Seafloor acoustic logger unit: preliminary notes about a new tool for remote underwater bioacoustic studies

Abstract

Since their beginning in 1999, local cetacean passive hydroacoustic studies have increased in Chile, but have been limited to the traditional technique of deploying hydrophones from boats.
Some of the main problems of this technique are reviewed, based on a personal experience. Here are discussed the results of the first tests of a self-made remote autonomous underwater sound recorder following specific needs that may be shared by other colleagues, but have not been fully addressed by other contemporaneous projects. The Seafloor Acoustic Logger is composed of two parts, a physical unit (SALu) to be installed on the seabed and a Java-based software interface (SALi) that allows for the recording schemes of SALu to be graphically setup from any personal computer, netbook or even smartphone that has an available USB port, in the lab or the field. Recording schemes include: Continuous, automatically creating successive sound files
of 1.9 GB; user defined Periods; up to 18 Timers that can be set up by date, time and duration; and Trigger, the innovation for a microcontroller-driven device of a signal-based recording mode that provides SALu with real-time analysis capabilities to decide whether or not to record incoming sound, by comparing peaks longer than 30 ms with a threshold preset by the user. Trigger is proposed as a feature to increase the efficiency of signal analysis in the lab, as it produces sound files composed primarily of the sought data as opposed to time-based events that may collect mostly environment background sound.
Based on a modified Microtrack II (M-Audio) digital sound recorder connected to a
SQ26-8 hydrophone and controlled by a custom made printed circuit board, SALu was designed to collect underwater bioacoustic data in the form of digital sound files up to 24 bits of depth and 96 kHz of sample rate, and from <0.020 to 50 kHz, during a period depending on the preset recording mode. The endurance of recording can range between 14 and 86 days and nights, without the presence of the researcher nor his/her boat close to the sensor. By eliminating the behavioral reactions of animals studied to the presence of a boat and noise usually transferred to the sensor due to sea surface motion, SALu is presented as a valid complement to traditional hydroacoustic techniques, collecting data in remote locations or even when ports are closed due to weather conditions. SALu is recommended to support opportunistic and systematic behavioral and ecological bioacoustic studies, as well as assessments on cetacean interactions with fishing and aquaculture human activities. With simple modifications, SALu can also support inland bioacoustic studies on herpetology, mammalogy and ornithology, among others possible fields.