What probably happened is the "surface duct" phenomena. First:
1. In warm water (positive gradient) sound bends upwards.
2. In cold water (negative gradient) sound bends downwards.
Then:
1. Sound is radiated from source in all directions.
2. Sound radiating downward from from source will bend upwards.
3. When it hits the surface, it is reflected in a downward direction.
4. When you have warm water (positive gradient) the sound will bend upwards again.
5. The sound will hit the surface, is reflected downward, and the process repeats.
That's a "surface duct" and is probably the mechanism of your long detection range.
You take advantage of the surface duct when you are in shallow water yourself.
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