The location of earthquakes is shown as swarms of dark circles.
27
In both cases, the eruptions resulted from the intrusion of an 810 km long, north-striking
feeder dike into the shallow crust that vented several places along strike. Intrusion of a shallow
dome beneath Mono Lake 250 years ago uplifted the lake-bottom sediments to form Pahoa
Island and vented in a small eruption of andesitic lavas from vents on the north side of the island
(Lajoie, 1968; Stine, 1987). The eruptive history of the Mono-Inyo volcanic chain over the past
5,000 years includes some 20 small eruptions at intervals ranging from 250 to 700 years.
3.2
Seismic Activity in the Long Valley Caldera
Earthquake and volcanic activity in this part of California and the Owens Valley corridor to
the south reflect the long-term interaction between tectonic and magmatic processes in the
Earth's crust and upper mantle underlying the Sierra Nevada and the Basin and Range Province.
Seismic studies in LVC over the last two decades have been very instrumental in understanding
the deep structure of the caldera (Hill et al., 1985b, Rundle et al., 1985, Kissling et al., 1984,
Sanders 1993, Sanders et al., 1995 and Steck and Prothera, 1994). All these studies show some
form of magmatic intrusion at a depth of 6 to 8 km. Of particular interest is the understanding the
nature of the heat source for the hydrothermal system, Steck and Prothero (1994) estimated an Swave velocity reduction of about 30% associated with the postulated magmatic body that could
provide the heat source to the hydrothermal system.
Current seismicity and offsets along the faults indicate that the faults are active. For the past
two decades, the LVC has been the center of increased seismicity, ground deformation, localized
increases in concentrations of volcanic gases, and postulated subsurface magma movement
(Langbein et al., 1995; Sorey et al., 1998). In 1978 seismicity in LVC began to increase and by
1980 a series of M6 earthquakes occurred to the south of the caldera. After 2 M5 earthquakes in
1983, a high precision ground deformation monitoring network was installed by the USGS. The
28
monitoring network included strainmeters, tiltmeters, and differential GPS. Leveling between
1980 and 1982 found 20 to 30 cm uplift of resurgent dome. This uplift was rapid at first but has
since slowed down. The cumulative uplift since 1975 is about 70 cm.
In 1989, an elevated level of carbon dioxide was the first indication of intrusion of magma
beneath Mammoth Mountain. By 1997, there occurred renewed uplift and seismicity inside the
caldera, which produced over 10,000 microearthquakes. Hypocenters of several hundreds of
swarm earthquakes were located precisely using multiplet location techniques (Stroujkova,
2000). That study showed that earthquakes were caused by displacements along sub-vertical
faults. In addition, focal mechanism analysis showed that the seismicity is compatible with the
regional stress field. Analysis of source-time functions of some “usual events” showed that these
events were triggered by a fixed length/time scale process, such as the length/inflation rate of a
magmatic or hydrothermal flow structure (Stroujkova, 2000). In this study, further analysis of Sto-P converted reflections located at a depth of about 7.6 km were postulated to be either
partially molten or fluid saturated.
Despite the numerous studies, there are still major unanswered questions about what started
the seismicity and whether it is now decreasing, in a steady state, or increasing. Furthermore, it is
unclear whether there is an intrusion in the Mono-Inyo series of craters. It is therefore important
to understand intrusion mechanisms in order to establish the hydrothermal fluid flow in LVC.
3.3
Hydrothermal activity and exploration history of LVC
In the early 1970's, intensive studies of LVC began through the USGS Geothermal
Investigations Program due to the existence of a large young silicic volcanic system (heat source)
29
and an active hydrothermal system which includes hot springs and fumaroles (Muffler and
Williams, 1976., Sorey et al., 1991). Hot springs are mainly found in the eastern part of the
caldera in areas where the land-surface elevations are relatively low. On the other hand
fumaroles exist primarily in the western part of the Caldera in high elevation areas. There are
two conceptual models for the hydrothermal activity of LVC.
The first model by Bailey, 1987, proposes that LVC has an active hydrothermal system
under the Resurgent Dome which feeds hot springs. The largest springs are in Hot Creek Gorge
where about 250 liters per second of thermal water discharge. This accounts for about 80% of the
total thermal water discharge in the caldera (Figure 3). The hydrothermal system is recharged
primarily from snow-melt in the highlands around the western and southern rims of the caldera.
The meteoric water infiltrates to depths of a few kilometers where it is heated to at least 220°C
by hot rock near geologically young intrusions. However, drilling in the Resurgent dome does
not confirm the postulated upflow.
