1. Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson tom.wilson@geo.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Terrain Conductivity Methods (cont.)

2. Tom Wilson, Department of Geology and Geography Objectives for the day Brief discussion of units Context for AMD problem Rule of thumb – general utility and limitations Profiling versus sounding with examples from the Greer mansion site A couple computer modeling examples Additional examples illustrating terrain conductivity/resistivity combinations Around 1:55 or so begin problem discussions Problem 8.4 (discussion only) Questions about problems 8.6 – 8.7

3. Tom Wilson, Department of Geology and Geography Express 15  -m resistivity in terms of mmhos/m conductivity units

4. Tom Wilson, Department of Geology and Geography Response Functions -  & R Consider the following two-layer problem - Over this simple two-layer earth model what is the measured apparent conductivity -  a – the composite ground conductivity measured by the conductivity meter be? How do you compute  a ?

5. Tom Wilson, Department of Geology and Geography  1 =20 mmhos/m  2 =2 mmhos/m  3 =20 mmhos/m Z 1 = 0.5 Z 2 = 1 Given the above diagram, could solve the equation below? Review the 3-layer (2 z) earth model problem Z R V R H.000 1.000000 1.000000.200.9284767.6770329.400.7808688.4806249.600.6401844.3620499.800.5299989.2867962 1.000.4472136.2360680 1.200.3846154.2000000 1.400.3363364.1732137 1.600.2982750.1526108 1.800.2676438.1363084 2.000.2425356.1231055 2.200.2216211.1122055 2.400.2039542.1030602 2.600.1888474.0952811 2.800.1757906.0885849 3.000.1643990.0827627 3.200.1543768.0776539 3.400.1454940.0731363 3.600.1375683.0691128 3.800.1304545.0655074 4.000.1240347.0622578 4.200.1182129.0593147 4.400.1129097.0566359 4.600.1080592.0541887 4.800.1036061.0519428 5.000.0995037.0498762 5.200.0957124.0479660 5.400.0921982.0461979 5.600.0889320.0445547 5.800.0858884.0430231

6. Tom Wilson, Department of Geology and Geography The in-class AMD problem is based on reclaimed strip mine near Morgantown

7. Tom Wilson, Department of Geology and Geography How many different conductivity layers need to be considered? How many different values of z are needed? Does it matter whether d (depth) and s (intercoil spacing) are in feet or meters? Set up your equation following the example presented by McNeill and reviewed in class, and solve for the apparent conductivity recorded by the EM31 over this area of the spoil. 20’ 30’ Pitfloor Let’s make the model a little more complicated

8. Tom Wilson, Department of Geology and Geography Z R V R H.000 1.000000 1.000000.200.9284767.6770329.400.7808688.4806249.600.6401844.3620499.800.5299989.2867962 1.000.4472136.2360680 1.200.3846154.2000000 1.400.3363364.1732137 1.600.2982750.1526108 1.800.2676438.1363084 2.000.2425356.1231055 2.200.2216211.1122055 2.400.2039542.1030602 2.600.1888474.0952811 2.800.1757906.0885849 3.000.1643990.0827627 3.200.1543768.0776539 3.400.1454940.0731363 3.600.1375683.0691128 3.800.1304545.0655074 4.000.1240347.0622578 4.200.1182129.0593147 4.400.1129097.0566359 4.600.1080592.0541887 4.800.1036061.0519428 5.000.0995037.0498762 5.200.0957124.0479660 5.400.0921982.0461979 5.600.0889320.0445547 5.800.0858884.0430231 The equation you solve should look like this. where -  1 =  3 = 4 mmhos/m  2 = 100 mmhos/m  4 = 10 mmhos/m z 1 = (20’/12’) = 1.67 z 2 = (30’/12’) = 2.5 z 3 = (60’/12’) = 5

