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Methods
Multibeam Echosounder
Multibeam echosounder systems (MBES) first appeared in the 1970s and primarily consisted in an extension of a singlebeam echosounder. Echosounders are capable to transmit a pulse of controlled length, repetition rate and frequency to an underwater acoustic transducer and to accurately time the returning echoes (propagation velocity of sound waves in water: ~1,500 m/s). Instead of transmitting and receiving a single vertical beam, the MBES transmits several tens of beams (typically 100 to 200) with small individual widths (1° to 3°) in the form of a fan perpendicular to the navigation line. This configuration provides depth informations out to several hundred meters each side of the vessel which allows to survey large areas of the seabed with a higher density and a better accuracy than singlebeam echosounders.
During recent years MBES have greatly evolved and nowadays they are a broadly accepted tool for seabed mapping. Acoustic transducers were developed with frequencies ranging between a few hertz and several megahertz depending on the region and the water depth surveyed. A low frequency of 12 kHz is used for deep sea research, whereas recently developed MBES with high frequencies between 200 and 300 kHz are applied for the investigation of shallow-water areas.
Seabed Classification System
Conventional methods to classify the seabed are in situ sampling of bottom sediments or optical methods like analyzing of photos and videos. However, remote acoustic classification techniques are becoming of increasing importance because they are less expensive and time-consuming and provide a higher spatial resolution in determining the seabed characteristics. Especially, echosounder-based techniques, first developed for fishery purposes , are a useful tool to classify a big area of the seabed in a relative short time period with a high spatial resolution. In general, echosounders are used for bathymetric surveys. However, acoustic signals reflected by the seabed contain more information than just the water depth. The intensity and shape of a returning acoustic signal is affected by a number of factors, primarily sediment grain size and sorting, seabed roughness, bedforms, and presence, concentration and type of benthic fauna and flora. For instance, the harder or rougher the seabed, the more energy is scattered back to the transducer and vice versa. Therefore, echosounder-based classification systems are utilized to reveal geological structures of the seabed composed of various types of sediments and rocks.
QTC VIEW
The QTC VIEW system uses the first returning echo from the seabed only and analyzes the shape of each echo with a series of five algorithms. These algorithms characterize the waveform by using energy and spectral components, yielding 166 descriptors of each echo. Principal-Component-Analysis (PCA) reduces the large quantity of information to three most useful descriptors (Q1, Q2, Q3), which prove enough to recognize the different types of seabed. When plotting the points defined by Q1, Q2, and Q3 on a three-axis plot, echoes of similar character form clusters that stand for distinct acoustic classes. To correlate each acoustic class with seabed characteristics groundtruthing by sampling bottom sediments has to be carried out. The result is a georeferenced trackplot classified by sediment types.
Sidescan Sonar
A basic Sidescan Sonar System consists of a topside processing unit, a cable for electronic transmission and towing, and a subsurface unit (a towfish) that transmits and receives acoustic energy for imaging.
Sidescan Sonar is a method of underwater imaging using narrow beams of acoustic energy (sound) transmitted out to the side of the towfish and across the bottom. Sound is reflected back from the bottom and from objects to the towfish. Certain frequencies work better than others, high frequencies (>500 kHz ) give excellent resolutions but the acoustic energy only travels a short distance. Lower frequencies (50-100 kHz ) give lower resolution but the distance that the energy travels is greatly improved.
The towfish generates one pulse of energy at a time and waits for the sound to be reflected back. The imaging range is determined by how long the towfish waits before transmitting the next pulse of acoustic energy. The image is thus built up one line of data at a time. Hard objects reflect more energy causing a lighter signal on the image, soft objects that do not reflect energy as well show up as darker signals. The absence of sound such as shadows behind objects show up as very dark areas on a sonar image.
Multiparameter Probe with inductive Current Meter
The Multiparameter Probe Series 2001compact consists of 1+4 sensors, e.g. an inductive current sensors, as well as of an internal compass and memory, cable interface and internal batteries.
for more information contact
Dr.-Ing. Helmut Schlüter VDI, HS Engineer
www.hs-engineers.de
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