Mapping Of A Stratigraphic Boundary By Its Seismoelectric Respons

Experiments carried out at our Haney test site near Vancouver, Canada, have shown that<br>some boundaries in unconsolidated sediments may be mapped on the basis of their seismoelectric<br>response. Using a sledgehammer source and electric field receivers, we have observed that an<br>electromagnetic signal is generated when a seismic wave impinges upon a boundary between<br>organic-rich fill and impermeable glacial till. The depth to the interface, verified by drilling,<br>ranges from about 1 to 3 m. The electrical response is believed to be a transient streaming<br>potential produced by a seismically-induced flow of pore water at the interface.<br>The seismoelectric response is linear in that its frequency content is comparable to that of<br>the seismic wave. Surface measurements exhibit radial symmetry about the shotpoint, and the<br>arrival time of the signal corresponds to the seismic traveltime from the shotpoint to the<br>subsurface boundary. The response cannot be attributed to seismic shaking of the dipole receivers<br>because it is observed simultaneously by receivers in different locations.<br>The maximum amplitude of the electrical response observed at the surface, following a<br>sledgehammer blow 2 m above the boundary, is about 1 mV/m. This is measured by a horizontal<br>dipole at an offset of 4 m from the shotpoint. At greater distances, the amplitude of the first<br>arrival decays rapidly with offset and the waveform broadens. The change in waveform with<br>dipole offset suggests that the effective area of the interface contributing to the response may be<br>greater at more distant observation points.<br>Measurements were also made by detonating blasting caps at various depths in boreholes<br>penetrating the geologic boundary. The results of these tests support the conclusion that it is the<br>boundary, rather than the water table, that is responsible for the seismoelectric conversion.<br>Although the scale of our experiments was relatively small, the results are significant in<br>that they suggest that the seismoelectric method may be used to map aquitards or the boundaries<br>of permeable formations.