Towed bodies are commonly used for transducer deployment in fisheries assessments. As the transducers are not affixed directly to the vessel, towed bodies have the advantages of providing reduced aeration and pitch/roll in rough weather and being relatively easy to install and remove. Towed bodies present a greater risk for cable breakage than do hull mounted systems and deployment/retrieval can be a safety hazard in rough sea conditions.
Deep towed bodies are used when the species of interest is found only in deep water. By deploying a transducer at a depth closer to the targets of interest, this approach improves spatial resolution of targets, reduces noise, and improves detection of the bottom in bathymetric areas such as canyons. Although this approach is primarily used in marine systems, it may be useful for studies of deep species in deep lakes (e.g., Lake Superior during the day) or in cases of low SNR.
Two styles of towed bodies are currently used in the Great Lakes (Fig. 8). Each is a suitable design for towing at 5-7 knots under typical survey conditions. Other towed body designs exist and may also be suitable for inland lake surveys. As with any towed body, the operator must ensure that the transducer face is parallel with the surface while at survey speed. If stationary sounding is to be performed, stabilizing lines may be needed.
Figure 8. Two towed body styles currently used in the Great Lakes. The towed body on the left is 4’ long and the one on the right is 8’ long.
A pole mounted transducer bridges the gap between towed bodies and hull-mounted systems. In this set-up, a pole is affixed to the vessel at a distance from the side and the transducer is mounted on a plate at the base of the pole (Fig. 9). Pole mounting provides greater transducer stability than towed bodies, but more flexibility than hull mounting for deployment and removal. On small vessels, some pole-mount systems can consume critical deck space or present an obstacle if a stabilizing mount is used across the deck. Pole mounts have the same disadvantage as hull mounts of being more susceptible to the pitch/roll of the vessel than towed bodies.
Figure 9. Photo of the Lake Erie pole mount.
A sonar tube deployment is similar to a pole mount, but differs in that there is a tube that runs from the deck to the bottom of the vessel. The transducer is mounted on a pipe or ram that is lowered through the sonar tube to a depth appropriate for the vessel. Advantages of this approach are 1) it is less susceptible to vessel movement than other athwartship pole mounts because the transducer is at the most stable portion of the vessel, 2) the transducer can be deployed and retrieved easily and quickly while underway at full speed, and 3) the presence of paired sonar tubes allows for easy calibration, as one can lower the sphere down one tube while the transducer is in the other tube.
Hull mounted systems have the advantage of reducing the frequency of cable breaks and of remaining safely deployed at all times. As a result, hull mounted systems reduce deployment and removal times on vessels frequently used for acoustics. Data quality from hull-mounted transducers may be reduced in rough weather due to vessel pitch/roll and aeration. But, incorporating pitch/roll sensor data into post-processing routines may minimize the effect of pitch/roll on the data. Many marine vessels use drop-keel systems instead of hull-mounted to reduce the probability that transducers will be damaged or lost if the vessel encounters a shoal or seamount.
A Sea Chest is an alternative to external hull mounting that consists of a chamber that is mounted on the inside of the hull with a rubber diaphragm to the outside (Fig. 10). The transducer sits within this chest and the chest is filled with USP grade caster oil. Advantages to the Sea Chest are the reduction of noise in rough seas and the ability to service the transducer without haul-out. Disadvantages include 1) loss of signal with higher frequencies in cold water, 2) cost and labor required to replace the castor oil as it degrades, 3) limitations on transducer size, and 4) more complicated calibration procedures than for a towed body or pole mounted system.
Figure 10. Sea Chest. Schematic from Fleischer et al. (2002).