The Use of Geophysics In Ground Investigation


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The Use of Geophysics In Ground Investigation

  1. 1. The use of Geophysics in Ground Investigation Presented by Claire Graham
  2. 2. Outline: <ul><li>Who we are </li></ul><ul><li>What we do </li></ul><ul><li>The techniques that we employ </li></ul><ul><li>Which techniques are appropriate where </li></ul><ul><li>Case studies </li></ul>
  3. 3. Who we are: <ul><li>Company started in 1990 </li></ul><ul><li>Private and independently run </li></ul><ul><li>Based in south Worcestershire </li></ul><ul><li>Close to motorways so good communications nationally </li></ul><ul><li>Currently 17 staff with ability to put three teams into field </li></ul><ul><li>Most members of staff are graduates </li></ul><ul><li>Good skills base in the collection, processing and interpretation of data </li></ul><ul><li>We own all of our equipment giving us speed and flexibility of response </li></ul>
  4. 4. What we do: <ul><li>We are geophysical survey contractors. </li></ul><ul><li>All of our work is non-intrusive </li></ul><ul><li>Most of our work is shallow normally looking between 0 and 5m into the ground </li></ul><ul><li>We also use radar to survey structures and buildings </li></ul><ul><li>As we have good CAD skills needed for our reporting we offer CAD drafting to other organisations </li></ul><ul><li>We operate in a number of sectors including: </li></ul><ul><ul><li>Utility surveys </li></ul></ul><ul><ul><li>Archaeology </li></ul></ul><ul><ul><li>Engineering / Shallow Geology </li></ul></ul><ul><ul><li>Environmental </li></ul></ul><ul><ul><li>Forensic </li></ul></ul><ul><ul><li>Buildings </li></ul></ul>
  5. 5. What sort of applications do our services cover:
  6. 6. Who we work for: <ul><li>Since establishment in 1990 we have carried out over 1800 commissions from some 800 clients </li></ul><ul><li>We have had enquiries from over 1900 different organisations </li></ul><ul><li>These clients include </li></ul><ul><ul><ul><li>Consultants </li></ul></ul></ul><ul><ul><ul><li>Contractors </li></ul></ul></ul><ul><ul><ul><li>Archaeological units </li></ul></ul></ul><ul><ul><ul><li>Municipal, county and district authorities </li></ul></ul></ul><ul><ul><ul><li>Government agencies </li></ul></ul></ul><ul><ul><ul><li>Universities </li></ul></ul></ul><ul><ul><ul><li>Architects </li></ul></ul></ul><ul><ul><ul><li>Cathedral and church authorities </li></ul></ul></ul>
  7. 7. Where we work: A distribution plot of survey locations • Utility Mapping Jobs • Other Jobs (e.g. Forensic) • Archaeology jobs • Engineering Jobs
  8. 8. Commitment to quality: <ul><li>ISO 9001.2000 Quality Assurance Accredited </li></ul><ul><li>ISO 14001.2004 Environmental Management Accredited </li></ul><ul><li>CHAS accredited </li></ul><ul><li>A Registered Organisation with the Institute for Archaeologists (IfA) </li></ul><ul><li>Member of The Survey Association (TSA) </li></ul><ul><li>Member of the European Ground Probing Radar Association (EuroGPR Assoc) </li></ul><ul><li>Achilles UVDB Registration No. 80959 </li></ul><ul><li>8 members of staff are members of the IfA </li></ul>
  9. 9. How we can help you: With nearly 20 years of experience in carrying out non-intrusive shallow investigations we are happy to offer impartial advice on solutions to a wide range of problems. If we feel we cannot offer a solution ourselves we will always suggest other organisations who may be able to help <ul><li>This advise would be backed up with: </li></ul><ul><ul><li>methodologies </li></ul></ul><ul><ul><li>assessment of the likelihood of success </li></ul></ul><ul><ul><li>addressing H&S issues </li></ul></ul><ul><ul><li>risk assessments </li></ul></ul><ul><ul><li>cost and time scale estimates </li></ul></ul>
  10. 10. The techniques: Techniques typically utilised during shallow geophysical surveys include: <ul><ul><li>Magnetic susceptibility </li></ul></ul><ul><ul><li>Detailed magnetic survey </li></ul></ul><ul><ul><li>Earth resistance </li></ul></ul><ul><ul><li>Electromagnetic methods </li></ul></ul><ul><ul><li>Ground penetrating radar (GPR) </li></ul></ul><ul><ul><li>This is not an exhaustive list! </li></ul></ul>
  11. 11. Archaeological Surveys Pre-development Investigations <ul><ul><li>A requirement of planning approval in the UK is that archaeological investigations are carried out in advance of construction works. These will include both buried remains and also above ground structures. </li></ul></ul>For large sites, perhaps new housing or infrastructure, where there is no known archaeology a general purpose survey is needed. This will often involve an initial reconnaissance survey.
