Insights into Optimal Soft Start and Shutdown Procedures for Stationary or Moving Sound Sources

Insights into Optimal Soft Start and Shutdown Procedures for Stationary or Moving Sound Sources

Ainslie, M.A.
von Benda-Beckmann, A.M.

2012

To reduce the risk of impact on marine life of underwater sound generated by anthropogenic activities (such as offshore construction, seismic surveys and sonar searches), various mitigation measures are often put in place. Two commonly adopted mitigation measures are the soft start (or "ramp-up" – the two terms are used interchangeably) and “shut-down”’ procedures. During a soft start procedure the source level of the sound source is increased gradually before use at operational power; the expectation is that nearby animals respond to the increasing sound level by swimming away from the sound source. Soft starts are used out of a precautionary principle, although no clear evidence exists for their effectiveness for mitigation [1, 2]. In order to quantify the possible benefit of a ramp-up period, we derive analytical expressions for sound exposure resulting from the use of a sound source (the "operation") for stationary sources. A methodology is proposed to quantify the efficacy of soft start procedures by assessing the impact on a population of animals, instead of nearby animals that are at highest risk. The simplicity of the resulting expressions makes them suitable for tuning the soft start with the aim of minimizing the environmental cost of the operation. A shut-down procedure might be initiated when one or more animals enter an area where they are considered to be at high risk. An analytical expression is derived for the shut-down zone for stationary and moving sources, ensuring the cumulative exposure that an animal receives from multiple sound events is less than a predefined threshold. Sound exposure is used as a measure of risk on the grounds that increased exposure is thought to increase the risk of hearing impairment and injury in marine mammals [3] and in fish [4]. For simplicity, no hearing-related weighting is used, and spherical spreading propagation conditions are assumed, with no absorption. Except where stated otherwise, the animal is assumed to swim with constant speed and direction (vertical motion is not considered). It is further assumed that sound pressure level does not change significantly from pulse to pulse. The purpose of these simplifying assumptions is to gain insight into the effect on risk of changes in parameters like duration and source level, depending on assumed swim velocity and (where relevant) source platform speed

Physics & Electronics
AS - Acoustics & Sonar
TS - Technical Sciences
Defence Research
Defence, Safety and Security
Marine life
Mamals
Underwater sound
Adverse effects
Man made noise
Sonar
Offshore activities
Surveys
Mitigation
Soft Start
Shut Down

http://resolver.tudelft.nl/uuid:e61a5841-8615-4121-bdbe-91f85855c9f4

461899

Proceedings of the 11th European Conference on Underwater Acoustics - ECUA 2012, 2-6 July 2012, Edinburgh, UK

conference paper