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


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