  12. 12. Magnetic Susceptibility A reconnaissance technique which being rapid and cost effective allows large areas to be surveyed in a very short space of time. Areas can then be targeted with more intensive and higher resolution techniques such as gradiometry and earth resistance. Applications (archaeological): for large rural sites to measure the level of past human activity and inform detail survey.
  13. 13. Magnetic Susceptibility
  14. 14. Detailed Magnetic Survey A detailed technique which is still relatively quick and cost effective. The most common technique for locating buried remains in a rural context. Limitations: Strongly affected by magnetic material so can not be used near buildings, fences or in many urban contexts. Applications (archaeological): Locating archaeological features (ditches, pits and other cut features, kilns, hearths etc) on development sites for solar farms, wind farms, housing development etc. Applications (engineering): Locating buried ferrous objects (unexploded ordnance etc).
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  16. 16. Earth Resistance A detailed archaeological technique which is ideal for locating buried masonry remains. Not as quick and cost effective as detailed magnetic survey and therefore not as widely used. Limitations: Probes need to be placed into the ground so this technique can not be used over hard surfaces. Applications (archaeological): Locating buried masonry remains (walls, rubble/hardcore, made-ground, roads or trackways) and cut features such as ditches, pits and graves.
  17. 17. Survey to locate the foundations of a section of an old manor that had been demolished due to fire damage. Data collected at 0.5m centres
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  21. 21. Electromagnetic Ground Conductivity Survey <ul><li>EM has the advantage of not requiring probes to be placed into the ground. This means that soil conductivity (and hence resistivity) can be measured over hard or high resistance surfaces. </li></ul><ul><li>Limitations: Can be affected if too close to buildings , fences etc; large piece of equipment that does not fit into small spaces and this technique does not produce depth information. </li></ul><ul><li>Applications: </li></ul><ul><ul><li>Detection of buried metal objects </li></ul></ul><ul><ul><li>Detection of cavities </li></ul></ul><ul><ul><li>Buried foundations and mine shafts </li></ul></ul><ul><ul><li>Geological feature mapping </li></ul></ul>
  22. 22. EM61 Survey – to locate buried drums
  23. 23. EM61 Survey – to locate buried drums
  24. 24. EM31 Survey – to locate buried foundations
  25. 25. Ground Probing Radar (GPR) GPR is often the only geophysical technique that can be used in environmental, engineering and archaeological applications on busy urban and industrial sites. It is capable of working through a wide variety of surface materials both inside and outside buildings and structures. Typical locations can range from car parks, highways and waste ground through to factory floors and basements.
