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Electrical prospecting is a method usually attempted for groundwater exploration. However, the method requires the stability of electrical contact resistance of electrode to the surrounding earth materials. Therefore, it sometimes becomes difficult to conduct a survey in areas where the contact resistance stability could not be assured. In this study, we propose to employ the electromagnetic propagation, which could be detected without any use of electrode, and to apply the phase-shift method as in seismic data processing. We apply the method to the horizontal magnetic components with multiple frequencies in order to image subsurface resistivity structures to locate groundwater. The survey is conducted more easily and shortly, if only the magnetic sensors above the surface are enough for estimating the structures. As in the seismic migration, both upward/downward imaging and the exploding reflector concepts can be applied to the horizontal magnetic components. The synthetic data examples show that the migration method is effective for imaging the conductive anomaly. However, it is necessary to select appropriate frequency bands and to understand correct resistivity structures to some extent in order to estimate the subsurface ones. We conclude that this technique gives an approximate resistivity structures quickly and that the migration of magnetic components is expected to provide information on the subsurface. This method is also useful for geological interpretations and for an initial model of the more complicated inversion method.

Subsurface Imaging with EM Migration of Magnetic Fields from Multiple Frequencies