Does anyone have experience of recording underwater at depth? I am
interested in recording at around 100 m in one study, and up to 2 km in
another.
Hello Steve,
I'll assume you're operating from a boat. To record at 100 m depth, by far the
simplest thing to do is just to get a hydrophone with a cable 100 m long.
(Most commercially-available hydrophones will work at 100 m depth, but be sure
to check!) Dangle it off the side of the boat, possibly with a weight if the
hydrophone doesn't sink by itself. It helps to have some means to stop or
reduce the up-and-down motion of the cable and hydrophone caused by the rolling
and pitching of the boat. Two techniques you can use for this are (1) to
attach a 1-m long elastic piece (bungee cord, surgical tubing, etc.) at two
points in the cable 2-3 m apart, so that the elastic bears the weight of the
hydrophone and you have a large loop of cable out to the side where the elastic
is; and (2) to run the cable through the center of a flat disk, 30-50 cm in
diameter, which is attached to the cable and which inhibits any up-and-down
cable motion. If you use both of these methods, put the disk b
elow the cable loop.
You might also have a problem with cable strum or vibration, which comes from
relative motion of the cable and water; this can occur either because wind
pushes the boat with respect to the water, or because the boat is anchored and
there is a current. (Differential currents at different depths can also be a
problem, but probably not much of one at only 100 m depth.) If strum is a
problem, two things you can do are (1) pay out cable as the boat drifts, so
that the cable down to the hydrophone is not moving relative to the water, and
(2) use a cable with a faring, which is basically a wrapper of some sort that
allows it to move through the water without strumming.
Recording at 2 km depth is significantly harder. You can, of course, use a
2-km-long cable and proceed as above; you'll also need a hydrophone which works
at 2 km. Another option is to use one of the sound-recording systems that have
been developed for use on whales, seals, etc. These are compact little (~20 cm
long) packages that incorporate a hydrophone and other sensors, electronics,
and a hard disk for the data. The two I know about are the Bioacoustic Probe
(or B-Probe), developed by Bill Burgess at Greeneridge Sciences
(http://www.acoustimetrics.com/bprobe/), and the DTAG, developed by Mark
Johnson and Peter Tyack at Woods Hole Oceanographic Institution
Good luck,
Dave
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