We have developed a new marine magnetotelluric (MT) measurement system that can reduce noises caused by sea wave motions and can be applied to measurements under very shallow seawater areas, such as a coastal region with a sea depth of 10 to 100. m. The difficulties of geophysical exploration in shallow water and coastal areas include (1) fishery activity, (2) limitations of survey vessel size, and (3) motion noise caused by sea waves. In order to overcome these difficulties, we selected a MT method that uses natural EM fields without transmitting an electric current in seawater, which enables the method to be used in areas with active inshore fisheries. In addition, the developed marine MT system is very short, which reduces motion generated by sea waves, and compact, which enables light-draft small survey boat operation. We conducted offshore data acquisition using a new MT measurement system and an onshore MT survey at the Horonobe coastal area, Hokkaido, Japan. High-quality data were successfully obtained in both onshore and offshore field surveys. Two-dimensional (2D) inversion for field data from onshore to sea bottom reveals that a quaternary sedimentary layer of a few hundred meters in thickness, which was determined by well logging to be a freshwater layer, extends horizontally offshore for several kilometers under the sea. The results obtained herein demonstrate that the newly developed marine MT measurement system can be used to clarify the geoelectrical structures of brackish/fresh groundwater distributions and in coastal areas.
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