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Old 17-12-2013, 10:08   #1
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mystery depth soundings

While sailing in deep water between St. Martin and the BVIs the depth sounder would periodically show depths between about 38 and as little as 12 feet. It can be a little disturbing - you think whale? shipping container? The unit seems to be working normally when in depths within reading range. Any idea what causes these strange readings?
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Old 17-12-2013, 10:22   #2
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Re: mystery depth soundings

Ours sometimes does that off soundings. I have always put it down to harmonics when specific depths cause the sounder to receive previous signals, which are interpreted as a shallower depth.

I made this all up out of thin air, but the same phenomenon occurs for us at certain times/depths. The depths are never consistent or long-term like a real depth, however.

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Old 17-12-2013, 10:25   #3
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Re: mystery depth soundings

Thermoclines, (water temperature differentials), water depths greater than your unit is capable of, school of fish, disturbed water caused by the wake of another vessel, and about a hundred other things.
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Old 17-12-2013, 13:53   #4
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Re: mystery depth soundings

Quote:
Originally Posted by captain465 View Post
Thermoclines, (water temperature differentials), water depths greater than your unit is capable of, school of fish, disturbed water caused by the wake of another vessel, and about a hundred other things.
What he said.
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Old 17-12-2013, 14:35   #5
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Re: mystery depth soundings

Air bubbles will give them fits too
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Old 17-12-2013, 14:39   #6
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Re: mystery depth soundings

Quote:
Originally Posted by captain465 View Post
Thermoclines, (water temperature differentials), water depths greater than your unit is capable of, school of fish, disturbed water caused by the wake of another vessel, and about a hundred other things.
Thermoclines would be my guess as well. Submarines use them to hide from surface ship sonar. Depth soundings are... well sonar signals that bounce off the bottom.
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Old 17-12-2013, 14:55   #7
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Re: mystery depth soundings

Thermoclines are that shallow only in lakes and arctic regions in specific times. Schools of fish is my bet. Have seen also algea bloom/plankton to give false bottom signal..
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