Progressive inversion for hypocenters and P wave and S wave velocity structure: Application to the Geysers, California, Geothermal Field

Abstract

Seismicity at The Geysers is induced by some aspect of steam production. Accurate earthquake locations, particularly hypocentral depth, are needed to determine the relationship between geothermal energy production and seismicity. Progressive P and S wave velocity‐hypocenter inversions were done using data from 39 microearthquakes at The Geysers to estimate microearthquake locations and determine if the geothermal field has a distinctive seismic signature. Comparable final Vp, Vs and Vp/Vs velocity models were obtained from four different starting velocity models. The elevation interval of maximum steam production coincides with minimum observed Vp/ Vs and Vp/ Vs increases below the primary production zone, suggesting that reservoir rock becomes more fluid saturated with increasing depth. A Vp/ Vs peak at the condensation zone‐production zone elevation delineates the top of the stream reservoir. Earthquake locations are confined to two depth intervals. A zone of shallow seismicity occurs immediately below the condensation zone and above maximum depths of steam production. A more arealy restricted zone of seismicity is located below maximum production depths and is abruptly terminated at an elevation of −3.7 km.

Source Mechanisms of Induced Earthquakes at The Geysers Geothermal Reservoir

“…The choice made in the present study is to use a onedimensional model that allows straightforward Green function calculations and then make adjustments to approximate some of the three-dimensional effects. The chosen base model is a layered model that was developed by O'CONNELL and JOHNSON (1991) for The Geysers region, and it is shown in Fig. 6.…”

Fault zone structure determined through the analysis of earthquake arrival times

“…A topic which has not been addressed analytically in this thesis is the covariance on the hypocenter location estimates as function of both statistical errors in the observations and non uniqueness of the velocity Inodel adopted for their locations. Pavlis and Booker (1980), O'Connell (1986), and O'Connell and Johnson (1991)are among the few authors that have addressed this topic, relying on the formulation of errors given by Backus (1970 Step size Throughout the iterative inversion, one of the most critical parameters to be set is the non-negative damping constant t.Lp(both l*P and tLs when the inversion is performed for both P-and S-model), The value of til.is parameter determines both t,he direction and the size of the adjustments in the velocity coefficients, lt stabilizes the solution by limiting the effect of small eigenvalues of the matrix of the travel time partial derivatives of the annulled data set. These small eigenvalues produce large perturbations of the velocity model, deteriorating and often reversing the descent path toward the minimum of the objective functional, Selection of an appropriate value for the damI_ing parameter has been addressed in different ways.…”

The Variability and Interpretation of Earthquake Source Mechanisms in The Geysers Geothermal Field From a Bayesian Standpoint Based on the Choice of a Noise Model

“…O'Connell and Johnson (1991) obtained vertically heterogeneous one-dimensional P and S wave velocity models. The complexities in geological conditions are reflected in strong lateral variations in seismic velocities, the v P ∕v S ratio and attenuation.…”

“…The complexities in geological conditions are reflected in strong lateral variations in seismic velocities, the v P ∕v S ratio and attenuation. O'Connell and Johnson (1991) obtained vertically heterogeneous one-dimensional P and S wave velocity models. They found a low v P ∕v S ratio at depths of maximum steam production and high v P ∕v S above it.…”