Electrical Resistivity Lab

1. Electrical Resistivity Lab HWR 532 Josh Clemons

2. The following graphics were generated using “Model 41” consisting of 41 electrodes spaced 1 meter apart with 4 nodes per electrode spacing.

3. Buried TargetSmall target, shallow depth

4. Buried TargetLarge target, shallow depth

5. Buried TargetSmall target, intermediate depth

6. Buried TargetLarge target, intermediate depth

7. Buried TargetSmall target, deep depth

8. Buried TargetLarge target, deep depth

9. 2 Targets, Different rSmall, high r target above large, low r target

10. Note how the following two graphics are almost identical, although the underground situation is different. The small target below the large target is virtually invisible.

11. 2 Targets, Different rSmall, high r target below large, low r target

12. 1 TargetLarge, low r target

13. 1 TargetSmall, high r target

14. Values Used for Clay/Sand Slides • EC of water = 800 mmho/cm • Porosity of sand = 0.4 • Porosity of clay = 0.45 • Archie equation: ECbulk = (ECwater)(water content)2(porosity)1.2

15. 1 m sand above clayq = 0.05

16. 2 m sand above clayq = 0.05

17. 3 m sand above clayq = 0.05

18. 4 m sand above clayq = 0.05

19. 1 m sand above clayq = 0.15

20. 2 m sand above clayq = 0.15

21. 3 m sand above clayq = 0.15

22. 4 m sand above clayq = 0.15

23. 1 m sand above clayq = 0.25

24. 2 m sand above clayq = 0.25

25. 3 m sand above clayq = 0.25

26. 4 m sand above clayq = 0.25

27. 1 m sand above clayq = 0.35

28. 2 m sand above clayq = 0.35

29. 3 m sand above clayq = 0.35

30. 4 m sand above clayq = 0.35

31. Notice in the following four slides that it is difficult to tell the difference between various water contents in the same beds.

32. 2 m sand above clayq = 0.05

33. 2 m sand above clayq = 0.15

34. 2 m sand above clayq = 0.25

35. 2 m sand above clayq = 0.35

36. Inversion ModelingWenner array, 1 m spacing

37. Inversion ModelingWenner array, 1 m spacing • RMS error after 3 inversions is 29.7% - not very good. • The model indicates a relatively resistive formation or group of formations just below the surface, underlain by less resistive layers.

38. Inversion ModelingWenner array, 3 m spacing

39. Inversion ModelingWenner array, 3 m spacing • RMS after 3 inversions is 35.7% - even worse than for the 1 m spacing. • Drawing conclusions from this model would be risky. However, it does appear that there is something relatively resistive just below the surface, underlain by less resistive material.

40. Inversion ModelingWenner array, 5 m spacing

41. Inversion ModelingWenner array, 5 m spacing • RMS after 3 inversions is 64.1% - the worst yet. The RMS has increased with electrode spacing for these models. • As with the 3 m spacing, drawing a conclusion is risky. However, the apparent relatively resistive body appears again at right.