Temporal Variation of Seismic-Wave Velocity Associated with Groundwater Level Observed by a Downhole Airgun near the Xiaojiang Fault Zone

摘要

Subsurface velocity variations may reflect subtle stress changes in the medium and hence can be used as a "stress gauge." However, it is challenging to continuously monitor temporal changes in subsurface velocities with high precision, due to the lack of reliable repeatable sources. In this article, we report an experiment on continuous velocity monitoring, with a borehole airgun as the repeatable seismic source. These active source excitations are recorded by nine short-period seismometers that were deployed at a distance of ~3-900 m from the source. The experiment was conducted in the east branch of the Xiaojiang fault (XJF) zone in Xundian County, Yunnan Province, China, and it lasted for one week. The relative velocity (dv/v) changes are measured with a precision of ~2.0 x 10-3. Consistent velocity variations ranging from 5% to 20% are observed at four stations. The velocity variation agrees well with the groundwater-level change of several meters, which is interpreted as the velocity response to the effective stress and fluid saturation in the shallow layer. The velocity change within the XJF zone is slightly greater than that outside of the fault zone. This study suggests that such a monitoring system could potentially be used in other regions to monitor temporal changes in shallow seismic velocities along active fault zones.