USE OF THE RADIO-MAGNETOTELLURIO METHOD F OR RESEARCH AT AQUATIC SITES

The radio-magnetotelluric method (Radio-MT) can be operated in water t hanks to an adaptation of the electric field transducer. Measurements are very easy to carry out: the set of transducers (electric and magne tic field transducers) is towed on a raft behind a boat. The small dim ension of the system makes it possible to perform resistivity profilin g across rivers and thus to asses lateral variations of the undergroun d proprieties across the watercourse with a better accuracy than is po ssible with the direct current prospecting method where the total leng th of the measurement array is over about 200 m. With the latter metho d only profiles parallel to the watercourse are possible if the river is not wider than a few hundred metres. This means that the two method s are complementary. We have performed several experiments and survey campaigns with this new method. With one of them, we have compared the resistivity results obtained by both methods along a 7 km long profil e. The second survey campaign has helped to understand the geological structure of the surroundings of a river across which a dam constructi on is planned. Lastly, we have carried out resistivity measurements al ong the same profile, some 50 m long, three times, in a lake with thre e different water depths: dry lake, and waterdepths of 30 cm and 1 m. With this comparison we have been able to experimentally confirm the t heoretical modelling of the effect of water on the Radio-MT resistivit y measurements. The applications of Radio-MT in water are the same as those of DC prospecting: geological surveying and survey of the sedime nt on the water floor (mud, sands and gravels). Of course, unlike DC r esistivity prospecting, Radio-MT cannot be used at sea where the skin depth of the electromagnetic waves is too small.