The Ambient Seismic Field at Groningen Gas Field: An Overview from the Surface to Reservoir Depth

The long-term exploitation of the Groningen gas field led to compaction at reservoir depth, subsequent ground subsidence, and recently earthquakes. As part of an ongoing effort to quantify the hazard and risk in the region, several permanent and temporary seismic arrays have been deployed. As a result, the Groningen area is one of the seismologically best-instrumented areas worldwide. In this article, we describe several seismic experiments that were conducted in the region and take advantage of the numerous possibilities they offer to characterize the ambient seismic wavefield at the surface, in the shallow subsurface, and at reservoir depth. By means of beamforming, analysis of cross-correlation functions, surface-wave eigenfunction analysis, and correlations of neighboring frequencies, we are able to determine the main characteristics of the ambient seismic field (ASF), including the predominant propagation modes and phases. We retrieve clear multimode Rayleigh and Love waves, as well as and P waves, from cross correlations of the ASF. At reservoir depth, we show that the wavefield is largely trapped and reflected between geologic boundaries above and below the reservoir. This article reviews the characteristics of ASF observations with the goal of guiding future investigations of shallow structure of the Groningen area.