Mapping geothermal reservoirs using broadband 2-D MT and gravity data

Geothermal energy is playing a larger role as an alternative energy source for both electricity<br>generation and for space heating. Our recent magnetotelluric (MT) and gravity surveys in<br>Iceland and Hungary have both characterized known geothermal reservoirs and identified new<br>drilling opportunities. The success of these surveys has resulted in additional 2D and 3D MT<br>and gravity data acquisition and the onset of a drilling program to evaluate the identified<br>geothermal potential.<br>Higher temperatures and salinity of the pore water, as well as the concomitant increased rock<br>alteration associated with geothermal areas, often contribute to a decrease in the bulk<br>resistivity in a rock mass. The zones of low resistivity that are associated with geothermal<br>reservoirs can be detected by electromagnetic techniques such as the MT method.<br>We used MT/AMT measurements to acquire natural time varying electrical and magnetic<br>fields at frequencies of 10,000 Hz ~ 0.001 Hz. The EM field propagates into the Earth as<br>coupled electrical and magnetic fields and these fields are commonly represented in the<br>frequency domain as a four element impedance tensor. The characteristics of the MT<br>resistivity curves are analyzed to extract structural information (associated with resistivity<br>contrast) that is used to determine high-permeability zones and up flow zones of hydrothermal<br>systems (Malin, Onacha, and Shalev, 2004).<br>To complement the MT data, gravity surveys were acquired along the MT survey lines to<br>assist in detecting fault systems below the surface. Fault system information can be used to<br>analyze and to understand groundwater channels and water flow directions. At the same time,<br>gravity data may be used to interpret the subsurface and to aid in locating prospective heat<br>sources. Integrating the MT and gravity data reduces the intrinsic ambiguity of either dataset<br>and produces a more robust interpretation.