The tripod frame: mooring acoustic receivers on the seabed

Post provided by Jolien Goossens

Jolien Goossens tells us about the challenges of installing acoustic receivers on the seabed and the tripod they designed to overcome them.

sea

Installing scientific instruments in the marine environment comes with many challenges. Equipment has to withstand the physical forces of tides, currents and storms. Researchers have to take into account the effects of biofouling, corrosion and human activities. Even access to the study site can pose its difficulties, as diving is limited by depth and weather conditions. Practical deployment mechanisms are therefore needed to sustain consistent data flows.

Acoustic telemetry enables the observation of animal movements in aquatic environments. Individual animals are fitted with a transmitter, relaying a signal that can be picked up by acoustic receivers. To facilitate a convenient installation of these instruments, we developed and tested a new design, mounting a receiver with an acoustic release on a tripod frame. This frame enables the recovery of all equipment and better yet, improves the quality of the data.

Let’s get technical

The frame consists of an 80 kilogram galvanized-steel tripod with an orange, floatable collar mounted on the top platform. The hollow central beam of the tripod contains a coiled Dyneema rope that connects the frame to the collar. In this collar, a VR2AR receiver is fixed. This acoustic receiver has a built-in acoustic release system that, when activated, detaches the receiver from its acoustic release pin. In our design, this release pin is attached to the steel tripod.

Figure1

Technical drawing of the tripod frame design (©VLIZ)

Once the tripod is correctly prepared, it is ready for deployment. The installation is fairly straightforward: the frame is hauled from the research vessel into the water and placed on the seabed. Recovery on the other hand, is a bit more elaborate. From a small manoeuvrable boat, the acoustic release is activated with a hydrophone transceiver that communicates acoustically with the receiver. This mechanism separates the release pin (fixed to the frame) from the receiver (attached to the collar). As the collar floats, it surfaces together with the receiver, and is collected with the small boat. However, the collar is still attached to the frame with the rope, allowing for the retrieval of the tripod frame with the research vessel’s winch. As all equipment is recovered, the tripod can be redeployed after reassembly!

Frame_release2

Underwater footage of activated release (©VLIZ)

Less tilt, better data

The tripod and its installation protocol proved to be practical and efficient during a field trial in the North Sea. Out of forty deployments, recovery failed twice. In one instance, an error in the receiver firmware disabled the release mechanism. The other issue was caused by strong currents during retrieval. Therefore, weather conditions and tides should be taken into account for the recovery procedure.

Circboxplot

Orientation of receiver in an anchor line (left) and tripod frame (right).

The performance of the receivers in this tripod field trial was compared to a commonly used anchor line design. In the latter set-up, an acoustic receiver is fastened to a line that is attached to a float on one end and anchored to a weight on the other. Whereas the stability of the frame makes for a consistent orientation of the receiver, the anchor line moves with tides and currents. Strikingly, every receiver on an anchor line reached a maximum tilt angle of 90°, indicating they hit the seabed. The tripods on the other hand maintained a smaller and more constant inclination. As a result, the tripod frame rendered a higher data quality. The detection efficiency of receivers was evaluated by analysing how well receivers would detect the built-in transmitters of receivers at varying distances. The tripod frame was proven to detect more transmitted signals at greater distance than the anchor line design. As the detection efficiency of tripod was higher and more consistent over time, the design would better capture biologically driven patterns in presence of tagged animals, rather than variation in receiver performance.

Applications to other instruments

The acoustic release system and principle of the tripod frame can serve other instruments as well, alongside the acoustic receiver. In the context of the Belgian LifeWatch project, the frame has been modulated to hold a C-POD, a hydrophone and an ADCP. It has also been used in the framework of the ARMS program. Such a multidisciplinary, ambulatory mooring allows for simultaneous observations to answer a range of integrated research questions.

20190214_090145

Deployment of a tripod frame, modulated to hold a C-POD (©VLIZ)

To find out more about this method for mooring acoustic receivers, check out our open access Methods in Ecology and Evolution article, ‘Mooring scientific instruments on the seabed – Design, deployment protocol and performance of a recoverable frame for acoustic receivers’.

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