Bottom-Penetrating Sounders use acoustic technology for mapping both the surface of the ocean floor and structures of different density below the floor. Rocks, sunken ships, mud, sand, and masses of animals all have different densities. When sound reaches these features, the reflected signals create a discrete image. This is useful not only for the study of physical attributes of the ocean floor, but also for studying the distribution of organisms that live on or burrow below the benthos. The distance of penetration of sound waves into the seafloor increases as the frequency of the sound decreases, but the decreasing frequency also reduces the ability of sounders to profile small objects and structures.
An airgun array, such as this, shoots compressed air that makes noise used to map the sea floor and what lies beneath it. (Discovery of Sound in the Sea)
Reflection of sound waves off of hard surfaces and back up to a receiver can be used to accurately determine the shape of the sea floor. (Discovery of Sound in the Sea)
Refraction of sound waves are good for more accurate study of what lies below the surface because sound travels through, and bounces off, different seafloor sediments in very different ways. (Discovery of Sound in the Sea)
An image created by profiling below the seafloor (called sub-bottom profiling) using a Bottom-Penetrating Sounder. (NOAA Ocean Explorer)