MEASUREMENT OF THE SEISMOELECTRIC RESPONSE FROM A SHALLOW BOUNDARY

Field experiments carried out at a site near Vancouver, Canada have sh own that a shallow lithologic boundary can be mapped on the basis of i ts seismoelectric response. As seismic waves cross the boundary betwee n organic-rich fill and impermeable glacial till, they induce electric fields that can be measured at the surface with grounded dipole recei vers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conver sions. Two types of seismoelectric signals are observed. The primary r esponse is distinguished by near simultaneous arrivals at widely separ ated receivers. Its arrival time is equal to the time required for a s eismic 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 di poles located within a few meters of the shotpoint. At greater distanc es, the amplitude of the primary arrival decays rapidly with offset, a nd 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 attr ibuted to piezoelectricity or to resistivity modulation in the presenc e of a uniform telluric current. We infer that seismically induced ele ctrokinetic effects or streaming potentials are responsible for the se ismoelectric conversion, and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experiments wer e small in scale, the results are significant in that they suggest tha t the seismoelectric method may be used to map the boundaries of perme able formations.