A2.4VF2 Applied Environmental Geoscience 2 Lecture 7

1. A2.4VF2 Applied Environmental Geoscience 2Lecture 7 GEOPHYSICS CASE STUDIES

2. Contents • Rational choice of methods • Case history 1: A waste disposal site • Case history 2: An archaeological site • Overall conclusions

3. RATIONAL CHOICE OF METHOD

4. Choice of method • A wide variety of geophysical methods is available, many at modest cost. • They are usually offered by specialist contractors who inevitably promote their benefits. • Thus the user has considerable scope for both choice and cost-effectiveness. • However, geophysical results can be ambiguous at best and misleading at worst.

5. Choice of method • Several factors should be considered when planning a survey. These broadly fall into: • The physical contrast that is being used to form the image • The expected geometry of the underground structure • The survey pattern and type of output • Practical aspects of the survey

6. Choice of method 1. The physical contrast forming the image • What physical properties will show the best contrast? • Are there any strong but irrelevant contrasts that will mask the required information? • Would a second survey using different properties enhance the results or just duplicate them?

7. Choice of method 2.The geometry of the subsurface • What is the likely shape of feature being imaged? • Is it linear, compact or extended over an area? • How deep is it?

8. Choice of method 3. The survey pattern • Is the basic survey to be 1-D, 2-D or 3-D? • Is the result a profile, a map, a cross-section or a volume? • Is an area or line survey better? • What spatial resolution is needed?

9. Choice of method 4. Practical aspects • What are the time or cost constraints? • How portable is the equipment? • Are there any special restrictions eg on access or damage?

10. A WASTE DISPOSAL SITE

11. Waste disposal sites • If we consider the above factors in the context of a waste disposal site: • The contrasts variously involve disturbed ground, groundwater, aqueous leachate and VOCs • The geometry is relatively confined, although any plumes may be laterally extensive • A volumetric (3D) survey is needed, with cross-sections and maps derived from it • There may be access restrictions, gases etc • Ground truth may be limited since the aim will be to contain the problem, not dig it up.

12. Example: The University of Connecticut landfill site investigated by the USGS Water Division • This former landfill was investiaged by a combination of DC resistivity, EM conductance and georadar • The aim is to map the extent of the landfill, define the exact positions of waste trenches, locate possible leachate plumes and comment on the integrity of the bedrock. • http://water.usgs.gov/ogw/bgas/publications/wri994211/wri99-4211.html

14. Comparison of DC resistivity cross sections showing low conductivity leachate areas

15. Anisotropy of electrical conductivity related to bedrock fracture patterns

16. Example of synthetic resistivity cross sections to test possible configuration of bedrock fractures

17. Identification of conductive fracture zone from EM data

18. Modelling inclined fracture zone using EM data

19. AN ARCHAEOLOGICAL SITE

20. Archaeological sites • If we now consider the above factors in the context of an archaeological site: • The contrasts principally involve disturbed ground and stonework • The geometry is laterally extensive but usually shallow • A plan (2D) survey is needed, with the emphasis on maps • Some urban sites may have access restrictions but in general access is easy • Ground truth is plentiful since the result will be an excavation

21. Example: The Forum Novum site in the Sabine Hills east of Rome • Investigations centre on a prominent villa and the settlement’s granaries (horrea) • http://www.arch-ant.bham.ac.uk/bufau/research/forum_novum/fieldwork.htm

22. The aim is to map the outlines of the building and their internal partitions, as a guide to excavations. • Like most archaeological sites, the preferred methods are DC resistivity and proton magnetometry, backed up by selective georadar.

24. Overall magnetic map of the Novum Forum site

25. Aerial photograph of the horrea

27. Comparison of the magnetic (left) and resistivity (right) maps of the villa

28. Detailed magnetometer survey showing excavated areas

29. Georadar results from the villa site (note the use of different time-slices to give differential depth information)

30. Excavations at the villa site

31. OVERALL CONCLUSIONS

32. Conclusions • There are four keys to the successful use of all geophysical methods: • Use a method based on properties that have good contrasts across subsurface boundaries • Apply processing techniques that will minimise the inevitable background noise in the signal • Relate the results to known subsurface materials and structure, usually via nearby boreholes • Use two or more methods in combination.

33. Summary • Rational choice of methods • Case history 1: A waste disposal site • Case history 2: An archeological site • Overall conclusions

34. THE END