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TECHNICAL REVIEW OF BERGERMEER SEISMICITY STUDY

TNO REPORT 2008-U-R1071/B

6 NOVEMBER 2008

    

Professor Bradford H. Hager and Professor M. Nafi Toksöz

Department of Earth, Atmospheric and Planetary Sciences

Massachusetts Institute of Technology

Cambridge, MA 02139

USA

8 October 2009

TABLE OF CONTENTS

SUMMARY………………………………………………………
…………………………………….....	  3							

INTRODUCTION……………………………………………………
…………………………………	  6

TECHNICAL
REVIEW…………………………………………………………
……………………..	  7

    
Background……………………………………………………
………………………………………..	  7

     Geologic
Model…………………………………………………………
……………………………… 9

     Subsidence
Modeling………………………………………………………
……………………….	10

     Reservoir
Engineering……………………………………………………
……………………….. 10

     Geomechanical
Model…………………………………………………………
……………………10

     Seismic Hazard
Analysis………………………………………………………
…………………..14

CONCLUSIONS AND
RECOMMENDATIONS………………………………………………..1
6             

REFERENCES……………………………………………………
………………………………………18

ANSWERS TO THE QUESTIONS OF GASALARM
2………………………………………20 

ANSWERS TO THE QUESTIONS OF 

           SOIL MOVEMENT TECHNICAL
COMMITTEE……………………………………..25	

APPENDICES

     A.  Project Description – Technical Review (original
document)………………... 26

     B.  Questions I and II (original
document)………………………………………………….28

     C.  Brief Resumes –
Hager/Toksöz…………………………………………………
…………..32

SUMMARY

The scope of the TNO study is to assess the risks of seismic activity
induced by proposed gas storage and injection at the Bergermeer field. 
This activity would involve both pressure and temperature changes
resulting from injection of cold gas (in the initial stages of the
project) and from gas production.  These temperature and pressure
changes generate both changes in local thermoelastic and poroelastic
stresses and changes in stresses associated with differential compaction
and expansion.

The TNO report “Bergermeer Seismicity Study” is a comprehensive
document.  We reviewed the report, its conclusions and recommendations,
and an extensive list of related references.  The report can be divided
into three parts: Reservoir Modeling, Geomechanical Modeling, and
Seismic Hazard Analysis. Before addressing the main conclusions of the
TNO study below, in the order in which they were given in the report, we
state here what we believe is the most important conclusion:  We agree
with the result of the TNO study that the maximum magnitude of an
earthquake that could occur in the Bergermeer field during the proposed
injection and production phase is ML = 3.9.

Reservoir Modeling

In a reservoir modeling study, temperature changes were simulated for
one production cycle.  The two reservoir modeling conclusions,
paraphrasing the TNO summary, along with our responses (in italics),
are: 

The injection of cold cushion and working gas and one production phase
show a decrease in temperature localized around the wells, with the
temperature of the rest of the field substantially unchanged.

The reviewers concur with this conclusion.

The initial two years of injection lead to the largest temperature
decreases.  Afterwards the reservoir pressure has increased to the point
that the gas needs to be compressed, increasing the injection
temperature.  Subsequent gas production leads to the reheating of the
previously cooled regions.

The reviewers concur with this conclusion.

Geomechanical Modeling

The pressures and temperatures obtained from the reservoir model are
used in two-dimensional geomechanical models to calculate changes in
stress, deformation and fault stability.  The geomechanical models
depend on a large number of input parameters about the geologic
structure (in both two and three dimensions), stress state, initial
conditions and material properties, not all of which can be specified
accurately. The reviewers believe that the results of geomechanical
modeling alone, by themselves, cannot quantify the seismic hazard.  They
can, however, provide a useful picture of the processes in the reservoir
and can contribute to the understanding of induced earthquakes.   The
four main conclusions from the geomechanical modeling, paraphrasing the
TNO summary, along with our responses, are: 

During modeling of depletion (1971-2006), only fault segments
intersecting or bounding the reservoir showed the potential for
reactivation.  Large fault movements occurred on the central fault of
the reservoir where reservoir rocks on both sides of the reservoir
overlap.  At the end of depletion, calculated stress conditions on the
central and bounding faults are close to failure.

The earthquakes that happened during production provide important
observational evidence that these faults were indeed reactivated.  Since
2001, the reservoir pressure has decreased by an amount comparable to
that during the period between the two sets of earthquakes - 1994-2001. 
The significance of the conclusion is that, even if no
injection/production activity were undertaken, the Bergermeer field
could have earthquakes comparable in magnitude (ML=3.0-3.5) to those
that occurred in 1994 and 2001. 

During injection, the main parts of the faults intersecting and bounding
the reservoir are calculated to stabilize.  Locally some fault slip is
calculated on the central fault directly above and below the overlap of
the reservoir.  Calculated fault movements during injection are an order
of magnitude smaller than during depletion.

Production-induced stresses have accumulated since their potential
release in the 2001 earthquakes.  Therefore the reviewers agree that the
initial pressure build-up during the early part of the injection may
reduce the shear stresses on and contribute to the stabilization of
faults. However, the results of the geomechanical models may not be
reliable indicators of the range of fault slips that might occur during
injection, because the stress conditions calculated at the time of the
initiation of injection are not consistent with the observed reverse
faulting.   The change in pressure between 1994 and 2001 is comparable
in magnitude to the change in pressure planned for reinjection.  This
amount of pressure change might result in fault slips comparable to
those that generated the ML = 3.5 earthquake in 2001.

During production of the working gas, no fault slip occurs in the
geomechanical models.

As was the case for conclusion 4, the calculations may not be reliable
indicators of the fault slip that might be triggered during production
of the working gas.

The localized temperature decreases that occur during the initial
reinjection do not affect the stability of known faults if the injection
wells are at least 200 m (uncertainty included) from these faults.

The reviewers agree with this conclusion.

Seismic Hazard Analysis

Magnitudes of potential seismic events were estimated from fault
movements derived from the geomechanical models.  The TNO study
conclusions and our responses, in italics, are:

7)  During injection, the largest slip observed in the geomechanical
models corresponds to seismic magnitudes ranging between 2.4 and 2.7.

Because geomechanical models depend on large numbers of parameters with
various levels of uncertainties, the reviewers find that magnitude range
of 2.4 to 2.7 is too restrictive.  Events larger that ML=2.7 cannot be
ruled out.

8)  The maximum possible seismic magnitude is 3.9.  Larger magnitude
earthquakes are improbable due to the limited dimensions of the faults. 


The reviewers agree with the maximum magnitude ML=3.9

Recommendations

The reviewers find the 6 recommendations listed in the TNO report to be
reasonable and support them, except for one caveat concerning
recommendations 3 and 4:  We recommend that consideration be given to
modifying the geomechanical models so that they explain the reverse
faulting earthquakes that occurred in 1994 and 2001 if these models are
to be used to evaluate uncertainties in reservoir conditions. 

Introduction

We were asked by the Ministry of Economic Affairs, Netherlands, to
conduct a   technical review of the report “Bergermeer Seismicity
Study, TNO Report 2008-U-R1071/B, 6 November 2008.”  The TNO report
was prepared to assess the seismic risk due to injection and production
activities if the depleted Bergermeer natural gas field were to be used
in the future as an underground gas storage facility.  We were
specifically instructed to submit a report containing:

A critical technical review of the assumptions, conclusions and
recommendations of the TNO Report

Answers to the questions raised by the Gasalarm2 Foundation and the Soil
Movement Technical Committee.  (APPENDIX B)

We were provided with the TNO report and supporting confidential and
public (published) documents.  (See REFERENCES.)  In addition, we
studied a number of articles relevant to both the general theme of the
report and to the specific methodologies and analyses used in the study.

