P wave travel time changes in the Groningen reservoir

Abstract

After the M 3.6 Huizinge earthquake of 2012, the Groningen gas field in the Netherlands has become one of the most densely instrumented regions of induced seismicity in the world. In 2013 two former production wells were equipped with geophone strings at the reservoir level at 3 km depth. In a previous study Zhou & Paulssen (2017) showed that anthropogenic noise is the dominant noise source at the reservoir level, and they found that the P and S wave velocity structure along borehole SDM-1 could be accurately retrieved from noise interferometry by cross-correlation. In this study we show that signals from nearby passing trains can be used to infer time-lapse variations in the reservoir. High-frequency (30-100 Hz) train noise is identified in the spectrograms of the 10 geophones in borehole SDM-1. Deconvolution of 20 s of train signal recorded on the vertical component of the top geophone with that of the other geophones produces a clear downgoing P wave as well as weak bottom and top reflections from the reservoir (Fig. 1). The travel times of the downgoing P wave allowed very accurate estimates of the inter-geophone velocities in the reservoir. In addition, small time-lapse variations were measured in the data of the two geophone deployments that were analyzed: Jan-Jun 2015 and Jul-Dec 2015. The travel times within the reservoir decrease by 0 – 0.1% per half year with a larger decrease in the second half of 2015. This observation correlates with bigger surface subsidence for the second half of 2015. Albeit tiny, the travel time variations cannot be explained by vertical shortening only, so an additional P wave velocity increase related to compaction is required to explain the data. Furthermore, we found a clear travel time anomaly (up to 0.8 ms) in the time span Jul 17-Sep 2 between the bottom geophone, located just below the gas-water-contact in the reservoir, and the one above (or any of the other geophones). This anomaly is associated with the drilling of a deep borehole at 5 km distance (Jul 9-Aug 28)