Keun-Pil Park

HIGH-FLUX GAS VENTING IN THE EAST SEA, KOREA, FROM ANALYSIS OF 2D SEISMIC REFLECTION DATA.

Seismic reflection data from a multi-channel streamer deployed offshore Korea reveal evidence of hydrateforming gases being vented into the ocean. Numerous, localised vent structures are apparent from reduced seismic reflection amplitude, high seismic velocities, and reflector pull-up. These structures penetrate upward from the base of the gas hydrate stability zone (GHSZ) and are typically several hundred metres wide, and only a few hundred metres high. Underlying zones of reduced reflection amplitude and low velocities indicate the presence of gas many kilometers below the seabed, which migrates upward through near-vertical conduits to feed the vent structures. Where the local geology and underlying plumbing indicates a high flux of gases migrating through the system, the associated vent structures show the greatest change of reflector pull-up (the greatest concentration of hydrate) to be near the seabed; where the local geology and underlying plumbing indicates a moderate flux of gases, the greatest change of reflector pullup (the greatest concentration of hydrate) is near the base of the GHSZ. The distribution of gas hydrate in the high-flux gas vent is consistent with the recent salinity-driven model developed for a rapid and continuous flow of migrating gas, while the hydrate distribution in the lower-flux vent is consistent with a ∗ Corresponding author: Phone: +1 205 363 6302 Fax +1 250 363 6565 E-mail: rhaacke@nrcan.gc.ca Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

SEISMIC REFLECTION BLANK ZONES IN THE ULLEUNG BASIN, OFFSHORE KOREA, ASSOCIATED WITH HIGH CONCENTRATIONS OF GAS HYDRATE

It has recently been recognized that abundant gas hydrates occur in localized zones of upwelling fluids, with concentrations much higher than in regional distributions associated with bottomsimulating reflectors (BSRs). We report a study of multi-channel seismic reflection data across such structures in the Ulleung Basin, East Sea backarc offshore Korea, an area with few BSRs. The structures are commonly up to several km across and a few hundred meters in depth extent, and are characterized by reduced reflectivity and bowed-up sediment reflectors on time-migrated sections. The seismic pull-up mainly results from higher velocities, although physical deformation due to folding and faulting is not ruled out. Some of the features extend upward close to the seafloor and others only partway through the gas hydrate stability zone. The base of gas hydrate stability zone (BGHSZ), calculated assuming a regional average constant heat flow of 110 mW/m, is confirmed by the presence of gas inferred from reduced instantaneous frequencies and high instantaneous amplitudes, and from a decrease in seismic velocities. The ∗ Corresponding author: Phone: +1 250 721 6188 Fax +1 250 472 4620 E-mail: istoian@uvic.ca Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.

Source estimation and direct wave reconstruction for the Laplace‐domain waveform inversion of deepwater seismic data

We propose an alternative strategy for overcoming the high sensitivity of the Laplace-domain waveform inversion to early time noises, especially of deepwater seismic data. We estimate the source wavelet from the raw direct wave using the full Newton method in the frequency domain and reconstruct the direct wave without noise using the estimated source wavelet and the Green’s function in a constant velocity medium. The data set adequate for the Laplace-domain waveform inversion can be made by merging the reconstructed direct wave with the original data set without direct wave. Our strategy is applied to field data from deepwater environments and a realistic velocity model can be recovered from the Laplace-domain waveform inversion of the reconstructed seismic data.