Measurement of the seismoelectric response from a shallow boundary

摘要

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response. As seismic waves cross the boundary between organic-rich fill and impermeable glacial till, they induce electric fields that can be measured at the surface with grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P-wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) is measured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primary arrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increase with dipole offset, and they appear to be generated in the immediate vicinity of each dipole Sensor. Our studies show that the responses cannot be attributed to piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streaming potentials are responsible for the seismoelectric conversion, and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experiments were small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.