  26. 26. Ground Probing Radar (GPR) <ul><li>A GPR antenna consists of a transmitter and a receiver </li></ul><ul><li>GPR works by emitting an electromagnetic pulse into the ground. </li></ul>
  27. 27. Ground Probing Radar (GPR) <ul><li>Each material has a different ‘dielectric constant’, the property that determines the velocity of the electromagnetic pulse through that material. </li></ul><ul><li>A GPR antenna consists of a transmitter and a receiver </li></ul><ul><li>GPR works by emitting an electromagnetic pulse into the ground. </li></ul>
  28. 28. Ground Probing Radar (GPR) <ul><li>Where a change of dielectric constant occurs a percentage of the energy is reflected back to the GPR. </li></ul><ul><li>The amplitude of this reflection is determined by the change in dielectric constant. </li></ul><ul><li>A GPR antenna consists of a transmitter and a receiver </li></ul><ul><li>GPR works by emitting an electromagnetic pulse into the ground. </li></ul><ul><li>Each material has a different ‘dielectric constant’, the property that determines the velocity of the electromagnetic pulse through that material. </li></ul>
  29. 29. Ground Probing Radar (GPR) As the shape of the electromagnetic pulse is a cone the antenna will actually ‘see’ an object before it crosses it.
  30. 30. Ground Probing Radar (GPR) The distance to the object effectively decreases as the antenna gets closer, reaching a minimum when the antenna sits directly above the object.
  31. 31. Ground Probing Radar (GPR) <ul><li>This produces a characteristic diffraction in the data. </li></ul><ul><li>For small services these diffractions form a hyperbola which allows an average velocity to be determined for the survey. </li></ul><ul><li>This velocity is then used to calculate depths for the anomalies observed </li></ul>diffraction
  32. 32. Ground Probing Radar (GPR) Different frequency antennas can be used for different applications: High frequency antennae have a high resolution, but shallow depth of penetration, whilst low frequency antennas have a lower resolution but can penetrate to much larger depths. 1500MHz antenna 400MHz antenna 200MHz antenna
  33. 33. Ground Probing Radar (GPR) <ul><li>Applications: </li></ul><ul><ul><li>Air voids - solution features, subsidence investigation, mineshafts, vaults, chambers etc </li></ul></ul><ul><ul><li>Service location </li></ul></ul><ul><ul><li>Archaeology- structures, ditches and pits </li></ul></ul><ul><ul><li>Structural Survey of buildings </li></ul></ul><ul><ul><li>Forensic applications (clandestine burials etc) </li></ul></ul><ul><ul><li>Reinforced concrete inspection </li></ul></ul><ul><ul><li>Bedrock determination </li></ul></ul>Multi-channel, multi-frequency antenna (200MHz & 600MHz)
  34. 34. GPR Example – Air Raid Shelter
  35. 35. GPR Example – Air Raid Shelter
  36. 36. Case Study – Locating buried fuel tanks
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  42. 42. Geophysical Survey doesn’t always work <ul><li>Why is this? </li></ul><ul><li>Not every technique is suitable for the surroundings- e.g. Resistivity over concrete </li></ul><ul><li>Geology can influence results- GPR absorbed by clay, detailed magnetic survey is poor over deep alluvium and bedrock which has a high iron content </li></ul><ul><li>What can we do to prevent this? </li></ul><ul><li>Thoroughly study the site- the more information we have on a site the more likely we are to use the appropriate technique. </li></ul><ul><li>Use more than one technique </li></ul>
  43. 43. Timescales <ul><li>The client should be aware of the time scales and constraints involved in carrying out geophysical surveys. A typical programme should include the following key stages: </li></ul><ul><ul><li>Pre-fieldwork preparation including search of utility companies record </li></ul></ul><ul><ul><li>Commencement of fieldwork </li></ul></ul><ul><ul><li>Completion of fieldwork </li></ul></ul><ul><ul><li>Commencement of reporting </li></ul></ul><ul><ul><li>Completion of reporting and submission of draft drawings for approval by the client if required </li></ul></ul><ul><ul><li>Submission of final approved drawings </li></ul></ul>
  44. 44. Any Questions? Claire Graham – Sales Manager Stratascan Ltd Vineyard House Upper Hook Road Upton upon Severn Worcestershire WR8 0SA UK Tel 01684 592266 Fax 01684 594142 Email [email_address] Website Contact details: