Enhanced Characterization of Induced Seismicity

Abstract

At the most geothermal fields, the activities associated with geothermal energy production and or enhanced geothermal system (EGS) development will cause an increase seismicity, which the physical mechanisms are still not fully understood. For the definition of the nature of induced seismicity, a second-order dynamic moment tensor analysis may be used to ascertain if seismicity is arising from stress release along preexisting faults or from fracture permeability creation associated with EGS activities. With dynamic moment tensor analysis an accurate estimation of the source parameters, including location, in the presence of complex geological media with highly variable seismic velocity properties is a non-trivial task. Here, we employ the full elastic waveform inversion (FWI) in Laplace-Fourier domain to estimate the location and seismic moment tensor parameters of sources embedded in 3D isotropic heterogeneous media. Forward modeling is carried out with the 3D finite-difference code that generates P- and S-waves from the point sources defined by second-order moment tensors. We apply the nonlinear conjugate gradient method (NCG) for the minimization of the objective function and source model updating. The FWI algorithm is shown to be stable in the presence of complex geometry including faults and random Gaussian noise. We present results of testing the FWI methodology on the synthetic dataset for the Raft River geothermal field, Idaho. The detectors were placed on the earth surface and seismic events were suggested at the depth about 2000 meters. Matching the waveforms from seismic events provides the improved source location along with estimates of the pertinent components of the moment tensor. Investigation of the influence of velocity model and an initial position of an event are performed.