Exploring For Geothermal Reservoirs Using Broadband 2-D MT And Gravity In Hungary

Summary

Geothermal energy is playing a larger role as an alternative energy source for both electricity generation and for space heating. Magnetotelluric (MT) and gravity surveys were conducted throughout Hungary in basins which were felt to be prospective for geothermal exploration. Integrated interpretations of this data have identified new drilling opportunities. The success of these surveys has resulted in additional 2D MT and gravity data acquisition and the onset of a drilling program to evaluate the identified geothermal potential. Introduction

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. MT/AMT measurements were used 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.

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.

Methodology

The magnetotelluric method utilizes natural variations in the Earth’s magnetic and electrical field as a source. Natural MT signals come from a variety of natural currents, including thunderstorms and solar winds. The total frequency range of MT data can be from 40 kHz to less than 0.0001 Hz. Data is acquired in a passive mode using a combination of electric sensors and induction coil magnetometers and can detect changes in resistivity to great depths. The electric sensors are used to determine the electric field which is derived from measurements of the voltage difference between electrode pairs Ex and Ey. The induction coils are used to measure the magnetic fields Hx, Hy and Hz in 3 orthogonal directions. The ratio of the recorded electric and magnetic fields [Ex/Hy] gives an estimate of the apparent resistivity of the Earth at any given depth. The audio frequency magnetotellurics (AMT) method is a subset of the MT sounding technique for audio frequencies from 1 Hz to 20 kHz and higher. It achieves moderate exploration depths to about 2,000 m with higher vertical resolution, whereas the exploration depth with MT can exceed 10 km.

Gravity based geophysical methods are usually applied in order to provide additional support for the definition of geological structures at a regional scale.