A number of email exchanges and telephone conversations were held with
Economic Affairs Ministry staff members (Drs. D. Voskuil, C. deZwaan) to
clarify the nature and the scope of the review.  Because of some
difficulties in communication the reviewing process did not start until
11 September 2009.  A teleconference was held between the reviewers
(Drs. B. Hager and M. N. Toksöz) and representatives from the EA
Ministry, KNMI, TNO and TAQA.  A set of Power Point presentations that
was emailed prior to the teleconference gave further details about the
material included in the report and helped to clarify some points.

Additional email communications and documents transmitted to the
reviewers by the participants of the teleconference were helpful for the
review.  In summary, the EA Ministry and the team of
scientists/engineers that contributed to the report, “Bergermeer
Seismicity Study,” have been very responsive to the questions and to
the requests of the reviewers.  The only difficulty for the reviewers
has been the very tight schedule. 

The TNO Report “Bergermeer Seismicity Study” is a well-written,
well-documented study.  Most of the major aspects of assessing induced
seismicity are addressed.  Relevant assumptions, data and parameters
used in the study are presented clearly.  Well-tested modeling codes are
used for reservoir simulation and geomechanical modeling.  We consider
the TNO report a very good study, as far as it goes.

In studies of this nature there are never sufficient data to produce
models with complete certainty.  It is not possible to characterize all
aspects of the geologic models and conditions that give rise to tectonic
forces, deformation and earthquakes.  There are two approaches to deal
with uncertainties.  One is deterministic modeling, where an informed
judgment is made about the best parameters (e.g., fault geometries,
elastic moduli, fault slip, fault area) and the seismic moment and
magnitude for a representative earthquake are calculated.  

A few models may be generated based on “normal” conditions and on
other plausible models to determine sensitivity to various parameters. 
Then a final model is chosen based on some selection criteria.

An alternative method is a probabilistic approach.  Uncertainties are
assigned to all parameters and are incorporated into the calculations
with many thousands of models, generally done using a Monte Carlo
simulation.  The results of such an approach are a “best estimate”
and statistical confidence bounds.  The choice of deterministic or
probabilistic calculation depends on the nature of the problem, types
and amounts of data, and the computational resources. 

In the TNO study the deterministic approach was used because the problem
is confined to a single field, with a relatively well-known structure. 
In addition, a limited amount of earthquake data is available.  Also,
making thousands of reservoir simulations and geomechanical calculations
would have been prohibitive in time and in cost.

In deterministic modeling, the geomechanical model and parameters used
are based on available data and experts’ opinions.  In that sense, it
is subjective.  There could be differences of opinions between experts. 
In fact, the reviewers faced this issue on some topics.  

In the technical review given in the next section, we cover the topics
in the order that they are discussed in the TNO report.

TECHNICAL REVIEW

The TNO Report provides detailed information about the field, the
methodology, and the models and data used for the seismicity study. 
Chapters 2 through 7 describe the general background, subsidence
modeling, reservoir engineering modeling, geomechanical analysis and
seismic hazards analysis.

Background 

Background material given in chapter 2 is very important because it sets
the framework for the study and describes the primary data used to
constrain the geomechanical models.  Four earthquakes that occurred in
the Bergermeer field (Table 1) provide critical data for the seismic
hazard study.  Table 1 lists the dates, magnitudes and intensities of
the four earthquakes in the Bergermeer field and a fifth (October 10,
2001) in the neighboring Bergen field.  Figure 1 shows the epicenters of
the events (Ref. Haak et al., 2001; KNMI Technical report:  TR-239).  

Table 1. Seismic moments, magnitudes of earthquakes considered in the
report

Date	Moment M0 (Nm)	KNMI

 ML1	Reamer & Hinzen ML2	Intensity

6 August 1994	4.0 x 1013	3.0	3.2	IV-V

21 September 1994	7.0 x 1013	3.2	3.4	V

9 September 2001	1.9 x 1014	3.5	3.8	VI+

10 September 2001	6.3 x 1013	3.2	3.4	IV-V

10 October 2001	1.8 x 1013	2.7	2.8	III+



1 Data from Haak, 1994a, 1994b and Haak et al., 2001.

 2 Calculated using equation (9) of Reamer and Hinzen, 2004

Figure 1 (from Haak et al., 2001). Epicenters of the five earthquakes
discussed in the report. Epicenters of events 1-4 are on the Central
Reservoir fault of the Bergermeer Field. Event 5 is associated with the
neighboring Bergen field.

Seismic moment (M0) is the most robust measure of the size of an
earthquake.  It is given by

	M0=A·d·µ

where M0=scalar moment, A=fault (rupture) area, d=fault slip
(displacement) and µ=shear modulus.  The moment can be determined from
the displacement spectra (low frequency limit) of seismograms.  The
shear modulus is obtained from seismic velocity and density or from
geomechanical modeling.  Fault area and slip cannot be obtained
independently without either using a simplified source model or
synthetic seismograms using finite source models to match the observed. 
The KNMI reports used a Brune source model (Brune, 1970).  This model
assumes a circular fault rupture surface (radius r) and determines r,
stress drop (∆() and fault slip (d) from the moment, displacement and
corner frequency.  The KNMI reports clearly mention the models and the
equations used to calculate the radius (r) and fault slip (d).

However, the Brune model, which uses a circular fault, is not an exact
match for the long rectangular faults.  Faults used in the Bergermeer
field geomechanical modeling are long (>2 km) and thin (width 200-450
m).  Source parameters (e.g. fault slip) obtained by the Brune model
provide a good approximation, but not rigid constraints, for the slip
for geomechanical models.  To obtain better constraints, it is necessary
to calculate synthetic seismograms using a rectangular fault geometry
and distributed slip to match the recorded near-field seismograms.  The
required geologic structure and information about seismic velocities, as
well as appropriate computational algorithms, are available for such
calculations. 

Geologic Model

The geologic model of the Bergermeer field is well defined by ample
geologic, 3-D surface seismic and well-log data.  Figure 1 shows the
faults that define the structure of the field and the epicenters of the
Bergermeer earthquakes.  The field is an elongated feature and lies on a
horst block trending in the NW-SE direction.  The model used for
reservoir simulation and geomechanical modeling is a model that combines
inputs from Horizon Energy Partners (2006 report) and from TNO and TAQA
scientists.  The reservoir, the Slochteren sandstone, is a fine-grained,
competent sandstone.  The overlying seal is the Zechstein formation,
which consists of a series of evaporites.  There are a number of faults
trending in the NW-SE direction.  The ones most relevant to the
seismicity study are faults that define the NE and SW boundaries of the
reservoir and one internal fault, that we call here the Central
Reservoir fault.  The Central Reservoir fault may be viewed as a
“scissors” fault because slip on it decreases to the north, becoming
too small to be imaged seismically in the middle of the field.  Note
that the hypocenters of the four earthquakes with magnitudes ML=3.0 or
larger (nos. 1, 2, 3, 4 in Figure 1) appear to lie on the Central
Reservoir fault close to the “hinge of the scissors.”  The fifth
earthquake, located about 5 km away, is associated with the neighboring
Bergen field.  (Haak et al., 2001, KNMI Technical Report TR-239).

