Towed hydrophone are usually made of more than one sensor, in order to receive information about the direction from where a signal arrives. Two main techniques are used to get this information: beamforming and time delay measure.
Hope they may be of some help if you are thinking about towed hydrophones for Marine Mammal research.
FarONE - 2018
We have a "single element" towable hydrophone.
Assembled with a 50kHz sensor and low noise preamp electronics, powered via standard phantom power, using a long (150 meters as standard) balanced signal transmission line (to give reasonable immunity to electric noise), to be connected to anything with a standard professional MIC input with phantom power.
This system is aimed at helping "low-budget" researchers willing to get an underwater acoustic snapshot of the study area, operating from slowly moving sailing boats or from fixed coastal stations up to 500 meters from the listening point.
This hydrophone, codenamed FarONE, is branded by Nauta-rcs.
Based on the same sensors, manufactured in the UK, they differ in cable lenght (110m and 220m) and in additional features (a pair of extra hydrophones for stereo listening while free drifting, different plastic cases, different audio amplifiers).
This is our standard setup for 2009/2010.
Do not even remember how many times we have changed the sensors on this.
In 2008 it got entangled into a propeller and was lost. It had four sensors, and could be equipped with up to eight. One pressure gauge, and a nice modification to accomodate a temperature transmitter.
Lessons learnt: on a prototype you will need more wires than how you have.
Lessons learnt: Let's provide training. Longer tow cable means lower noise and deeper sensor. Where thermocline is an issue, you need deep sensors.
Manufactured with three elements (two wideband, high-pass filtered; one wideband, no filters), is our configuration for low budget systems during spring 2006.
Here you can see the manual.