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

Abstract

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