Bottom-simulating Reflector Amplitude Research Articles

Seismic analysis and distribution of a bottom-simulating reflector (BSR) in the Chilean margin offshore of Valdivia (40° S)

Multi-channel seismic (MCS) reflection data recorded offshore from Valdivia (40° S), in the Chilean margin, were processed to obtain a seismic image to establish structural characteristics and relate them to the presence of the bottom-simulating reflector (BSR). Seismic structure velocity of the BSR was determined using 1-D forward modeling. Recorded seismograms for two representative common mid-point (CMP) gathers were compared with synthetics, using different physical parameters to fit the waveforms. Our results confirm the presence of gas hydrates above the BSR. The BSR spatial continuity appears to be either interrupted or irregular due to the presence of faults. Tectonic movements can change the gas hydrate stability zone and consequently the BSR disappears or becomes weaker. Structural and topographic factors, differences in concentration, vertical distribution characteristics and internal structure of gas hydrates can influence BSR amplitude behavior. Variability in the concentration, volume, and extra supply of free gas coming from faults could be the main factors in the change of BSR amplitudes. The inclusion of the attenuation factor in the modeling supports the existence of free gas below the BSR. It is possible that the free gas below the BSR is distributed in bubbles or “bags”.

Seismic Data Processing and the Characterization of a Gas Hydrate Bearing Zone Offshore of Southwestern Taiwan

Various seismic attributes of gas hydrate bearing sediments were analyzed in the accretionary prism offshore of southwestern Taiwan utilizing seismic imaging, velocity analysis, AVO analysis, and AVO inversion of large offset seismic data. A bottom-simulating reflector (BSR) is clearly observed on the seismic section with a reversed polarity compared to that of the seabed reflection. Instantaneous amplitude sectioning clearly shows lateral variations of the BSR. The zero-phase waveform of the BSR is distinct and the weak reflectors above the BSR can be observed on the instantaneous phase section. AVO analysis shows the absolute value of the negative BSR amplitude increasing with offset. A low P-wave interval velocity layer was found below the strong BSR by detailed velocity analysis. Both the P (normal incident P-wave reflection coefficient) and G (AVO gradient) values are highly negative for the strong BSR on the P and G sections, and they lie in the third quadrant of the P and G cross-plot section. The P+G (reflectivity of Poisson's ratio) value is also negative on the P+G section and the P-G (normal incident S-wave reflection coefficient) value is approximately zero on the P-G section along the same strong BSR. All the seismic characters described above suggest that a gas hydrate layer exists together with a free gas layer below it along strong BSRs in the area offshore southwestern Taiwan.

Probable gas hydrate/free gas model over western continental margin of India

Multi-channel seismic reflection data from the western continental margin of India (WCMI) have been analyzed to construct a plausible model for gas hydrate formation. A reflector at 2950 ms two way travel time (TWT) on one of the sections is interpreted to represent the base of the layer of the methane hydrate, identified by a bottom simulating reflector (BSR) that lies almost 500 ms beneath the sea floor. BSRs of similar origin are common world wide, where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment, with or without underlying free gas. In this study we present a model with the contrasting physical properties that produce synthetic wavelets that match with the observed BSR amplitude and waveforms for varying source-receiver offsets of multi-channel seismic reflection data. The preliminary results presented here put important constraints on models that predict the distribution and formation of hydrate. Offset-dependent amplitude recovery also gives an appropriate response for hydrate characterization.

Seismic Data Processing of Western Nankai Trough and the Character of its Bottom‐simulating Reflector

AbstractReprocessing of NT62‐2 seismic line of western Nankai Trough has been conducted to focus on the character of the distribution of bottom‐simulating reflector(BSR). The appearances of BSR on the near‐offset profile, the stack profile, the migration profile, the instantaneous amplitude profile and the instantaneous phase profile are compared and described. They are also distinguished in shot gathers and CMP gathers. There are some evidence of BSRs on the near‐offset profile, and better images of BSR can be obtained by stack and migration. The character of strong amplitude of BSR can be easily seen on the instantaneous amplitude profile. And they are clearly mapped on the instantaneous phase profile.

Seismic velocity studies of a gas hydrate bottom‐simulating reflector on the northern Cascadia continental margin: Amplitude modeling and full waveform inversion

On the northern Cascadia subduction margin, the multichannel seismic amplitude‐versus‐offset (AVO) behavior of a bottom‐simulating reflector (BSR) suggests a P wave velocity change from high‐velocity hydrate‐bearing sediment to lower velocity sediment containing a small amount of free gas. The observed nonlinear AVO behavior, constant or slightly decreasing amplitudes at near‐to‐mid offsets and a large‐amplitude increase at far offsets, can be reproduced in the models if an S wave velocity enhancement is assumed as expected from hydrate cementation. The AVO behavior of the hydrate BSR is found not to be as useful as was earlier thought for determining the amount of free gas below the BSR. This is because Poisson's ratio change below the BSR due to gas is likely very small in high‐porosity unconsolidated sediments. The uncertainty in the models is large, as there is no reliable S wave velocity information for the sediments containing hydrate or gas. AVO modeling alone is not sufficient to distinguish different velocity models across the BSR. Our interpretation of the BSR amplitude behavior is that a P wave velocity increase above the BSR is the main cause of the BSR reflection amplitude increase at large incidence angles. Caution must be taken in applying AVO analysis, as little is known about S wave velocities. However, subtle differences in BSR amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion. A well‐defined reference velocity‐depth profile is also required to represent water‐saturated sediment unaffected by either hydrate or free gas. Using full waveform velocity inversion, a high‐resolution velocity model for the hydrate BSR has been derived. The best fit model for the seismic data near the Ocean Drilling Program (ODP) sites 889/890 consists of a high‐velocity zone above the BSR and a thin low‐velocity layer below, in agreement with the ODP downhole velocity data.

An Analysis of a Seismic Reflection from the Base of a Gas Hydrate Zone, Offshore Peru (1)

Seismic reflection data recorded near ODP Site 688, offshore Peru, exhibit a persistent bottom-simulating reflector (BSR) from a depth corresponding to the theoretical base of the gas hydrate stability field. Although gas hydrate has been recovered in cores from scientific drilling in both the Pacific and the Atlantic, drilling in areas exhibiting a BSR has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments. To carry out a quantitative analysis of the BSR, the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques. Synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from Site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free-gas zone. Results indicate the BSR is discontinuous laterally. Where the BSR is of high amplitude, free gas in a zone 5.5-17 m thick beneath the hydrated sediments provides the observed waveform; where the BSR amplitude is low, the free-gas zone is much thinner than 5.5 m or is entirely absent.