GEOPHYSICAL METHODS
Originally
appreared in:
January 2007 issue, pgs 47–52.
Posted with permission.
Passive low frequency spectral analysis:
Exploring a new field in geophysics
This pathfinder/DHI technique is under rapid development and uptake
for exploration.
René Graf, Spectraseis Technologie AG, Zurich; Dr. Stefan M. Schmalholz, Swiss Federal Institute
of Technology, Zurich; Dr. Yuri Podladchikov, University of Oslo;
Dr. Erik H. Saenger, Freie Universität Berlin
Amplitude, arbitrary units
In 2003, a group of scientists in Swit- Hz as noise, and for good reason: conven- A strong scientific team, substantial rezerland set out to answer some intriguing tional geophones are insensitive in this do- search funding and the support of credquestions with implications for the way main and little useful data can be expected. ible operating partners would also be reoil and gas reserves are discovered and As one geologist put it, “All my career I’ve quired. All of this came at a time when
produced. Research conducted by Dr. been fighting noise. Now you want me to investment in geophysical services, not
Stefan Dangel at the University of Zur- believe the noise is information?”
to mention funding for technology startich had highlighted a strong and consisYet, low-frequency waves are less sus- ups, was at a low ebb.
tent empirical relationship between low- ceptible to many of the problems that
With barely a scent of the present exfrequency spectral anomalies in seismic plague conventional seismic and electro- ploration boom in the air, Spectraseis Techbackground wavefields and geological magnetic methods, particularly in areas nology Inc. was founded in early 2003 to
characteristics of a collection of reservoirs, with poor seismic response or obstacles begin the task of acquiring low-frequency
mainly in the Middle East. Similar ob- such as thick basalt or conglomerate lay- seismic data and to develop industrial
servations have also been reported in the ers. Successful unraveling of these pat- applications as the research progressed.
Russian literature since the early 1990s.
terns observed in the sub-10-Hz domain Promising early work with Petrobras in
Dangel’s research was robust by any would be a valuable new contribution to Brazil and a Shell affiliate in Austria drew
standard, but focused on one feature in exploration geophysics.
Swiss government funding for a dedicated
particular: curious amplitude peaks clusA high-quality effort would need to research group at ETH Zurich. An intered around 3 Hz in surface velocity data acquire new, high-quality datasets and vestment by the new technology ventures
measured above hydrocarbon reservoirs.1 tackle the physical mechanisms behind group of Norsk Hydro in 2005 helped to
The possibility of a universal hydrocar- these “hydrocarbon micro-tremors” that accelerate and expand the development of
bon-indicator, while attention-grabbing, Dangel1 and others2,3,4,5 have found. commercial acquisition systems and data
did not sit well with the realprocessing software.
world complexities that the
Today, with a research and
3.0
industry confronts day-to-day.
development team of 10 sciNot above oil reservoir
Moreover, the reasons for such
entists and commercial land
Above oil reservoir
features were left largely open.
surveys planned or in progress
2.5
The question was whether
with Petrobras in Brazil, Pemex
Dangel’s research pointed more
in Mexico, Norsk Hydro in
2.0
generally to coherent patterns
Libya and KOC in the Arabian
in low-frequency background
Peninsula, it is evident that
1.5
waves. If so, could these be dilow-frequency analysis will be
rectly related to reservoirs and
part of the exploration and res1.0
other subsurface structures in
ervoir characterization toolkit
a way that would provide new
of the future. The questions
data for exploration and pronow are which applications
0.5
duction decisions?
will prove most useful and how
An accumulating body of
quickly the rest of the industry
0.0
knowledge in the earth science
will embrace them.
