Development Of An Active Source For Detecting Whales In Airgun Safety Zones

Visual detection of marine mammals is limited by wave height, light level, storms, and fog. Passive Acoustic Monitoring (PAM) is not so limited but cannot detect the presence of silent animals. Active Acoustic Monitoring (AAM, also known as sonar) produces a sound and detects animal presence by the echoes returning from their bodies. AAM functions in all conditions regardless of whether animals are vocalising. AAM supplements visual observations and PAM as mitigation measures, and is essential for ensuring that operations can commence under low visibility conditions.

Two projects on active sources were funded. Project 1 (SIMRAD) tested two models of commercial fish-finding sonar for their ability to detect killer whales within 2,000 m horizontally from a test vessel, and for the strength of the returned echoes. Project 2 (Defence R & D Canada) conducted a survey of presently-available active sources and methods that industry could adopt with little or no modification.

Project 1 found that the SIMRAD SP90, operating at 20-30 kHz, made clear killer whale detections at 1,500 m at source levels between 206 and 218 dB re 1 μPa, but did not make good detections vertically at depths greater than the 200 m thermocline. Whales showed no behavioural reactions to these active sources. The SP90 detected the artificial target at 2,000 m horizontal range, probably because of a surface duct that enhanced sound propagation.

The Defence R & D project surveyed the types and locations of industry operations that would benefit from using AAM. Of 13 existing AAM systems surveyed, six were rated as appropriate for industry use without any needed investment in research and development.

Objective and methods

Project 1:

• test the ability of the SIMRAD SP90 and SH80 to detect killer whales as a function of detection range and depth, setting criteria for whale identification and effects on whale behavior
• investigate the ability to detect an artificial whale target under controlled conditions at different water depths and ranges
• model sonar sound transmission using ray tracing, sound propagation and probability of detection based on recorded sound speed profiles and estimated whale echo strength.

Project 2:

• survey the types of industry operations and the areas of the world in which they occur to identify which existing AAM systems offer the most suitable performance
• survey the physical size of marine mammal species and the diving characteristics that AAM systems must be able to detect
• survey the factors that affect AAM system performance
• identify the AAM systems that match the above needs with no modification, with some modification, or that would not fit the above needs.

Importance

These studies showed that commercial sonar units exist that may help industry meet regulatory requirements for monitoring marine mammals at sea during poor visibility conditions. Developing entirely new systems specific to industry use may not be necessary because existing technology may suffice with little or no modification. Using AAM systems could eliminate delays in startup caused by poor visibility conditions and could thereby decrease the costs and increase the efficiency of offshore surveys. These projects gave industry adequate guidance on which active sources to select for industry purposes.

Institutions/PIs

• Project 1: SIMRAD (Frank Reier Knudsen)
• Project 2: Defence R & D Canada (Jim Theriault)