The sea noise loggers used were designed at Curtin University in the early 2000's and are well proven instruments. The housing (stainless, 1.2 m long x 115 mm dia., ~ 34 kg in air) lies on the seabed in a frame with an external, weighted hydrophone (Massa TR1025C or HiTec U90 types). A pre-amplifier (20 dB gain) inputs to a signal conditioning card where a low frequency rolloff is applied to frequencies below 8 Hz to flatten the naturally high levels of low frequency sea noise and increase the system dynamic range. The signal is digitised (16 bit), a further 20 dB of gain applied and samples saved to a small capacity flash card in PC format. When the small flash card is around 3/4 full files are copied to either a 128 GB hard drive or flash card (128 GB set by the 32 bit operating system). The systems were designed to sample intermittently and to use small flash cards initially, in order to reduce power consumption and system self-noise.
The moorings are designed to be deployed from commercial fishing vessels hence use rope, and to isolate the sea noise logger as best as possible from the mooring to reduce noise artefacts. A mooring comprises the sea noise logger, a ground line around twice the water depth laid on the seabed with attached weights, floats and an anchor, which couples into a swivel 5 m above an acoustic release, with 175 kg of dump weights below this and a series of floats above the swivel. An image of a mooring is displayed below. We have had some peculiarities with the moorings, in the NSW and southern Australia moorings leatherjacket fishes eat the ropes and hydrophone cable, we have to armour the hydrophone cable and use a hard lay thick rope. We prefer to liaise with professional fisherman, they offer a local base, a wealth of local knowledge, can do anything at sea and enjoy learning about their environment. An image of two sea noise loggers and the staff involved with gear recovery and deployment in 2013 out of Portland is shown below.
The sea noise recorders are calibrated for system gain with frequency by inputting white noise of known level (always -90 dB re V2/Hz) in series with the hydrophone. This calibrates the full system response and accounts for the impedance matches of various components. The hydrophones used and the calibration technique give a calibrated response over 1-2 Hz up to the anti-aliasing filter frequency used, 2.8 kHz. The calibration files are available through IMOS on request. The instrument clocks are set to GPS transmitted UTC time before deployment and the clock drift read after recovery. Due to often sharp jumps in temperature during deployment and recovery we quote an absolute clock accuracy of ± 250 ms.
Metadata lists for each deployment are available from IMOS. These list location, sample times, calibration information (calibration file, hydrophone details and clock synchonisation) and notes,
Sea noise files contain header and footer information in text format, with 16 bit unsigned integer data in binary format inserted between (0-5 V internal rail). The header contains information on sample set up and scheduling, the footer the time sampling started and ended (noting a 'tick' is 1/65536 of a second and is always added to the seconds displayed). Note that the time sampling started is not the same as the time the file was opened (encoded in the file names as seconds from 01-Jan-1970 in hexadecimal format).