Full waveform inversion applied to a bottom simulation reflection in NIGHT Line 46, Hawke Bay

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Pecher, I.A. 2002 Full waveform inversion applied to a bottom simulation reflection in NIGHT Line 46, Hawke Bay . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/08 17 p.

Abstract: Bottom simulating reflections (BSRs) in seismic data are generally thought to be caused by free gas beneath the gas hydrate stability zone resulting in a significant drop in compressional-wave velocity. A full-waveform inversion scheme was applied to a BSR in Hawke Bay in data acquired during the North Island GeopHysical Transect (NIGHT) in 2001. The analysis of a common-midpoint gather included some initial processing, extraction of source signal, . Lower Hutt: Institute of Geological & Nuclear Sciences. r- p transform, large-scale velocity analysis in the . Lower Hutt: Institute of Geological & Nuclear Sciences. r- p domain, Fourier transformation, and full waveform inversion. Results show that the BSR is caused by a thin low-velocity layer, most likely free gas. The BSR is relatively weak compared to other BSRs worldwide suggesting only little free gas. The base of free gas cannot be identified indicating that it is either gradational or the layer is relatively thin compared to seismic wavelengths. A high-velocity bulge above the BSR may indicate relatively high amounts of gas hydrates. The waveform inversion converged reliably. Some recommended improvement would include modifications to the . Lower Hutt: Institute of Geological & Nuclear Sciences. r-p transform to allow dip corrections, a robust technique to obtain source wavelets, and ultimately, an inversion for shear-wave velocity based on amplitude-versus-offset characteristics. (auth)