Eletromagnetic Tomography Using Surface Sources and Borehole Receivers in Kakkonda

Abstract

NEDO has been undertaking the Deep Seated Geothermal Resources Survey Project in Kakkonda (Iwate, Japan) since 1992. The pilot survey well WD-1a reached 3,729 m and over 500 degrees C in FY1995. In FY1996, NEDO drilled a sidetracking well WD-1b and hit several high permeability zones. We developed a new exploration technique using borehole to surface electro-magnetic tomography system named VEMP (Vertical ElectroMagnetic Profiling) under this project (Miura et al., 1995). We applied it to a test survey at the open-hole section (2,250 to 2,850 meters) of WD-1b in 1997. We measured magnetic fields (horizontal fields toward transmitters and vertical fields) at six frequencies (1,4,16,32,64 and 128Hz) at 13 depth levels in the hole. Signal sources were grounded-wires at six locations on the ground surface. For interpretation of the data, we are developing 2.5-D FEM nonlinear least square inversion code including robust weighting for the field data. The initial model for the inversion was based on the 2-D inversion result that was obtained by MT/CSAMT sounding in 1994 (Yamane et al., 1996).