Spontaneous potential surveys for geothermal exploration in Tenerife and La Palma (Canary Islands)

The spontaneous-potential (SP) method is a passive geophysical technique that measures naturally occurring voltage differences on the Earth's surface. This method is capable of identifying geoelectric anomalies which can be generated by different sources. In active volcanic areas, these geoelectrical anomalies may be related to thermoelectric and electrokinetic processes caused by the circulation of hydrothermal fluids in subsurface porous materials. The sensitivity of the SP method in characterizing hydrogeologic and hydrothermal circulations, together with its simplicity and non-intrusive nature, has made this method widely used for geothermal exploration in the last decades.

In the Canary Islands, the surface geothermal manifestations are less evident than in other active volcanic systems worldwide. Thus, exploration techniques used to study the geothermal potential of the Canaries must focus on investigating the possible presence of deep-seated hydrothermal reservoirs. For this purpose, self-potential surveys were conducted on the Tenerife and La Palma islands to determine the spatial variations of the electrokinetic potential related to the geothermal and volcanic-structural characteristics of the study areas. The choice of these two islands to promote the search for geothermal resources lies mainly in their historical volcanism, characterized by five well-documented historical eruptions on Tenerife and up to 8 on La Palma, where the most recent and voluminous eruption occurred in 2021.

The SP campaigns were carried out in two volcanic areas: the NW rift zone of Tenerife and the west flank of the Cumbre Vieja rift zone of La Palma. The instrumentation consisted of several V-FullWaver devices from IRIS Instruments, equipped with Cu-CuSO4 non-polarizable electrodes and copper wire reels ranging from 60m to 250m. The methodology consisted of measuring the potential difference of the electric field along different profiles. These profiles are divided into sections where the reference electrode remains at the beginning of the profile. At the same time, the other is moved, measuring on points spaced of about 60 m m until the maximum length of the cable is reached. Then, a new reference electrode is established, and the measurements continue along the profile. To obtain continuity in the data set along each profile, the reference correction is applied to connect all sub-sections of a single SP profile.

Measurement points were located along several trails within the geothermal prospecting areas. Preliminary results show anomalies ranging between -281 and 198 mV in Tenerife and between -234 and 256 mV in La Palma. The main objective of the SP application is to contribute to delimiting those areas of hydrothermal interest associated with the presence of geothermal resources. Although this study is in its initial stage, it promotes a more sustainable and resilient future for the Canary Islands, in which geothermal resources could provide a reliable and renewable energy source.