The continuous use of the oceans, in particular the sound generated from seismic sources, has the potential to cause disturbance to marine mammals such as whales, dolphins, porpoises and seals. A series of guidelines have been produced by the UK based Joint Nature Conservation Committee (JNCC) in order to mitigate harm to cetaceans.
These guidelines, which have been adopted by the international community, are designed to ensure that Operators involved in seismic activities do not adversely affect the behaviour of, or cause damage to, marine mammals.
Marine Mammal Observers (MMO) are now increasingly present on seismic vessels for visual cetacean monitoring during daylight hours and to ensure compliance with JNCC guidelines.
Submerged cetaceans, however, are more at risk than those at the surface. Marine mammals use a variety of sounds including trains of clicks, whistles and moans for communication, echolocation and foraging, which can be heard over several kilometres. Passive acoustic monitoring (PAM) systems allow the detection of vocalising marine mammals, especially useful at night or in poor visibility. The JNCC strongly encourage the use of these systems for all seismic surveys, as a mitigation tool, to accompany visual observations conducted by MMO. The PAM system monitors marine noise from a towed hydrophone system.
Fugro Survey’s Environmental Department, based in the UK, has a number of trained and experienced Marine Mammal Observers that work offshore in the North Sea and worldwide. Fugro’s MMOs are also experienced in operating and interpreting PAMs in order to ensure compliance with regulations.
The Passive Acoustic Monitoring system comprises an array-tow cable with four hydrophones, deck cable, an electronics processing unit containing audio output unit connected to two laptops that support the detection software, PAMGuard. The hydrophones transmit an analogue signal through the cable which is converted to a digital signal and split into high and low frequency via the processing unit. One laptop runs PAMGuard to detect high frequency signals such as occurs with porpoise vocalisations using a click detector module and sound recorder. The second laptop runs PAMGuard to detect low frequency signals associated with dolphin whistles and beaked or toothed whale clicks. The marine noise is expressed visually and audibly via stereo headphones.
Known distances between hydrophones enable predictions on source location to be made. Thresholds of frequency can be adjusted to account for ‘background’ noise. This relies on source detection and visual confirmation.
PAM systems are in their infancy, but are quickly being implemented as a marine mammal monitoring technique within the oil and gas industry.
Subsea World News Staff , June 15, 2012