Vertical seismic profiling with distributed acoustic sensing images the Rotliegend geothermal reservoir in the North German Basin down to 4.2 km depth

Abstract. We performed vertical seismic profiling at the Gros Schonebeck site in order to gain more detailed information on the structural setting and geometry of the geothermal reservoir, which is comprised of volcanic rocks and sediments of Lower Permian age. During the four-day survey, we acquired data for 61 source positions with the novel method of distributed acoustic sensing (DAS), using hybrid wireline fiber-optic sensor cables deployed in two 4.3 km deep wells. We show that wireline cable tension has a significant effect on data quality. While most of the recorded data has a very good signal-to-noise ratio, individual sections of the profiles are affected by characteristic coherent noise patterns. This ringing noise is a result of how the sensor cable is mechanically coupled to the borehole wall, and it can be suppressed to a large extent using suitable filtering methods. After conversion to strain rate, the DAS data exhibits a high similarity to the vertical component data of a conventional borehole geophone. Upgoing reflections are nevertheless recorded with opposite polarity, which needs to be taken into account during further seismic processing and interpretation. We derived accurate time-depth relationships, interval velocities, and corridor stacks from the recorded data. Based on integration with other well data and geological information, we show that the top of a porous and permeable sandstone interval of the geothermal reservoir can be identified by a positive reflection event. Overall, the sequence of reflection events shows a different character for both wells, which is explained by lateral changes in lithology. The top of the volcanic rocks has a somewhat different seismic response, and no stronger reflection event is obvious at the postulated top of the Carboniferous. The thickness of the volcanic rocks can therefore not be inferred from individual reflection events in the seismic data alone. The DAS method has enabled measurements at elevated temperatures up to 150 °C over extended periods and has led to significant time and cost savings compared to deployment of a conventional geophone chain.