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[SEM12-02] Near-surface geothermal fluids conduit imaged by audio-frequency magnetotteluric survey in a hydrothermal area, Indonesia
Keywords:geochemical survey, EM survey, magnetotellurics
Natural radioactive gases in volcanic areas (such as radon) are used as indicators of fluid discharges coming from deep geothermal reservoirs. Therefore, soil gas sampling is often used in exploration for geothermal resources. We found the radon concentration in the soil air was found at a hydrothermal site in the Wayang-Windu region, Indonesia. It is probably associated with concealed fracture zones. However, due to a limitation of sampling points, the locations of fracture zones and geothermal fluid conduits were unknown.
The magnetotelluric (MT) survey, one of the electromagnetic (EM) geophysical exploration, is a powerful method for imaging geothermal fields. It requires the natural fluctuation of geomagnetic and electric fields. The observed EM responses at a number of MT sites can be used for quantitative analysis of subsurface resistivity distribution. In this study, we applied the short-dipole audio-frequency magnetotelluric survey (SAMT) in the Wayang-Windu geothermal field. The horizontal spatial resolution is higher than that of the conventional MT exploration. Using the observed data, an inversion algorithm of two-dimensional resistivity structure was performed to clarify the cross section of resistivity distribution down to the depth of 500 meters.
In our result, it was found that a low resistivity layer spreads commonly in the survey area at a depth of several hundred meters or more. This can be interpreted as a hydrothermally altered (clay-rich) layer. This low resistivity layer reaches the surface in some areas; one of the areas has indicated the high radon concentration in the soil air. This near-surface low resistivity zones possibly corresponds to the geothermal alteration zones along the faults. The good coincidence between high Radon and low resistivity implies that the fault is a fluid conduit from a deep reservoir. Thus, we successfully demonstrated the usefulness of combination of geochemical and geophysical surveys for finding the steam spots in the geothermal field.
The magnetotelluric (MT) survey, one of the electromagnetic (EM) geophysical exploration, is a powerful method for imaging geothermal fields. It requires the natural fluctuation of geomagnetic and electric fields. The observed EM responses at a number of MT sites can be used for quantitative analysis of subsurface resistivity distribution. In this study, we applied the short-dipole audio-frequency magnetotelluric survey (SAMT) in the Wayang-Windu geothermal field. The horizontal spatial resolution is higher than that of the conventional MT exploration. Using the observed data, an inversion algorithm of two-dimensional resistivity structure was performed to clarify the cross section of resistivity distribution down to the depth of 500 meters.
In our result, it was found that a low resistivity layer spreads commonly in the survey area at a depth of several hundred meters or more. This can be interpreted as a hydrothermally altered (clay-rich) layer. This low resistivity layer reaches the surface in some areas; one of the areas has indicated the high radon concentration in the soil air. This near-surface low resistivity zones possibly corresponds to the geothermal alteration zones along the faults. The good coincidence between high Radon and low resistivity implies that the fault is a fluid conduit from a deep reservoir. Thus, we successfully demonstrated the usefulness of combination of geochemical and geophysical surveys for finding the steam spots in the geothermal field.