For reservoir simulation and subsidence modeling, a three-dimensional
geological model was used.  For the geomechanical calculations a
two-dimensional model based on a NE-SW cross section, perpendicular to
the strike of the structures, was used.  We discuss the potential effect
of assuming a two-dimensional structure in the geomechanical model later
in the report.

Subsidence Modeling

The surface subsidence due to gas production and resulting reservoir
compaction was monitored by frequent leveling campaigns (1980, 1981,
1984, 1992, 1997, 2001, 2006).  The maximum subsidence observed for the
Bergermeer field is       10.5 cm.  The shape of the subsidence bowl and
its maximum amplitude depend on the reservoir pressure drop (compaction)
and elastic properties of the overlying strata. Elastic moduli were
determined for each layer in the geological model using its density and
seismic velocities and using a static/dynamic correction factor.  To
determine the compaction parameters from the observed subsidence,
elaborate forward modeling and inversion methods were used.  The
sensitivity of the subsidence to varying the elastic parameters of the
overlying geological units and to the effects of the compaction of
neighboring gas fields was investigated.

The reviewers find this study to be comprehensive and credible.  The
study results, which state that some uplift is expected from
repressurization of the reservoir and that,  “Based on the subsidence
data the range (of compaction parameter) is between 0.3 10-5 and 1.1
10-5 bar-1,” are reasonable.  The reviewers are not aware if there
were borehole markers to monitor compaction in the Slochteren reservoir
unit.  If such data existed, it could reduce the estimated range of the
compaction parameter.

Reservoir Engineering

Dynamic reservoir modeling to determine pressure and temperature during
injection and production was done using state-of-the-art reservoir
simulation codes.   Both Eclipse 100, used for isothermal flow, and
Eclipse 300, used for composition and thermal simulation, are leading
reservoir simulators used world-wide.   The reviewers find this study to
be well done and comprehensive.  The results are intuitive and logical. 
Important insight was gained into the thermomechanical response of the
reservoir to depletion and repressurization.  The temperature changes,
localized around the boreholes, are most significant within about a 100
m radius of the injection wells.  Keeping the injection wells at least
150 m from faults and monitoring the well temperature and pressure are
sound recommendations.  

Geomechanical Model

TNO calculated the stresses and fault slips accompanying reservoir
production and injection using the reservoir modeling software, DIANA. 
The model domain assumed for computational efficiency is a
two-dimensional cross section, chosen to contain sandstone in both Block
1 and Block 2 in contact across the Central Reservoir fault.  The
seismic hazard associated with gas storage was calculated under the
assumption that the predicted fault slips and associated fault areas
could be converted to moment and then to magnitude using the parameters
estimated by Hanks and Kanamori (1979).  The local magnitude, ML, is
assumed to be equal to the moment magnitude, Mw.  (We note that the
scatter in the data upon which the Hanks and Kanamori scaling
relationship is based is typically  ± 0.5 magnitude unit. The resulting
uncertainty is not discussed in the TNO report.) 

In order to have confidence in predictions of the geomechanical models
during the planned injection phase, the models should be tested by
comparing their predictions to available observations of subsurface
conditions during the production phase.  In the TNO report, model
predictions were compared to the stress estimates from post-production
minifrac tests in well BGM#8, several hundred meters (~ 1 ½ times the
reservoir thickness) away from the Central Reservoir fault.  Model
predictions of surface subsidence were compared with the observed.  The
geomechanical parameters and the “scenarios” that matched the
maximum subsidence values were selected.

In our opinion, because the purpose of the study was to assess the
potential for earthquakes, the most important observations for testing
the geomechanical models are the earthquakes that were associated with
production.  The only such events detected have fault slip in the
reverse sense.  In our view, there is no reason to doubt the reverse
faulting focal mechanisms of the 2001 events determined by Haak et al.
(2001).  The suggestion by Dost and Haak (2007) that this reverse
faulting resulted from differential compaction that reactivated
preexisting normal faults is plausible.  Although there is uncertainty
in the hypocenter locations of the 1994 and 2001 events, their most
likely location is on the Central Reservoir fault.  In our opinion, it
is important that a geomechanical model used to predict future fault
movements explain these events.  The TNO geomechanical model predicts
that the Central Reservoir fault should have slipped in a normal, not a
reverse sense, during the production phase.  For this reason we do not
believe that it makes reliable predictions of fault motions expected
during reinjection.

 

We believe that one reason that the TNO geomechanical model does not
predict the reverse fault motions associated with differential
compaction of the reservoir is that it does not include the faults or
other means to accommodate this motion where it would be expected to
occur.  Specifically, Fault 4 (see Figure 2) terminates just above the
top of the Block 1 reservoir.  As noted by Roest and Kuilman (1994),
boundary effects occurring at the contact between compacting and
non-compacting formations are large near the top of a reservoir.  In
addition to the creation of differential normal stresses, shear stresses
along a fault plane extending above the reservoir would increase,
leading to conditions that promote slip.  However, there is no fault in
the TNO model in the location most likely to have reverse motion induced
by compaction of the reservoir.  Alternatively, a ductile region could
accommodate the compaction-induced shearing by deformation distributed
over a shear zone.  Because the Zechstein formation is an evaporite, it
might be expected to deform in this way, but such ductile deformation
was permitted in only one of the seven geomechanical models presented.
(The single model with ductile deformation did not match the minifrac
stress estimate, but the effects of different flow laws were not
reported.  This single model does not provide a basis for rejecting all
models with ductile deformation.)

With a structure (fault or ductile region) to accommodate deformation,
compaction of the reservoir could lead to down dropping of the footwall
block above the reservoir, reducing the normal stress clamping the fault
and causing shear stress in an orientation that promotes motion in a
reverse sense on the preexisting (normal) Central Reservoir fault.

Figure 2 (modified from Figure 6.7 of TNO, 2008).  Fault 4, the Central
Reservoir fault, separates the reservoir sandstones (yellow) of Block 1
and Block 2.  Faults 3, 4, and 5 terminate in the Zechstein formation,
where it is assumed that the rocksalt composition makes seismic slip
less likely than in the reservoir sandstone.

An additional important limitation of the TNO geomechanical model is
that it assumes a two-dimensional geometry in a region where the
structure is changing substantially along strike.  A simple
visualization is provided in Figure 3, modified from Haak et al. (2001),
illustrating that the central fault is a scissors fault, with the
separation between the reservoir rocks in Block 1 (east) and Block 2
(west) varying along strike.  This three-dimensional variation in
structure could be important because slip on one segment of the fault,
perhaps aseismic, could transfer stress to an adjacent segment of the
fault, amplifying the stress on the fault generated by differential
compaction alone.  

Figure 3.  Block diagram of the three-dimensional structure of the
Bergermeer reservoir (modified from Figure 6 of Haak et al., 2001).  The
dashed lines indicate the projection of the “plane” of the Central
Reservoir fault above the compacting reservoir in Block 1.  Compaction
of the reservoir in Block 1 causes subsidence of the region in the
“footwall” along the projection of the Central Reservoir fault.  The
resulting loading generates shear stress in a reverse sense, as
sketched.  Because of the properties of the Zechstein, it is likely that
the region loaded by compaction of the reservoir would shear, either by
local or distributed deformation, shedding stress onto the portion of
the fault where more brittle sandstone is on both sides of the fault. 
It is in the latter region where the 1994 and 2001 reverse faulting
earthquakes probably occurred.

