Summary of DART Geophysical Methodology 100111

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A summary of the proposed Geophysics methodology to be used on the DART project. Presented at the academic and stakeholder meetings (10th and 11th January 2011 respectively) by Rob Fry (Bradford University).

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Summary of DART Geophysical Methodology 100111

  1. 1. Geophysics and DART Moisture variation through the soil OR ‘Contrast is Everything’ Robert Fry Division of Archaeological, Geographical and Environmental Science
  2. 2. Geophysical methods (especially Radar and Earth Resistance) are reliant on the local ground conditions to provide a contrast between target archaeological features and the soil matrix, it is this contrast between archaeology and soil which makes it detectable. By understanding the effect of the environmental impact over the study sites we will create a conclusive model of how environmental conditions effect the detection of features, and how this information can lead to better detection in the future. Investigation into the effects of soil moisture variation and seasonality on the contrast properties of archaeological residues. 
  3. 3. From: Carr 1982. Fig 10 Soil and resistivity Image redacted
  4. 4. Al Chalabi and Rees (1962) Clark (unpublished1980) Cott (unpublished1997) Parkyn (unpublished 2010) Why a need for further understanding? Conclusions of previous studies inconsistent with each other No correlation with in-situ probes No hard soil science Not very local weather measurements (usually dependant on weather stations of about 10-20km away) No comparisons with Hyper-Spectral datasets Past studies do not show the dynamic action between feature and soil matrix in profile No investigation into the immediate effect of changing weather on a daily scale Past Geophysical Studies on Seasonality
  5. 5. Methodology: establishing test areas Test Sites have been selected at both Diddington and the Royal Agricultural College which are situated on differing geologies These will undergo a large-scale geophysical investigation in Jan / Feb / March to ascertain the main areas of focus: Fluxgate gradiometer / earth resistance survey / Ground Penetrating Radar Archaeologically sparse areas with a main feature running through – e.g. old field boundary / ditch Monthly geophysical surveys following the internment of TDR sensors, over the small targeted areas – currently 10x10m areas Fluxgate gradiometer, earth resistance, electrical resistance tomography, ground penetrating radar surveys undertaken every month, as close to hyper-spec flying times as possible Full blog: http://dartproject.info/WPBlog/?p=861
  6. 6. Survey equipment: Earth Resistance Geoscan RM15 Earth Resistance Meter Passes electrical current through one ‘probe’ to another. The potential difference between the two current probes is measured. This changes dependant on the sediments and moisture content in the soil. Relies on ‘ideal’ underground soil conditions (e.g. pore size, archaeological sediments, moisture content etc.) for adequate contrast between the soil matrix and the archaeological sediments. Will give a plan view of the local resistance variation of the sites on a monthly timescale
  7. 7. Methodology: Earth Resistance Initial survey of the test sites. C. 0.25-0.5 ha. Resolution of 0.5m X 1m Identify the best-fit areas for monthly surveying. Monthly survey 10x10m grid at each site Resolution of 0.5x0.5m
  8. 8. Survey equipment: Electrical Resistance Tomography (ERT) ZZ Geo FlashRes64 resistivity instrument ALL possible electrode configurations of data, combined into a singular dataset (C.64,000 different readings) Used monthly over the test areas to track how moisture travels through designated sections of the study zone, to fully understand the relationship between the retention of water particles between the target feature and surrounding soil matrix Image: ZZ Geo FlashRes64 Brochure 2010 Image redacted
  9. 9. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Section of earth
  10. 10. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Ditch cut
  11. 11. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Image: Fry. R. 2010 MSc. Dissertation Fig. 6.20 Abandonment / Filling
  12. 12. Full blog: http://dartproject.info/WPBlog/?p=995
  13. 13. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. First measurement datapoint