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10
Frequency, Hz
world suggested this might be
the case, but the subject had
HYDROCARBON
never been seriously tackled Fig. 1. Data from a survey in Brazil showing consistent
MICROTREMORS
with an eye on oil and gas. The anomalies in the Fourier spectra of surface velocities,
The starting point for our
within and outside the boundaries of a known oil
seismic industry systematically measured
work
has been the empirical
reservoir.
disregards seismic data below 10
observations of Dangel, et al.,1
JANUARY 2007 World Oil
GEOPHYSICAL METHODS
and since then, augmented with larger
scale surveys. More than 20 studies at
different oil and gas fields around the
world have shown characteristic spectral
anomalies with a high degree of correlation to the location and geometry of hydrocarbon reservoirs.1,2,3,4,5,6
The studies focus on passive seismic data in the 1- to 10-Hz frequency
range, acquired using high-sensitivity
broadband seismometers instead of conventional geophones. The key observation is that modifications of the seismic
background spectrum are different for
interactions with geological structures
containing hydrocarbon-filled pores
compared to interactions with similar structures containing water only. In
other words, “hydrocarbon tremors”
may be viewed as a frequency dependent
“scattering” of the incoming background
waves. Subsurface structures, such as
hydrocarbon reservoirs, generate characteristic patterns in the Fourier spectra of
surface velocities, Fig. 1.
The assessment of subsurface structures by analyzing the Fourier spectra of
surface velocities is in line with an increasing number of methods that investigate
ambient seismic waves to get information
about subsurface structures. For example,
earthquake hazards for buildings are assessed in this way, by deriving site-specific
amplification factors (micro-zonation)
for incoming earthquake waves.
Power spectral density, (cm sec)2 Hz
DATA ACQUISITION
Passive low-frequency surveying, as
the name implies, relies on ambient seis-
mic waves, a continuous natural wave
field found with varying amplitudes at
every location on earth.7 The main driving force for these waves are believed to
be the ever-present ocean waves. They
have a peak around 0.2 Hz, as can be
seen in Fig. 2.
As with other passive methods, lowfrequency surveys enjoy significant environmental, logistical and cost advantages over conventional active seismic and
electromagnetic methods that require
a powerful artificial source. They also
have an inherently low health-and-safety
risk profile.
Spectraseis’ proprietary approach,
dubbed HyMAS, uses ultra-sensitive,
portable 3C broadband seismometers
to acquire complex data, which can be
filtered to separate artificial and surfacegenerated signals from spectral patterns
related to subsurface structures. The instruments are installed in shallow holes
about 0.5-m deep for weather shielding
and to improve coupling, Fig. 3.
Others have experimented with different equipment. A low-frequency survey led by Shell in Saudi Arabia’s Rub
Al-Kali Desert employed amplified 3C,
4.5-Hz geophones, surface deployed and
equipped with a low-noise amplifier to
increase sensitivity and integrate passive
recordings within a conventional 2D
seismic acquisition workflow.8
Ideally, a grid layout is used with node
spacing ranging from 250 to 1,000 m.
Several monitoring stations are installed
for the duration of the entire survey. The
crew moves the recording stations from
point to point, leaving them at a given
location to measure and record between
3 and 24 hours, depending on local
noise conditions (high cultural/industrial noise levels require longer measuring
times), Fig. 4.
Quality control is applied in the field.
Data points must meet certain specifications. An efficiently managed survey of
100 sq km can be acquired by a crew of
20 to 30 people in about 30 days. This
is significantly faster than the acquisition
of active seismic data. Specialized data
management software and field stations
launched by Spectraseis in 2006 have
moved low-frequency surveys from the
realm of scientific experiments to a tightly controlled and highly scalable commercial operation. What does this mean
for an operator? Low-frequency data can
be acquired, processed, interpreted and
used in decision-making in as little as
10% of the time needed to acquire and
process a conventional seismic survey.
CAUSES OF 1 TO 10 HZ
SPECTRAL ANOMALIES
It has been established in many parts
of the world that coherent patterns relating to oil and gas reservoirs exist in the
low-frequency domain. Identifying the
underlying physical mechanisms of these
so-called Direct Hydrocarbon Indicators (DHIs) is the key challenge for us
as researchers. Success will enable us to
perform more realistic numerical simulations and sensitivity studies. Three
candidates are described below and illustrated in Fig. 5.
