2014 Volume 36 Issue 1 Pages 21-31
The electromagnetic (EM) method, used to investigate subsurface resistivity structures, is one of the primary survey tools in geothermal exploration because resistivity is sensitive to temperature and fluid state. The magnetotelluric (MT) method, which is usually applied to deep geothermal exploration, uses a low-frequency natural EM field as its signal source. Methods that use artificial EM fields, such as the controlled source audiomagnetotelluric (CSAMT) and transient electromagnetic (TEM) methods, which can acquire high S/N ratio data, have been used often in relatively shallow geothermal exploration. However, recently, the measurement precision of the audio-magnetotelluric (AMT) method, which utilizes high-frequency natural EM fields, has been improved. Therefore, the AMT method is expected to be applied more widely to geothermal exploration. To verify the effectiveness of the AMT method, a demonstration experiment was undertaken in the Ogiri geothermal field, Kagoshima, Japan. The principal results are as follows.
(1) To acquire AMT data equivalent to CSAMT data, it is necessary to adopt the remote reference method. Moreover, it is desirable that the AMT measurements be performed at night when noise level is low and generally, signal level is high.
(2) If 2-D inversion using the TE mode or 3-D inversion is applied, it is recommended that the magnetic fields be measured as close as possible to the electric fields.
(3) Although the AMT method is effective in investigations of shallow geothermal reservoirs, it must be used in conjunction with the magnetotelluric method for geothermal exploration at depths in excess of two kilometers.
(4) The AMT method does not need an artificial signal source and therefore, repeated surveys can be performed easily. This means that the AMT method is suitable for the monitoring of geothermal reservoirs, as well as hot-spring reservoirs and volcanic activity.
