Zetica - e-news
June 2006 Volume 2, Number 7

In this issue

 

Useful Links & Recent Publications:

Benefits of high speed GPR to manage trackbed assets and renewal strategies
PWI paper presented in Sydney, June 2006

Solution matrix for road or motorway applications
The US Federal Highways Dept has published a very useful matrix of geophysical and NDT methods for solving engineering problems during SI, construction and maintenance of road infrastructure.

WW2 bomb risk maps
Download risk maps for your region (28 regions across the UK)
WW2 bomb risk maps image

Limits of detection of unexploded ordnance – are you getting what you paid for?
White paper on physical limits of detection for surface- and borehole-based geophysical methods

Statistical assessment of the risk of unexploded bombs
Paper published in Ground Engineering, May 2006

UXO frequently asked questions
Download a list of FAQ demystifying various aspects of unexploded ordnance

Integrated geophysical survey to target boreholes for mineshaft investigation – Lindal Tunnel, Cumbria

Zetica Ltd was commissioned by Mott MacDonald on behalf of Network Rail (LNW Territory) to undertake an integrated geophysical survey at Lindal Tunnel. The tunnel is located near Dalton-in-Furness in Cumbria. A combination of in-tunnel ground penetrating radar (GPR), and over ground 2D resistivity imaging, magnetic profiling and electromagnetic profiling was utilised.

The objective of the survey was to determine the locations of four suspected hidden shafts within the lined section of the tunnel ahead of ground investigation works aimed at facilitating remedial design work.

The results of the integrated survey have identified a number of anomalies that could relate to the possible hidden construction shafts and has provided a number of recommended locations for the drilling program. These anomalies have subsequently been investigated and 3 out of 4 were verified to be mineshafts.

Entrance to portal
Entrance to portal

Models of true resistivity for over ground sections parallel to the tunnel
Models of true resistivity for over ground sections parallel to the tunnel

Anthony Drake of Mott Macdonald commented:
"The geophysics surveys were an integral part of the phased approach to this ground investigation attempting to identify the location of a number of hidden shafts. The combination of geophysical methods used helped to provide extra confidence so that intrusive works could be targeted at the anomalous areas minimising the time spent on site."

Click here to request information on mineshaft detection services.

Cost benefit of geophysical surveys

We are often asked the question – "Why should I spend money on a geophysical survey?". There are numerous examples of site wide geophysical surveys saving our customers money. 3 are presented below demonstrating areas where risk could have been or was mitigated and significant savings resulted.

The first relates to a site investigation where boreholes were required in a busy city centre and despite a recommendation to carry out a full Utilities survey including ground penetrating radar (GPR) for non-metallic services, only a CAT survey was conducted. The result was an expensive repair bill.

The other two examples are self explanatory with only a fraction of the contingency budget having to be spent for potential causes of delays in construction / piling.

Project Description

Utilities

 

Estimated geophysical cost for 3D mapping

Actual cost of repair

Clearance of boreholes for Site Investigation. BT cable cut because of inadequate survey.

£2,000

£30,000

 

 

 

 

Underground storage tanks (UST’s)

 

Geophysical survey cost

Contingency delay cost

Investigation of car dealership forecourt. Unmapped buried tanks located.

£2,000

£10,000

 

 

 

 

Buried reinforced foundations

 

Geophysical survey cost

Contingency delay cost

Investigation of quayside property to install new services. In-ground foundations and other obstructions mapped.

£3,500

£30,000

Depending on the target and depth of burial plus the burial setting we can advise whether geophysics is worth carrying out at all and if it is, to maximize the cost benefit by designing a sufficient survey to meet the survey objectives. The following graphic shows the % of a site investigation budget used by a customer to investigate a petrol station forecourt. The object of the exercise was to contribute to the site investigation so the customer gets better value for money.

Value of information vs % of budget expended on non-invasive methods

Click here to request more information.

Measuring compaction of materials in backfilled trench

Zetica Ltd was requested by a client to carry out an integrated surface wave seismic profiling and soil impact testing survey on an infilled trench in a car park. The objective of the survey was to establish confidence in the compaction of crushed concrete fill within a 20m x 7m x 4m deep backfilled trench.

The results of modelling the Continuous Surface Wave (CSW) data shows crushed concrete fill thickness varying from roughly 5m at the western end to 3m in the east. The five CSW profiles acquired over the trench suggest that the stiffness of the crushed concrete varies from 150MPa to 300MPa. The CSW results showed that the material filling the trench is of higher stiffness than the ground surrounding it.

The results of soil impact testing showed that the top 300mm of compacted earth was everywhere greater than an RIV of 25 indicative of a well-compacted back fill.

Model of shear modulus (stiffness profile) for one of the sounding locations
Model of shear modulus (stiffness profile) for one of the sounding locations

Click here to request more information.

Metronet trials non-invasive geophysical methods for routine embankment investigations

Zetica Ltd was requested by Metronet Rail, the company responsible for the maintenance and renewal of two-thirds of London's Tube, to carry out a trial geophysical survey on embankments located between Southfields and East Putney stations on the Wimbledon Branch of the District Line. The objective of the survey was to characterise the embankment materials and underlying geology in order to provide target locations for a follow up intrusive investigation.

