The New Energy and Industrial Technology Development Organization (NEDO) has been conducting the "Deep-Seated Geothermal Resources (DSGR) Survey" project aiming at deep-seated geothermal resources lying below the depth of 3,000 m. As a part of the DSGR project, a "Multi-Frequency Array Induction Logging (MAIL)" tool has been developed to estimate the resistivity structure accurately around a wellbore.
The MAIL method, which has been developed as a downhole surveying technique, is an approach to estimate the resistivity structure within a few meters from the wellbore by employing an electromagnetic logging approach. The tool has one transmitter and several receivers for magnetic field of multiple frequencies. Observed vertical component magnetic responses are analyzed using one- and two-dimensional inversion methods. The conductivity model is cylindrically symmetrical for the borehole axis. Although there is a similar tool such as the AIT tool of Schlumberger, both sensor arrangement and analyzing method are different.
This paper describes 2D inverse modeling based on a cylindrical model using MAIL logging data obtained from a DSGR well. Result of the inversion showed good correlation with the granite distribution. Thus the MAIL tool was demonstrated to have resistivity resolution for radial and axial directions of a wellbore

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