Technique for Localization of Ocean Bottom Seismometers and Airgun Sources

In any seismic survey a number of factors contribute to the total uncertainty of the data including the uncertainty of the position for the seismic sources and receivers. This may be not a major issue for a conventional seismic survey using a towed array, because the relative geometry is usually known accurately enough and is easily obtained. For applications using deployed autonomous receivers (such as Ocean Bottom Seismometers (OBS)) for seismic tomography, the absolute positions must be known with high accuracy in order to achieve optimal results. This paper describes a technique for localizing seismic sources and receivers using regularized inversion of the travel times of the direct water wave and it has the following features: a) inversion of the full ray-tracing equations for a general sound velocity profile b) simultaneous inversion for all unknowns; c) smoothest solution for source ship track lines; d) correction for non-constant drift for the OBS clocks (approximated by two linear segments); e) correction for random time-base errors in OBS data; f) correction for a bias in the water column velocity profile. The technique will be presented along with results for synthetic tests including solution uncertainty estimation. Results will also be shown for localization of sources and receivers in a gas hydrate seismic survey off Vancouver Island (COAMS'99) consisting of 5 OBS's and more than 18000 shots to demonstrate the contribution of each feature of the algorithm into the elimination of the misfit between measured and calculated travel times.