Acquiring Surface Seismic Without Surface Receivers: A 3D DAS-VSP Case Study Offshore Abu Dhabi

Abstract

Abstract While shooting a dense ocean-bottom nodes (OBN) survey offshore Abu Dhabi, 3D Distributed Acoustic Sensing (DAS) Vertical Seismic Profiles (VSP) were recorded utilizing semi-permanent optical fibers present in wells across the field. The objective was not only to provide the benefits generally afforded by 3D VSPs, but to use the surface-related multiples and obtain a 3D image of the subsurface comparable to (if not better than) the OBN survey. The OBN survey recorded a high density shot carpet (25m × 25m) into 4C nodes on a 25x150m grid using a wide azimuth, long offset geometry. It was shot over a producing field with four artificial islands used as drilling pads. Optical fibers clamped to the upper completion tubing are present in the majority of wells drilled from these islands. The team decided to use DAS interrogators to "eavesdrop" on the OBN seismic sources while the seismic sources operated in the vicinity of two of the artificial islands. VSPs provide local measures of attenuation, velocity, anisotropy, and multiple content that can aid surface seismic processing. Furthermore, a 3D VSP can provide an image of the subsurface that has higher resolution than surface seismic, but which does not extend too far from the well. A recent imaging technology for offshore 3D VSP uses seismic multiples to reconstruct the subsurface as if it were acquired by surface seismic. This imaging technology is very promising but suffers from weak signal-to-noise ratio, especially in ultra-shallow water. To mitigate this issue, the team used a total of 16 wells to reconstruct the subsurface in a 300km2 combined area around the two islands. The wells were chosen for the length of their usable fiber and their diverse trajectories.