Processing and Inversion
In contrast to seismic, all Towed Streamer EM projects are acquired with a single streamer. Line spacing of <1.5 km permits anisotropic inversion of both 2D resistivity sections (one per sail line), as well as a 3D resistivity volume. Projects with a line spacing of >1.5 km are considered 2D, with resistivity sections generated from 2.5D inversion.
Unconstrained, seismically guided 2.5D anisotropic inversion is undertaken by our EM imaging and interpretation specialists. Where EM data has a line spacing of 1.5 km or less, then a separate 3D inversion may be performed in parallel.
PGS starts the 2.5D inversion process with a value assigned to a half space, which in the Barents Sea can be between 5 - 20 Ωm.
Due to the density of data acquired by the Towed Streamer EM system, varying the initial value of resistivity does not have a significant effect on the final output of the inversion. The only significant impact is the number of iterations the inversion has to run through before it reaches the final model, the closer the initial value is to reality, the fewer the number of iterations required. This reliable approach to the inversion process is just one of the advantages which are a direct result of the significantly higher volume of data acquired using Towed Streamer EM.
While seismically guided inversion can improve resolution, unconstrained inversion significantly improves subsurface understanding, and is a necessary step in the process of conducting guided inversion. When interpreted in conjunction with dual-sensor broadband GeoStreamer data, unconstrained inversion can target prospective structures identified on the seismic, enabling a level of prospect de-risking that is not possible using seismic alone.
~9000 sq. km horizontal resistivity depth image from the Barents Sea