Depth sounder doesn't work on muddy bottom

[Sonrisa]

Hi All,


Our boat has an old B&G depth sounder. We're currently cruising in French Polynesia.


In some bays here, the sounder stops reporting depth. This seems to happen near a river when there is a lot of sediment in the water, and presumably a soft muddy bottom.



Just curious if anyone has ever experienced this. It's difficult to source new parts here, so I'd love any ideas on debugging.


Thanks for your time.

[Ballsnall]

Clean the transducer?
Check the connections?
Change power settings?
Stay away from rivers?

[JPA Cate]

There are some conditions when there is a current running through where your depth sounder will see the anticline where the fresh water meets the salt and read that instead of the actual bottom. We have had inconsistent readings off very soft mud, also. There are lots of imperfect tools in the world. Also, you can get marine biofouling on the transducer, which will confuse it.

Ann

[Don C L]

I have not experienced it yet myself but I have heard of depth sounders losing the bottom in certain areas where there may be very soft mud, especially where there may be a kind of thick gelatinous goo on the bottom. I did see it once when diving in freshwater and I would not be surprised at all if that could not reflect anything. Not a bad idea to keep a lead line handy if you are in soft muddy areas if you are concerned about depth.

[GordMay]

Originally Posted by JPA Cate

There are some conditions when there is a current running through where your depth sounder will see the anticline* where the fresh water meets the salt and read that instead of the actual bottom...

I suspect Ann may be referring to estuaries [&/or thermoclines], which are the borderlands between salt- and freshwater environments, and they are incredibly diverse, both biologically and physically. When fresh river water meets [salt] sea water, the lighter fresh water rises up, and over, the denser salt water. Sea water noses into the estuary, beneath the outflowing river water, pushing its way upstream along the bottom.
When this occurs at an abrupt salt front, the salt content (salinity), and density may change from oceanic to fresh in just a few tens of meters horizontally, and as little as a meter vertically. This can create a sonar reflection.
Accompanying these strong salinity and density gradients are large vertical changes in current direction, and strength.

Thermoclines, which can also create a sonar reflection, are due to the density of the differing water temperatures. Since the density of the water changes with it's temperature, the sonar signal will reflect off the change in water density. This stems from a discontinuity in the acoustic impedance of water, created by the sudden change in density.

When the boat [or current] is moving faster, another common cause for depth sounders and fish finders to fail is turbulence.
Transmitted signals reflect back off the air bubbles, and this causes garbled ‘noise’, and masks the acoustic signals.
Underwater turbulence is caused by the boat's hull form, or underwater currents or obstructions, as well as the actual water flow over the transducer, and from propulsion.

* An anticline is a geological [rock] fold, that is convex upward, and a syncline is a fold that is concave upward.

A false shallow reading might be caused by a salt front, a thermocline, turbulence, or any combination thereof.

A false deep reading might be caused by a [sonar penetrable] soft [silty] bottom.

[Mike OReilly]

I’ve experienced both situations. Sailing into some fjords around here (Newfoundland) we’ll see the sounder go from 1000 ft to 10 ft instantly as we pass into the freshwater flow. A bit unnerving, but harmless.

I’ve also had my sounder go into error mode, or report erratic soundings, in shallow silty/muddy conditions. This happens most often to me when digging in the anchor at shallow depths. Sometime my reversing prop will stir up enough of the bottom to blind my sounder.