During production, two earthquakes occurred in 1994, followed by two
more earthquakes in 2001.  The change in pressure between 1994 and 2001
is comparable in magnitude to the change in pressure planned for
reinjection.  This amount of pressure change might result in fault slips
comparable to those that generated the ML = 3.5 earthquake in 2001.

Seismic Hazard Analysis

The Bergermeer region is relatively aseismic.  There is no evidence of
earthquakes in the historic seismicity data of the region (de Crook,
1993).  It is reasonable to assume that the earthquakes in 1994 and 2001
were induced events associated with gas production.  The TNO report
states that production in the Bergermeer field began in 1971 and
continued until its depletion in 2006.  Empirically, induced earthquakes
in oil and gas fields are more likely to occur during the “maturity”
phase of the fields.  In that sense, the history of Bergermeer
seismicity conforms to the general pattern of gas fields.  An unusual
feature of the seismicity is that four events of magnitude ML=3.0 to 3.5
have occurred since 1994, but there are not the large number of smaller
events that are typical of natural and induced seismic patterns.  The
limited number of events and lack of data for a broader magnitude range
limit the application of statistical analysis methods for hazard
assessment.

Table 1 shows moments and local magnitudes of the four earthquakes that
occurred in the Bergermeer field.  These data are taken from detailed
technical reports of KNMI (Haak 1994a, 1994b; Haak et al., 2001).   The
table shows differences between empirical relations used to convert
seismic moments to magnitudes.  The KNMI uses conversion calibrated for
the Netherlands.  In general, KNMI magnitudes agree with those
calculated using the empirical relationship of Hanks and Kanamori (1979)
based on a global average.  Reamer and Hinzen (2004) have a somewhat
different relationship based on data from southern Netherlands and
Germany.  Their moment-magnitude relationship gives slightly larger
magnitudes, as shown in Table 1 and in Figure 4.  The purpose of this
comparison is to demonstrate that there could be differences in
magnitudes assigned to a given event by different observatories.  
Typically, uncertainties for a given magnitude should be about ( 0.1
magnitude. 

One way to estimate the maximum magnitude of a likely event is a common
sense approach:  “Any future event could be at least as large as an
event that occurred in the past.”  For conservative estimates,
generally, a safety factor is added to the observed maximum magnitude. 
With the data in Table 1, a conservative estimate of the maximum
magnitude would be ML=3.9.

The approach taken in the TNO study is to use geomechanical modeling to
calculate the deformation, stress evolution and fault slip during
pressurization and depletion of gas.  This approach is useful for
evaluating the impact of various conditions in the reservoir on fault
movements.  However, the modeling requires many inputs about the initial
conditions, fault parameters, failure criteria and rheological
properties, all of which have uncertainties that could affect the
seismic event magnitudes.  The maximum magnitude is limited by the size
of the fault and the displacement (i.e. fault slip) that could occur on
the fault.  It cannot exceed the value that would result from the
rupture of the whole fault.

The maximum magnitude ML=3.9 cited in the TNO report (p. 87, conclusion
#8) is an appropriate value.  The probability of an event of this
magnitude is extremely low (Ref. van Eck, et. al., 2006; Figure 3).  A
magnitude of ML=3.9 would correspond to a peak intensity VI+, a value
between intensities VI and VII, but closer to VI.

Figure 4:  Illustration of local ML vs. Mw and Mo, from Reamer and
Hinzen (2004)

CONCLUSIONS AND RECOMMENDATIONS

The TNO report, “Bergermeer Seismicity Study,” is a comprehensive
document.  It utilizes large amounts of data combined with elaborate
modeling of phenomena related to seismicity and potential earthquake
hazard during gas injection and production.  Based on our review of the
report and on the extensive list of related publications, we conclude:

 The Report addresses, broadly, the issues related to the seismicity and
seismic hazard at the Bergermeer field.

The computations and numerical models are done with
“state-of-the-art” computer codes.

The results of the subsidence study and the reservoir simulations, for
flow and for temperature, are clearly presented.  We agree with their
conclusions and recommendations.

Geomechanical analysis for stress and deformation modeling is done by
finite element modeling, including poroelasticity and frictional fault
behavior.  The models provide insights into potential deformation and
fault slip (i.e. earthquakes) on the faults included in the mesh. 
However, as elaborated earlier in this report, the approach has some
serious shortcomings for the prediction of location and magnitudes of
potential earthquakes.

It is a two-dimensional model dealing with a three-dimensional reservoir
that varies substantially along strike of the model, particularly along
the Central Reservoir “scissors” fault.

Fault structures or other means of accommodating anelastic deformation
are not included in the regions immediately above the reservoir, where
large stresses are generated by production and injection.  (In only one
model was this region allowed to deform by ductile flow, and this model
was discounted for other reasons.)

It assumes two-dimensional planar fault surfaces without
heterogeneities, asperities or stress concentration from slip variations
in the third dimension.

It relies heavily on fault displacement (slip) for determining the
seismic moment and hence the magnitude of induced earthquakes.  The
moment depends on the product of fault slip times fault area. 
Independent knowledge of the fault area is needed to determine the slip.

 

Because of the limitations of the geomechanical modeling, the reviewers
suggest relying more heavily on available earthquake data for estimating
the maximum magnitudes of potential earthquakes.  For the maximum
magnitude, the reviewers agree with the value of ML=3.9 cited in the
report. 

A detailed analysis and modeling of seismic records from close-in
stations of the 2001 Bergermeer earthquakes would provide more detailed
information about their source mechanisms.   The reviewers do not expect
further analysis to change the conclusion that these are reverse
faulting events.  However, more accurate determination of the depth,
amount of fault slip, and dimensions of the faults that slipped could be
obtained.  The reviewers recommend that this be done.

Probabilistic seismic hazard estimate for induced earthquakes in the
Netherlands has been done for gas fields in the Netherlands  (van Eck et
al., 2006).  This includes the Bergermeer field, albeit with few data. 
The results are consistent with those of the TNO report in that the
probability of any event of ML=3.9 or greater is extremely low.

References

BGM_4 and 8 analysis-Final rev26-11-08 .pdf (2008)

Brune, J.N. (1970). Tectonic stress and the spectra of seismic shear
waves from earthquakes, Journal of Geophysical Research 75, 26,
4997-5009.

de Crook, T. (1993). Probabilistic seismic hazard assessment for The
Netherlands, Geologie en Mijnbouw 72, 1-13.

de Crook, Th., H.W. Haak, B. Dost (1998). Seismisch risico in
Noord-Nederland, KNMI Report TR-205, 24 pp.

Dost, B., and H. W. Haak (2007).  Natural and induced seismicity, in
Th.E. Wong, D.A.J. Batjes, and J. de Jager, eds., Geology of the
Netherlands, Royal Netherlands Academy of Arts and Sciences, 223-229.

Grasso, J.R. and G. Wittlinger (1990). Ten years of seismic monitoring
over a gas field, Bulletin of the Seismological Society of America 80,
2, 450-473.

Haak, H.W. (1994a). Seismische analyse van de aarbeving bij Alkmaar op 6
augustus 1994, KNMI Report TR-166, 17 pp.

Haak, H.W. (1994b). Seismische analyse van de aardbeving bij Alkmaar op
21 september 1994, KNMI Report TR-167. 21 pp.

Haak, H.W., B. Dost and F.H. Goutbeek (2001). Seismische analyse van die
aardbevingen nij Alkmaar op 9 en 10 september en Bergen aan Zee op 10
oktober 2001, KNMI Report TR-239. 24 pp.