9. Tom Wilson, Department of Geology and Geography Z R V R H.000 1.000000 1.000000.200.9284767.6770329.400.7808688.4806249.600.6401844.3620499.800.5299989.2867962 1.000.4472136.2360680 1.200.3846154.2000000 1.400.3363364.1732137 1.600.2982750.1526108 1.800.2676438.1363084 2.000.2425356.1231055 2.200.2216211.1122055 2.400.2039542.1030602 2.600.1888474.0952811 2.800.1757906.0885849 3.000.1643990.0827627 3.200.1543768.0776539 3.400.1454940.0731363 3.600.1375683.0691128 3.800.1304545.0655074 4.000.1240347.0622578 4.200.1182129.0593147 4.400.1129097.0566359 4.600.1080592.0541887 4.800.1036061.0519428 5.000.0995037.0498762 5.200.0957124.0479660 5.400.0921982.0461979 5.600.0889320.0445547 5.800.0858884.0430231 The EM31 has a 12 foot intercoil spacing hence - z 1 = (20 feet/12 feet) = 1.67 z 2 = (30 feet/12 feet) = 2.5 z 3 = (60 feet/12 feet) = 5 Given also that  1 =  3 = 4 mmhos/m  2 = 100 mmhos/m &  4 = 10 Given the tables of R values at right R V (1.67) ~ 0.288 (about a third or 0.1 less than 0.298) R V (2.5) ~ 0.197 (average of R’s for z = 2.4 and 2.6) R V (5.0) ~ 0.0995

10. Tom Wilson, Department of Geology and Geography Recall those “rules of thumb” regarding the optimal sensing depth or exploration depth. For the EM31 operated in the vertical dipole mode the “ROT” says exploration depth is 18feet. Examining the terms in the equation you computed - How does the middle term - which arises from an average depth of 25 feet - contribute to the apparent conductivity measured at this location. About 70% of the value of ground conductivity comes from the layer centered at depths well beyond (almost twice) the optimal exploration depth. This is a point to keep in mind especially when trying to locate contamination zones which may have abnormally high conductivity. We might normally exclude use of the EM31 in attempts to detect something at depths greater than 20 feet or so. See 3LayerTCModel.xls

11. Tom Wilson, Department of Geology and Geography Greer Mine Spoil Terrain Conductivity Study The production of acid mine drainage (AMD) from surface and underground coal mines in the Appalachian region has been a major environmental problem since mining began in the region and continues to receive much attention in affected communities. Untreated AMD entering surface and ground water degrades the water quality and reduces the value of affected lands. The Surface Mining Control and Reclamation Act (SMCRA, 1977) requires that if mining activity contaminates or interrupts the ground water or surface water supply of adjacent users, the mine operator must remediate or replace the water supply. Remedial procedures are often set up in response to the need to be in compliance of SMCRA water quality standards and are frequently extensive and costly. Lack of site-specific subsurface information often limits the effectiveness and increases the cost of these techniques. From Fahringer 1999

12. Tom Wilson, Department of Geology and Geography The water is treated with anhydrous ammonia and calcium hydroxide (lime) in the southwest corner of the site (Sincock, 1998). Treated water collects in settling ponds before being discharged into a tributary of the Cheat River. From Fahringer 1999

13. Tom Wilson, Department of Geology and Geography Efforts to treat the AMD in-situ have taken place in the last three years and have included injection of sodium hydroxide (NaOH) into the spoil as well as surface applications of post-treatment alkaline sludge and lime slurry into ditches. From Fahringer 1999

14. Tom Wilson, Department of Geology and Geography On the surface of the mine three of trenches were dug to dispose of treated sludge and AMD. These trenches are located near a groundwater divide (Sincock, 1998) and trend northwest-southeast in the western portion of the site. From Fahringer 1999

15. Tom Wilson, Department of Geology and Geography EM 31 field measurements taken around sludge-filled trenches at the Greer site in the fall of 1998 and EM 34 measurements taken in the spring of 1999 show conductivity highs extending from the trenches. These conductivity highs originate at the trench and extend along pathways through the surrounding spoil. From Fahringer 1999

16. Tom Wilson, Department of Geology and Geography This map shows the general flowpaths inferred from Sincock's single-salt tracer test, as straight line vectors of flow from 10 wells to 5 springs. From Fahringer 1999