10-8
10-10
Noisy
10-12
Quiet
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10-16
0.001
0.01
0.1
1.0
Frequency, Hz
10
100
Fig. 2. A survey of seismic stations worldwide shows
that seismic background waves vary in amplitude but are
continuously present. The so-called “ocean wave peak”
is seen around 0.2 Hz. Source: Aki, K., Richards, P.G.,
Quantitative Seismology: Theory and Methods. Freeman 1980.
Fig. 3. A portable broadband seismic station installed at a
Pemex field in the Burgos basin, Mexico.
JANUARY 2007 World Oil
Standing wave resonance.
frequency dependence of the
This occurs on a macro-scale,
reflection coefficient.
where characteristic maxima are
generated due to reflections beResonant
amplification.
tween the reservoir and the surResonant amplification effects
face, and within the reservoir,
of ambient seismic waves are
caused by complex impedance
also promising candidates for
contrast due to the reservoir.
explaining hydrocarbon miWhen seismic waves propagate
cro-scale tremor signals. These
from one medium into another
effects will behave like a driven
medium with different complex
source and they are supported
impedance, then a part of the
by the following observations1:
wave is reflected. The charac1. An often narrow-freteristic two-way travel time, or
quency range of the signal
resonance frequency, between
(1.5–4Hz)
the Earth’s surface and the bot2. The mean absolute powtom of a low-velocity surface
er of the hydrocarbon tremor
layer or the top of a reservoir,
depends on the level of the engenerates characteristic spectral Fig. 4. A survey layout for Norsk Hydro over a field in Libya
vironmental noise
anomalies. Importantly, the ef- includes monitoring stations, a 1,000-m grid and two densely3. The power of the signal
fective impedance contrast can spaced line profiles for the non-permanent stations.
is proportional to the total hybe enhanced or solely generated
drocarbon-bearing layer thickby high attenuation in reservoir
ness of the reservoir
rocks.9 We study spectral anomalies gener- the dominant mechanism between 1
4. 3C recordings show a trough inated by standing wave resonance for elastic and 10 Hz is the so-called patchy satu- stead of a peak in the H/V-ratio
and poroelastic media.
ration model.12,13,14 We study patchy
5. Investigations on the wave-field
saturation effects within the reservoir propagation directions (using a direcSelective attenuation. Characteristic to determine under what conditions a tionally sensitive sensor setup) showed
minima are due to frequency-dependent selective, frequency-dependent attenu- that the signals causing the anomaly
attenuation within the reservoir. Fre- ation could generate spectral anomalies originate from the reservoir direction.
quency-dependent reflections take place similar to the observed hydrocarbon miDirect numerical simulations using
if the seismic waves hit a layer with dif- crotremor signal.
Navier-Stokes equations show that pores,
fusive attenuation properties or if waves
Wave attenuation on the small-res- which are partially saturated with oil and
propagate from an elastic into a poroelas- ervoir scale will be approximated by an gas, exhibit a resonance frequency. This
tic medium.9,10 There exist several physi- effective model, capturing the essentials resonance mechanism can be approximatcal models to describe the attenuation of of wave attenuation and dispersion, at ed by a damped-oscillator model. Dependseismic waves due to wave-induced po- the larger scale (top 10 km) to evaluate ing on pore geometry, the oscillator modrous fluid flow.11 These models describe the transfer of the spectral anomalies to- els are either linear or nonlinear.15 Similar
wave attenuation on different spatial and ward the Earth’s surface. We particularly resonance effects have been described for
temporal scales.
focus on the differences between gas and capillary trapped oil blobs.16,17
A model that presumably describes oil pore-fill and the consequences on the
We couple the oscillator model to a
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Fig. 5. Three possible mechanisms that generate DHIs in the background spectrum: standing wave resonance, selective attenuation,
resonant amplification.
