Seismic Reflection Exploration of Karst Phenomena of a Geothermal Reservoir in Southern Germany

Abstract

A geothermal power plant in Unterhaching, south of Munich/Germany, generates 3.36 MW of electrical power. Two boreholes (doublet) penetrated the top of the Upper Jurassic (Malm) at a depth of approx. 3000 m. The Malm which is present in most parts of the Southern German/Upper Austrian Molasse Basin is a highly-productive aquifer with increasing depths and temperatures from north (Danube River) to south (Alps). Information from boreholes in the eastern Molasse Basin indicates that the most prospective sites are in the immediate vicinity of faults. Optimal development of geothermal productivity therefore requires exploration of the geological structure, as well as information on the karstification of the Malm. Available seismic profiles, originally carried out in the framework of hydrocarbon exploration, were reprocessed with the aim of interpreting the facies and thus the degree of karstification within the Malm.
The most prospective areas are, besides of faults, those where seismic diffractions (indicators for reef facies and karstification) occur together with low seismic velocities (indicators for large amount of water). Location and the deviation of the boreholes were defined from these 2D seismic profiles. Both boreholes, Gt 1 and Gt 2, about 4 km apart, were successful and supplied at least 118 l/s water of temperatures of 130.5 oC. Supplementary vertical seismic profiling (VSP) and moving source VSP-measurements were carried out in the production well Unterhaching Gt 1 for a more detailed investigation of its vicinity. A high-resolution 4 x 5 km 3D seismic reflection survey, scheduled for summer 2009 is used to explore a fault structure which was the target area of the injection well Unterhaching Gt 2. The interpretation of this dataset serves as a basis for a regional geological 3D model and for subsequent hydrological and numerical modeling.