Geological studies invariably starts at the incipient reconnaissance stage, which entails preliminary mapping of the lithologic units and structures, mapping of thermal surface manifestations and pos-sibly relate to the structures and or volcanism in the prospect of interest. Detailed geological studies is performed in the geothermal field and its surroundings..
Detailed mapping of geothermal alteration on the surface is performed and alteration minerals analysed using XRD. The chronology of the surface alteration is studied in order to understand the temporal variation in the surface activity. An exhaustive mapping of thermal manifestations is per-formed and the physical properties of surface manifestations are measured and recorded, including temperature, flow rate, conductivity etc..
Photogeological maps and remote sensing images are used to carry out mapping of geological structures during the reconnaissance and/or the detailed mapping.
The imagery of optical, near-Infrared and thermal-Infrared region of the electromagnetic spectrum can be used to identify surface expressions of geothermal resources. The geothermal manifestations such as sinter/tufa, hydrothermal alteration products (clays, sulfates), thermal anomalies have distinct spectral signatures that can facilitate recognition on the remote sensing imagery.
Geochemical exploration for geothermal resources involves sampling, analysis and interpretation of discharge of thermal fluids from geothermal manifestations. The main objectives of geochemical studies are to characterize the thermal fluids, establish their origin, flow direction (upflow, outflow), evaluate mixing scenarios, estimate the equilibrium reservoir temperature and determine the suitability of the fluids for the intended use. The following subsection provides more details on the information obtained from geochemical surveys.
Geothermal anomalies are linked to geophysical anomalies because changes in temperature and geothermal gradient can change subsurface physical properties that influence measurements at the surface. In this respect, a geothermal system generally causes heterogeneities in the physical properties of the subsurface, which can be observed to varying degrees as anomalies measurable from the surface. The changing physical properties can be measured using different geophysical methods and instruments (Table 1).
Physical Property Geophysical Method Common Equipement Electrical conductivity/resistivity
Electromagnetic / Electric
MT / TEM
Physical properties and the corresponding geophysical methods and instrument
Heat flow measurements
The primary objective of heat flow measurements is to estimate amount of heat energy being lost naturally, analyse the distribution of heat loss features and locate hidden fracture zones. Heat loss is related to temperature gradients. Heat loss mechanisms are conduction, convection and radiation. In geothermal settings, heat is lost through conduction-mainly in contact with the system rock bodies, through convection: mainly through discharging fluids. High heat loss anomalies usually coincide with the structural trend and/or areas with thermal manifestations.
Conceptual model and well siting
Conceptual model is a descriptive or qualitative model incorporating, and unifying, the essential physical features of the systems in question. A geothermal conceptual model can incorporate information such as size, temperature, flow patterns including upflow, outflow and recharge (indicated with arrows), processes like mixing, chemical buffering or boiling, internal flow barriers like dikes or sealing, the cap-rock, and the nature of the heat source.