RSK Group - Geophysics Focus newsletter

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Editorial

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We hope everyone had a pleasant May Day Bank Holiday and is enjoying spring. With daylight-saving time upon us, opportunities increase for outdoor survey work during the longer days, and we’re raring to go!

A highlight this month is the latest in STATS' half day seminar series. This event provides updates on the latest developments in non-destructive investigation techniques, including a section by RSK STATS Ltd (RSK STATS) on the advantages of using ground-penetrating radar (see below for more details).

As ever, let us know if you have any comments or questions related to our work or this newsletter.

George Tuckwell, RSK Geophysics director and team-leader

Latest news

‘Scene Beneath the Surface’ seminar success

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RSK STATS is delighted with the success of its latest afternoon seminar, held on 27th April at Hertfordshire University’s Fielder Centre, entitled ‘Scene Beneath the Surface: an update on the latest non-destructive investigative techniques for structures’.

Non-destructive testing (NDT) is a highly-valuable technique that can save both money and time in evaluating flaws in materials. NDT is an increasingly commonly-used tool for civil engineering, structural (eg. concrete) and forensic applications. The seminar focused on some of the latest developments in NTD techniques, presented by presented by guest speakers who are acknowledged specialists in their fields.

Among the topics discussed was a talk on ‘Geophysical investigation and non-destructive testing’ by Dr George Tuckwell, RSK Geophysics director, including the use and benefits of ground penetrating radar and impact echo techniques. Further lectures included ‘High Definition Laser Scanning’ by Mark Hudson of Coastway Ltd, ‘Infra-red thermography’ by Nick Selves of Building Sciences Ltd, ‘Non-invasive detection of embedded ferrous corrosion’ by Prof. Peter Hancock of Keele University, and a talk on ‘In-situ Condition Assessments of Timber using NDT’ by Dr John Williams of Trada.

Dr George A Georgiou, President of the British Institute of Non-Destructive Testing, chaired the event. Dr Georgiou said afterwards: "this was an excellent set of talks, on a diverse range of applications. A number of techniques discussed were new to me, and raise the possibility of us publishing a special edition journal on the topics discussed".

The event was well attended by civil, structural and geotechnical engineers, surveyors, architects, forensic consultants, and developers, as well as many other interested parties.

Delegate copies of the presentations and literature are available on request by emailing Claire Ferguson.

 

Shallow Structural Void Detection with Ground-penetrating Radar

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The RSK geophysics team  has successfully used ground-penetrating radar (GPR) to delineate the locations of voids beneath a marble-tiled floor of one of London’s most prestigious hotels. 

GPR is typically used to image buried features at shallow depths, providing continuous, real-time profiles of the subsurface. For example GPR can provide an excellent means of accurately locating pipes and tanks. Using higher frequency antennas, GPR applications can include mapping voids beneath concrete slabs and, in the case of this unusual study, features beneath a tiled floor of a hotel lobby.

The hotel suspected that not enough grout had been placed under the newly laid tiles resulting in a network of sub-tile voids, which were causing the tiles to move and crack. Our team was commissioned to test whether the voids under the tiles could be imaged.

A high frequency 2.6GHz antenna was used to scan the floor along lines spaced at 10cm intervals (see Fig 1). The resultant radargrams were analysed from which a detailed map of the grout was produced identifying areas that were interpreted to be underlain by voids.

Below in Fig 2, a GPR profile across a tile is shown. Each void produces a strong reflection (coloured red). In 3D, the void areas can be clearly mapped (see Fig 3).

In order to test whether the survey was successful, tiles were lifted for inspection. The grouting and void patterns matched the GPR extraordinarily well. Therefore, our maps of the voiding are vital to assist in designing effective remedial measures to stabilise the floor. 

We produce solutions and consultancy to overcome a wide range of structural void problems. For further information on mapping voids in concrete, or in other floor materials, please contact us.

 

Fig 1

Fig 2

Fig 3

 

Electrifying Geophysics

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Our engineers hold Basic Electrical Safety Competence (BESC) for work within High Voltage electricity substations and Person/ Competent Person for work specific to National Grid sites. This training enables RSK Geophysics to carry out geophysical survey work in proximity to operational plant and equipment connected with the UK’s electricity transmission system.
 
