RSK Group - Geophysics Focus newsletter

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Editorial

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Welcome to another edition of Geophysics Focus. This summer has already seen a number of notable events such as the World Cup, a change in government and the tightening of public sector purse strings. The pressure on public sector building projects such as schools and affordable homes also has knock-on effects in the private sector. Luckily for us, our portfolio of projects is diverse and growing – particular in the energy market, which has weathered the financial storm well in recent years. Read on for news of our recent work – undertaken in the Channel Islands and London – and for some insight into the use of the Resistivity Imaging Technique.

George Tuckwell, RSK Geophysics director and team leader

Latest news

Thameside focus for SafeGround

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Our SafeGround team has recently completed several sizeable projects in the London area. One site comprised a large land holding owned by the London Development Agency (LDA) located in the Beam Reach area of the Thames Gateway.  SafeGround completed a full topographic survey of the 11ha site, which will be promoted for high-profile employment use.  In addition, we supported the proposed £325-million Tideway Wharf waterside redevelopment in Central London.SafeGround undertook a full utility survey comprising a complete ground-penetrating radar (GPR) survey and radiodetection tracing using the latest equipment and inspecting all covers.  The information will aid the planning and design of the site, and complements work already undertaken at the site by other RSK Group teams.

SafeGround’s team leader, Gerwyn Leigh said, “As the UK's leading utility mapping and geophysics specialists, we are delighted to be working to help bring underdeveloped land back into prime use in the capital.”

In addition, SafeGround has also completed work at numerous National Grid sites for the installation of boreholes.  A raft of services provided by SafeGround included service clearance around the boreholes; this included the use of radar to scan for obstructions and utilities, unexploded ordnance (UXO) clearance, and a topographic survey. 

A fact sheet describing our new UXO clearance service can be viewed here.

For more information on SafeGround, contact Gerwyn Leigh

SafeGround Geophysical engineer using the latest SmartNet topographic survey equipment

Figure 1. SafeGround Geophysical engineer using the latest SmartNet topographic survey equipment

We will bring you a fully updated SafeGround website shortly, so watch this space.

 

Near-Surface Geophysics Group meeting

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The Annual General Meeting and a geophysical field exhibition of the Near-Surface Geophysics Group (NSGG) (formerly known as the Environmental and Industrial Geophysics Group EIGG), was held on 13th July 2010.  The event was well attended by RSK Geophysics staff, several of whom hold positions on the NSGG committee. As of 2011, RSK principal geophysicist Timothy Grossey will be NSGG treasurer.  Tim announced, “After a relatively quiet 2009, the group is looking forward to several high-profile meetings during the next 18 months, including a joint archaeological and forensics meeting at the Geological Society’s Burlington House in December”. For more information on the NSGG, visit www.nsgg.org.uk.

 

RSK Geophysics in Jersey

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The States of Jersey Transport and Technical Services Department (TTS) commissioned RSK Geophysics to undertake a non-intrusive Ground Penetrating Radar survey of the promenade at La Collette in St Helier. Parts of the promenade are now over 100 years old and are suffering from voiding from within the structure. The works are part of an investigation into the possible use of the promenade as an emergency vehicle access route.

The radar survey revealed a large number of possible discrete voids along a 400m section of the coastline. As the facing revetment is still in good condition, the material beneath the promenade is not being lost at a high rate, and it is considered that the integrity is sound enough for its continued use by the public.

Andy Downie, TTS senior engineer said, “RSK’s report was clear, concise and informative … very helpful, and [provided] strong evidence for using non-intrusive techniques in the future”.

For more information contact Matt Stringfellow.

GPR survey being conducted along the promenade.

Figure 2. GPR survey being conducted along the promenade.

 

Can you guess what this is?

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Take a look at the data plot below and guess what it is.  On the surface it is a plain brick wall (in an attic) which to the naked eye had no obvious features or indications of what lay beneath.  The data plot is neither a Magic Eye image from the 90s nor have our engineers been over enthusiastic with the colouring crayons.  Figure 3 actually illustrates radar data of a 3D ‘slice’ through the attic wall revealing the hidden chimney flues.  The voids in the flues are causing high-amplitude resonance as shown by the warm colours.  On-site engineer Stephen Owen said, “Its always pleasing to see hidden features so clearly”. 

For more information on the capabilities of non-intrusive void detection with geophysics, view our fact sheet here.

Radargram within the wall

Figure 3. Radargram within the wall

Geophysical Techniques Explored - GPR

 

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In this issue, we take a look at Resistivity Imaging (also referred to as Electrical Resistivity Tomography or ERT). This technique is a particular useful tool in geological and contaminated land/brownfield investigations where it can be used to provide additional ground-truth information about the subsurface between boreholes.

The electrical properties of the subsurface vary with the ground material, the presence and saturation level of fluids, and the presence of buried objects (natural or man-made). The resistivity imaging technique is used to describe the distribution of these properties as a function of depth and horizontal distance by acquiring electrical measurements taken at the surface.

Measurements of ground resistance are taken by introducing an electric current into the subsurface via two metal stakes (electrodes) planted into the ground (see Figure 4 below). If the surface comprises hardstanding, small holes are drilled through it to allow contact with the soil beneath. The current passing through the ground sets up a distribution of electrical potential in the subsurface. Using Ohm’s law, this voltage can be converted into a discrete, apparent resistance reading.

The data set is then put through a sophisticated processing stage called inversion where a detailed ground model is iterated to fit the recorded data. The result of this is a final image of the distribution of resistivity with depth, along the line of the survey (see Figure 5). The investigation depth is governed by the electrode spacing and the length of the survey line. As a rule, the depth imaged is about a third of the line length, depending on the line configuration.

Advantages

Applications

§  Non-intrusive

§  Greater depth than ground penetrating radar (up to a maximum depth of approximately 40m)

§  Low cost compared to microgravity

§  Provides fully 2D spatial or 3D volumetric geological models of the ground

§  Complements discrete borehole locations

§  Depth to bedrock

§  Stratigraphy (mineral and overburden thickness)

§  Fault zones and voids

§  Landfill extents

§  Chemical contamination

§  Leachate migration

§  Buried obstructions

 

Figure 4. High-resolution set-up Figure 5. Typical 2D resistivity model of a landfill site

Figure 4. High-resolution set-up         Figure 5. Typical 2D resistivity model of a landfill site

 

For more information, download the resistivity fact sheet from our site at:
www.environmental-geophysics.co.uk/tech_res.htm

New sector sheets

 

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You can find out more about RSK Geophysics involvement in your business sector by clicking below to reach a range of our new service sector sheets:

§  Retail

§  Road & Rail

§  Land & Property

§  Education

§  Renewables

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.