_{AVO and Seismic Waveform Inversion in the Plane Wave Domain: Application to Gas Hydrate Data}

SUMMARY

            _{AVO analysis has been used with some success in seismic exploration to directly detect the presence of hydrocarbons. AVO inversion essentially implies a least squares fitting of reflection coefficients (seismic amplitude) as functions of source-receiver offsets in the moveout corrected seismograms assuming that the background velocity is known accurately. Unlike the conventional approaches, we carry out the background velocity and AVO inversion in the plane wave (intercept time - ray parameter or t-p) domain. Normal moveout analysis in the plane wave domain results in interval velocity estimates and the t-p data are closer approximations to the plane wave reflection coefficients. Having determined the background velocity and fractional changes from an AVO inversion, we carry out a full waveform inversion in which we use full elastic waveform modeling that includes all internal multiples and converted waves.}

            _{We apply this multi-stage seismic waveform inversion approach to a suite of CMP gathers from a 2D seismic line collected offshore of the east coast of the United States; a region in which the occurrence of gas hydrates has been reported. Gas hydrates have the economic potential of being tapped as a fuel source and also have the potential as a greenhouse agent if freed into the atmosphere. In seismic sections, the base of gas hydrate zone is marked by bright high amplitude reflections, which follow the sea floor topography and are called bottom-simulating reflectors (BSR). The BSRs have negative polarity with respect to the seafloor reflection and in a common shot or a CDP gather; the amplitude increases with offset.}

            _{Our analysis was aimed at deriving a high resolution seismic velocity structure for the gas hydrates and the sediments below. At locations where a BSR exists, we identify a low velocity zone that coincides with the BSR. We also identify several thin low velocity zones beneath the BSR interpreted to be due to the presence of free gas. We compare and contrast our results with the velocity function derived from zero-offset VSP data collected during the ODP drilling Leg 164 at holes 997 located NE of our seismic line. The general trend of the two independent estimates of velocity is in good agreement. The low P-wave velocity zones show no change of shear wave velocity indicating the presence of free gas, which is confirmed by drilling in the nearby area. However, the VSP derived velocity model was obtained by the application of smoothing in the traveltime inversion of the VSP data. The resulting VSP derived velocity model shows a nearly 200 m thick low velocity zone (continuous free gas) which may be caused by artifacts due to smoothing. Unlike the VSP model, our results shows several thin low velocity layers.}

_{PHILOSOPY OF THE MULTI-STAGE WAVEFORM INVERSION APPAROACH}

_{FLOW CHART OF THE INVERSION ALGORITHM}

_{Inversion of Seismic Data from a 2D line from the Carolina Trough (East coast): the bright reflection that is roughly parallel to the topography of the seafloor is the BSR (the bottom simulating reflector). The inverted velocity profiles are superimposed on the stack section. Notice the low velocity zones below the BSR.}

 _{The inverted velocity profiles are superimposed on the fluid factor (Smith-Gidlow) section. Note that the low velocity zones coincide with the low-velocity zones.}

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