Hanks, T.C. and Kanamori, H. (1979). A moment magnitude scale, Journal
of Geophysical Research 84, 2348-2350.

Horizon Energy Partners BV, Report, 2006.

Logan, J.M., N.G. Higgs, J.W. Rudnicki (1997). Seismicity risk
assessment of a possible gas storage project in the Bergermeer field,
Bergen concession, Report to BP, 137 pp. (confidential).

McCurdy, P. (2008).  High Level Geomechanics Review for Bergermeer Gas
Storage Project, Senergy, Technical Note P\Project\W07TBV01L

Reamer, S.K. and K. Hinzen (2004). An earthquake catalog for the
Northern Rhine area, Central Europe, Seismological Research Letters 75,
6, 713-725.

Roest, J.P.A., and W. Kuilman (1994).  Geomechanical analysis of small
earthquakes at Eleveld gas reservoir, Eurock ’94, 573 – 580.

TNO presentation (2008). BGM-4 and BGM-8 Mini Frac Analysis, TNO Final
Presentation Rev01, 23 October 2008.

TNO report (2008). Bergermeer Seismicity Study, TNO Report
2008-U-R1071/B.

Van Eck, T., F. Goutbeek, H.W. Haak, B. Dost (2004). Seismic hazard due
to small shallow induced earthquakes, KNMI Report WR 2004-01. 52 pp.

Van Eck, T., F. Goutbeek, H.W. Haak, B.Dost (2006). Seismic hazard due
to small-magnitude, shallow-source induced earthquakes in The
Netherlands, Engineering Geology 87, 105-121.

Van Lith, J.C.G. (1983). Gas fields of Bergen concession, The
Netherlands, Geologie en Mijnbouw 62, 	63-74.

Van Wijhe, D.H., M. Lutz & J.P.H. Kaasschieter (1980). The Rotliegend in
The Netherlands and its gas accumulations, Geologie en Mijnbouw 59,
3-24.

Wald, D.J., V. Quitoriano, T.H. Heaton and H. Kanamori (1999).
Relationships between peak ground acceleration, peak ground velocity and
modified Mercalli Intensity in California, Earthquake 	Spectra 15, 3,
557-564.

ANSWERS TO THE QUESTIONS OF THE GASALARM2 FOUNDATION

1. TNO uses elasto-plastic geomechanical models to calculate potential
slip on a fault plane. A critical geometry of reservoir and fault
structure is chosen, which is sensitive for reactivation of the fault.
Plastic (reversible) slip is calculated on the fault, during depletion
and injection assuming permanent equilibrium conditions (implying the
assumption that all potential slip created by the preceding depletion of
the field has been accommodated).

Gasalarm2 is of the opinion that in reality discrepancies from the ideal
shape of the fault plane as used in the model may be present, in the
following called obstructions, preventing incremental a-seismic movement
along the fault, and that therefore it cannot be excluded that the
reservoir fault(s) are (is) in a meta-stable condition (“hanging
earthquake” that could be triggered).

In the opinion of Gasalarm2 the model predictions in the TNO study
concerning maximum possible slips that could be created by the,
relatively small, pressure changes during one injection-production cycle
(corresponding to M = 2.4 - 2.7, should they be accommodated in an
non-elastic (seismic) manner) are self-evident given the model
assumptions and, therefore, provide no proof that no larger event can be
triggered.

Question: Is the above-mentioned TNO approach a complete and reliable
way to explore maximum potential slip during the project phase?

Answer:  The TNO approach assumes that all of the yielding calculated in
the models on a given fault could occur during a single earthquake. 
Strong asperities that did not fail as the result of loading that has
already occurred would tend to reduce the amount of coseismic slip. 
Because the stress loading is cyclical during depletion and injection,
stress would not be expected to accumulate on asperities in the same way
it does on tectonic faults, where the loading stress is always applied
in the same direction.  Thus the TNO conclusions might appear to be
conservative from the standpoint of the effect of slip hanging up on
obstructions (or asperities). 

However, the two-dimensional models might underestimate the stress that
could accumulate on asperities if loading is transferred “out of the
plane” along strike of a fault, as shown in Figure 3

It therefore seems plausible from a geomechanical perspective that
larger magnitude events than those predicted by the TNO geomechanical
analysis could occur.  This conclusion agrees with both the TNO seismic
hazard analysis and our inference from seismicity models that a maximum
expected earthquake of magnitude 3.9 might occur.

2. (With reference to the calculations in Chapter 7 of TNO (2008))

Gasalarm2 states (see Q1) that part of the calculated slip, which did
not show up in the 4 (historic) earthquakes may still be present as a
‘hanging’ quake. On the basis of Table 7-1 of TNO (2008) the
magnitude of such a quake can be 3.8 (taking the dynamic shear-modulus
for the estimate of slip/magnitude, rather than the static as in Table
7.1), or larger, given the uncertainty in parameters and dimensions (see
e.g. Q4). 

Question: What is the opinion of the expert(s) about this issue?

Answer: We agree with the TNO report that the static shear modulus is
the appropriate modulus to use to calculate the stress state on faults
caused by quasi-static loading.  It is this stress, associated with slow
loading, that is released during an earthquake.  Increasing the
magnitude of a potential earthquake by using the dynamic modulus is not
appropriate.  

Other uncertainties, which are addressed in the answer to the first
Gasalarm2 question, support the Gasalarm2 estimate that a magnitude 3.8
event is plausible, but not for the reasons stated in posing the second
question.

3. Figure 3,2 of the Seismicity Report shows a 3D view of the Bergermeer
gas field (based on a model of Horizon 2006). From this view Gasalarm2
concludes, that the main (internal) fault may be longer than
anticipated. According to Gasalarm2 the length of the fault is probably
4.1 to 5.9 kilometres and not 2.5 kilometres. Consequently, Gasalarm2
assumes, that the probable size of the reactivated part of the fault
plane may be much larger than is stated in table 2.2 of the TNO report
(page 18) and therefore the potential magnitude of earth tremors may be
much higher (M=4.1).

Question: What is the relation between the length of the fault plane,
the probable activated part of the fault plane during the events and the
maximum magnitude of a seismic event? How important is the estimation of
the total length of the central fault?

Answer: The magnitude of a seismic event is proportional to the area of
the part of the fault that ruptures during an event, not the total
length of the fault.  Thus it is the estimate of that part of the
Central Reservoir fault that would break, not the total length of the
fault, that is important.  We agree with the TNO report that the part of
the Central Reservoir fault that cuts through the rocksalt of the
Zechstein formation is unlikely to slip in a seismic event.  Therefore,
the length of 2.5 km is appropriate.

4. Gasalarm2 assumes that the stabilisation of the fault structures at
reservoir level due to pressure increase during injection will be of
minor importance as compared to potential previously created unreleased
tensions (see Q1 and Q2).

TNO assumes that the re-pressurization of the reservoir will lead to a
more stable fault structure (see chapter 6.3 of the TNO Seismicity
report).

Question: what is the opinion of the expert(s) about these views?

Answer:  While repressurization will generally tend to reduce the
stresses caused by production, the amount of repressurization planned is
substantially less than the amount of depressurization, so stresses on
some faults might not be completely reversed.  There is often a time
delay between when a fault is stressed and when it eventually ruptures
in an earthquake.  Indeed, it is fairly common for earthquakes to occur
in a reservoir even after production ceases.  Thus, earthquakes at
Bergermeer might well occur even if repressurization did not proceed. 

