This thesis presents the analysis and interpretation of passive seismic data
collected in a 20-month monitoring period. The investigation is divided into four studies,
each focusing on a different aspect of the seismic data to infer the reservoir properties.
First, I applied three different methods (the iterative linearized, nonlinear grid-search, and
double-difference methods) to relocate 405 microearthquakes that occurred between
October 1999 and June 2001 in a producing field in Oman. A numerical technique is
applied to "collapse" the relocated hypocenters and to find the simplest structural
interpretation consistent with the data. Comparing the methods, the applicability of
waveform correlation methods such as the double-difference in this case is limited by the
relatively large number of events with dissimilar waveforms. Unlike the iterative
linearized method, the nonlinear grid-search method gives the best results with the
smallest average rms error of the absolute locations because it avoids the local minimum
problem. The relocated hypocenters clearly delineate nearly vertical, northeast-southwest
striking faults near the crest of the field, which is consistent with the graben fault system
mapped by surface geologic surveys and reflection seismic interpretations. I also
performed statistical tests to estimate location errors, and found that the station geometry
is the major factor that limits the accuracy of focal depths.
Secondly, this thesis presents a non-linear wavelet-based approach to linear
waveform inversion of high-frequency seismograms for the estimation of a point source
mechanism and its time function. For earthquake mechanism inversions, it is important to
stabilize the problem by reducing the number of parameters to be determined.
Commonly, overlapping isosceles triangles or boxcar functions are used for the
parameterization of the moment tensor rate functions (MTRFs). Here, I develop a
wavelet-based strategy that allows us to construct an adaptive, problem-dependent
parameterization for the MTRFs employing fractional spline wavelets. Synthetic results
demonstrate that the adaptive parameterization improves the numerical approximation to
the model space and therefore, allows more accurate estimations of the MTRFs. The
waveform inversion is performed in the wavelet domain and leads to a multiresolution
sparse matrix representation of the inverse problem. At each resolution level a
regularized least-squares solution is obtained using the conjugate gradient method. The
wavelet-based waveform inversion method has been applied successfully in three real-
data examples: the April 22, 2002 Au Sable Forks, New York earthquake, the September
3, 2002 Yorba Linda, California earthquakes, and 11 M>1 microearthquakes in a
producing field in Oman. In the Oman field, the dominant styles of focal mechanism are
left-lateral strike-slip for events with focal depths less than 1.5 km, and dip-slip along an
obliquely trending fault for those with focal depths greater than 2.0 km.
Thirdly, the covariance matrix method of shear-wave splitting analysis is
presented. Different from conventional methods that usually analyze only two horizontal
components, this method processes all three components of the seismogram
simultaneously, allowing not only orientation but also dip information of fractures to be
resolved. Synthetic test results show that this method is stable even for high noise level.
The method is applied to the Oman microearthquake records that display distinctive
shear-wave splitting and polarization directions. From the polarizations, I estimate the
predominant subsurface fracture directions and dipping angles. From the time delays of
the split wave I determine the fracture density distributions in the reservoir.
Finally, I examine the spatio-temporal characteristics of the microseismicity in the
producing reservoir. The frequency-magnitude distribution measured by the b-value is
determined using the maximum likelihood method. I found that b-values are higher for
events below the deeper Shuaiba oil reservoir than those above. Also, the feasibility of
monitoring the temporal change of b-values is demonstrated. The analysis of production
and injection well data shows that seismicity event rates in the field all strongly
correlated with gas production from the shallower Natih Formation. Microseismicity,
focal mechanisms, GPS analysis, and production / injection well data all suggest the NE-
SW bounding graben fault system responds elastically to the gas-production-induced
stresses. Normal faulting is enhanced in the reservoirs by the compaction related stresses
acting on the graben fault system
This thesis presents the analysis and interpretation of passive seismic data
collected in a 20-month monitoring period. The investigation is divided into four studies,
each focusing on a different aspect of the seismic data to infer the reservoir properties.
First, I applied three different metho...