This invention provides a system apparatus and method for a low-drag ship-towed deployment of a non-linear volumetric array of hydrophones, allowing line-intersect or line-transect sampling of marine mammal populations, where “ship-towed” is defined as any type of vehicle capable of moving the apparatus through the water.
Passive acoustic monitoring (PAM) is the preferred technique for detecting marine mammals, because the marine mammals use low-frequency sounds for their own echo-location and communication, and the passive monitoring does not interrupt nor distort those sounds. The low-frequency sounds are very efficiently transmitted in seawater and travel great distances because the hydroacoustic impedance properties of seawater favor lower frequencies and disfavor higher frequencies.
Fixed-location hydrophones have been used to monitor marine mammals, but cannot monitor the vast amounts of deep-ocean habitat as required. Presently, line arrays of hydrophones, towed one behind the other, are used. In order to obtain useful location information, the line array needs to detect more than a single isolated click in order to obtain enough information to make a triangulation. A massive amount of data from line arrays must be processed and analyzed after the fact in order to derive the location information.
The community organization PAMGUARD has been established to address the fundamental limitations of existing cetacean passive acoustic monitoring (PAM) software capabilities, and continues to develop open-source PAM software for acquiring and analyzing hydroacoustic data related to marine mammals. Any equipment or methods developed for this field should be operable or interoperable with this evolving PAMGUARD software.
Sound will reflect at the interface boundary of materials with differing acoustic impedance. Sound will also attenuate in materials. Hydrophones, when towed in the ocean, need a boundary layer to separate the hydrophone from the water flow or significant flow noise will result. In order to optimize hydrophone measurements in the ocean environment, one should surround the hydrophones with materials having low attenuation and close acoustic impedance matches to seawater.
The towing of a hydrophone array across great distances at a reasonable speed to cover those distances, which is about 10 knots, puts a huge amount of stress on any hydrophone array, whether a line array or otherwise, and on any tow cable used. In order to tow the hydrophone array far enough behind the towing ship to avoid the hydroacoustic noise of the ship, the tow cable needs to be at least 100 meters long, and preferably 300 meters long. Such tow cables, which are integrated with data-transfer cables, are known, and any new equipment or methods should make use of such existing tow cables, if possible. The existing cables can withstand 1000 pounds of tension, and that breaking point could be reached if the towed hydrophone array generated even a few hundred pounds of hydrodynamic drag force.
The monitored sounds are very faint, and can be overwhelmed by turbulence in the vicinity of the hydrophones. Existing practice of the PAM technique suffers from turbulence caused by cable drag and by hydrodynamic drag or turbulent flow around whatever housing is provided for a hydrophone array being towed at 10 knots at the end of a 300 meter tow cable.
Because the PAM technique requires detection of the small differences in the time of a given sound reaching each hydrophone in an array, the latitude, longitude, and depth position of the whole array, plus the position of the individual hydrophone with respect to one another, must be known in order to analyze the data, and preferably should be stable in order to avoid additional complexity in the analysis, and because such instability is likely correlated with hydrodynamic drag, turbulence, self-noise, and strain on the hydrophone array structure and the tow cable.
The advantages of an array of hydrophones arranged in a tetrahedron are known. Each hydrophone is equidistant from each other hydrophone, simplifying the calculations, and the relative position of each hydrophone to the others is fixed. However, a tetrahedron is difficult to tow underwater at the end of a long cable, and any roll, pitch, or yaw in the travel of the hydrophone array will alter the relative position of each hydrophone with respect to the underwater sound source. Any such change of orientation or attitude of the hydrophone array must be captured and accounted for in the analysis of the data. Such changes of orientation or attitude are also likely to correlate with increased hydrodynamic drag, self-noise, and strain on the hydrophone array structure and the tow cable.
There is a need for a small, low cost volumetric array, integrated with PAMGUARD, for the use of the government, military, and universities for marine mammal population studies, mitigation for military and commercial activities in the ocean, and detection and localization of submerged assets such as downed planes, moorings, AUVs or ROVs.