電磁気法探査（EM法探査）技術の現状と展望

Electromagnetic (EM) methods have been developed for estimating the apparent resistivity or conductivity of material composing subsurface structure depending on survey objectives, while seismic methods are applied mainly to understand geological structure or stratigraphy. Magnetotelluric subsurface sounding (MT) methods are proven tools detect high-conductivity anomalies in the Earth's interior; controlled source EM (CSEM) methods detect high-conductivity anomalies; and, nuclear magnetic resonance (NMR) methods estimate “the mean free path” of hydrogen atoms in various molecules composing underground materials. Regarding methodologies, EM methods frequently applied in practice are: (1) natural source MT methods, (2) artificial source MT or EM methods, and (3) borehole EM methods. The fundamental principles of these methods can be summarized as measuring the induction effects of a survey target using a natural or artificial electromagnetic source. Finally, we investigate examples of recent EM approaches in connection with seismic methods and discuss the integration of survey data from both methods as a key to exploring underground structures not only in terms of stratigraphic interpretation but also for estimating material physical properties.