William Scott Phillips

Using Automated, High-precision Repicking to Improve Delineation of Microseismic Structures at the Soultz Geothermal Reservoir

Abstract — An automatic, adaptive, correlation-based algorithm for adjusting phase picks in large digital seismic data sets provides significant improvement in resolution of microseismic structures using only a small fraction of the time and manpower which would be required to re-analyze waveforms manually or semi-automatically. We apply this technique to induced seismicity at the Soultz-sous-Forêts geothermal site, France. The method is first applied to a small, previously manually repicked subset of the catalogue so that we may compare our results to those obtained from painstaking, visual, cross-correlation-based techniques. Relative centroid-adjusted hypocenters show a decrease in median mislocation from 31 to 7 m for preliminary and automatically adjusted picks, respectively, compared to the manual results. Narrow, intersecting joint features not observed in the preliminary hypocenter cloud, but revealed through manual repicking, are also recovered using the automatic method. We then address a larger catalogue of ∼7000 microearthquakes. After relocating the events using automatic repicks, the percentage of events clustering within 5 m of their nearest neighbor increases form 5 to 26% of the catalogue. Hypocenter relocations delineate narrow, linear features previously obscured within the seismic cloud, interpreted as faults or fractures which may correspond to fluid propagation paths, or to changes in stress as a result of elevated pore pressures. RMS travel-time residuals for the larger data set are reduced by only 0.2%; however, phase-pick biases in the preliminary catalogue have influenced both the velocity model and station correction calculations, which will affect location residuals. These pick biases are apparent on the adjusted, stacked waveforms and correcting them will be important prior to future velocity model refinements.

Application of regional phase amplitude tomography to seismic verification

Abstract — We have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0 Hz, Pg/Lg amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative Pg/Lg attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for Pg and Lg. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.

Chapter 12 Source Effects From Broad Area Network Calibration of Regional Distance Coda Waves

Abstract We have applied regional coda techniques to a network of 64 stations across central and east Asia to isolate source effects for bands from 0.03 to 8 Hz. The heterogeneity of the study region required us to determine two‐dimensional (2‐D) path and transfer function corrections. The importance of the 2‐D path corrections increased with frequency and distance, and for continental paths, became critical beyond 500 km for 1 Hz data. We propose a new spreading model for coda amplitudes, termed the extended Street‐Herrmann (ESH) model, to which attenuation can be added, facilitating the use of tomographic techniques for path correction. The 2‐D transfer function varied between continents and oceans, as well as within continents in areas of poor Lg propagation, reflecting differing excitation of Lg and Sn coda. We also demonstrate the use of empirically determined coda shapes, or type curves , to measure coda amplitudes, adding precision and flexibility for source regions of special interest. We applied these techniques to 112,000 records from 35,000 events, magnitudes 2–7, depths 0–50 km, between latitudes 0° and 60° and longitudes 60° and 150°. The resulting coda source spectra were used to derive moments and, for the better recorded events, corner frequencies, allowing computation of apparent stress for just under 6700 earthquakes. Preliminary apparent stress results ranged from 10 −2 to 1 MPa and showed some regional variation. For example, stress increased from south to north across the Tian Shan, perhaps reflecting deformation in varying crustal rheology or effects of prior slip history. Low stress observed in Tibet could be an artifact of under correction for high attenuation; however, the correlation also could be physical. Low stress observed in oceanic regions is inconsistent with local studies and indicates that upgrades to the coda methodology to more explicitly account for mixed Lg and Sn coda will be needed. The regional network coda results should be further tested by comparing to ground‐truth spectra obtained by applying coda techniques to data from local scale networks within the study region.