Sorey, 1985 have postulated the upflow to occur in the west moat where the heated water
with lower density rises along steeply inclined fractures. The hydrothermal fluid flows laterally,
down the hydraulic gradient, from the west to the southeast in Casa Diablo and then eastward to
discharge points along Hot Creek and around Crowley Lake. Reservoir temperatures in the
volcanic fill decline from 220°C near the Inyo Craters to 50°C near Crowley Lake due to a
combination of heat loss and mixing with cold water. The upflow has not been confirmed.
30
Figure 3 Location of wells, hot springs, streams fumaroles and precipitation
recording stations in LVC for studying the natural hydrological variations and
the response of the hydrothermal system to volcanic and tectonic processes
(http://lvo.wr.usgs.gov/HydroStudies.html)
The hydrologic monitoring program by the USGS around the caldera has detected
changes in the hydrologic system caused by geothermal development and variations in
precipitation and recharge (USGS). Decreases in thermal-spring discharge caused by subsurface
pressure declines has been observed at sites within about 5 km to the east of Casa Diablo
geothermal field. There has also been an increase in steam discharge at Casa Diablo and sites
farther west due to increased boiling in the geothermal reservoir caused by geothermal
production. The long-term response of water levels in the wells is dominated by variations in
annual precipitation, which caused relatively high water levels in 1983-1986, an extended period
31
of declining water levels in 1987-1994, and a period of steadily increasing water levels from
1995 to the present (USGS).
Exploratory drilling to a depth of 3 km on the resurgent dome and subsequent
instrumentation of the Long Valley Exploratory Well (LVEW) has led to some understanding of
the tectonic and volcanic processes in LVC (Hill et al., 1998; Sorey et al., 2000; Sorey et al.,
2003). Additional drilling has provided useful information on the stratigraphy of the rocks
infilling the Caldera. The post caldera lavas occur upto a depth of 600 m in the central resurgent
dome and the syn-caldera Bishop tuff. Basement rocks occur below the caldera fill below a depth
of 1800m. The temperature gradient in the LVEW well is lower that expected considering that
the postulated uplift due to magma injection was anticipated to give higher temperatures. Results
from deep drilling has shown that the deeper hydrothermal reservoir which is associated with
Bishop Tuff shows no significant heat flow anomaly in the central Resurgent Dome (Finger and
Eichelberger, 1998). The existing down hole temperature measurements indicate that
temperatures at a depth of 6-8 km could be between 300˚and 400˚C (Sorey et al., 1991). Wells
drilled on the southwest side of the resurgent dome at Casa Diablo tap into the hydrothermal
system at 170°C to supply three hydrothermal power plants that generate about 40 megawatts of
electricity feeding into the Southern California Edison grid (Duffield et al., 1995).
Although extensive geophysical and hydrological studies have been carried out in LVC
over the last two decades, a conceptual model for the hydrothermal system and the nature of the
heat source have not been conclusively established. The upflow zones for the hot hydrothermal
system are not known. It is also not clear whether there is one major upflow zone or several
upflow zones. It is postulated that the heat source is located beneath the Mono-Inyo craters in the
western part of LVC (Sorey et al., 1991, Sorey, 1985). However, well INYO-4 (Figure 4) drilled
32
close to the craters show low temperatures with a maximum of 80˚C recorded at a depth of 490m
(Sorey et al. 1991).
Figure 4 Map of LVC showing major faults, postulated ring fracture (RFC), the Resurgent
Dome, Rhyolite Plateau (RP) and the Temperature exploration wells. Rhyolite Plateau is in
the Discovery Fault Zone (DFZ) with a NE-SW trend. HSF is the Hartley Springs Fault,
LCF is Laurel-Convict Fault, FLK is Fern Lake Fault and HCF is Hilton Creek Fault
(From Sorey et al., 1991)
33
3.4
Rhyolite Plateau previous geophysical and geological work related to
geothermal exploration.
Several integrated geothermal exploration programs have been carried out in the Rhyolite
Plateau geothermal lease area (Figure 5) in the western part of the LVC to determine the best
targets for drilling wells to supply adequate hydrothermal fluids to the generate electricity and
also determine the characteristics of the hydrothermal reservoir. Previous geological and
geophysical work has been summarized by Teplow (2002).