JANUARY 2007 World Oil
World Oil JANUARY 2007
3
GEOPHYSICAL METHODS
Data storage
Normalization
Signal cleaning
Calibration
Analysis
Mapping
QC and management
e.g. time and frequency
filtering
Final values
Homogenization
Information consistency
Hydrocarbon
potential and thickness
Point data QC
Raw data
Processing
sensor
MDA
Point data analysis
Interpretation
Outputs maps
Reporting
Dataset analysis
Templates
Central
database
Information archiving for 4D
Fig. 6. RIO’s integrated workflow automates many processing
functions and has drastically cut the turnaround time and
strengthened quality control for low-frequency data sets.
one-dimensional wave propagation equation and study under what conditions the
resonance of the oil-filled pores is activated, and under what conditions the resonance frequency can be measured at the
surface. In particular, we investigate the
coupling between the porous reservoir
material and the ambient rock material,
and the subsequent propagation of the
resonant waves to the Earth surface.
An alternative explanation of the
mechanism is the so-called “drop-bubble model”, which considers non-equilibrium phase transitions in
hydrocarbon reservoirs.5
DATA PROCESSING AND
INTERPRETATION
The aim of processing the
data is to remove or attenuate
all signals that are not related to
subsurface structures—mainly
surface noise, generated by
road traffic, industrial activities,
wind and rain—and to correct
the dataset for inter-temporal
and near-surface geology-related variations. For this purpose,
a proprietary toolbox has been
developed that encompasses
data storage, data management
and data-selection routines. A
range of techniques can be applied to the selected data encompassing time domain, frequency domain and combined
temporal-spectral analysis.
The company’s processing
software suite, called RIO (Fig.
6), contains more than fifty processing and analysis routines,
Fig. 7. A mapping tool for multi-attribute and geostatistical
interpretation of low-frequency passive seismic data.
several of which are the subject of key patent applications. One recently developed,
patent-pending technique applies an innovative auto-normalization method to
resolve signal variations over time.
Processing has been largely automated so that relatively simple cases can be
processed with a minimum of human
interaction. More difficult problems
still need an experienced analyst to solve
them. Overall, the RIO software package
has drastically reduced the turn-around
time for data sets and strengthened qual-
ity control by moving stable processes
from the test bench to a well structured
and controlled workflow.
The company has also developed a
specialized mapping and geostatistical
program for interpretation, which it will
be providing to survey customers under
a limited license, Fig. 7. It allows an efficient interpretation of low-frequency
survey results. The low-frequency data
can be easily and interactively overlaid
with other geological and geophysical
information, i.e., seismic contour maps,
fault maps, etc. In addition,
a variety of mathematical
operators for multi-attribute
cross-correlation can be applied. The program also allows a visual inspection of
various data sets.
Fig. 8. The maximal value of the V/H ratio within the 1- to 6-Hz
range for each sensor is shown over the southern part of a
fully explored reservoir in Voitsdorf, Austria. This alternative
to the standard H/V technique is an additional, proprietary
attribute for microtremor hydrocarbon detection.
JANUARY 2007 World Oil
APPLICATIONS
Low-frequency data can
be applied in exploration,
field appraisal and production. It is an additional tool
to reduce the risk of drilling a
dry hole by providing a DHI.
It should not be viewed as a
stand-alone tool, as it cannot
provide the detailed geometrical information necessary
for planning wells.
This new passive technique is particularly efficient
for exploration. It allows
large exploration areas to be
screened quickly and at a low
cost to identify areas with a
high hydrocarbon potential.
More expensive 3D seismic
can then be limited to these high-potential areas. This saves time and money and
can significantly shorten the time span
from exploration to production.
The technique is also suitable to identify stratigraphic traps that are normally
not mappable on 2D- or 3D-seismic
data, an increasingly important application as the oil industry runs out of classical structural traps.
Another interesting area of application is in deep offshore exploration,
where large fields are still expected to be
found. 3D seismic normally cannot identify whether mapped structures contain
hydrocarbons. The new method can.
CASE STUDIES
The volume of data supporting lowfrequency spectral analysis is now accumulating rapidly. At least nine new surveys will be completed in the first half
of 2007, which will more than double
the volume of data available for further
research. While the usual limitations on
disclosure of customer data have constrained the number of published case
studies, two can be mentioned here.
