| Authors: |
Jean-Francois GIRARD, Nicolas COPPO, Pierre WAWRZYNIAK, Bernard BOURGEOIS, Jean-Michel BALTASSAT, Alain GADALIA, BRGM, ORLEANS, FRANCE; |
| Abstract: |
Resistivity imaging is a key parameter in most geothermal exploration programs, and particularly in volcanic environment. It is assumed that resistivity variations allow imaging the caprock, but also hydrothermal weathering or preferential water flow. The geothermal fluid is generally brine water which drastically decreases the electrical resistivity and the temperature increase also decreases resistivity. The resistivity signatures for several conceptual geothermal reservoirs can be found in the literature. Magneto-telluric (MT) is generally used to image resistivity variations down-to a relevant investigation depth of several kilometers. Based on natural source signal, MT prospection efficiency depends on the Sun activity during the survey but moreover depends on the noise conditions. In many cases, because industrial activities have been developed close to the geothermal resource, the noise conditions make it difficult to obtain reliable MT tensors and hence a good resistivity image. Generally, the only way to counter is recording longer time series, to apply robust noise filtering and combining with a remote filtering. The noise issue in MT prospection is even more drastic in Island context. We investigated the substitution of controlled-source EM instead of MT measurements close to urbanized area. The distance constraints to respect the far field conditions are so challenging that they are often logistically not possible. Indeed, when it is possible to measure the receiver stations far enough from the source, the distortion due to the controlled-source disappear. But when the conditions for applying standard processing of CSAMT are not fulfilled, it is necessary consider the source effect to interpret properly the EM response. We have studied and performed EM imaging in the near field, using a pair of pre-existing boreholes to inject high current into the ground through metallic casings. Based on numerical modeling, we propose a methodology to obtain maps and section of apparent resistivity. The results obtained were confronted to previous geophysical campaign results performed in the 1980’s and the prospected area has been enlarged. This measuring protocol was applied to prospect the Lamentin area (Martinique, Lesser Antilles, France). This survey was performed in 2013 within the framework of a large exploration program founded by the FEDER, ADEME, Regional Council and SMEM designed to explore the geothermal potential of Martinique. |