The photograph (in Fig 4) below shows a member of the RSK geophysics team deploying the Geophysical Exploration Equipment Platform (GEEP) at a large substation site. The system is equipped with a differential global positioning system (dGPS) and multiple instruments can be mounted onto the platform, which is towed behind a vehicle enabling a fast and more efficient survey. The data plot in Fig 5 shows EM61 data collected over the area, with the hot colours representing buried features. Detailed interpretation of the data provided valuable information on the subsurface that has greatly reduced the risks posed by buried obstructions at this site.

 Fig 4

Fig 5

Geophysical Techniques Explored - GPR

 

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In this edition we highlight another common technique – EM mapping, in which the electrical conductivity of the ground is measured as a function of depth and/or horizontal distance. Mapping variations in conductivity can indicate anomalous areas worthy of further geophysical or intrusive investigation. The technique is cost effective and, ideally, should be used for site-investigation mapping, borehole and trial-pit programme design, and for filling gaps between boreholes.

EM conductivity measurements are taken by inducing an electromagnetic current into the ground from a transmitter coil above surface, and recording the resulting secondary electromagnetic field at a receiver coil a fixed distance away. The strength of the secondary field depends upon the conductivity of materials in the ground. The transmitter and receiver coils are usually mounted on a single device, which is carried along parallel survey lines across the survey area.

Different rocks and buried structures/objects exhibit different values of conductivity. Large increases in conductivity may be caused by increases in ion concentration (total dissolved solids) in the groundwater. The measurements of differences are useful in environmental applications such as mapping shallow contamination plumes and detecting the extent of landfills. In geotechnical applications, such measurements can be used to map sinkhole locations and soil thicknesses.

In addition to the noting of conductivity readings, measurements of the in-phase component of the electromagnetic field are recorded. The in-phase measurement reflects the presence of metallic conductors and is used for metal detection. Abrupt spikes are indicative of metallic conductors such as buried cables, pipes, piles, drums, unexploded ordnance (UXOs) and tanks.

Fig 6

In this example (left), the extent of unknown buried landfill waste is clearly shown in red. 

 

Fig 7 shows an EM survey being done at a petrol station.

Fig 8 - Buried tanks and pipework are indicated using warmer colours (yellows and reds).

 

The RSK geophysics team deploys a variety of EM instruments from which the best is selected for finding the target objects:

Instrument

Depth
(metres below ground level)

Typical Applications

EM31

Down to 6m

To detect/map: geological variations, groundwater contaminants, any subsurface feature associated with changes in the ground conductivity, buried metalwork and ore bodies

EM34

Down to 40m

To detect/map contaminant plumes and to undertake groundwater exploration

EM38

0.75 to 1.5m

For agricultural surveys, and to map soil property variation

EM61
(Advanced metal detector)

Down to 3–4m

To detect ferrous and non-ferrous metals, buried structures, underground storage tanks (USTs) and UXOs

All instruments are fully portable and data collection is rapid (at walking speed) utilising accurate dGPS positioning. At larger sites, increased data-collection rates can be gained using towed systems (at ~6Km/hr).

For more information, download the EM fact sheet from our site, at:
www.environmental-geophysics.co.uk/documents/ref_manual/em.pdf

FREE Reference Guide to Geophysics

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We have a number of general and topic-specific seminars and short courses on the application of geophysics, and the services that the RSK geophysics team offers to clients. We deliver these on site to interested parties, and are able to tailor presentations to specific needs and interests of particular audiences. Email us for more details

FREE Reference Guide to Geophysics

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RSK’s free reference guide or manual to geophysics, A Reference for Geophysical Techniques and Applications, is a first point of reference used by increasing numbers of industry professionals, and is used by several university academics as a teaching resources.

Find out what all the fuss is about by downloading your free copy here. Hard copies are also available free of charge. Email us for more details.

RSK Group: the one-stop-shop for environmental support services

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RSK Group is the UK’s largest, privately owned, environmental consultancy that provides over 200 individual engineering, scientific and environmental services across 17 broad categories. The company employs 750 consultants in 35 worldwide locations, and last year reported a turnover of £63m. For more information, visit www.rsk.co.uk.