Reverse-time migration of a methane gas hydrate distributed acoustic sensing three-dimensional vertical seismic profile dataset

Abstract

Distributed acoustic sensing (DAS) has emerged as an important technology for seismic investigations for downhole vertical seismic profile (VSP) surveying. DAS fiber has high-spatial and -temporal resolution resulting in higher-quality and finely sampled P- and S- wavefields which motivates the development and refinement of three-dimensional (3-D) seismic imaging framework capable of realizing the full potential of DAS data for generating high-resolution images of subsurface geological structure. The work reported in this thesis outlines the development of a full-wavefield seismic imaging framework using an acoustic reverse-time migration (RTM) procedure adapted from more standard surface-based seismic usage. The key advantages of using an RTM imaging procedure over other seismic imaging methods is in its ability to handle complex earth models, sharp velocity contrasts, and significant structural dips. The procedure was developed using synthetic velocity models and data, with imaging challenges (i.e., irregular surface topography, borehole deviations, and modeling frequencies to 100~Hz) examined prior to working with field data. The framework was then applied to a DAS 3-D VSP data set acquired on the North Slope of Alaska as part of a multi-partner methane-gas hydrate investigation. Preprocessing was conducted to isolate and improve the signal-to-noise ratio of compressional-wave (P-wave) data components that are used in the acoustic RTM framework. In addition, velocity model refinement was conducted to produce an accurate 3-D P-wave model required for RTM imaging. The 3-D RTM code, utilizing graphical processing units for faster wavefield computations, was used to develop high-quality images of the two target hydrate sand reservoirs. The overall work shows that using a high-frequency 3-D RTM imaging procedure with DAS 3-D VSP data results in high-quality and interpretable subsurface images, which were validated through and accurate well-tie analysis. Suggested future research activities include development more advanced inversion approaches (e.g., acoustic or elastic least-squares RTM) that could generate higher-resolution images and lead to more quantitative subsurface reflectivity estimates allowing for better geologic interpretation and understanding of target hydrate reservoirs.