Overall report of ground geophysical survey using electromagnetic method

Hasegawa, Ken; Yamada, Nobuto; Koide, Kaoru

Tono Geoscience Center conducted CSMT and MT surveys in the region of the Toki granite from 1997 to 1999, as a part of the Regional Hydrogeological Study. After these surveys were performed, applicability of MT method to prospecting of the deeper part of the granite was evaluated. As a result, several problems of CSMT and MT surveys were pointed out. Accordingly, we checked the quality of data obtained through the surveys and found out that the interpretation results of the underground resistivity distribution are deficient in reliability because almost all data contain large artificial electromagnetic noise. Major reason behind its poor results is adoption of the high-frequency tensor CSMT system. Because of its ease of data acquisition, it was adopted without investigation of electromagnetic noises around the survey area. This fact indicated that we must investigate details of the noise around survey area in advance and select optimum equipment and survey specifications, which can distinguish a signal from data containing such large noise.