  14. 14. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Second measurement datapoints
  15. 15. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. And so on…
  16. 16. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. End of the line…?
  17. 17. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Start again, BUT BIGGER!
  18. 18. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Image: Fry. R. 2010 MSc. Dissertation Fig. 6.20 Datapoints collected over the section
  19. 19. Survey equipment: Electrical Resistance Tomography (ERT) A resistance pseudosection over a ditch feature using a Wenner Array. Image: Fry. R. 2010 MSc. Dissertation Fig. 6.20 Inverted apparent resistivity pseudosection Sides of ditch
  20. 20. Survey equipment: Fluxgate Gradiometer Bartington 601/2 Initial stage of investigation on a large Potential for monthly investigation.. Whether this is worth it or not..? Initially, the survey will target an area of around 1ha, around the suspected target zones. This will be at a resolution of 0.125 x 0.5m. Due to speed of survey, a gradiometer survey could also be conducted monthly, to assess if there is much magnetic variation over the target area throughout the year. This would also be at a resolution of 0.125 x 0.5m.
  21. 21. Survey equipment: GPR Transmitter sends a high frequency electromagnetic pulse into the ground This is reflected at interfaces of different dialectic permittivity. Relative permittivity (dialectic constant) of: Fresh water = 81 Air = 1 Dry clay = 3 - 5 Wet clay = 5 - 40 © A. Schmidt 20/5/95 Image redacted
  22. 22. Survey equipment: GPR Pulse Ecco 450 MHz University of Bradford IDS Dual frequency antenna 250MHz / 700MHz University of Birmingham
  23. 23. Before the monthly surveys…
  24. 24. Preliminary Study: How does immediate environmental weather conditions effect geophysical detection? To understand the immediate effect of weather on geophysical detection Check for the validity of results taken on monthly basis If there is significant variation over the month what will be the impact on the monthly measurements? How will this change the methodology for the monthly test sites? Full blog: http://dartproject.info/WPBlog/?p=995 Scope for a preliminary study testing the immediate effect of weather on geophysical detection. All seasonality studies have focused on once-monthly surveying. Is there much day-to-day variation? What happens to geophysical readings after a big storm? Etc. Pilot (month-long) study proposed:
  25. 25. Where? Constrained by logistical issues: Potential site: Full blog: http://dartproject.info/WPBlog/?p=995 Preliminary Study: How does immediate environmental weather conditions effect geophysical detection? Leeds / Bradford locality essential Access to geophysical equipment essential Some seasonality tests already performed ideal ‘ The Amphitheatre’, University of Bradford Campus
  26. 26. Preliminary Study: How does immediate environmental weather conditions effect geophysical detection? Methodology This study will use the same instruments and sampling strategies as used by the DART team for the monthly surveys. Earth Resistance GPR ERT (Electrical Resistance Tomography) Gradiometer Full blog: http://dartproject.info/WPBlog/?p=995
  27. 27. Fin. Thank you for listening Full blog: http://dartproject.info/WPBlog/?p=995
  28. 28. References Al Chalabi, M. M. and A. I. Rees. 1962. An Experiment on the effect of rainfall on electrical resistivity anomalies inthe near surface. Bonner Jahrbucher, (162). Carr, C. 1982. Handbook in Soil Resistivity Surveying. Centre for American Archaeology Press. USA. Clark, A. J. 1980. Archaeological detection by resistivity . PhD. The University of Southampton Cott, P. J. 1997. The effect of weather on resistivity measurements over a known archaeological feature . M. Phil. The University of Bradford. Fry, R. 2010. An archaeological and geophysical integrated approach to problem-solving: The Defences of Silchester. MSc. Dissertation, The University of Bradford Fry, R. 2010. Ideas on Geophysical Methodology (V2) DART Project Website . http://dartproject.info/WPBlog/?p=861 Fry, R. 2010. Preliminary Study DART Project Website. http://dartproject.info/WPBlog/?p=995 Parkyn, A. 2010 . ‘It Never Rains but it Pours’ Earth Resistance Seasonality Testing In Bradford Poster: NSGG Conference on Recent Work in Archaeological Geophysics. December 2010.The University of Bradford. All photos used are the authors own.

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