Volcanoes and geothermal and mineral resources – Exploring subsurface geology
Most of the economic and energy resources are found in the subsurface. Exploration in the search of water reservoirs, hydrocarbons, mineral deposits, and geothermal underground resources is therefore of greatest societal relevance.
The knowledge of the existing resources in the subsurface and their potential exploitation is based on the characterisation of rocks and fluids and their properties. This task is commonly carried out by indirect geophysical methods or by direct in situ observations and measurements by means of drilling and downhole logging tools, which provide in situ rock properties and images, up to several kilometres depth.
Volcanoes are one of the main geological systems hosting economic and energetic reservoirs. Volcanoes are the main source of geothermal energy. Compared to the normal geothermal gradient of about 25°C per km of depth in most of the World, when magma (i.e.: molten rock generated at the Earth interior) enters the crust, for example, as a shallow intrusion beneath a volcano, this normal gradient is perturbed locally as temperature rises around the intrusion. The extent and duration of such a thermal anomaly depend mostly on the temperature and volume of the intruded melt. The presence of hot magmas below the surface of active volcanic regions offers the prospect of harnessing a huge amount of geothermal energy. The geothermal energy is a renewal resource, as it exploits the abundant Earth’s interior heat and water, which once used and cooled, is then piped back to the reservoir. Having the use of this natural energy source has important implications for preserving the environment.
When exploring the subsoil for geothermal or mineral resources through boreholes, geophysical logging tools are used to obtain in situ oriented images and measurements at depth, from few meters to several thousand meters. Complete sets of different physical properties and borehole wall oriented images are obtained with downhole logging tools equipped with sensors that provide the evidence to construct a detailed picture of the subsurface rocks and fluids.
Boreholes are frequently equipped with monitoring sensors to obtain a continuous record of ongoing processes in the subsurface, such a fluids flow (water, hydrocarbons, gases) and migration and also seismicity and deformation and temperature changes, among other. The boreholes then become subsurface monitoring labs and are especially critical in places where active subsurface processes are relevant, either for economic resources or, to mitigate natural hazards for the population safety. This type of research is carried out at the downhole research lab facilities at the ICTJA used for the long term and continuous monitoring experiments. This facility is equipped with a complete system of geophysical logging tools, and borehole monitoring data loggers in the frame of ongoing research projects.
The characterisation of the reservoirs in the subsurface results not only in an improved subsurface resources evaluation but also on the knowledge of the existence of faults, fractures and other potential sources of geotechnical and natural hazards. The detailed picture of tectonic, structural and geotechnical properties and characterisation of rocks and fluids in the subsurface can help to minimise potential hazards and risks. Geological, geophysical and geochemical information on the subsurface is used to obtain reliable 3D simulation and visualisation models for the exploration and improve the efficiency of both exploration and exploitation strategies with a significant costs reduction.
At the Institute of Earth Sciences Jaume Almera, CSIC, of Barcelona, Spain, the Group of Subsurface geology and downhole measurements, lead by Dr. María José Jurado, in collaboration with the group of Volcanology (GVB-CSIC) lead by Prof. Joan Martí, addresses different aspects of the subsurface structure and resources, including active faults and processes, hazard assessment and characterisation of subsurface fluids and petrophysics and resources. Their work on subsurface geology and active subsurface processes aim at improving the imaging of the earth’s subsurface dynamics. Thus, helping to understand the distribution of minerals and fluids, geothermal reservoirs, as a way to define better exploration and exploitations guidelines.
Research on energy and mineral deposits related to active and past volcanic systems are required to improve and standardise the geophysical, geological and geochemical technologies needed to characterise and monitor subsurface geological reservoirs, in order to minimise the exploration and exploitation costs by reducing the drilling uncertainties. A better characterisation of such reservoirs implies the systematic application of drill hole geophysical logging techniques during and after drilling, which will help to correlate and constrain existing data from surface geology and geophysical exploration and combine all the available information in order to obtain reliable 3D simulation and visualisation models. These should help to improve the efficiency of exploration and exploitation strategies significantly reducing their costs.
Professor Joan Martí
Professor of Research
Spanish Research Council (CSIC) Group of Volcanology of Barcelona