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GEOPHYSICAL SERVICE FACT FILE

WATER ENGINEERING GEOPHYSICS 2


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Tel: +44-1905-619531


SUB-BOTTOM PROFILING WITH SONAR

Sonar is a high resolution, water-borne, geophysical method used for investigating the geology and bathymetry of the sub-bottom environment. It operates beyond the depth range of Ground Probing Radar in marine or freshwater settings. The method has many useful applications for water engineering including profiling water depth and sediment thickness in rivers and reservoirs, assessing scour around quays and bridge piers, and locating buried foundations and other large submerged features on sea and lake floors.



sonar geophysical survey

Scour investigation on a River Thames crossing



EQUIPMENT & OPERATION

High resolution sonar uses impulses of acoustic energy to produce graphic depth sections of the sub-bottom environment. A typical sonar system consists of an acoustic transducer incorporating a seismic transmitter and hydrophone receiver, signal control console and output display module. The transducer is normally towed behind the boat to allow direct coupling with the water; the remaining equipment is stored inside a boat. The transducer sends out a stream of acoustic impulses through the water and into the sub-bottom. Reflections generated at boundaries between materials of contrasting density such as soft sediment and rock are detected by the transducer hydrophone. Higher frequency sources are used for shallow depth investigations of muds and silts to maximum sediment thicknesses of 10 metres. Lower frequency sources allow enhanced depth penetrations, but lower resolutions with operating ranges of 50 -100 metres in a wide variety of sub-bottom conditions.




sonar section by bridge pier



DATA PROCESSING & INTERPRETATION

Signal reflections are recorded in real time to produce a cross-sectional image of the sub-bottom below the survey line. Data interpretation correlates reflection events with real interfaces such as the water-sediment or mobile bed-permanent channel boundaries. Using sonar sections allows continuous interpolation of conditions between boreholes and bottom grab samples.



APPLICATIONS FOR WATER ENGINEERING


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