| Abstract |
Electromagnetic exploration techniques such as magnetotellurics (MT) have been successfully used in shallow geothermal exploration for many years. However, they can also be used in deeper studies of the crustal and mantle structures that control the location of shallow geothermal resources. During MT fieldwork in 1995-9, the geoelectric structure of the Tibetan Plateau was studied from the Himalaya in the south to the northern margin of the plateau. Using both broadband and long period MT systems, the electrical resistivity was imaged. These electrical resistivity models exhibit a widespread zone of low resistivity (below 10 ohm-m) in the midcrust (typically around 20-50 km). In some locations, this low resistivity horizon coincides with seismic bright spots. The negative polarity of these reflections indicates that a fluid phase is present. Shallower conductive zones may be explained on the basis of saline fluids, but at greater depth, partial melt is required. In Southern Tibet, the Yangbaijin geothermal field is located above a zone of low electrical resistivity and bright seismic reflections. It is located in a graben formed by the east-west extension of Tibet. |