Petrobras Mossoro. A 100 sq km
blind test for Petrobras in 2004, covering a known complex producing field in
the Potiguar basin in northeastern Brazil,
clearly identified two, and partly revealed
the third, producing zones within the
block. The results also showed a strong
positive correlation between signal amplitudes and oil column thickness measured by eight logged wells. Subsequent
surface corrections and reprocessing,
using newly developed 2006 processing techniques, resolved the discrepancy
with respect to the third area. Further
surveys starting this month in the same
region will provide a comprehensive set
of new results.
RAG Voitsdorf. An experimental survey in collaboration with Shell-affiliate
RAG in Austria correctly predicted the
company’s first successful oil well in ten
years. Data from an exploration target
was calibrated with the results from an
existing producing area some 5 km to the
north. The results (Fig. 8) indicated that
an oil column at least twice as thick as
the 15-m payzone of the northern field
could be expected from the alreadyplanned exploration well, and in fact,
some 32 m of pay was logged.
A number of major operators have also
reported encouraging results from experimental surveys focusing on low frequen-
cies, including Kuwait Oil Co. (KOC)
and ADCO in the Middle East.2,4
New studies from surveys in Mexico
and Libya should be released in the near
future, supported by both seismic and
drilling results. Field development and
reservoir characterization applications
are the focus of a five-year technical
co-operation agreement recently signed
with KOC.
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OUTLOOK
2007 promises to be a breakthrough
year for the development of both our scientific understanding of low-frequency
behavior and interest in commercial
applications of passive seismic techniques. An EAGE workshop on passive
seismic methods and applications held in
Dubai from December 10–14 attracted
some 120 professionals from various oil
companies and contractors.
Developments in the next six months
will include a groundbreaking marine
trial in the North Sea, in collaboration
with Norsk Hydro and Scripps Institution of Oceanography in San Diego.
The marine survey will use recoverable
ocean bottom sensors deployed over a
proven non-producing field on the Norwegian Shelf.
Our scientific strategy is to use wellestablished and state-of-the-art theories
and tools—including data analysis, normal-mode analysis, poroelastic theory,
attenuation, inversion, and numerical
modeling—to evaluate and tune the
technique to particular campaigns, to
improve methods for finding oil and gas.
Results of this research will be used to incrementally improve the data processing
work flow.
Although many important questions have now been answered, others
remain and will no doubt be the subject
of research and debate for some years to
come. The challenge for low-frequency
spectral analysis, as for any new geophysical technology, will be to mature
and establish the method’s technical
limits while channeling development efforts toward applications with the highest payoff for the industry.
WO
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LITERATURE CITED
Dangel, S., et al., “Phenomenology of tremor-like signals observed
over hydrocarbon reservoirs,” Journal of Volcanology and Geothermal
Research, 128(1-3): pp. 135–158, 2003.
Akrawi K. and G. Bloch, “Application of passive seismic (IPDS)
surveys in Arabian Peninsula,” EAGE Workshop: Passive Seismic:
Exploration and Monitoring Applications, Dubai, United Arab
Emirates, 2006.
Birialtsev, E. V. and I. N. Plotnikova, I. R. Khabibulin, N. Y. Shabalin, “The analysis of microseisms spectrum at prospecting of oil reservoir on Republic Tatarstan,” EAGE Conference, Saint Petersburg,
Russia, 2006.
Rached, G. R., “Surface passive seismic in Kuwait,” EAGE Workshop: Passive Seismic: Exploration and Monitoring Applications,
Dubai, United Arab Emirates, 2006.
Article copyright © 2007 by Gulf Publishing Company. All rights reserved.
Not
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15
16
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Suntsov, A. E. and S. L. Aroutunov, A. M. Mekhnin, B. Y. Meltchouk, “Passive infra-frequency microseismic technology–Experience and problems of practical use,” EAGE Workshop: Passive Seismic: Exploration and Monitoring Applications, Dubai, United Arab
Emirates, 2006.
