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Topography and Bathymetry
Published in Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste, Experimental Hydraulics: Methods, Instrumentation, Data Processing and Management, 2017
Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste
Side-scan sonar efficiently covers a large area on the bottom of the water body by taking advantage of the differing sound reflecting and absorbing characteristics of different materials (L3-communications, 2000). The side-scan technique uses a sonar device which is either towed from a vessel or mounted to the hull of the vessel. The device emits conical or fan-shaped pulses across a wide angle perpendicular to the path of the sensor. The resulting acoustic beam is wide in the across-track direction and narrow in the along-track direction (Johnson & Helferty, 1990). The strengths of acoustic reflections from the bottom are recorded in a series of cross-track slices. When these slices are combined they form an image within the coverage width of the beam (swath). The sound frequencies used in side-scan sonar usually range from 50 to 500 kHz; higher frequencies yield better resolution but with less range. This technique can be used to detect debris and other obstacles in the water body. Side-scan sonar is also used for fisheries research, dredging operations and environmental studies. Figure 4.3.3 illustrates the resulting bathymetry map of a 250 kHz side-scan sonar of several river arms in a freshwater delta in the south of Norway.
Offshore site investigation
Published in White David, Cassidy Mark, Offshore Geotechnical Engineering, 2017
Side-scan sonar is a method of underwater imaging using narrow beams of acoustic energy (sound) transmitted out to the side of the ‘towfish’ (or equivalent such as an ROV) 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 such as 500 kHz–1 MHz give excellent resolution but the acoustic energy only travels a short distance. Lower frequencies such as 50 kHz or 100 kHz give lower resolution but the distance that the energy travels is greatly improved.
Relationship between geoacoustic properties and chemical content of submarine polymetallic crusts from offshore Brazil
Published in Marine Georesources & Geotechnology, 2020
Arthur Ayres Neto, Vanessa Alves da Costa, Clara Pinto Ferreira Maia Porto, Thais Cristina Vargas Garrido, Jean-Pierre Hermand
This condition of normal incidence only exists in very rare circumstances and, due to ship movements (pitch, roll, heave and yaw) and seafloor irregularities (at different scales), it is rarely fulfilled. Computations of the reflection coefficient must take into account the angle of incidence. Higher incidence angles (relative to normal incidence) are indicative of greater loss of acoustic energy in the opposite direction and reduced reflection back to the sensor. Surveys using high-resolution seismic and single-beam backscatter can be approximated as having a normal incidence geometry (zero offset), especially if the distance between the sensor and the seafloor is small (i.e., a small signal footprint). The incidence angle becomes a more important factor affecting echo intensity when multi-beam backscatter and side-scan sonar are used. The strong anisotropy observed for our samples, associated with the inverse correlations between acoustic impedance and metal content depending on the measurement direction, suggests that this correlation would be close to zero at a specific incidence angle and, consequently, metal identification and content evaluations would not be possible.
Geotechnical considerations for the concept of coastal reservoir at Mangaluru to impound the flood waters of Netravati River
Published in Marine Georesources & Geotechnology, 2019
C. R. Parthasarathy, T. G. Sitharam, S. Kolathayar
Review of the available geotechnical information helped in deriving a generalized geological model as a useful preliminary design profile for a quick check on stability and settlement of the proposed coastal reservoir, which is good enough for the feasibility study stage, although, site-specific detailed integrated site survey is warranted during detailed project report stage. A detailed integrated site survey comprising of geophysical investigation (bathymetry, side-scan sonar & sub-bottom profile) and geotechnical investigation (boring, in situ testing & laboratory testing) shall be planned to be undertaken to derive the geological model appropriate for the location demarked for the coastal reservoir.
BSEE decommissioning cost estimates in the shallow water US Gulf of Mexico
Published in Ships and Offshore Structures, 2022
Platforms and single-well caissons in water depths less than 91 m (300 ft) are trawled for clearance verification. The BSEE preferred verification technique is to drag a standard trawl net across 100% of the site in two directions. In some cases, alternative site verification techniques such as side scan sonar or documentation of sweep assembly results have been used.