Integration of VES and TEM Methods to solve Static Shift Technique, a Case Study at Shalateen, Red Sea Coast, Egypt.
The accumulation of charges at resistivity boundaries causes the electric field not to be continues close to these boundaries, so the measured electric field is affected by a constant shift on a log scale called Static Shift. The coastal fan in Shalateen area is representing a good case study for static shift in vertical electric sounding caused by geological heterogeneity in the near subsurface layers. The study area located along the Red Sea coast at the Northern corner of Halayieb - Shalateen triangle. Integration of data from the various electrical and electromagnetic (EM) sounding techniques is critical. Models of all the methods show concordant response patterns. In Meju (2005) experiment, the geological model was characterized by the existence of a conductor of low resistivity layer between two relative highly resistive layers and resistivity is increased with depth. That conditions are quite similar to the current study area where the conductor is the salt water aquifer and then the resistivity increased to reach the basement rock. TEM data are used to correct static shifts in DC resistivity sounding stations, where the static shift is a galvanic effect that affects electrical resistivity soundings. The electrical resistivity sounding curve is generally vertically shifted but parallel DC sounding curves (in the log domain). Static shift corrections are made to match the overlapping TEM sounding curves at the unchanged curve segments, but there is uncertainty if the two curves are really coincide within the given sounding bandwidth.
The vertical electrical sounding stations were measured using Schlumberger array with the maximum AB/2 range between 250 m and 300 m. Transient electromagnetic (TEM) survey was conducted in the studied area using Sirotem MK3 conductivity meter. The field data have been recorded close to the VES sites, and away from any external electrical source or high potential lines to minimize the noise effect. The square lope size used during acquisition was 50m. The concordance in the curves pattern increases with increasing depth because of difference in vertical resolution in shallow depth between both methods. The VES curves show shifting upward or downward from the TEM curves which known as static shift. The static shift is a result of distortion of the electric field caused by the presence of shallow and local small bodies of heterogeneity nature.