Equipment Used to Examine Area Below the Seafloor
Sub-bottom profiling systems are employed to identify and characterize layers of sediment or rock under the seafloor. These systems can also be helpful in locating hard objects that are buried beneath the seafloor, such as shipwrecks.
How Sub-Bottom Profiling Works
With this technique, a sound source directs a sound pulse toward the seafloor. Parts of this sound pulse reflect off of the seafloor, while other parts penetrate deeper. The portions of the sound pulse that penetrate the seafloor are both reflected and refracted as they pass into different layers of sediment. These signals return toward the surface, where they are detected by hydrophones that are towed by a surface vessel.
The time it takes for the reflected sound pulses to return to the surface vessel can be used to determine the thickness of the sub-bottom layers in the seafloor and how the layers are positioned (e.g., level or sloped). The reflected sound also gives some limited information about the composition of the various layers.
Refracted sound pulses follow a complex path and give more detailed information about the sub-bottom layers. Analysis of the seismic refraction allows a more comprehensive understanding of the density of various sub-bottom layers to be developed.
The Trade-Off Between Penetration & Resolution
Implementing a sub-bottom system involves an inevitable trade-off between penetration into the seafloor and resolution.
Sound sources that produce lower frequency pulses achieve greater penetration though the seafloor, but produce a picture of lower resolution. Higher frequency pulses achieve higher resolution, but they do not penetrate as deeply into the sub-bottom strata.
Although the relationship between penetration and resolution is not linear, it is fairly constant. The key to obtaining optimal results from a sub-bottom survey is matching the objective of the survey with the appropriate equipment.
