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Mapping Techniques: Acoustics

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Summary Bathymetry Surveys Side-scan Sonar

Sub-bottom Profiling Acoustic Seafloor Classification Systems (ASCS)

Sub-bottom Profiling

Diagram of a combined sub-bottom profiling system and side-scan sonar. Courtesy: Science Applications International Corporation

Sub-bottom profiling systems identify and measure various sediment layers that exist below the sediment/water interface. These acoustic systems use a technique that is similar to simple echosounders. A sound source emits a signal vertically downwards into the water and a receiver monitors the return signal that has been reflected off the seafloor. Some of the acoustic signal will penetrate the seabed and be reflected when it encounters a boundary between two layers that have different acoustical properties (acoustic impedance). The system uses this reflected energy to provide information on sediment layers beneath the sediment-water interface.

Acoustic impedance is related to the density of the material and the rate at which sound travels through the material. When there is a change in acoustic impedance, such as the water-sediment interface, part of the transmitted sound is reflected. However, some of the sound energy penetrates through the boundary and into the sediments. This energy is reflected when it encounters boundaries between deeper sediment layers having different acoustic impedance. The system uses the energy reflected by these layers to create a profile of the sub-bottom sediments.

Several sonar parameters (output power, signal frequency, and pulse length) affect the instrument performance.

• An increase in output power gives better penetration into the sub-bottom layers. This will usually provide deeper penetration into the sub-bottom layers. Sometimes however, if the bottom is very hard or not very deep, the increase in power will cause more signal to be reflected back off the seafloor. The signal might then be reflected off the sea surface, leading to multiple reflections and "noise" in the data.
• Signal frequency also has an effect on system performance. Higher frequency systems (2 to 20 kHz) will produce high definition data of the upper seafloor sediment layers. These higher frequency signals have shorter wavelengths, and they are able to discriminate between layers that are close together. Lower frequency systems will give greater penetration but at a lower resolution.
• Longer sound pulse length transmits more energy and yields deeper seabed penetration. However, a long pulse length may decrease the ability to discriminate between adjacent reflectors, thus decreasing the system resolution.

Advantages and Limitations

Sub-bottom profiling systems can be useful for characterizing benthic habitats, since they provide information about sub-surface sediment structure. No other acoustic techniques provide this type of information, and only physical sampling via cores or in-situ photography via sediment profile imaging will allow for characterization of subsurface structures. Sub-bottom profiling systems may penetrate as deep as 30 meters (approximately 90 feet) into the seafloor, which is much deeper than most cores and Sediment Profile Imaging (SPI) can penetrate. However, the penetration depth depends on the hardness of the overlying layers and the presence of gas deposits, such as methane.

Sub-bottom systems are limited by a narrow swath width, so continuous coverage of the seafloor is time-consuming and expensive to obtain. As with other single-beam acoustic methods, the footprint is relatively small and dependent on depth.

How do sub-bottom profiles and side-scan images compare?

Uses

High-resolution sub-bottom systems have been used to detect and measure the thickness of dredged material deposits, detect hard substrate that has been covered by sedimentation, identify buried objects (such as cables and pipelines), and define the basement (or bedrock) layer for potential confined aquatic disposal sites for dredged material.

Researchers at Louisiana State University (LSU) combined seafloor acoustic patterns from side-scan sonar and the sedimentary profiles provided by the sub-bottom system to identify oyster habitat. Using these data, they are able to evaluate habitat quality and select areas suitable for oyster reef relocation.

The Hudson River Estuary Program uses a combination of acoustic and physical sampling, including sub-bottom profiling, to map benthic habitats of the Hudson River Estuary. Researchers are using sub-bottom data to study the depositional history of the estuary and to examine the relative age of buried oyster reefs and depositional sediments.

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