oa Delineating Lake Bottom Structure by TEM on Water Surface

Recently the development of TEM (Transient Electromagnetic) provides higher resolution and greater exploration depth than ever before and it is time to consider whether this advanced TEM technique can be applied to under-water mapping. About 250 TEM soundings have been measured within a lake of about 120m by 60m in a muddy deposits environments in southern Taiwan. For the TEM measurements the terrTEM unit (Monex GeoScope) was used with an inloop configuration, for which the receiver spacing is about 5 m and the transmitter loop is about the same size as the lake. For imaging the vertical and horizontal resistivity distribution within the deposit and the surrounding area, all sounding results were interpreted using a smooth inverse procedure. Some auger drilling and sounding were available for correlation. This led to a final model. Underneath the water bottom is mud/shale inter-bed with a depth greater than 50 m and resistivity ranging from 1 to 5 ohm-m. The depth of the clayey layer predicted by TEM data is comparable to that of drilled wells nearby, and the thick mud to the southeast suggests the lake dipping to the southeast. The success of the method depends upon the appropriate design of survey parameters; this new application is open to further refinement, both in instruments and in survey techniques.