Waveform inversion applied to a bottom simulating reflection on the eastern Lord Howe Rise

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SR_2004-027-pdf
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Pecher, I.A. 2004 Waveform inversion applied to a bottom simulating reflection on the eastern Lord Howe Rise . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2004/27 24 p.

Abstract: Full waveform inversion is applied to study the nature of a widespread bottom simulating reflection (BSR) at a typical location on the eastern flank of Lord How Rise. The BSR displays positive polarity, indicative of a velocity increase, in contrast to typical gas hydrate-related BSRs which are caused by a sharp velocity decrease at the top of free gas zones beneath gas hydrates. Positive-polarity BSRs are commonly interpreted as being of diagenetic origin, usually silicate phase transitions. Silicate diagenesis, however, is unlikely to form a BSR in the study area based on sediment composition. Diagenesis of the silica-deficient pelagic carbonates in this region depends mostly on the assemblages of the nannofossils that dominate the carbonates. These assemblages are controlled by surface water temperatures. Warm-water loving discoasters, which are much more subject to diagenetic dissolution and re-precipitation, alternate with resistant cooler-water assemblages. Therefore, reflections from diagenetic carbonate horizons should parallel bedding, unlike the BSRs that cut through stratigraphic horizons in the study area. It is speculated that two scenarios could generate positive-polarity BSRs in gas hydrate settings, a thin gas hydrate layer with a sharp top and without any significant amount of free gas beneath it, or a free-gas layer with a gradational top but a sharp base. Best-fit velocity models were obtained by inverting a common-depth point gather for its full waveform. Four starting models were used, (1) a gradient model as commonly used for BSR inversion, (2) a single velocity step, mimicking a diagenetic BSR, (3) a thin high-velocity layer, simulating a gas-hydrate layer, and (4) wedge-shaped low-velocity layer caused by a free-gas zone with a gradational top. Results suggest that a gas-free zone with a sharp base is unlikely to cause this BSR. Although the full waveform inversion cannot unambiguously distinguish between a velocity step and a thin high-velocity layer, results do favour the presence of a thin high-velocity layer. It is cautioned, however, that the superposition of stratigraphically controlled and BSR-related velocities may lead to a complex velocity function. (auth)