The geoelectric methods of DC resistivity and induced polarization (IP) have had a long history as complementary geophysical tools with which to remotely explore the earth. Pioneering work on both techniques dates to earliest part of the last century. Resistivity had gained industry acceptance well before the middle of last century; while due to its more sophisticated hardware requirements, IP's commercial establishment lagged until the later half of the last century. The ability of marine towed resistivity systems to quickly and effectively categorize sediments in the near subbottom has been commercially demonstrated in the telecommunications industry. The application of DC resistivity to other geotechnical problems is discussed. Also discussed are the limitations of DC resistivity and the application of marine towed IP to problems where DC resistivity falls short.
Williamson & Associates has figured prominently in the relatively brief history of the application of electrical methods to marine geophysical survey. First, in the early 1990's, in partnership with Cable & Wireless Marine, Ltd. (now Global Marine Systems, Ltd.), Williamson & Associates launched the first commercial foray into marine resistivity with the commissioning of two C-BASS (Cable Burial Assessment Survey System) sleds. The C-BASS program was followed by second partnership, this time with Sage Engineering, Ltd., (now Thales, Inc) leading to the development of two deeptowed ReDAS (Resistivity Data Acquisition System) systems.
Both the C-BASS and ReDAS survey platforms are bottom-towed sleds capable of taking continuous multielectrode, resistivity measurements of the top few meters of seafloor while working in water depths exceeding 2000m.
The C-BASS weighs approximately 1ton and when compared to ReDAS, is a more fully instrumented platform incorporating a sub-bottom profiler, lights and a video camera, a CTD and a mini-CPT (Cone Penetrometer). ReDAS is primarily a resistivity data collection system, and is smaller and lighter than the C-BASS, weighing approximately 1/2ton. Both are easily deployed from survey-class vessels using standard "deep-tow" oceanographic winches and A-frames.
With regard to their resistivity measurement systems, though different in detail, C-BASS and ReDAS have very similar performance characteristics. This is due largely the fact that both systems were targeted at the telecommunications industry and were therefore tuned to requirements of the cable burial assessment (BAS) surveys.
Both C-BASS and ReDAS can be considered marine Towed Direct Current systems and are treated the same for purposes of this discussion-in the details of the systems lie subtle differences.
Land-based towed array resistivity systems have been available for some time. However, the performance of these land-based systems is often quite poor, with the data quality being limited due to inadequate coupling of the electrodes to the ground.