Soil and soil-gas surveys for geothermal exploration at Cumbre Vieja volcano, La Palma, Canary Islands

The most common types of survey in known geothermal areas are water and gas sampling of natural discharges. However, where wáter and gas discharge features are few and where the extent of the field is not known, soil and soil-gas surveys can prove useful. These surveys can identify areas in a field that are permeable and potential upflow or boiling zones. They can also outline the boundaries of a geothermal system, often in conjunction with geophysical surveys to provide more comprehensive understanding, especially when geophysical data is difficult to interpret. For all of the above, a soil and soil-gas survey for geothermal exploration had been carried out at Cumbre Vieja volcano that is characterized by neither having visible manifestations of volcanic gases nor zones of hydrothermal alteration on its surface. We have divided the soil and soil-gas survey for geotermal exploration into four categories based on the number of sampling sites per square kilometer: 1) Regional exploration survey (1 sampling site/km²), 2) Regular exploration survey (about 5 sample sites/km²), 3) Detailed I exploration survey (between 10 and 100 sampling sites/km²) and 4) Detailed II exploration survey (more than 500 sampling sites/km₂). We have classified our soil and soil-gas survey for geothermal exploration at Cumbre Vieja volcano as a regular exploration survey due to the fact that 1200 observation or measurement points were selected for its 220 km² area, resulting in approximately 5.5 sampling sites per km². Similar soil gas surveys had been performed in other study áreas of the Canarian archipelago (Rodríguez et al. 2021). At every sampling site, we performed in-situ measurements of soil CO₂ efflux and ²²²Rn activity in the soil gas atmosphere. In addition, soil gas samples were collected at a depth of 40 cm for further chemical and isotopic analysis in our geochem lab. Simultaneously with the soil gas survey, soil samples were collected from the base of the B Horizon-Subsoil (30–50 cm) which were subsequently dried and sieved (80 mesh) for subsequent leaching in order to analyze those volátiles (Hg⁰, As, B and NH₃) of interest that were fixed in the organic-clay fraction of the soils. Some preliminary results of this soil and soil-gas survey for geothermal exploration at Cumbre Vieja are presented.

Rodríguez F., Pérez N. M., Melián G. V., Padrón E., Hernández P. A., Asensio-Ramos M., Padilla G. D., Barrancos J. and D’Auria L. (2021). Exploration of deep-seated geothermal reservoirs in the Canary Islands by means of soil CO2 degassing surveys. Renewable Energy, https://doi.org/10.1016/j.renene.2020.09.065