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GEOPHYSICAL SURVEY FACT FILE

WATER ENGINEERING GEOPHYSICS 1


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Tel: +44-1905-619531




INTRODUCTION

The Ground Probing Radar (GPR) method has been customised by Subsurface Geotechnical for water engineering geophysical investigations. Applications include the concrete testing of barrages, weirs, locks and dams. Water-borne geophysical surveys can also profile the thickness of mobile bed in river channels and assess the extent of siltation in canals and reservoirs. The geophysical survey results are presented in an engineering compatible format readily understandable by the client.

GPR survey of a gauging station

Investigating a gauging station by GPR



SURVEY OPERATION

For water-borne surveys only the GPR transducer is actually towed over the water surface. The remaining equipment is either set-up on land, or mounted inside a boat for larger scale operations. Pulses of radio frequency energy are transmitted through the water by the transducer, propagating downwards into the river bed or underwater engineering structures like concrete aprons. At the boundary between electrically contrasting materials such as water-concrete; water-sediment or concrete-substrata the pulses are reflected back towards the surface, where they are detected by the receiver component of the transducer for digital storage by the control module.




ground penetrating radar survey of barrage

Testing a barrage apron on the River Indus




DATA PROCESSING & INTERPRETATION

The acquired data can enhanced by processing, prior to output as a digital display or hard-copy graphic section of the reflection interfaces beneath the survey line. Expert interpretation procedures are then applied to identify critical reflection events on the radar trace. Key reflections are correlated with real boundaries associated with the water engineering structure. These can include the base of the water column; the top and bottom of a concrete apron and internal defects such as voids and honeycombing.




ground probing radar survey of a lock towpath

Towpath GPR survey using low frequency antenna unit



APPLICATION 1
LOCK & WEIR INVESTIGATIONS

GPR surveys are particularly useful for locating voids hidden beneath a towpath or behind a lock chamber. Longitudinal profiles can be surveyed along the towpath to produce sections behind the lock wall. The internal structure of a lock wall is investigated by vertical profiles down the face of the wall. Results can be used to determine the wall profile and thickness and to locate internal defects. Water-borne profiles locate voids within or beneath a weir apron. If a heavy crane is required for remedial operations such as gate replacement, a GPR survey is carried out over the proposed track and station of the crane.





GPR profile across a silted-up navigation channel



APPLICATION 2
CANAL SILTATION

GPR can be readily applied to investigate canals for quantifying the extent of siltation. Data is acquired by towing transverse profiles across the surface of the canal. Interpretation procedures determine the profile of the original navigation channel, so that the thickness of accumulated silt can be measured. Wrecks and other obstructions buried within the silt can also be located and mapped, to provide vital information on potential hazards to dredging and other restoration operations




results of a ground radar profile across a river bed


APPLICATION 3
SCOUR ASSESSMENT

Fluvial investigations assess scour potential on the upstream side of bridge piers and weirs by measuring the thickness of the mobile bed, and by profiling the permanent river channel. The survey results allow accurate targeting of follow-up intrusive and diving investigations.





APPLICATIONS FOR WATER ENGINEERING


Phone us at: +44-1905-619531 or e-mail us at: info@geophysical.biz


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