High‐Resolution Ambient Noise Imaging of Geothermal Reservoir Using 3C Dense Seismic Nodal Array and Ultra‐Short Observation. Issue 8 (22nd August 2021)

• Record Type: Journal Article
• Authors: Cheng, Feng; Xia, Jianghai; Ajo‐Franklin, Jonathan B.; Behm, Michael; Zhou, Changjiang; Dai, Tianyu; Xi, Chaoqiang; Pang, Jingyin; Zhou, Changwei
• Is Part Of: Journal of geophysical research. Volume 126:Issue 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-22
• Subjects: ambient noise tomography -- dense array -- ambient interferometry -- geothermal imaging -- ultra‐shot observation; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB021827
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Tomographic imaging based on long‐term ambient seismic noise measurements, mainly the phase information from surface waves, has been shown to be a powerful tool for geothermal reservoir imaging and monitoring. In this study, we utilize seismic noise data from a dense nodal array (192 3C nodes within 20 k m 2 ) over a ultra‐short observation period (4.7 days) to reconstruct surface waves and determine the high‐resolution (0.2 km) three‐dimensional (3‐D) S wave velocity structure beneath a rural town in Zhejiang, China. We report the advantage of cross‐coherence over cross‐correlation in suppressing pseudo‐arrivals caused by persistent sources. We use ambient noise interferometry to retrieve high quality Rayleigh waves and Love waves. Body waves are also observed on the R‐R component interferograms. We apply phase velocity dispersion measurements on both Rayleigh waves and Love waves and automatically pick more than 23, 000 dispersion curves by using a Machine Learning technique. 3‐D surface wave tomographic results after depth inversion indicate low‐velocity anomalies (between −1% and −4%) from the surface to 2 km depth in the central area. Combined with the conductive characteristics observed on resistivity profile, the low‐velocity anomalies are inferred to be a fluid saturated zone of highly fractured rock. Joint interpretation based on horizontal‐to‐vertical spectral ratio (HVSR) measurements, and existing temperature and fluid resistivity records observed in a nearby well, suggests the existence of the high‐temperature geothermal field through the fracture channel. Strong correlation between HVSR measurements and the S wave velocity model highlights the potential of extraction of both amplitude and phase information from ambient noise. Plain Language Summary: The geothermal energy potential beneath our feet is vast and clean. Increased development of geothermal energy provides people a promising solution for the nation and the world as we become ever more concerned about global warming, as well as air pollution. However, the development of national geothermal resources in China is far behind the target. We utilize seismic ambient noise to produce a high‐resolution three‐dimensional (3‐D) tomographic image of the geothermal system beneath a rural town in Zhejiang, China. This technique allows us to detect velocity variations/contrasts due to subsurface inhomogeneity over an ultra‐short time window which offers the opportunity for high temporal‐resolution (several days scale) geothermal reservoir imaging and monitoring/tracking. We identify low‐velocity variations outlining a highly fractured zone from the surface to 2 km depth in the central area. We interpret the low‐velocity/high‐conductivity/high‐temperature fracture zone as a heat transfer channel of the local geothermal system. Key Points: Ambient noise data have been recorded using a dense nodal array (192 3C nodes within 20 k m 2 ) over ultra‐short observation period (4.7 days) Both surface waves (Rayleigh and Love waves) and P waves are identified in the cross‐coherence functions S wave velocity model is consistent with existing geophysical data and suggests the existence of high‐temperature geothermal resources