Special Report - Bottom-Simulating Reflections(BSR).

Seismic lines from deep continental shelves all around the world contain anomalous reflections known as bottom-simulating reflections(BSR). The reflections mimic the sea-floor topography at a near constant depth below the surface, and commonly cut across geological layers. The nature of the reflection indicates a horizon across which seismic velocity dramatically decreases. At one time, scientists thought the reflection must be due to some mineralogical alteration in the sediment due to heat and pressure. Once the existence of natural methane hydrate was established, BSRs were thought to record the decrease in velocity when passing from hydrate-bearing sediments to those containing only water. Therefore, BSRs were thought to be a direct indicator of hydrate: no BSR meant no hydrate. However, the velocity contrast between hydrate and no-hydrate was determined to be insufficient to cause BSRs. Today, scientists have established that BSRs are an indication of concentrations of free methane gas that is blocked from further upward migration by the presence of methane hydrate in the overlying layers. Consequently, the distribution of BSRs may mark only a subset of the areas containing hydrate.

Seismic line with position of BSR indicated