Sub-bottom profiling systems are used to identify and characterize layers of sediment or rock under the seafloor. The technique used is similar to a simple echosounder. A transducer emits a sound pulse vertically downwards towards the seafloor, and a receiver records the return of the pulse once it has been reflected off the seafloor. Parts of the sound pulse will penetrate the seafloor and be reflected off of the different sub-bottom layers. The data that is obtained using this system provides information on these sub-floor sediment layers.
Sub-bottom profiling systems utilize the principle of seismic reflection. Seismic reflection uses a stronger sound signal than echolocation and lower sound frequencies. The sound pulse is often sent from an airgun towed behind the ship. An airgun uses the sudden release of compressed air to form bubbles. The bubble formation produces a loud sound. The sound from the airgun travels down to the seafloor. Some of the sound reflects off the seafloor but some of the sound penetrates the seafloor. The sound that penetrates the seafloor may also reflect off layers of material within the seafloor. The reflected sounds travel back up to the surface. The ship also tows a number of hydrophones (called a towed array or streamer) which detects the reflected sound signal when it reaches the surface. The time it takes the sound to return to the ship can be used to find the thickness of the layers in the seafloor and their position (sloped, level, etc). It also gives some information about the composition of the layers.
- NOAA Coastal Services Center, "Benthic Habitat Mapping: Sub-bottom Profiling." (Link)