By Yvonne Barkley
Erin Oleson, lead of the PIFSC Cetacean Research Program, and I recently attended a 10-day workshop at the Southwest Fisheries Science Center to develop standardized methods for passive acoustic data collection. This workshop focused intense effort and funding to produce equipment that will form a solid foundation for cooperation between NMFS Science Centers. Representatives from each Science Center participated in the workshop and learned the basics of array design and construction and built two of their own oil-filled towed hydrophone arrays. These arrays will be used for collecting acoustic data during NOAA shipboard line-transect surveys to assess abundance estimates of whale and dolphin species throughout each NMFS region.
Towed array acoustics is used in concert with visual observations to detect and locate cetaceans (whales and dolphins) in a study area. Acoustics assists with tracking animals in order to estimate the number of animals and identify the species. Technology has advanced over the years to improve methods for acoustics data collection and increase the quality and quantity of data. However, each NMFS Science Center acoustics program has not progressed at the same rate resulting in different methods and hardware for each Center. This has made it difficult to borrow, swap, and repair equipment in the past.
This recent workshop allowed each acoustics program to build the same modular towed hydrophone array system, including high-quality underwater connectors by Impulse. In the past, we have spliced arrays directly to our tow cable, which is labor intensive and makes it difficult to change arrays if necessary. The new modular system enabled by underwater connectors allow each piece of the system to be easily detached and interchangeable between all Science Centers. The equipment includes two different oil-filled hydrophone arrays.
The ‘end array’ is consistent with the standard design that has been used during line-transect cetacean surveys in the past. The array is mainly constructed out of polyurethane plastic tubing and then filled with castor oil once the preamps and hydrophones are in place. We built the end arrays with 5 hydrophones, 3 mid-frequency, with listening capability up to 96kHz, and 2 high-frequency, with listening capability up to 250 kHz. These hydrophones allow us to collect acoustic data using a broad frequency range, and thus monitor for all odontocete cetaceans. The main difference in the design from past end arrays includes a brass head connector instead of PVC. The brass will act as a saltwater ground for the entire system and hopefully minimize introduced electrical noise that we experience when working on NOAA ships.
The second array we built is an ‘inline array.’ The inline array is a newly designed array with connectors at both ends. This allows the inline array to be connected directly to both the tow cable and a ‘modular baseline’ that essentially acts as an extension between arrays. The end array connects to the modular baseline cable completing the modular array system. This system will improve the ability for acoustics to locate animals in the field using a larger separation distance between the hydrophones for localization. Localization is achieved using a time-difference of arrival algorithm that outputs an angle to the sound source (dolphin) based on when the sound (i.e. whistle) reached the hydrophones. By crossing bearings from the end array with bearings from the inline array, we can compute animal location more accurately and more quickly than during past surveys, where we had to wait for convergence of several bearings form a single array. Now each Science Center will be able to tailor their localization methods based on their focus species and collect more precise and accurate acoustics data.
Two days were spent testing each Science Center’s arrays; everyone’s equipment worked great with no repairs necessary. Everyone entered the workshop with various skill levels, but with the dedicated time and effort, the construction of the modular towed hydrophone array system was successful.