The results of the trial survey have provided information on the variation in both the compressional and shear wave velocities of the materials within and beneath selected embankments. In addition to small variations in P-wave and S-wave velocity within the embankment materials, more significant localised changes in shear wave velocity in the underlying geology were also identified.

Metronet image 1

Metronet chart

On the basis of the results of the trial, it was recommended that the surveys of additional embankments are undertaken using a combination of the seismic refraction and surface wave seismic techniques. The superior quality of the seismic data compared, for example, to electrical resistivity tomography (ERT) data over the same embankments suggests that the seismic inversion models are likely to be more reliable for routine use over similar materials.

Neil Esslemont, Metronet’s Earth Structures Assessment Team Leader, comments "It’s still early in the evaluation process, but if the positive results to date are reproduced on other embankment sites, we will be allowing for more routine use to help locate intrusive site investigation boreholes where they can be most representative of embankment materials and integrity."

Click here to request more information on Zetica’s methodology for embankment investigations.

Contact Us:
Zetica logo

www.zetica.com
info@zetica.com
Tel: 01993-706767
Tel: 01322-612140

CEO:
Asger Eriksen

Managing Director:
Mike Sainsbury

Technical Manager:
Jon Gascoyne

Rail Services Manager:
Ben Venables

Copyright © 2006

Network Rail awards Zetica contract for train mounted high speed GPR contract

Zetica has just been awarded a contract by Network Rail to carry out high speed train-mounted GPR surveys, data processing and interpretation across 8,000km of the UK network. Competition was tough with bidders from Germany and the United States.

This development will build on the work Zetica has done to date producing high speed multichannel GPR solutions mounted on measurement coaches and road-railers. Zetica has carried out extensive trials of a range of antennae and GPR hardware in a rail setting and offers the flexibility of alternative configurations including single channel and multichannel options using a range of antenna frequencies for different GPR manufacturers.

CEO Asger Eriksen:
"Over the past 2 years, Zetica has undertaken comprehensive product development specifically for high speed GPR on track. This work was partially funded by the DTi through the Knowledge through Partnership (KTP) program and combined the resources of Liverpool University and a dedicated research team at Zetica.

The research team has evaluated, developed, tested and improved methods of high speed radar surveys in the UK, including the automation of data processing through new software development. The developments from this research will be used to assist Network Rail in their high speed radar objectives."

Click here to request more information on Zetica’s trackbed investigation services.

Click here to access more information on Zetica’s existing high speed rail radar product – ZARR.

Technical note – Crosshole seismic velocity tomography

Borehole methods such as crosshole seismic or resistivity tomography have a role to play in targeting features at depths beyond the detection limit of surface methods. Crosshole methods are preferable to drilling using individual boreholes to detect hazards such as mineshafts / adits as the probability of detection is improved and important information about the intervening ground and the properties of ground overlying features can be derived.

Crosshole seismic tomography surveys involve the measurement of the travel times of seismic ray-paths between two or more boreholes in order to define seismic velocity and rock structure in the intervening ground. Any one-source borehole could be used to shoot into multiple receiver holes.

Crosshole seismic velocity tomography image

For a p-wave engineering survey use is generally made of a high voltage p-wave Sparker system as the source and a 24-channel hydrophone streamer as receivers. The Sparker produces a seismic source within a water-filled borehole through rapid heating of the water column.

The work would be carried out to the following industry standard: ASTM D4428M-00. The boreholes should be drilled to below the anticipated depth of the lowest target. The survey resolution is generally estimated to be between 1/5 and 1/10 the borehole separation but will depend on the source frequency and the velocity of the formation materials, which determine the signal wavelength in the ground. Typically the minimum target size for a 10m borehole separation and 1m sampling interval will be 2m.

Boreholes should be watertight plastic cased and bottom-capped to maintain a constant water level. Cement bonding is required to ensure a continuous good contact with the formation. Downhole verticality is measured in each borehole.

Example crosshole seismic tomogram (2D left and 3D right) to image buried cavity.
Example crosshole seismic tomogram (2D – left and 3D – right) to image buried cavity.

Click here to request more information.

Research partners

Zetica is ideally placed to undertake important applied geophysics research through a combination of relevant skills and experience and close links to the University of Liverpool. Zetica provides a state-of-the-art test site in Enstone, Oxfordshire with unique facilities for testing surface and borehole geophysical equipment and methodologies.

Particular areas of research expertise include:

  • Unexploded ordnance detection methods (Zetica's test site is the UK's only civilian proving ground for UXO detection technologies)
  • Utilities detection using integrated methods such as GPR and RFL
  • Buried hazard mapping using integrated surface geophysical methods
  • Trackbed investigation using high speed GPR
  • Engineering geophysics applied to geotechnical design

Click here to access list of publications.

Click here if you are interested in partnering to research and develop new non-invasive investigation methods.

Lunchtime Seminars

Lunchtime seminars imageZetica offer a popular Geological Society registered seminar, with CPD points, on the uses and abuses of geophysics. Engineers are brought up to date on the latest geophysical methods available in the market place and interesting areas of research and development. The presentation normally lasts 30 – 40 minutes and is case history-based with a 15 minute discussion session following.

Click here to email Ellen Stevens to discuss your requirements and arrange a seminar at your offices.