5. Gasalarm2 observes that for the operating phase of the BGS only the
first production/injection cycle has been modelled by TNO. In
particular, the recovery phase of the cushion gas has not been covered
(based on a realistic estimate of the then prevailing reservoir
conditions).  Apart from risks resulting from phenomena such as erosion
of the fault plane and fatigue, (see TNO recommendation page 87, #3),
the seismic risks associated with final cushion gas recovery should not
be ignored.

Question: What is the opinion of the expert(s) about the missing
analysis?

Answer:  In our view, including the final recovery of the cushion gas
would not change the conclusions in an important way.  The recovery
phase is expected to be similar to the second half of the initial
production phase.

6. According to Gasalarm2 the temperature effects are not fully
addressed in TNO (2008).

In particular did Gasalarm2 expect an estimation of the effect of
potential preferential flow as a result of the presence of cracks, minor
faults and flow channels generated in the past gas production phase and
a judgement on the necessity of a corresponding additional safety margin
for the distance between well and fault.

Question: 

a. What is the opinion of the expert(s) on this subject? (distance to
the faults, heating by compression, cooling by expansion, long-term
effects; overall size of the surface area of the reservoir influenced by
temperature effects)

b. Practical detail: What, in this respect, is the opinion of the
expert(s) about the use of the existing wells to inject the cushion gas
(taking into account their proximity to the internal fault)?

Answer:  The volume of rock affected by the temperature changes
associated with the initial storage is small compared to the source
dimensions of damaging earthquakes.  Also, since the permeability of
reservoir rock is high, flow along fractures may not be critical. 
Preferential flow paths are likely to be oriented parallel to faults
along the tectonic fabric.  In our opinion, the thermal models are
sufficiently conservative.

7. Hypothesis: based on reservoir dimensions, parameters and history
there is a probability ≥15% that, due to project activities, the
central part of the municipality of Bergen NH will during the project
life-time (including final depletion at the end of the project) be hit
by an earthquake with a 10 times stronger impact than the one
experienced in 2001, and that this will cause severe financial damage
(given the fact that the 2001 event already caused significant damage (M
= 3.5, EMS intensity VI+ near epicenter; KNMI (2001)).  (Approx. 4x
stronger event (3.9 vs. 3.5), approx. 2x stronger felt in Bergen due to
epicenter extending immediately within the build-up area). Ref. to
European Macro Seismic scale:   HYPERLINK
"http://www.gfz-potsdam.de/portal/-?$part=binary-content&id=1883158&stat
us=300" 
http://www.gfz-potsdam.de/portal/-?$part=binary-content&id=1883158&statu
s=300  	

Question: Does, in the opinion of the expert(s), the TNO study present
convincing evidence to reject this hypothesis?

Answer:   In our opinion, given the uncertainties in relating fault
parameters to local magnitude (see Figure 4 for typical scatter), the
occurrence of a ML = 3.9 earthquake associated with the Bergermeer
reservoir is possible, but it is unlikely, with a probability much less
than the 15% probability that the scenario posed by Gasalarm2 in this
question suggests.  First, the probability that a ML = 3.9 event would
occur in the Bergermeer field is less than 1% over the life of the
project (van Eck et al., 2006).  Second, the region covered by the built
up area of Bergen covers only a small fraction of the area affected by
production of the Bergermeer reservoir.  Third, slip on the scissors
fault dies out towards Bergen and the geometry of the reservoir appears
to be simpler there.

8. Further to Q7: Hypothesis: given the uncertainty margins in reservoir
rock parameters, precise fault dimensions, reservoir rock homogeneity,
uncertainty about the precise mechanism underlying the 2001 earthquake
and other uncertainties (e.g. concerning thermal effects during
injection/production, effects of water injection), there is a
probability ≥5% of an earthquake with an even 20x stronger impact (M =
4.1) than the 2001 event.

Question: Does, in the opinion of the expert(s), the TNO study present
convincing evidence to reject this hypothesis?

Answer:  In our opinion, the probability of such a high impact event is
substantially less than 5%.  As stated above, the probability of a
magnitude 3.9 event in the field is less than 1% and the probability of
a magnitude 4.1 event is even lower.

9. Question: what is the expert(s) opinion about the treatment/reporting
of uncertainty/error margins and confidence intervals in the model
calculations and scenario choice in the TNO study? 

Answer:  The TNO study addresses the effects of many of the
uncertainties in material properties by running a substantial number of
models assuming different properties. It does not discuss some other
sources of uncertainty/error propagation that could, influence the
interpretation of some of the results.  However, their most important
conclusion, that the maximum local magnitude expected is less than 3.9,
is robust and sufficiently conservative. 

As indicated in our report, the uncertainties in the geomechanical
models associated with fault geometry are not adequately addressed.  In
particular, the possible effects of three-dimensional structure are not
investigated.  As indicated in Figure 3, shearing in a reverse sense in
the region of the projection of the Central Reservoir fault in the weak
Zechstein formation driven by differential compaction of the reservoir
in Block 1 could load that part of the Central Reservoir fault at the
pivot point of the “scissors,” where sandstone is present on both
sides of the fault.  We emphasize that this could affect whether or not
smaller earthquakes occur during storage, but would not affect the
conclusions about earthquakes with magnitudes greater than ML = 3.9
being extremely unlikely.

ANSWERS TO THE QUESTIONS OF THE SOIL MOVEMENT TECHNICAL COMMITTEE 

The following questions were asked by the Tcbb (Technische commissie
bodembeweging; english:  Soil Movement Technical Committee):

What is the opinion of the evaluator on the risk estimates and are they
compatible with the physics (ref. TNO report and KNMI risk reports)?

Answer:  In the view of the reviewers, the estimate in the TNO report
that the maximum magnitude earthquake that could be expected is ML = 3.9
is compatible with the physics. 

The fault dissecting the Bergermeer field is (partly) sealing: what
pressure difference between the hanging- and foot-wall may cause
earthquakes?

Answer:  Determining a quantitative estimate of the pressure difference
across this fault that could lead to earthquakes is beyond the scope of
this review.  However, the 1994 and 2001 earthquakes appear to have
occurred on this fault, so pressure changes associated with seven years
of production may have been sufficient to trigger earthquakes.

The Tcbb considers the possibility of seismic monitoring at reservoir
level, since only larger events (M>3) have been recorded with the
current monitoring system. Is this a justified approach or are there
alternatives?

Answer:  This approach is justified by the importance of monitoring the
behavior of the reservoir.  In addition, we recommend a more
comprehensive geodetic monitoring including the use of GPS to measure
horizontal motions, in addition to vertical motions.

How is excessive movement to be prevented? Can this be done by changing
the rate or volume (maximum pressure difference) of production?

Answer:  The probability of triggering earthquakes depends upon both the
stress level and the rate at which the stress changes.

 

APPENDIX A

Project Description

Technical Review of TNO’s Bergermeer Seismicity Study 

Introduction

In the near future TAQA Energy B.V. wants to utilize the depleted
Bergermeer gas field as an Underground Gas Storage facility. The
Netherlands Organisation for Applied Scientific Research (TNO) has
performed a study regarding the seismic risk of the injection/production
activities and is called the Bergermeer Seismicity Study. Assumptions
made in the report have raised questions and concern among the local
community. The local community fears that the gas storage activity will
cause a considerable earthquake resulting in substantial damage to their
homes and other buildings. Therefore, the Minister of Economic Affairs
has been asked to have the report of TNO reviewed by an independent
expert. This Project Description contains the scope of work for this
technical study.

