1. Introduction
The Haro Strait is a complex, shallow water acoustic environment with steep bathymetric relief combined with an active shipping channel, frequent small boat activity, and Naval operations. The western side of San Juan Island is also a primary foraging area for the Southern Resident killer whales[1]. Consequently, these animals are of unique public concern in this area because of the potentially high impact of human activity on their environment. Questions regarding the acoustic environment of these animals have arisen as recreational whale watching, commercial shipping, and Naval activity[2] have grown in this area. A reasonable question to ask in this context is whether increasing underwater noise levels affect the killer whale�s ability to forage for prey by echo-location. For example, the analysis of the echo-location signals from killer whales[3] indicate that backscattered signal levels from salmon can be very low and comparable in level to natural background noise levels.
This report will address specific aspects of modeling the propagation of sound sources in the Haro Strait, focusing on the numerical estimation of transmission loss in the open channel. In particular, we will investigate the propagation of sound generated by large commercial ship traffic in the Strait and the estimation of sound source levels of individual ships. We will illustrate the role of modeling as a tool for model/data comparisons and the interpretation of field measurements of underwater sound. In this process we will employ a variety of compiled databases of the environment, information on ship traffic and vessel tracking, and field measurements of underwater noise collected recently in the Haro Strait in an area frequented by killer whales.
For purposes of this report, acoustic modeling is used to complement field measurements, as the shallow water environment of the Haro Strait is far too complex, and the geo-acoustic parameters of the area are not characterized well enough to rely on modeling alone. When modeling is constrained by measurements it can provide a useful tool to fill the gaps in measurements in both space and time. For example, measured data will be shown for a specific receiver location and time, and modeling results will be compared with this data to infer source levels of individual large commercial ships. If model results compare favorably and confidence is developed in the modeling strategy for this particular area, then the model may be used to estimate sound pressure levels at other locations in the region where measurements are not available.