Three-Dimensional Electrical Resistivity Tomography And Its Application To Larderello-Valle Secolo Geothermal Field In Tuscany, Italy

Abstract

The Valle Secolo region in the Larderello geothermal field in western Italy is a vapor-dominated reservoir producing steam primarily from shallow, highly fractured Miscan anhydrites. In this area, water re-injection into various wells has been carried out for many years. During this period, electrical resistivity surveys have been conducted for the purpose of monitoring distribution of re-injected water and steam displacement through changes in subsurface electrical properties. This paper describes a 3-D d.c. electrical resistivity inversion algorithm and its application to data obtained from two surveys conducted in 1991 and 1993, respectively. The objective of this effort is to relate the variations in resistivity with position and time to the injection history. Our inversion models indicate that the primary resistivity variations in the Larderello geothermal field are of structural origin, e.g., the variation of conductivity of the geologic section. However, the models from both surveys contain a low resistivity anomaly at a depth of a few hundred meters that does not correlate with structure. From its location relative to the injection wells, and from changes in its properties between the two surveys, we infer that the anomaly is related to the injection and is probably a zone of high permeabilitymore » or high water saturation. These preliminary results show that it is possible to detect and monitor the re-injection of fluid through the systematic observation of electrical resistivity at the site. The method is also suitable for the detection of environmental contaminant movement. However, field measurements must be repeated using the same geometry and uniform calibration over time.« less