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Our 3-D finite-element modeling scheme for transient electromagnetic (TEM) data consists of two parts: (1) calculation of the primary electric field of an electric bipole source on a layered earth, and (2) calculation of the secondary electric field caused by induced scattering currents in 3-D anomalous regions. The magnetic field is computed from derivatives of the electric field. The scheme works with harmonic fields; transient (vertical magnetic) fields are obtained by Fourier transformation over a wide frequency range. An aysmptotic boundary condition on the secondary field terminates the finite-element grid.

We constructed a 3-D model of the Mori geothermal field in northern Japan from TEM surveys with long-offset electric bipole sources at 86 sites and from a shallow controlled-source magnetotelluric (CSMT) survey. Joint inversion of the TEM and CSMT data gave a preliminary layered-earth model. Modeling of 3-D variations about the layered structure suggests that the reservoir should be developed around the vent of the caldera with hot fluid being supplied from the southwest of the vent.

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