Summary

Increasingly many agencies are showing greater interest in sound propagation impact to marine mammals from man-made sources. They are requiring the industries responsible for these sound sources to carry out suitable measurements in order to quantify this. Two effective methods are presented here that produce complete sound field maps of the actual survey location verifying the source level propagation. These methods have minimal impact on the environment and are easy to deploy and implement. The results obtained to date provide accurate representations of the sound propagation and are used to determine mitigation zone requirements for actual surveys.

Introduction

Worldwide government agencies and regulators are showing an increasing interest in Sound Source Verification (SSV) techniques, analysis and results in order to assess the impact of sound on marine mammals and the environment. The purpose of the SSV is to monitor, measure and record sound levels generated during man made operations within the seas and oceans. For example, agencies require an SSV to be undertaken for seismic survey source arrays, prior to full operations commencing. Broadband decibel (dB) levels are calculated for 190 dB, 180 dB, 160 dB and increasingly 120 dB and empirical distances are identified from the source location. This information is then utilized in order to define marine mammal mitigation zones to be applied to a particular survey and location.

Many different factors will affect the propagation of the sound from the source including bathymetry, salinity, water depth and temperature. Initially a model is produced based on the actual site location and will identify the theoretical mitigation zones. In order to ensure model accuracy and comparison with survey results a dense set of official bathymetry data parameters for the location, source position and performance parameters for the sources and hydrophones deployed has to be used along with environmental data.

Theory and/or Method

Modelling

Seiche utilize a fully range dependent, parabolic equation modelling solution based on RAMGeo code created by Naval Research Laboratory in Washington, DC.

Using this technique, many complex attributes can be applied to the model including, a large bathymetry data set to provide an authentic sea bed profile, specific sea bed sediment layers to define absorption, reflection and refraction characteristics and the water column density and salinity for accurate speed of sound definition.

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