Bsr

Reflection strength is proportional to the change of acoustic impedance, which is the product of velocity times density. The base of the hydrate-cemented zone, where it traps free gas bubbles, represents a very large change in velocity, and therefore produces a very strong reflection, the BSR (Fig. 1). This may or may not be a continuous reflection event in seismic profiles because the presence of gas will vary laterally due to variations in trapping configuration, porosity, gas supply, etc. Where it exists, however, the reflection is commonly very sharply defined, because the phase boundary is a distinct, not diffuse, limit to hydrate occurrence. In contrast, the top of gas hydrate within the sediments has no such precisely defined limit created by a phase boundary; actually the top of the gas hydrate stability zone exists above the sea floor in most locations. As a result, even where there is a discrete hydrate accumulation within the sediments, no sharp reflection boundary is observed at its top in seismic profiles because there is no large velocity contrast there. Free gas cannot exist within the GHSZ as long as water exists there because it would spontaneously form gas hydrate.

Figure 1. U.S. Geological Survey seismic reflection profile across the Blake Ridge on the continental rise off South Carolina (Fig. 3.), showing clear examples of the type of gas hydrate effects observed on seismic profiles.The base of gas bearing sediments in contact with water- or gas plus water-filled sediments forms a very strong reflecting horizon that parallels the sea floor, known as the "bottom simulating reflection", or BSR. Above the BSR, in the region where gas hydrate is stable, the amplitude of reflections is reduced, a phenomenon known as "blanking" at locations where gas hydrate is concentrated.

Figure 1. U.S. Geological Survey seismic reflection profile across the Blake Ridge on the continental rise off South Carolina (Fig. 3.), showing clear examples of the type of gas hydrate effects observed on seismic profiles.The base of gas bearing sediments in contact with water- or gas plus water-filled sediments forms a very strong reflecting horizon that parallels the sea floor, known as the "bottom simulating reflection", or BSR. Above the BSR, in the region where gas hydrate is stable, the amplitude of reflections is reduced, a phenomenon known as "blanking" at locations where gas hydrate is concentrated.

The base of hydrate stability, as disclosed by the BSR, occurs at an approximately uniform sub-bottom depth throughout a small area because it is controlled by the temperature; thermal gradients across an area tend to be consistent. In Figure 1 a very strong BSR approximately parallels the sea floor and cuts through reflections from sedimentary strata. The coincidence in depth of the BSR to the theoretical, extrapolated pressure/temperature conditions that define the hydrate phase boundary and the sampling of hydrate above BSRs and gas below give confidence that this seismic indication of the base of hydrates is dependable.

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