Selection of noise sources and short-time passive surface wave imaging——A case study on fault investigation

Abstract

As a comprehensive resource, geothermal energy plays an important role in local economic development. Faults that control the distribution of geothermal resources are often delineated by geophysical methods. Aiming at a geophysical task of detecting local faults, we used the multichannel analysis of passive surface-wave method and deployed three survey lines with 178 three-component seismic nodal stations in Qingtian town, Zhejiang Province, China, to record ambient noise for imaging local shear (S)-wave velocity structures. Results of time-frequency analysis and beamforming determined the periodic distribution of noise sources. Artificial noise generated around array stations during the daytime greatly reduced the quality of imaging results, while useful ambient noise for imaging came from the flowing river out of the array area at nighttime. We performed cross-coherence operation on the data recorded during nighttime. The signal of surface waves appeared clearly in the anti-causal part of virtual shot gathers, which was chosen for dispersion measurements. After inversion, totally 32 1-D S-wave velocity profiles were combined and interpolated to reveal the 2-D subsurface structure down to a depth of 1.5 km. Two main faults are observed and correspond well with the previous geological maps. Even with two-hour-long noise recordings during nighttime, the similar results are achievable. Our results demonstrated the accuracy and efficiency of passive surface wave methods in delineating local faults. In addition, the imaging test suggests that the stable and reliable result can be obtained using short-time noise recordings by choosing proper distribution of sources.