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Joint Hypocenter-Velocity Inversion of Local Earthquake Arrival Time Data in Two Geothermal Regionsby Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on April 3, 1991 in partial fulfillment of the requirements for the degree of Doctor of Science ABSTRACT
P wave and S wave arrival time data from local earthquakes in two geothermal
regions, the Los Alamos Hot Dry Rock Reservoir in New Mexico and the Larderello
Geothermal Field in Italy, are inverted to simultaneously determine the hypocenter
parameters and the three-dimensional velocity structures. In both areas, geothermal
production is associated with fractured zones in crystalline rock. The joint
hypocenter-velocity inversion yields three-dimensional images of these fractured
zones. Constraints are applied which not only stabilize the inversions, but
also help the inversions to converge to the most geologically reasonable velocity
models. These constraints include spatial velocity derivative regularization,
minimum and maximum velocity bounds, and constraints on the velocity values
at specified points using information from vertical seismic profiling (VSP)
and seismic reflection data.
The Los Alamos Hot Dry Rock Reservoir, located in north-central New Mexico,
was created by hydrofracturing in hot crystalline basement rock. The joint
hypocenter-velocity inversion is applied to microearthquake arrival time data
from nearly 700 events recorded at 4 borehole seismometers during an eight-hour
interval of the hydrofracturing. The dimensions of the fractured reservoir,
which is the region imaged during the inversion, are less than 1 km. The P
wave and S wave velocities are determined independently, except for the indirect
coupling through the hypocenters.
The inversion yields low S wave velocity anomalies which generally correlate
well with the earthquake locations. The percent velocity perturbations increase
as the velocity regularization weighting decreases. Studies of inversions
performed using different regularization weightings suggest that the S wave
velocities decrease by at least 13% in the most intensively fractured
regions of the reservoir. Since the travel time perturbations caused by the
fluid-filled fractures are much smaller for P waves than for S waves, the
P wave velocities are less constrained by the data than the S wave velocities.
For this reason, the P wave velocities are strongly influenced by the velocity
regularization and the final VP model is very smooth. Also, because
of the limited azimuthal ray coverage, the earthquake locations can trade
off with the VP / VS ratios. If the joint inversion
is performed without any constraint on the VP / VS
structure, then the VP / VS ratios computed from
the final velocity models decrease to unreasonably low values (1.1 - 1.3).
The earthquake locations are systematically biased by these poor VP
/ VS values. Inversions performed with a lower bound of 1.60
applied to the VP / VS ratios yield models which
satisfy the data as well as the models from the inversions performed without
the bound, and they yield more geologically reasonable VP /
VS structures. The residuals decrease 11 - 15%. The average
absolute change in the earthquake locations during the inversions with the
VP / VS bound is 20 - 27 m. The relative
earthquake locations are improved by all of the joint inversions performed,
even those inversions in which the absolute earthquake locations
are biased by poor VP / VS ratios.
The second data set is from the Larderello Geothermal Field, located in
west-central Italy. The joint inversion is applied to P wave and S wave arrival
times from 269 earthquakes recorded between 1977 and 1990. Most of the earthquakes
occur shallower than 8 km depth. The horizontal dimensions of the imaged region
are 40 km by 30 km, and the velocities are determined to about 20 km depth.
The complex, shallow velocity structure, to 1.5 km depth, is determined from
VSP and seismic reflection data. These velocities remain fixed during the
joint inversion of the earthquake data. Because there are not enough S wave
data to yield an independent, detailed S wave velocity model, the S wave velocities
are not directly determined. Rather, the VP / VS
ratios are found. Since the S wave velocity structure is not well-resolved
by the data, the S wave velocities are required to conform to the P wave velocities.
This constraint gives a geologically reasonable "default" model.
The inversion results show three distinct low P wave velocity anomalies
within the basement. Two of the anomalies are 5 to 10 km in width and occur
between 4 and 7 km depth. The P wave velocities decrease approximately 17
- 25% in these two regions. These features correlate well with the two
structural peaks of a strong seismic reflector known as the K horizon. The
cause of the low velocities may be hot fluids and/or steam trapped in fractured
rock beneath an impermeable zone, as suggested by observations from one well
which penetrated the K horizon. This proposed impermeable zone corresponding
to the K horizon may be caused by mylonitic deformation along a low-angle
normal detachment fault. The third low P wave velocity anomaly occurs at 8
km depth and deeper, and represents the intrusion which is the heat source
for the geothermal field. The computed P wave velocities within this feature
are as low as 4 - 4.5 km/s, but these values are not well-constrained due
to the poor ray coverage at these depths. The location and general shape of
the anomaly are consistent with observed gravity data and with an independent
velocity model computed from teleseismic arrival time data. The VP
/ VS model obtained from the joint inversion has poor spatial
resolution, due to the small number of available S wave arrival times. For
this reason, no distinct VP / VS anomalies are associated
with the two small, shallow VP anomalies. An increase in the VP
/ VS ratio is associated with the large, deep P wave velocity
anomaly. This increase in VP / VS, and the corresponding
decrease in VP, is consistent with partial melting.
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