Using reverse vertical seismic profiling (RVSP) to characterise the subsurface fracture system of the Seokmo Island geothermal field, Republic of Korea

Abstract

This paper presents a case study of fracture-system interpretation using reverse vertical seismic profiling (RVSP) with seismic reflection, refraction, and borehole televiewer data on Seokmo Island, South Korea. We first extracted fracture locations from a pre-existing image obtained by prestack phase-screen migration of RVSP data, and the strike and dip of each fracture from the borehole televiewer data. We established an initial velocity model using this fracture information and then generated synthetic common-receiver gather data through forward simulation. However, the synthetic data could not sufficiently reflect the characteristics of the field data. To resolve this problem, we added an upper alluvial layer, observed in the surface reflection and refraction data, to the velocity model. To improve the quality of the migrated image of the RVSP field data and the velocity model, we not only reprocessed the RVSP data, but also applied prestack generalized-screen migration, which is more accurate than phase-screen migration in computing steep fractures. The new synthetic data generated from the improved velocity model agreed well with the RVSP field data. As a result, we could describe a markedly improved subsurface structure including fracture locations. The workflow suggested in this study will be helpful for imaging fracture systems in oil and gas reservoirs as well as in geothermal reservoirs.