Seismic Modeling of Free Gas Occurrences and Bottom Simulation Reflection (BSR) Beneath Hydrate-bearing Sediments

Abstract

This master thesis study uses seismic modeling to better understand the seismic response of the free gas that accumulates beneath hydrate-bearing sediments. To this end, I developed 12 different geological models ranging from very simple geometries to anticline structures. Within these models, I varied the distribution of free gas in the layers beneath the BSR. Sediment properties were taken from velocity profiles of the Storegga gas hydrate system and the density approximately estimated from a computed Gassmann diagram. Norsar 2D, Norsar 3D and Seisrox modeling programs were used to generate PSDM seismic images. Higher frequencies, large incident angle and long horizontal sampling distances has resulted in the generation of BSR as termination of enhanced reflection. The termination of the enhanced reflections coincided with anticlinal and dipping BSR in this study. The presence of intercalation of gas free and hydrate bearing layers above the dipping BSR and gas free and gas bearing sediments below the dipping BSR led to the termination of enhanced reflection at the BGHSZ. Low frequency, near angle and small sampling distances resulted in proper BSR reflections for the geologic setting mentioned above. Lower frequency and large sampling distances results in termination of enhanced reflection of the BSR. A seismic section from Hydratech data was compared to the synthetic seismic model.