UNDERWATER CHARACTER AND HORIZONTAL PROPAGATION OF AIR-GUN SIGNALS

This paper describes work carried out as part of an APPEA/ERDC supported project, Investigation of the Environmental Effects of Offshore Seismic Survey Activities. In this project biological research concerning the interaction of acoustic signals with marine organisms is linked to sound propagation studies focussed on frequencies, water characteristics and seabed types of relevance to offshore exploration practice. A key feature of the propagation component of the project concerns the extent to which lateral propagation of airgun signals in the water column may be adequately predicted.The study of acoustic propagation in the sea is an integral part of both scientific and defence enquiry. In recent years, the comparatively complex issue of propagation in shallow water, here considered as involving depths of 200 m or less, has received increasing attention. The literature concerning low frequency propagation in shallow water has features in common with seismic literature. Different emphases, however, notably concerning applications, the role of shear waves in solids and the geometry of the propagation environment have largely resulted in separate development paths being adopted by these two related fields. This paper summarises the range of modelling approaches available to predict shallow water propagation, some key issues influencing propagation and the influence of water and seabed parameters on model performance.An illustration of propagation model performance is provided in this paper by comparing the output from a normal mode model KRAKEN, with experimental data obtained using a mono-frequency source in shallow water in Cleveland Bay, Queensland. This offers an opportunity to assess propagation in a complex and, in acoustic terms, poorly classified environment. These results will be complemented in the conference presentation by airgun data obtained in Exmouth Gulf, Western Australia.