PNNL

Abstract

Reflection seismic provides the primary technology for imaging subsurface geology between and distal to wellbores; and where rock physics properties are suitable, surface seismic and vertical seismic profiles (VSP’s) can provide important CCUS monitoring modalities. In addition, seismic data provide the subsurface framework for locating microseismic events and reducing risk of induced seismicity. Seismic surveys have traditionally been limited to acquisition and processing of compressional (P) -wave data, but modern technology includes the use of three-component receivers that can additionally record shear (S) wave modes that have been generated at the surface or converted from P-waves in the subsurface. Each collected waveform can potentially produce a separate data volume and new information on the subsurface. This paper reviews the insights gained through application of multicomponent VSP technology at the FutureGen2.0 site where, typical of many proposed CCUS and CCS sites, traditional P-wave surface seismic imaging is considerably impacted by seismic noise. Two 2D P-wave surface seismic lines, totaling 24 km, were acquired in 2011 as part of the screening process for the FutureGen 2.0 site selection. These data confirmed that no large scale faults are present at the site, but details of the images below about 500m are heavily degraded by unidentified sources of seismic noise. A multicomponent zero-offset and offset VSP program conducted in 2013 allows identification and determination of origin of individual seismic noise contributors. Initial scattering of surface-generated seismic energy at the site is caused by lateral variation in soil and unconsolidated glacial outwash. Seismic multiples create false event horizons in the consolidated rock, and in the Morgan County VSP data, originate at velocity contrasts between shales and limestones and at unconformities deeper in the section. Analysis of the signal loss caused by seismic attenuation indicates that there are two attenuative zones within the first 120m of the subsurface, probably related to thin coals and noncommercial traces of natural gas. Additional attenuation zones coincide with three deeper unconformities. Seismic mode conversion occurs at the surface, at the top of shallow bedock and at unconformities, including the top of the Eau Claire seal.