PLV-2
RHYOLITE PLATEAU
EXPLORATION AREA
INYO
CRATERS
44-16
PLV-1
RD-08
Figure 5 Rhyolite Plateau Exploration Area showing the location exploration
wells and faults. The faults have orientations in the NW-SE, EW and NE-SW
Detail studies on the structure of the western LVC (Suemnicht and Varga, 1988, Suemnicht,
1993) have led to the hypothesis that the upflow zone is the central part of the Rhyolite Plateau
about 1.5km to the north of the exploration well PLV-1(See Figure 4). Sorey et al., 1991,
provides additional information on the hydrothermal system in LVC based on downhole well
34
data, hydrothermal fluid sampling, electrical resistivity and age determination of hot spring
deposits. These studies indicate that a hydrothermal system might be found in the western part of
LVC. The evidence of temporal evolution of hydrothermal activity with two distinct times at
about 300,000 and 40,000 years ago might also contribute to the challenges of understanding the
evolution of the system and the fluid flow dynamics. The hydrothermal activity at about 40,000
years ago coincides with start of rhyolitic volcanism along the Mono-Inyo craters.
The structural trends identified by Suemnicht and Vaga (1988) show four distinct fault trends
in the northwest-southeast, northeast-southwest, north-south and nearly east-west. It is not very
well established which faults channel hot hydrothermal fluids. This research is therefore aimed at
partly analyzing the data to evaluate the role of the fractures. The east- west trending faults are
found close to the intersection of the northwest and northeast trending faults. The north-south
trending faults occur in short distances and are usually truncated by the northwest and northeast
trending faults.
By integrating geological, reservoir and geophysical data, Sorey et al., (1991) constructed
stratigraphic and hydrological controls of the hydrothermal system (Figure 6). Analysis of well
data in the Rhyolite area shows that the highest temperatures (more than 200˚C at an elevation of
1900 masl) were recorded in the exploration wells in the Rhyolite Plateau area (Figure 6).
Exploration wells 44-16 and M-1 show strong temperature reversals that indicate inflow of
cooler fluids perhaps controlled by faults. This is important because it outlines the complex role
of faults since they can act both as channels of hot or cold hydrothermal fluids. Analysis of the
water struck levels during drilling indicates that based on the available information, the water
table dips both to the northwest (44-16) and southeast (RDO-8) away from PLV-1.
35
The revised hydrogeological model by Teplow (2002) shows an upflow zone of
hydrothermal fluids at a temperature of over 200˚C is in the Rhyolite Plateau. It is postulated
that hydrothermal fluids flow laterally from WNW to ESE at a depth between 200-400 m. This
flow is postulated to be controlled by E-W trending faults. This study outlines the significance
of faults in controlling the flow hydrothermal fluids.
3.4.1 Ground Magnetic, Self Potential (SP) and Fracture Density
In this survey, the low magnetic susceptibility is attributed to demagnetization due to
circulation of hot hydrothermal fluids. The negative magnetic anomalies indicate either active or
relict hydrothermal systems. The negative anomalies show linear trends in the NW and NE
directions which seem to suggest strong structural control. The convecting hydrothermal fluids
may generate self-Potential in hydrothermal systems. The Rhyolite plateau is characterized by a
NE trending negative SP anomaly parallel to the NE trending Discovery Fault system (Teplow
2002). Although this might be interpreted as being indicative of the presence of hot hydrothermal
fluids, the data can be affected by local elevation and subsurface differences unrelated to
circulation of hydrothermal fluids.
3.4.2 Fracture Density
Analysis of microearthquakes shows that the clusters occur close to the faults in the
Rhyolite Plateau (Teplow 2002). Fracture density tomography by the shear wave splitting
method (Malin and Shalev 1999, Shalev 2001) indicates that the maximum fracture density at
shallow depths of 500m occurs in the Shady Rest area. Prejean et al., (2002) have analysis of
over 45,000 earthquakes that occurred between 1980 and 2000 in the Long Valley caldera area
36
using a double-difference earthquake location algorithm and routinely determined arrival times.
The locations reveal numerous discrete fault planes in the southern caldera. The faults in the
Caldera include a series of east-west striking right-lateral strike-slip faults beneath the caldera's
south moat and a series of more northerly striking strike-slip normal faults beneath the caldera's
resurgent dome. There is a strong correlation in the spatial distribution of the geophysical
anomalies which indicates that these anomalies might be caused by circulation of high
temperature hydrothermal fluids. These data sets indicate a possibility of several upflow zones
within the Rhyolite plateau area.
