Mapping water-bottom features involves in the fields of environmental geophysics and engineering structural studies. Standard direct current (DC) resistivity sounding is widely used to map subsurface structures on land, but it is seldom used to map underwater structures, because the deployment of underwater electrodes along the water-bottom is expensive and cumbersome. Therefore, a cost-effective and non-destructive survey technique is required to map underwater structures.

Recently the development of an automatic multi-electrode system for resistivity image profiling (RIP) has been successful in mapping two-dimensional (2-D) or three-dimensional (3-D) geological structures on land faster than standard DC resistivity soundings. In fact, the RIP method provides· higher resolution and greater exploration depth of electrical images. Therefore, it is time to consider whether this advanced RIP technique can be applied to underwater mapping.

The purpose of this paper is to describe the potential of the RIP technique conducted on the water surface to map the sub-water bottom geology. The electrode configuration used in this paper was a pole-pole array. The advantages of this survey technique are its convenience and low expense in operation. In addition, no previous water body and bottom topography corrections are needed for data processing as required for standard DC sounding data collected along the water bottom with underwater electrodes. To seek verification of this new application, a RIP survey was carried out on the water surface of Lake Chung-Dab in northern Taiwan. The shallow stratigraphy underneath the lake was delineated and compared with well data nearby. The RIP results did assist in interpreting the structures underneath the water bottom. The results clearly demonstrate the capability of the RIP technique to resolve an underwater stratigraphy, and the efficiency of the RIP method compared with standard DC resistivity method.