the issue is the thermocline which reflects sounds back either back to the surface or back to the ocean floor you need to get your hydrophone below the thermocline layer also the pinger is not very noisy meaning you need to get the hydrophone within nautical mile feet of the pinger what they end up doing is towing the hydrophone about feet above the ocean floor at feet you are able to scan a swath that's feet wide assuming the ocean floor is flat and level talk