Abstract
Convective hydrothermal systems have been extensively studied using electrical and electromagnetic methods given the strong correlation between low conductivity anomalies associated with hydrothermal brines and high temperature areas. However, studies addressing the application of similar geophysical methods to hot dry rock geothermal systems are very limited in the literature. The Timanfaya volcanic area, located on Lanzarote Island (Canary Islands), comprises one of these hot dry rock systems, where ground temperatures ranging from 250 to 605 °C have been recorded in pyroclastic deposits at shallow (<70 m) depths. With the aim of characterizing the geophysical signature of the high ground temperature areas, three different geophysical techniques (ground penetrating radar, electromagnetic induction and magnetic prospecting) were applied in a well-known geothermal area located inside Timanfaya National Park. The area with the highest ground temperatures was correlated with the location that exhibited strong ground penetrating radar reflections, high resistivity values and low magnetic anomalies. Moreover, the high ground temperature imaging results depicted a shallow, bowl-shaped body that narrowed and deepened vertically to a depth greater than 45 m. The ground penetrating radar survey was repeated three years later and exhibited subtle variations of the signal reflection patterns, or signatures, suggesting a certain temporal variation of the ground temperature. By identifying similar areas with the same geophysical signature, up to four additional geothermal areas were revealed. We conclude that the combined use of ground penetrating radar, electromagnetic induction and magnetic methods constitutes a valuable tool to locate and study both the geometry at depth and seasonal variability of geothermal areas associated with hot dry rock systems.
Highlights
One of the most remarkable features of active volcanic areas is the presence of high-enthalpyOne of the most remarkable features of active volcanic areas is the presence of high-enthalpy geothermal systems that exhibit high temperature zones (>150–200 oC) at ground level (e.g., [1])
Most of the literature on the exploration exploration of geothermal resources is related to hydrothermal systems and is primarily based on of geothermal resources is related to hydrothermal systems and is primarily based on electrical and electrical and electromagnetic methods due to the strong correlation between the low conductivity electromagnetic methods due to the strong correlation between the low conductivity anomalies anomalies associated with hydrothermal brines and the high temperature areas
Magnetic anomalies can be associated with variations in the magnetic susceptibility produced by high temperatures and the eventual alteration caused by the circulation of high temperature gases within the rocks
Summary
One of the most remarkable features of active volcanic areas is the presence of high-enthalpy. One of the most remarkable features of active volcanic areas is the presence of high-enthalpy geothermal systems that exhibit high temperature zones (>150–200 oC) at ground level (e.g., [1]). The most common are convective hydrothermal systems that consist of a magmatic body acting as a source, a groundwater system to transport the heat towards the surface and a confining, shallow, heat source, a groundwater system to transport the heat towards the surface and a confining, shallow, impermeable structure (e.g., [2]). Less common are hot dry rock (HDR) geothermal systems that consist consist of subsurface zones with very low fluid content (e.g., [3]). The use of such techniques for studying HDR systems studying HDR systems is very limited because HDR systems are not as common as convective is very limited because HDR systems are not as common as convective hydrothermal ones, and because hydrothermal ones, and because the presence of vapour as dominant phase instead of hot water the presence of vapour as dominant phase instead of hot water makes these kinds of systems more makes these kinds of systems more difficult to interpret using electromagnetic methods
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