THE USE OF A GAS BUBBLE CURTAIN AND ENCAPSULATED AIR FOAM TO REDUCE THE RECEIVED SOUND LEVELS FOR MARINE MAMMALS CLOSE TO IMPULSIVE SOUND SOURCES

摘要

The effects on many marine species from anthropogenic noise is coming under increasing scrutiny of the military, commercial and scientific communities [1,2]. With ongoing use of military sonar's and ordinance and offshore operations such as drilling, piling, bridge construction, acoustic surveying, seabed generators, etc. these various groups must increasingly look to assess and where necessary mitigate against the impact of there operations on the marine environment. Bubble curtains have been used to reduce damage from underwater explosives and prevent damage to underwater structures like dams for some time, however they are not currently widely used for operations such at impact piling and drilling. One of the first uses of gas bubble curtains to attenuate sound underwater was outlined by Love and Arndt [3] in 1947 when they used a air filled leather sock with micro holes to shield submarine noise measurements made in open docks from general harbour noise in Pearl Harbour. In 1982 Domenico [4] discussed the use of bubble curtains to reduce radiated ship noise and in [5] discussed field trials to measure the attenuation effects of a bubble curtain on a airgun transmission and modelled absorption as a function of gas bubble curtain width. More recently in 2000 Wursig et al. [6] demonstrated the use of a 25 m radius curtain surrounding a impact driven pile in 6-8 metres water depth in waters of western Hong Kong. In this case they noted broadband pulse levels were reduced by 3-5 dB due to the presence of the bubble curtain. Both the airgun and impact driven piling operation are relevant to the current study. However little data exists for the potential effects of bubble curtain and other methods to reduce the effect of these noise sources. In particular studies have shown that the low frequency hearing responses of species such as the harbour porpoise [7,8] can overlap with the contribution from both piling and airgun transmissions and mitigation methods are highly relevant to ongoing and future offshore operations