You are in search mode for an U-Boot reported or detected earlier...
A flat sea floor will show a constant line on your echograph. Suddenly the echo returns faster and you detect an object sitting down there. A few meters further the line stays constant as before.
You turn the vessel and pass over for a second time. Again you get that object scribbled on your graph. What would you do, Captain? I guess you'd tell your hydrophone guy to listen at this very special spot, but in the end you'd probably roll some ashcans overboard, just to make sure this structure receives a hit.
With a rocky seabed (big rocks, reefs and shelfs), it would be different. Your echograph could not differentiate between rocks and Uboat hulls, but it would be risky for a U-Boot to bottom down there...
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