What equipment can be utilized to examine areas below the seafloor?
Sub bottom profile data showing bedrock layer and protrusion at mud line. Courtesy USACE - Cold Regions Research and Engineering Laboratory.
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, such as shipwrecks, that are buried beneath the seafloor.
How Sub-Bottom Profiling Works
With this technique, a sound source directs a sound pulse toward the seafloor. Parts of this sound pulse will 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 identify the thickness of the sub-bottom layers in the seafloor and how the layers are positioned, e.g., level, sloped, etc. This reflected sound also gives some limited information about the composition of the various layers.
Refracted sound pulses follow a complex path, and these pulses give more detailed information about the sub-bottom layers. Analysis of this seismic refraction allows for a more comprehensive understanding of the density of various sub-bottom layers.
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.
Utilizing Magnetometers for Underwater Surveys
Another valuable tool for underwater surveys is the magnetometer. When a ferrous material is placed within an existing magnetic field (such as the earth’s magnetic field), it develops an induced magnetic field. An induced field that is superimposed on the earth’s field creates a magnetic anomaly. Magnetometers are employed to detect such anomalies.
Since magnetometers differ from many other instruments in that they do not directly measure the physical property of interest, the output signal of these sensors requires some signal processing for translation into the desired parameter.
Although magnetometers are somewhat more difficult to use, they do provide accurate and reliable data. Magnetometers are frequently utilized during pipeline surveys, shipwreck surveys, or whenever buried ferrous materials must be located. Additional information (such as burial depth) may be gathered by utilizing the gradient method.
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