Deliverables

The Ministry of Economic Affairs expects :

A report containing:

 a critical technical review of the assumptions, conclusions and
recommendations of the Bergermeer Seismicity Study, TNO report
2008-U-R1071/B, 6 November 2008.

answers to the questions raised by the "Gasalarm2 foundation" and the
Soil Movement Technical Committee (see appendices)

The report as mentioned should be submitted in both hard copy (20
copies) and in electronic form. The final report will be preceded by a
draft report.

 

Optional:

An oral presentation in the municipality of Bergen (The Netherlands) for
representatives of the local community.

Timing

The report is to be completed and delivered by September 21st, 2009.

Remarks:

Some of the questions raised in the appendices will need an explanation
from the governmental experts who are involved in the Bergermeer
project. The Ministry of Economic Affairs is willing to organize an
information meeting between the reviewer and these experts.

TAQA Energy B.V. supports the study and is willing to supply any
information needed.   

Reports supplied:

Logan, J.M.; Higgs, N.G.; Rudnicki, J.W.; Seismic risk assessment of a
possible gas storage project in the Bergermeer field, Bergen concession,
1997

Van Eck, Torild; Goutbeek, Femke; Haak, Hein; Dost, Bernard; Seismic
hazard due to small-magnitude, shallow-source, induced earthquakes in
The Netherlands; KNMI scientific report, 2004

  HYPERLINK "http://www.knmi.nl/~goutbeek/Submitted-seismic-hazard.pdf" 
http://www.knmi.nl/~goutbeek/Submitted-seismic-hazard.pdf 

Van Eijs, R.M.H.E.; Mulders, F.M.M.; Nepvue, M.; Kenter, C.J.;
Scheffers, B.C.; 2006; Correlation between hydrocarbon reservoir
properties and induced seismicity in the Netherlands. Engineering
Geology, 84, 99-111.

Reports or papers that need to be purchased can be reimbursed.

APPENDIX B

QUESTIONS I

Questions of the Gasalarm2 foundation

1. TNO uses elasto-plastic geomechanical models to calculate potential
slip on a fault plane. A critical geometry of reservoir and fault
structure is chosen, which is sensitive for reactivation of the fault.
Plastic (reversible) slip is calculated on the fault, during depletion
and injection assuming permanent equilibrium conditions (implying the
assumption that all potential slip created by the preceding depletion of
the field has been accommodated).

Gasalarm2 is of the opinion that in reality discrepancies from the ideal
shape of the fault plane as used in the model may be present, in the
following called obstructions, preventing incremental a-seismic movement
along the fault, and that therefore it cannot be excluded that the
reservoir fault(s) are (is) in a meta-stable condition (“hanging
earthquake” that could be triggered).

In the opinion of Gasalarm2 the model predictions in the TNO study
concerning maximum possible slips that could be created by the,
relatively small, pressure changes during one injection-production cycle
(corresponding to M = 2.4 - 2.7, should they be accommodated in an
non-elastic (seismic) manner) are self-evident given the model
assumptions and, therefore, provide no proof that no larger event can be
triggered.

Question: Is the above-mentioned TNO approach a complete and reliable
way to explore maximum potential slip during the project phase?

2. (With reference to the calculations in Chapter 7 of TNO (2008))

Gasalarm2 states (see Q1) that part of the calculated slip which did not
show up in the 4 (historic) earthquakes may still be present as a
‘hanging’ quake. On the basis of Table 7-1 of TNO (2008) the
magnitude of such a quake can be 3.8 (taking the dynamic shear-modulus
for the estimate of slip/magnitude, rather than the static as in Table
7.1), or larger, given the uncertainty in parameters and dimensions (see
e.g. Q4). 

Question: What is the opinion of the expert(s) about this issue?

3. Figure 3,2 of the Seismicity Report shows a 3D view of the Bergermeer
gas field (based on a model of Horizon 2006). From this view Gasalarm2
concludes, that the main (internal) fault may be longer than
anticipated. According to Gasalarm2 the length of the fault is probably
4.1 to 5.9 kilometres and not 2.5 kilometres. Consequently, Gasalarm2
assumes, that the probable size of the reactivated part of the fault
plane may be much larger than is stated in table 2.2 of the TNO report
(page 18) and therefore the potential magnitude of earth tremors may be
much higher (M=4.1).

Question: What is the relation between the length of the fault plane,
the probable activated part of the fault plane during the events and the
maximum magnitude of a seismic event? How important is the estimation of
the total length of the central fault?

4. Gasalarm2 assumes that the stabilisation of the fault structures at
reservoir level due to pressure increase during injection will be of
minor importance as compared to potential previously created unreleased
tensions (see Q1 and Q2).

TNO assumes that the re-pressurization of the reservoir will lead to a
more stable fault structure (see chapter 6.3 of the TNO Seismicity
report).

Question: what is the opinion of the expert(s) about these views?

5. Gasalarm2 observes that for the operating phase of the BGS only the
first production/injection cycle has been modelled by TNO. In
particular, the recovery phase of the cushion gas has not been covered
(based on a realistic estimate of the then prevailing reservoir
conditions).  Apart from risks resulting from phenomena such as erosion
of the fault plane and fatigue, (see TNO recommendation page 87, #3),
the seismic risks associated with final cushion gas recovery should not
be ignored.

Question: What is the opinion of the expert(s) about the missing
analysis?

6. According to Gasalarm2 the temperature effects are not fully
addressed in TNO (2008).

In particular did Gasalarm2 expect an estimation of the effect of
potential preferential flow as a result of the presence of cracks, minor
faults and flow channels generated in the past gas production phase and
a judgement on the necessity of a corresponding additional safety margin
for the distance between well and fault.

Question: 

a. What is the opinion of the expert(s) on this subject? (distance to
the faults, heating by compression, cooling by expansion, long-term
effects; overall size of the surface area of the reservoir influenced by
temperature effects)

b. Practical detail: What, in this respect, is the opinion of the
expert(s) about the use of the existing wells to inject the cushion gas
(taking into account their proximity to the internal fault)?

7. Hypothesis: based on reservoir dimensions, parameters and history
there is a probability ≥15% that, due to project activities, the
central part of the municipality of Bergen NH will during the project
life-time (including final depletion at the end of the project) be hit
by an earthquake with a 

10 times stronger impact than the one experienced in 2001, and that this
will cause severe financial damage (given the fact that the 2001 event
already caused significant damage (M = 3.5, EMS intensity VI+ near
epicenter; KNMI (2001)) 

(Approx. 4x stronger event (3.9 vs. 3.5), approx. 2x stronger felt in
Bergen due to epicenter extending immediately within the build-up area).
Ref. to European Macro Seismic scale:   HYPERLINK
"http://www.gfz-potsdam.de/portal/-?$part=binary-content&id=1883158&stat
us=300" 
http://www.gfz-potsdam.de/portal/-?$part=binary-content&id=1883158&statu
s=300  	

Question: Does, in the opinion of the expert(s), the TNO study present
convincing evidence to reject this hypothesis?

8. Further to Q7: Hypothesis: given the uncertainty margins in reservoir
rock parameters, precise fault dimensions, reservoir rock homogeneity,
uncertainty about the precise mechanism underlying the 2001 earthquake
and other uncertainties (e.g. concerning thermal effects during
injection/production, effects of water injection), there is a
probability ≥5% of an earthquake with an even 20x stronger impact (M =
4.1) than the 2001 event.

