A model for predicting propagation of anthropogenic acoustic noise in the sea

There is a growing environmental concern about the effect high level anthropogenic acoustic signals and noise can have on the life in sea in general, also noise in the form of transients signal generated by air guns used in marine seismic exploration. The signals from such air guns have the potential to travel to far distances and cause annoyance, discomfort or damage to, for instance, sea mammals. Effects could range from mild irritation through impairment of foraging or disruption of social interactions to hearing loss and in extreme cases physical injury or death. It is therefore of considerable interest to have tools for the predictions of long range sound propagation of air gun signals to use in the planning and execution of surveys. In this paper we will describe a new environmental impact analysis model for the prediction of the level and other characteristics of emitted man made acoustic noise in the sea. The main element of the model is an acoustic ray-tracing propagation program, called PlaneRay, which can treat range dependent bathymetry and take into account reflection loss from a layered sea floor with fluid and elastic layers. The sound source in the propagation model can be any transient acoustic waveform, for instance a recorded acoustic signature of an air gun. The model will then propagate the source signal out to a range of several kilometers and calculate the sound level as function of frequency for a large frequency band, typically 1000 Hz. The model will also calculate the complete broad-band time response of the received signal and thereby calculate important parameters, such as the peak-to-peak acoustic pressure amplitude, rms pressure level and time duration of the received signal. The main features of the model are described and examples of predicting long range propagation of an air gun signature under different oceanic conditions are shown.