HR: 10:50h
AN: S12A-03    [Abstracts]
TI: A New Method for MCS Refraction Data Analysis of the Uppermost Section at a Mid-Atlantic Ridge Core Complex
AU: Harding, A J
EM: aharding@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093,
AU: Kent, G M
EM: gkent@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093,
AU: * Blackman, D K
EM: dblackman@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093,
AU: Singh, S
EM: singh@ipgp.jussieu.fr
AF: IPGP, CNRS, Paris, 05, France
AU: Canales, J P
EM: jpcanales@whoi.edu
AF: WHOI, MGG, Woods Hole, 02543,
AB: The first refraction arrival or moveout of reflection arrivals are generally used to obtain velocity structure of the sub- surface. However, in deep-water environments and in the absence of near seafloor reflections, it is not possible to determine uniquely the velocity structure just below the seafloor from conventional methods. Here a new approach to analyzing seismic refractions recorded on a multi-channel streamer is tested with a subset of R/V Ewing data obtained over the central dome of Atlantis Massif in order to determine the high-resolution P-wave velocity of the uppermost ~1 km. This oceanic core complex on the Mid-Atlantic Ridge 30°N provides access to intrusive crust exposed at the seafloor via detachment faulting, thus eliminating the imaging problems due rough seafloor and heterogeneous basaltic carapace that can mask lower crustal structure at most mid-ocean ridges. In addition, IODP drilling results in the vicinity provide ground truth to depths of 1.4 km. Multi-channel seismic (MCS) data were downward continued to a new datum 1.5 km below the sea surface using a prestack phase shift approach. A filtered receiver wavefield from a single shot point was first extrapolated to depth, and this step was repeated for all shots. Next, the data were sorted into common-receiver location space and the 160 shots per common-receiver gather were downward continued to 1.5 km below the sea surface, producing a dataset that is equivalent to a reflection profile collected at a water depth of 1.5 km. Because the trace spacing in these two domains differ by a factor of 3 (i.e., 12.5 m versus 37.5 m), filtering parameters were changed prior to invoking a phase shift in the F-K domain, to minimize aliasing (with the common receiver domain trace spacing being most problematic at the 37.5 m distance corresponding to the shot spacing). The requirement of a second extrapolation in common-receiver location space reduces the full-fold line length, within the downward continued gathers, by an amount equivalent to a streamer length (6 km) from each end of the original record section. For most of the shots, clear refracted arrivals are present at ranges 0.2-4.5 km as compared to 1.5-6 km on original gathers, which should provide velocity starting from just below the seafloor down to about 0.5-1.5 km depth. Refracted arrivals have been picked for every shot trace and these data will allow us to invert for detailed velocity structure in the uppermost section. The data density and continuity is over an order of magnitude greater than a previous near-bottom-source OBS refraction experiment that obtained ambiguous results in this area. The presentation will include illustrations of this new methodology and the tomography results as well as comparison to results obtained using conventional methods in the same area (Canales et al., session T19 Fall AGU07).
DE: 7220 Oceanic crust
DE: 7250 Transform faults
SC: Seismology [S]
MN: 2007 Fall Meeting