High-Resolution Velocity-Depth Functions From a BSR Field at the Yaquina Basin off Peru

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Grobys, J. , Huebscher, C. P. , Gajewski, D. and Bialas, J. (2002): High-Resolution Velocity-Depth Functions From a BSR Field at the Yaquina Basin off Peru , AGU Fall Meeting 2002, 6 - 10. December 2002, San Francisco, USA. .
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In high-resolution MCS data from Yaquina Basin off Peru we observed various intricate reflection patterns related to gas and gas-hydrate. The BSR variations classify theprofile into five areas: Where the BSR follows stratigraphy, it is disrupted by several faults but strong in amplitude. Reflection amplitudes from the overburden are reduced.Blanking occurs beneath the BSR. Continuing down slope where the BSR crosses stratigraphy the BSR amplitude decreases, reflections from above and beneath haveregular strength. Further down slope BSR continuity dissolves into a patchy occurrence with blanking beneath. Up slope the BSR continues with reduced reflectionamplitude and touches the sea floor. Towards coastline the BSR disappears completely.One of the most important tools to unravel the origin of the reflections is the high resolution velocity analysis of OBS data. On top of the MCS line a OBS/OBH profile hasbeen shot. Sources were two 1.7 l GI-guns (Sodera) with a peak frequency of 80 - 100 Hz. Eleven OBS/OBH of the GEOMAR type recorded with a sampling rate of 1 ms. Thisdata set allows a high resolution velocity analysis. Two different processing flows have been applied to the data. The data have undergone a Kirchhoff wave-equationdatuming and adjacent coherence filtering to eliminate the one sided travel path through the water column. Both datumed and not datumed seismograms have beenexamined with a semblance supported interactive velocity analysis. Data have also been analyzed with high and low resolution in depth in order to find an optimumtrade-off between vertical resolution and minimization of errors caused by sensitivity of the DIX' formula regarding velocity variations at thin layers. The data help tounderstand the reflection pattern in terms of gas-hydrate occurrence and dissociation, fluid migration and anisotropic permeability of sediments.

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