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.
In order to elucidate whether polyacrylic acid (PAA) accelerates the deposition of silica scale from geothermal water at geothermal power plants, some laboratory experiments were conducted. All experiments were carried out at pH 8-9. The PAA alone or Ca did not affect the polymerization of silicic acid in solution. When silica gel was added into silicic acid solutions containing aluminum ion (Al) and PAA, the PAA forms a Al-PAA complex and the complex adsorbs on the surface of silica gel. The dissolution of silica gel was retarded by adsorption of the Al-PAA complex. The adsorption of monosilicic acid on the surface of silica gel was also accelerated. From the experimental results, it can be reasonably considered that the addition of PAA into geothermal water causes both the retardation of dissolution of silica scales and the acceleration of deposition of monosilicic acid (growth of the silica scales).