SV-P and S-S imaging at a CO2 storage site using vertical seismic profiling data

Abstract

Only compressional (P) waves are typically used to illuminate the geology of land-based carbon dioxide (CO2) storage sites because the costs associated with horizontal-force vibrators needed to emit shear (S) waves deters the use of that mode. However, S-waves are highly sensitive to fractures and provide clues to the porosity and permeability of the reservoir and seal. Shear wave energy may also be produced by vertical-force vibrators, and the resulting up-going direct-S waves may be recorded either as reflected S-S modes or as converted SV-P modes. These two data components can contribute information not available in P-wave components. In particular, the SV-P mode makes converted-wave analysis possible when working with seismic data generated by vertical vibrators and recorded with only vertical geophones.In this paper, we present a method for extracting S-waves emitted by vertical-force vibrators. This is a low-cost opportunity for full seismic wavefield analysis that enhances understanding of geologic reservoirs. We illustrate S-wave seismic imaging with a vertical seismic profiling dataset acquired at a CO2 storage site in Decatur, Illinois. We discuss a data processing flow that segregates direct-S modes. We compare SV-P and S-S images generated by a vertical vibrator source with P-P, P-SV, and P-SH images – the typical images made with vertical-vibrator sources. All images appear consistent and complement each other, indicating the method presented here allows the use of S-waves in CO2 storage site screening without requiring horizontal geophones.