Reflection Imaging of EGS Reservoirs at Soultz and Basel using Microseismic Multiplets as a Source

Abstract

The authors have been developing a new variation of passive reflection technique, where microseismic multiplets are used as a source (multiplet reflection imaging using microseismicity: multiplet RIM). Closely similar waveform and precisely determined hypocenter of the multiplet can be effectively used to image structures inside/around the seismically activated EGS reservoirs. In the multiplet RIM, artifacts and blurred reflection image due to limited number of sources and detectors is suppressed by identification of relative/absolute time of arrivals of reflected phase from the multiplets and new concept of inversion. The multiplet RIM has been applied to a data set collected at Soultz, France and Basel, Switzerland. The coherence index, which is a measure of arrival of reflected phase in the multiplet RIM, suggested that coherent reflected phase among the multiplets were successfully identified. The reflection image showed that there are possible reflectors at the bottom of shallow reservoir and surrounding zone at Soultz. Pre-existing fault around 1200m away from the stimulated zone was imaged at Basel. The indices of reflectivity along a borehole at both the sites were consistent with results from logging.