| Abstract |
From July 17-Aug. 19, 2020, we deployed and maintained a 1206 station array of 1C geophones over the footprint of the well-field at the Lightning Dock Geothermal (LDG) power plant in Hidalgo County, New Mexico. Such dense seismological data were collected for a variety of purposes, including to better characterize the shallow velocity structure. An initial P-wave velocity (Vp) model covering LDG was generated using reserse-VSP one-way travel times, sonic logs, well logs and gravity data (Edwards et al., 2021). However, such data only provided Vp velocity constraints and was mostly limited to the ray paths between sources within wells and array stations. Rayleigh waves can provide better azimuthal coverage and can constrain S-wave (Vs) velocity structure, especially within shallow depths ( less than 500 m), where large velocity changes can occur. With both Vp and Vs, we can then compute Vp/Vs ratio over LDG, which may yield insights into geothermal fluid upwelling and outflow over the shallow domain. To obtain the shear-wave velocity model, we applied ambient noise cross-correlation and imaging methods to the vertical component data recorded across the station array. The raw data were processed following the workflow described by Bensen et al. (2007), including four phases: 1) data pre-processing including temporal and spectral normalization at individual stations, 2) cross-correlation and temporal stacking, 3) measurement of the dispersion curves using automatic frequency-time analysis and 4) quality control. Noise cross-correlations were stacked over nighttime hours to avoid the strongly inhomogeneous noise field during the daytime resulting from operations at the geothermal field. The resulting noise cross-correlations reveal empirical Rayleigh waves traveling across the geothermal field ( less than 3 km) with well-defined average move-outs. Once the data were processed and quality dispersion curves in the frequency band of 1-4 Hz were derived, an inversion for Rayleigh-wave group and phase velocity maps was performed, and then the 3D Vs and Vp/Vs structure were obtained. The 3D Vp and Vs models and Vp/Vs ratios were then compared and were analyzed for their relationships to the geothermal reservoir and fluid migration. |