37
68-28
Figure 6 NW-SE geological section through the exploration wells in the western part of
Long Valley Caldera showing stratigraphy and faults. Maximum measured temperatures
measured in the wells are shown. The highest temperatures occur within the Rhyolite
Plateau (Modified from Sorey et al., 1991)
38
3.5
2004 Electrical Resistivity and Microearthquake studies
In the summer of 2004, Duke University deployed Twenty (20) new microearthquake
monitoring equipment and three (3) MTU-5A magnetotelluric equipment in the Rhyolite Plateau
(Figure 7). A total of 33 MT measurements were made at predetermined sites to fill in the
existing gaps in the data. Some of the MT sites were repeated in an attempt to improve the data
quality.
The careful evaluation of the data catalogue from the summer 2003 earthquake
monitoring campaign showed that the seismicity had been anomalously low when compared to
previous years. Difficulties with international business arrangements delayed the purchase of
both the earthquake monitoring equipment (provided through cost shares) and the
magnetotelluric profiling equipment (contributed to this project by the United Nations
Environmental Program and the Kenya Electricity Generating Company, KenGen), which made
it impossible to coordinate data collection with these two types of instruments. As a result, in
2003, we lacked sufficient data to accomplish our goal of creating a map of potential drilling
targets in our study area. Hence we completed the equipment purchases and planned a second
field campaign for 2004, using cost share funds.
The second field campaign took place mostly in June, 2004. A complement of 20 new
seismic recorders and 3 MT units were deployed in the Rhyolite Plateau area for several weeks
in the more promising areas found in the 2003 campaign.
3.5.1 Microearthquake Studies (MEQ)
In the summer of 2004, Duke University deployed 20 new Geospace GS-1 seismometers
in the Rhyolite Plateau at sites close to the MT measurements (see Figure 7). The GS-1 is a 339
component, 24-bit, 4-channel seismometer designed for seismic exploration in a variety of
terrains. The new seismometer is designed with a natural frequency of 1 Hz, well suited for
optimal response to microearthquakes
320000
322000 mi
ra R
dl e
Ca
324000
326000
328000
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395
P LV -2
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w 39
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I Y -4
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W 21
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4174000
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P LV -1
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US
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RD -08
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W 27
Tempwells
4168000
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DUKE04MEQ
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4166000
DUKEMT
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Wannamaker
S ca el 1 :100000
1000 2000 3000
m
4164000
318000
4164000
320000
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324000
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328000
330000
332000
Figure 7 Location of MEQ and MT stations in the Rhyolite Plateau area. Geothermal
exploration wells are shown in purple. MEQ stations from the 2003 deployment are as solid
black squares while the stations from the 2004 deployment are shown as labeled red
squares. The MT stations by CGG, Wannamaker and Duke are also shown in triangles of
different colors
The seismometers were anchored into the ground by 70 cm long auger (Figure 8). The
auger is a metal screw which couples the seismometer to the ground and increases security. The
seismometers were powered by lightweight alkaline batteries lasting between 40-60 hours. The
40
data was recorded on 2 Gigabytes flash cards. The PCMCIA flash storage card memories of 2
Gigabytes (GB) were changed every 2 days, at the same time the batteries were changed, even
though the flashcards had the ability to last up to 7-8 days while running at a sampling rate of
500 samples per second (2ms).
Figure 8 Components of the new GS-1 seismometer. The seismometer and the data
recorder couple together into one Unit which is anchored securely to the ground by the
ground auger
All events recorded during the field campaigns of data acquisition in the CEC supported
2003 and cost shared 2004 have been located these events using standard methods by utilizing 1D constant layer velocity structure. The events were located at a datum line which is the average
of the elevation of the recording sites. Station corrections were performed using the elevation
differences. A total of 434 events were located in the combined operation period of
41
approximately three months (Figure 9). About a quarter of these events were outside the limits
of the Caldera boundary.
Figure 3 Mapped locations for all events recorded in 2003 and 2004.
Colored dots represent earthquakes with specified depths. Dots and
triangles represent recording stations deployed in 2003 and 2004,
Most of the earthquakes are located to the south of the Caldera and occur at a depth of
more than 5km. Some shallow earthquakes occur at a depth of less than 2.5km. It is not very
clear why the earthquakes occur on the margin of the known geothermal resource. In some
geothermal fields for instance Krafla in Iceland, it seems that earthquakes occur at the boundary
of the low resistivity and high resistivity rocks (Onacha et al., 2005). Unfortunately for the LVC,
42