Many time Hydrophone recording consist of Chain noise at regular interval. How to minimize this unintended sounds?

Question

I was analyzing some marine data from the Red sea. I found some sound structures which didn't look like any biophony.

When I listen to this sound, it looked like some chain noise. since I didn't have much expertise in Marine acoustics, I was wondering if some marine animals do sound like that. After searching for literature and talking with a few folks who work in Marine bioacoustics, I realised this sound is of Chain only. I was not in the field. So I assume that the Chain is associated with Hydrophone with Buoy, causing the noise. Did anybody face this type of challenge? How to minimise this in their origin?

Here is the PSD of the 24-hour data.

The 24hours LTSA

Answer 1

Chains are used as anker and to attenuate potential vertical motions of the the recorder. If the sensor moves up more and more chain elements lift from ground and decelerate lift. Very often, recorders move up and down when the top float is on or too close to the surface and follow the surface waves.

The easy answer to the OP is: do not use chains as anchor or component of anchor, but use solid anchors.

The difficult part of the answer addresses the follow-on question on how to avoid vertical motion of the recorder. If the recorder is deep enough and a close to surface top float cannot be avoided, then one could try to decouple motion of top float from recorder with a construction like this:

where in green is the recorder, in blue are some floats, and in grey are weights.

There are certainly other measure to minimize mechanical system noise. For example:

To avoid sound coupling through cables and minimizing impact of cable vibration, attach recorder with rubber to the mechanical buoy structure.

Avoid any steel on steel shackles