Interpretation of 3D Magnetotelluric (MT) Surveys: Basement Conductors of the Menderes Massif, Western Turkey

Abstract

Geothermal resource exploration is enjoying increased activity in Western Turkey, following recent openings in geothermal permit licensing. As part of an assessment exercise of several license areas over the Menderes Massif, BM Holding has undertaken surface investigations including geology, geochemistry, and geophysics - gravity and broadband Magnetotelluric soundings (MT) - in addition to both shallow and deep drilling programs. While the Bouguer gravity anomalies respond mostly to graben structure (the dominant density contrast is between the basement and sedimentary cover) the MT soundings are sensitive to resistivity variations from the near-surface, including graben fill, through the deep structures within the basement. The deep, 3D resistivity distribution and drilling results are compared from two areas of the western Menderes Massif. The interpretation of the resistivity response in this setting differs from classical applications of MT surveys over conventional hydrothermal resources of volcanic settings. As the basement composition varies from coherent gneisses through pelitic schists, with significant bands of graphite related to inter-nappe shear zones, the successful integration of the MT resistivity results requires the building of a structural geological / conceptual model as the basis to geothermal exploration rather than simple “anomaly hunting”. The resistivity distribution can be used to predict deep basement structure, e.g. through the mapping of this regionally continuous graphite schist marker horizon, and this deep structural information used to update the conceptual geo-structural model.