Geophysical prospecting of high temperature geothermal reservoirs aims at identifying either fluid trapping structures or anomalies related to the properties of the hydrothermal fluid and rock to fluid interactions. Two types of reservoir environments can be characterized: (i) sedimentary reservoirs when a carbonate reservoir is generally capped by a dominantly argillaceous, hydraulically impervious and thermally insulating cover, and (ii) volcanic and volcano‐sedimentary reservoirs associated with hydrothermally altered areas.
Based on the aforementioned exploration goals and reservoir settings, a wide spectrum of geophysical methods can be applied whose selection is largely commanded by local geological conditions and expected reservoir morphology.
Major geophysical techniques are reviewed and their potential, as to geothermal reservoir prospecting issues, discussed.
Résumé
Il est maintenant courant d'employer l'ordinateur pour interpréter les sondages électriques. Cependant, le problème à résoudre en prospection électrique n'admettant pas une solution unique, le but des traitements automatiques consiste simplement à fournir plusieurs hypothèses qui aident à bâtir une interprétation géologique des mesures géophysiques. C'est, d'ailleurs, ce qu'on recherche aussi par la comparaison visuelle des courbes de terrain avec des abaques précalculés. Mais, tandis que l'interprétation manuelle, pour conduire à des solutions qui respectent les mesures faites sur le terrain, exige un temps très long, le traitement automatique fournit immédiatement des solutions compatibles avec les courbes àétudier.
Les procédés employés s'adaptent aux divers besoins de la prospection: ils peuvent traiter aussi bien la station isolée que les profils de sondages électriques. C'est ce qu'on se propose de montrer, à l'aide de quelques exemples tirés d'études récentes.
DUPRAT, A., ROUDOT, M. and SPITZ, S. 1986, Testing the TRANSIEL Method in Mineral and Geothermal Exploration, Geophysical Prospecting 34,[445][446][447][448][449][450][451][452][453][454][455][456][457][458][459][460][461][462] TRANSIEL is the name given to a qualitative, time domain induced polarization (IP) method. The aim of the method is twofold: to locate and to distinguish between deep and shallow polarizable media. This discrimination is based on the data analysis, which distinguishes TRANSIEL from the conventional IP methods.Two case histories are presented: one related to mineral prospecting and the other to geothermal exploration. The information supplied by TRANSIEL on the deep polarizable layers is checked at each test site by an independent inversion of the recorded transients.At the mineral test site, the method correctly predicts the target location. In this particular survey, the maximum penetration depth of the method is 700 m. At the geothermal test site, a fair correlation is found between IP anomalies and the productive wells. Since the method's penetration depth does not exceed 500 m, we believe that the observed IP anomalies are related to reduction phenomena occurring in the overburden and leading to pyrite formation. We suppose that these phenomena are caused by thermochemical exchanges between the reservoir and the overburden above zones of high reservoir permeability.
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