Question: Does, in the opinion of the expert(s), the TNO study present
convincing evidence to reject this hypothesis?

9. Question: what is the expert(s) opinion about the treatment/reporting
of uncertainty/error margins and confidence intervals in the model
calculations and scenario choice in the TNO study? 

QUESTIONS II

Questions of the Soil Movement Technical Committee

The following questions were asked by the Tcbb (Technische commissie
bodembeweging; english:  Soil Movement Technical Committee):

What is the opinion of the evaluator on the risk estimates and are they
compatible with the physics (ref. TNO report and KNMI risk reports)?

The fault dissecting the Bergermeer field is (partly) sealing: what
pressure difference between the hanging- and foot-wall may cause
earthquakes?

The Tcbb considers the possibility of seismic monitoring at reservoir
level, since only larger events (M>3) have been recorded with the
current monitoring system. Is this a justified approach or are there
alternatives?

How is excessive movement to be prevented? Can this be done by changing
the rate or volume (maximum pressure difference) of production?

 

APPENDIX C

Resumes

Bradford H. Hager

Professor, Department of Earth, Atmospheric and Planetary Sciences

Massachusetts Institute of Technology

Cambridge, MA 02139

Phone: (617) 253-0126

E-mail:   HYPERLINK "mailto:mcleod@usc.edu"  bhhager@mit.edu 

Web:   HYPERLINK "http://sir-lab.usc.edu"  http://geoweb.mit.edu 

Professional Preparation

Ph.D. in Geophysics, Harvard University,1978.

A.M. in Geology, Science, Harvard University, 1976.

B.A. in Physics, Amherst College, 1972.

Appointments

Associate Department Head, 2008 – present

Cecil and Ida Green Professor of Earth Sciences, MIT, 1989 - present.

Professor of Geophysics, Caltech, 1989.

Associate Professor of Geophysics, Caltech, 1984 – 1989.

Assistant Professor of Geophysics, Caltech, 1980 – 1984.

Assistant Professor, Department of Earth and Space Science, SUNY Stony
Brook, 1979 – 1980.

Selected Awards and Honors

Alfred P. Sloan Foundation Fellow, 1982 – 1986

American Geophysical Union – Fellow; James B. Macelwane Award, 1986

Orson Anderson Scholar, Los Alamos National Laboratory, 1996

Woolard Award, Geological Society of America, 2001

Fellow, American Academy of Arts & Sciences, 2009

Relevant Publications

Hager, B. H., R. W. King, and M. H. Murray, Measurement of crustal
deformation using GPS, Ann. Rev. Earth Planet. Sci., 19, 351-382, 1991.

Donnellan, A., B. H. Hager, and R. W. King, Discrepancy between
geological and geodetic deformation rates in the Ventura basin, Nature,
366, 333-336, 1993.

Hager, B. H., G. A. Lyzenga, A. Donnellan, and D. Dong, Reconciling
rapid strain accumulation with deep seismogenic fault planes in the
Ventura basin, California, J. Geophys. Res., 104, 25,207-25,219, 1999.

Hetland, E. A., and B. H. Hager, Postseismic and interseismic
displacements near a strike-slip fault: A two-dimensional theory for
general linear viscoelastic rheologies, J. Geophys. Res., 110, B10401,
doi:10.1029/2005JB003689, 2005.

Meade, B. J., and B. H. Hager, Block models of crustal motion in
southern California constrained by GPS measurements, J. Geophys. Res.,
110, B03403, doi: 10.1029/2004JB003209, 2005.

Meade, B. J., and B. H. Hager, Spatial localization of moment deficits
in southern California, J. Geophys. Res., 110, B04402, doi:
10.1029/2004JB003331, 2005.

Committee on Earth Science and Applications from Space: A Community
Assessment and Strategy for the Future, Earth Science and Applications
from Space: National Imperatives for the Next Decade and Beyond, Space
Studies Board, Division on Engineering and Physical Sciences, National
Research Council of the National Academies, The National Academies
Press, Washington, D.C., 2007.

M. Nafi Toksöz

Department of Earth, Atmospheric and Planetary Sciences

Massachusetts Institute of Technology 54-1814

Cambridge, MA 02139

Phone:  (617)253-7852

Email:   HYPERLINK "mailto:toksoz@mit.edu"  toksoz@mit.edu 

Professional Preparation

Colorado School of Mines		Geophysics 		B.S., 1958

California Inst. of Technology	Geophysics 		M.S., 1960

California Inst. of Technology	Geophysics 		Ph.D., 1963

Appointments

Robert R. Shrock Professor of Geophysics, EAPS, MIT, 2005–

Director, Earth Resources Laboratory, MIT, 1982–1998

Director, Wallace Geophysical Observatory, MIT, 1975–

Professor of Geophysics, MIT, 1971–

Associate Professor of Geophysics, MIT, 1967–1971

Assistant Professor of Geophysics, MIT, 1965–1967

Postdoctoral Research Fellow, Geophysics, Caltech, 1963–1965

Selected Awards and Honors

	Exceptional Scientific Achievement Award, NASA, 1976

Distinguished Achievement Medal, Colorado School of Mines, 1995

Honorary Membership, Society of Exploration Geophysicists, 1999

	Harry Fielding Reid Medal, Seismological Society of America, 2006

	

Principal Research Interests

Seismology, earthquake mechanisms, induced seismicity, reservoir
characterization and petrophysics

Relevant Activities

EPRI – Eastern U.S. Earthquake Hazard Study (for nuclear power plant
safety), Member, Senior Advisory Board, 1988-1995

Seismic hazard evaluation, gas field in Oman (PDO), 2001

Seismic hazard study evaluation for natural gas storage site in Turkey,
Technical Consultant to World Bank, 2003

Selected Publications

Gibson, R.L., Jr. and M.N. Toksöz, Permeability estimation from
velocity anisotropy in fractured rock, J. Geophys. Res., 95,
15643-15655, 1990.

Toksöz, M.N., B. Mandal and A.M. Dainty, Frequency-dependent
attenuation in the crust, Geophys. Res. Lett., 17, 973-976, 1990.

Toksöz, M.N., A.M. Dainty and E.E. Charrette, Coherency of ground
motion at regional distances and scattering, Phys. Earth Planet. Int.,
67, 162-179, 1991.

Cicerone, R.D. and M.N. Toksöz, Fracture characterization from Vertical
Seismic Profiling data, J. Geophys. Res., 100,  4131-4148, 1995.

Shen, F. and M.N. Toksöz, Scattering characteristics in heterogeneously
fractured reservoirs from waveform estimations, Geophys. J. Int., 140,
251-266, 2000.

Michelet, S. and M.N. Toksöz, Fracture mapping in the
Soultz-sous-Forets geothermal field from microearthquake relocation, J.
Geophys. Res., in press, 2007.

Burns, D.R., M.E. Willis, M.N. Toksöz and L. Vetri, Fracture properties
from seismic scattering, The Leading Edge, 1186-1196, 2007.

Sarkar, S., H.S. Kuleli, M.N. Toksöz, H. Zhang, O. Ibi, F. Al-Kindy and
N. Al Touqi, Eight years of passive seismic monitoring at a petroleum
field in Oman:  a case study, extended abstract, SEG Annual Meeting,
2008.

Zhang, H., S. Sarkar, M.N. Toksöz and H.S. Kuleli, Passive seismic
tomography using induced seismicity at a petroleum field in Oman,
Geophysics, in press, 2009.   

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