Scope of sub basalt imaging using locally generated converted waves and mixed mode depth imaging: A study on real data

Abstract

The seismic imaging below basaltic layer is extremely challenging. The attenuation of PP energy while penetrating through basalt section, very high absorption and scattering of PP waves and generation of multiples are some important impediments. The imaging problems are severe when basaltic layer has varying thicknesses (undulations) of multiple flows with inter layered sediments. Basalt has high velocity compared to surrounding sediment. Hence the point of critical reflection angle is reached at a smaller offset. Anomalous value of Vp/Vs generates converted waves from basalt top. Quantitative modelling has demonstrated the presence of PSSP energy up to a wide offset range (8+ Km). This has led us to explore the option of sub-basalt imaging using PSSP waves. We generated synthetic PP and PSSP gathers with one dimensional modelling. The PSSP energy is visible from an offset of 1800 m onwards from basalt top. In absence of multi component OBN or OBC data, an attempt has been made to extract PSSP waves from 2D seismic data (streamer data) for time and depth imaging of PSSP waves. In this paper, we also present mixed mode (PP + PSSP), depth imaging as an alternative option for more precise structural interpretation and mapping of sub-basalt reflectors.