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Introduction

Bottom simulating reflectors, so called BSRs, parallel the seafloor at subbottom depths of several hundred meters. Seismic investigations (Shipley et al., 1979; Miller et al., 1991; Hyndman and Spence, 1992) indicate that they are characterized by large negative reflection coefficients and increasing subbottom depth with increasing water depth. The base of the stability field for methane hydrates appears to be associated with these bottom simulating reflectors. Due to the enormous amount of methane that is apparently contained within hydrate structures, they are likely to have a significant ``greenhouse'' effect on future global climate, and might also represent an important future energy resource (Kvenvolden, 1993). Therefore, a good understanding of the origin and characteristics of the hydrate zones and BSRs is desirable. Only limited information is available from deep-sea drilling, as the risk of heating and destabilizing the initial hydrate conditions during the process of drilling is considerably high. Thus, the core samples and well-logs do not necessarily reflect the correct in situ hydrate characteristics and properties. Consequently, most information is inferred remotely from seismic reflection data (Singh et al., 1993; Shipley et al., 1979; Miller et al., 1991; Hyndman and Davis, 1992; Hyndman and Spence, 1992). Most of these investigations, which were based mainly on AVO responses and synthetic modeling, used primarily P-velocity information, accessible directly from the seismic data, and neglected possible important S-velocity effects entirely. The exact formation of the hydrate and its formation are still controversial, and different models have been proposed to explain the origin of the BSRs (Kvenvolden and Barnard, 983a; Hyndman and Davis, 1992).

In this study, we use both P- and S-wave information inferred from synthetic modeling, velocity and AVO analysis of marine data from the Blake Outer Ridge to explain the bottom simulating reflector. The validity of the different BSR models is explored and the effects of two proposed models can be clearly discriminated. The reflection amplitude variation with offset can be an important indicator of free gas at an interface (Shuey, 1985) and, together with the estimation of material properties at the interface, considerably limits the possible explanations of the physical origin of the BSR.

This paper discusses our work with preprocessing, modeling, inversion, and interpretation of the methane hydrate seismic data from the Blake Outer Ridge. Preliminary results of this study were presented by Ecker and Lumley (1993b,a). After careful preprocessing, including source wavelet deconvolution, trace interpolation, and amplitude and moveout traveltime calibration, a detailed velocity analysis is performed on the resulting CMP gathers. The estimated interval velocities are used to constrain a BSR model which can successfully reproduce the observed AVO amplitude responses. The impedance structure predicted by the modeled data is further reinforced by estimating the P- and S-impedance contrasts at all subsurface positions. Combining the results of the synthetic modeling and the impedance inversion, we give an integrated geophysical interpretation of the data.


next up previous [pdf]

Next: METHANE HYDRATES AND BSR Up: Ecker & Lumley: AVO Previous: Ecker & Lumley: AVO

2015-03-10