A towed seafloor source for seismic exploration

We have a new design for a pneumatically-activated, deep-towed seismic source that can be operated at or near the seafloor. This design overcomes some of the limitations of the seafloor imploder we developed three years ago. Our present working version - a 20-liter, 220 dB (re 1 /spl mu/Pa at 1 m) imploder - is robust and creates repeatable signals that excite shear waves. However, it is necessary to bring it up on deck and reseal it between shots. Our new design solves the long repeat interval problem while allowing for much more control on the signal bandwidth and amplitude. New petroleum and methane-hydrate deposits are being found in deep water at previously unexplored sites. To fully exploit these deep-sea resources, new tools and methods for imaging will be required. One such improvement needed is an active, seismic seafloor source. For deep-water seismic exploration, seafloor signal generation has several advantages over surface generation. The principal advantage is that seafloor sources directly excite shear waves. This allows differentiation of structures by shear velocity, adding another dimension to subsurface visualization, and is particularly useful in regions of unconsolidated sediments and gas. In addition, energy is focused a kilometer or more (depending on the water depth) closer to the zone of exploration with bottom sources, allowing better feature resolution. A secondary advantage is that an equivalent sea-floor source will have less effect on marine mammals, who, depending on their species spend all or a majority of their time near the sea surface - far removed from a working bottom source. A useful seismic sea-floor source will require the following characteristics: it produces signals of the desired bandwidth and amplitude for seismic prospecting, it is robust, the source signature is repeatable, firing time is controllable, and it can be fired repeatedly on the order of one or two minutes. We have made substantial progress in developing a se ismic bottom source that meets these requirements.