Using a seismic reflector for resolving streamer depth and sea surface profiles

Abstract

In this paper short paper we show how the depth profile of a towed streamer and the shape of the sea surface above the towed streamer, at a particular instant, can be obtained from travel time and ghost notch frequency analysis on an isolated seismic reflector. The resulting streamer profiles indicate smooth long wavelength variations that do not follow the sea surface profiles. The sea surface profiles exhibit a dominant wavelength and amplitude that is consistent with an ocean wave spectrum. Both streamer and sea surface results are consistent with observation logs. A nonhorizontal streamer depth profile introduces static shifts into the data and, combined with the sea-surface shape, perturbs the data bandwidth through changes in the ghost notch frequency. If both streamer and sea-surface profile were known, it might be possible to correct the data for these perturbations. Furthermore, detailed information on the streamer depth profile can be used for streamer depth quality control. Currently, depth sensors are used for monitoring the streamer profile. The depth sensors are widely spaced and filtered; the highest detected frequency of the hydrostatic depth sensors is about 0.05 Hz. The lowest frequency recorded from the seismic data is limited by low cut filters to about 3 Hz. This 'gap' in our data bandwidth (from about 0.05-3 Hz) is where the sea surface waves lie, ranging from about 0.05-0.5 Hz (Carter et al. 1986), i.e. there is no direct information on the sea surface waveband.