Holzner, R., et al., “Applying microtremor analysis to identify hydrocarbon reservoirs. first break, pp. 41–46, 23 May 2005.
Berger, J. and P. Davis, G. Ekstrom, “Ambient Earth noise: A survey
of the global seismographic network. Journal Of Geophysical ResearchSolid Earth, 109(B11), 2004.
Al Dulaijan, and P. Van Mastrigt, et al., “New Technology applications in the Rub Al-Khali Desert,” GEO 2006 Conference, Bahrain.
Korneev, V. A., and G. M.Goloshubin, T. M. Daley, D. B. Silin,
“Seismic low-frequency effects in monitoring fluid-saturated reservoirs.” Geophysics, 69(2): pp. 522–532, 2004.
Dutta, N. C. and Ode, H., “Seismic reflections from a gas-water
contact. Geophysics, 48(2), pp. 148–162, 1983.
Pride, S. R., and J. G. Berryman, J. M. Harris, “Seismic attenuation due to wave-induced flow,” and references therein, Journal of
Geophysical Research—Solid Earth, 109(B1), 2004.
Gurevich, B. and S. L. Lopatnikov, “Velocity and attenuation of elastic-waves in finely layered porous rocks.” Geophysical Journal International, 121(3), pp. 933–947, 1995.
White, J. E., and N. Mihailova, F. Lyakhovitsky, “Low-frequency
seismic-waves in fluid-saturated layered rocks,” Journal of the Acoustical Society of America, 57: S30–S30, 1975.
Johnson, D. L., “Theory of frequency dependent acoustics in patchysaturated porous media,” Journal of the Acoustical Society of America,
110(2): 682–694. 2001.
Holzner, R., and P. Eschle, M. Frehner, S. M. Schmalholz, Y.Y.
Podlachikov, “Hydrocarbon microtremors interpreted as oscillations
driven by oceanic background waves,” EAGE 68th, Vienna, Austria,
2006.
Hilpert, M. and G. H. Jirka, E. J. Plate, “Capillarity-induced resonance of oil blobs in capillary tubes and porous media,” Geophysics,
65(3), pp. 874–883, 2000.
Beresnev, I. A., 2006. Theory of vibratory mobilization on nonwetting fluids entrapped in pore constrictions. Geophysics, 71(6): N47–
N56.
ADDITIONAL REFERENCES
Biot, M. A., “Mechanics of deformation and acoustic propagation
in porous media,” Journal of Applied Physics, 33(4), pp. 1482–1498,
1962.
Carcione, J. M. and S. Picotti, “P-wave seismic attenuation by slowwave diffusion: Effects of inhomogeneous rock properties,” Geophysics,
71(3): O1–O8, 2006.
Carcione, J. M. and F. Cavallini, J. E. Santos, C. L. Ravazzoli, P. M.
Gauzellino, “Wave propagation in partially saturated porous media:
Simulation of a second slow wave,” Wave Motion, 39(3): 227–240,
2004.
THE AUTHORS
René Graf is chairman
of Spectraseis Technology Inc. He has more
than 30 years of oil and
gas industry experience
at Shell International and
as managing partner of
PROSEIS, an integrated
geological and geophysical consulting firm. He
holds a MSc in geophsysics from ETH Zurich.
rene.graf@spectraseis.com
Dr. Stefan M. Schmalholz is a senior research
scientist and lecturer at
the Geological Institute of
the Swiss Federal Institute of Technology (ETH)
Zurich, Switzerland. stefan.schmalholz@erdw.
ethz.ch
Dr. Yuri Podladchikov is a professor and PGP
Division lead at Norway’s Center of Excellence
for the Physics of Geological Processes at the
University of Oslo, Norway and a senior consulting scientist with Spectraseis Technology
Inc. iouri.podladtchikov@fys.uio.no
Dr. Erik H. Saenger is a senior research scientist and lecturer in geophysics at Freie Universität Berlin, Germany. saenger@geophysik.
fu-berlin.de
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