Electromagnetic Soundings, Pseudo-Resistivity Logs And Implications For Porosity And Ground Water Salinity

Sixteen electromagnetic (EM) soundings were obtained along the dike on the Fraser River Delta (south of Vancouver, B.C.) that goes from the B.C. Ferry Terminal near the U.S. border to the mouth of the south arm of the Fraser River (Canoe Passage). The data were collected using a Geonics time-domain electromagnetic (TDEM) EM- 47 system in the central sounding mode. These data were interpreted using the smooth model inversion option (Occam inversion) from the TEMIXGL software package of Interpex Limited, Golden, Colorado. The TEMIXGL smooth model inversion can tit up to 19 layers (thicknesses logarithmically increasing with depth) to the apparent resistivity versus time data. Once the layer thicknesses are fixed, the inversion computes the best fit for the resistivity of each layer. The smooth model therefore produces a plot of computed resistivity versus depth, somewhat similar to a resistivity log. It is not a true resistivity log since thicknesses increase with depth. In other words the resolution of the sounding decreases with depth; hence we shall call them pseudo-resistivity logs. The pseudoresistivity logs were compared with 3 resistivity (and gamma) logs within the same area. The agreement between the pseudo-resistivity logs and the downhole logs is remarkably good. Attempts to relate the 3 downhole logs and the 16 pseudo-resistivity logs to apparent porosity and ground water salinity using a modified Archie’s Law are under way. Preliminary results indicate that such a relationship may be possible as long as either the pore water salinity or the porosity is known. Even so, the relationship must be determined empirically. Once a relationship is developed for a given area the porosity and salinity versus depth can be obtained cheaply and quickly. Such information is essential for geotechnical applications and understanding ground water flow.