3D deep geoelectrical exploration in the Larderello geothermal sites (Italy)

The paper describes a new experimental deep electrical resistivity acquisition (down to 1600 m) for exploring deep and shallow geothermal systems. The test site is located in the Larderello geothermal area, the oldest geothermal field in the world under exploitation for power production. In this area, many data have been acquired in the frame of previous exploration projects but nowadays several critical issues are still matter of debate: permeability distribution, depth and volume of the magmatic heat source, supercritical fluid condition at depth, and the occurrence of low resistivity anomalies in a dry-steam crystalline and carbonate reservoir. In order to develop new methods for contributing to the hydrothermal reservoir issues, an experimental high resolution 3D Surface-Hole Deep Electrical Resistivity Tomography (SH-DERT) was designed and the Venelle2 well in the Larderello geothermal site, hosted in the crystalline units, was used for the experiment. The design of the in-hole experiment and the results of the deep geoelectrical survey are hereby presented. SH-DERT was properly designed to face extreme conditions at depth characterizing the geothermal well. It provided a 3D resistivity distribution. Transmitting and receiving electrodes were distributed on a large surface (6 km2) and in the Venelle2 well (down to 1600 m). The in-hole electrical cable was equipped to be able to operate in very high temperature conditions. The experiment represents a challenge and an opportunity for the applied geophysics in geothermal areas, where a lowest resistivity is highlighted in a zone above the reservoir and the resistivity of the reservoir is higher. Moreover, the relationship between temperature, clay alteration and resistivity can define a challenger to enable better prediction of reservoir temperature distribution from resistivity measurements. It is a potential improvement of the reservoir knowledge and a useful success for exploration drilling.