Advances of GREATEM and its application to geothermal resources

Studies have shown that Grounded Electrical-Source Airborne Transient ElectroMagnetics (GREATEM) is a promising method for resistivity structures investigating in some fields, in addition to inaccessible areas such as volcanoes, mountains and deep forest cover. To expand the application of the GREATEM system, a three-dimensional (3-D) resistivity model that considers large lateral resistivity variations is required. We developed a frequency- domain 3-D electromagnetic (EM) inversion approach that can be applied to time domain data from GREATEM. In the frequency-domain approach, TEM data were Fourier-transformed using a smooth-spectrum inversion method, and the recovered frequency response was then inverted. To deal with a huge number of grids and a wide range of frequencies in airborne datasets, a method for approximating sensitivities is introduced for efficient 3-D inversion. A conductive structure indicates an alteration zone, a fracture zone and a melting rock body in the geothermal field. Detectability of a conductor is important in a geothermal survey by EM methods. Synthetic model studies is carried out to investigate responses of GREATEM data for typical geothermal reservoir structures using the inversion codes. Especially, we discussed detectability of a structure under the conductive layers. Through the investigation, we discuss utilization of GREATEM for a geothermal survey. We also showed a geothermal structure formed at surroundings of volcanoes that imaged by GREATEM surveys.