High resolution velocity analysis of watergun wide angle reflection data

Abstract

Two seismic lines each approximately 10km in length were shot over an isolated approximately 250m thick sediment pond on the flanks of the mid-Atlantic Ridge. The source was a 1.32 liter watergun and the receiver was the Woods Hole Oceanographic Institution Digital Ocean Bottom Hydrophone (DOBH). The data were recorded at a sampling rate of 1800 Hz to take full advantage of the high band width source. Analysis was carried out by computing semblance and stack values along T-X trajectories defined in three ways: firstly the conventional hyperbolic stack, secondly the accurate ray traced travel time curve computed assuming isovelocity layering and finally the accurate travel time curve computed assuming a single layer of constant linear gradient. This last option yielded significantly higher values of coherency for the true basement reflection although all our analyses were marred by side reflection interference from the surrounding rough topography. The maximum semblance value obtained for the basement reflection in the linear gradient case was 0.38 for 120 shots, with the peak of both the semblance and the stack occurring at a gradient of 1.9s−1. This work illustrates the capability of a water gun source combined with a high bandwidth receiver and quantitative velocity analysis techniques, to produce high resolution determinations of sediment velocity structure.