17. Tom Wilson, Department of Geology and Geography A conceptual flow model based on observed potentiometric surfaces and the potentiometric map presented in Sincock's thesis (1998). Multiple flowpaths and extreme heterogeneity in the spoil are ignored mainly because they are poorly known. From Fahringer 1999

18. Let’s talk more about data acquisition and acquisition strategies Tom Wilson, Department of Geology and Geography 1. SOUNDING – Measurements centered at a point using the different intercoil spacings and dipole orientations (vertical or horizontal). This method of surveying is referred to as sounding. A sounding provides information about the variation of conductivity with depth. 2. PROFILING- One can collect data using a single coil spacing over a large area or along a survey line. This is referred to as profiling. Profiling provides information about the variation of conductivity throughout an area at relatively constant depths approximated by the coil separation optimized for the depth of interest. 3. One can also combine these methods to obtain profiles of conductivity variation with depth. The display of such data provide a quasi-cross sectional representation of conductivity variations with depth along a profile.

19. Using the rule of thumb to provide a rough portrayal of variations of conductivity with depth Tom Wilson, Department of Geology and Geography The rule of thumb suggests that depths being sensed using a vertical dipole orientation are localized about mean depths corresponding to 1.5 times the intercoil spacing. Actually depends on layer conductivity and weighting function The rule of thumb helps us display our results in cross section form!

20. Measure  a at different intercoil spacings and display  a ’s at respective exploration depths Tom Wilson, Department of Geology and Geography 40m 20m 10m 3.7m 60mdepth 30m depth 15m depth 5.5m depth ExplorationExploration DepthDepth Coil spacing EM34 EM31 EM34 Surface Vertical “exploration depths” What are the horizontal “exploration” depths?

21. Tom Wilson, Department of Geology and Geography 40m 20m 10m 3.7m 60mdepth Midpoint 30m depth 15m depth 5.5m depth ExplorationExploration DepthDepth Coil spacing Sounding EM34 EM31 EM34 Surface Vertical “exploration depths” What are the horizontal “exploration” depths?

22. Tom Wilson, Department of Geology and Geography “Exploration depth” remains constant and the measured variations in ground conductivity provide a view of relative variations in conductivity at the exploration depth Profiling DepthDepth Target Depth

23. Tom Wilson, Department of Geology and Geography Individual midpoints Combined profiling and sounding DepthDepth EM34 40m 10m 20m EM313.67m ExplorationExploration 5.5m 15m 30m 60m “exploration depths” at each survey point along the profile

24. Tom Wilson, Department of Geology and Geography Individual midpoints DepthDepth EM34V 40m 10m 20m EM31V3.67m Combined horizontal and vertical measurements pseudo cross section view

25. Tom Wilson, Department of Geology and Geography Survey layout

26. Tom Wilson, Department of Geology and Geography Note conductivity anomalies A, B, C and D. Initial EM31 survey over the trenches – a combination of profile data into a conductivity map Simple profile – data collected at points with constant intercoil spacing

27. Tom Wilson, Department of Geology and Geography Trench area was re-surveyed about 6 months later. Note reduction in anomaly magnitude from peaks of about 20 mS/m to 14mS/m.

28. Tom Wilson, Department of Geology and Geography Examination of Profile data

29. Tom Wilson, Department of Geology and Geography Line B Extends to pit floor shallow All we’re doing here is plotting the data at their exploration depths. These are not computer derived models. They are “pseudo” cross sections.

30. Tom Wilson, Department of Geology and Geography More Profiles

31. Tom Wilson, Department of Geology and Geography Reduced conductivity due to Injected sodium hydroxide? Something sitting on the pitfloor AMD?

32. Tom Wilson, Department of Geology and Geography Location of modeled profile shown by gold line Modeled Profile

33. Tom Wilson, Department of Geology and Geography 7.5m 15m, 20 meter horizontal 15m, 10 meter vertical 30m EM31 EM34 This line crosses the northeastern trench. Trench

34. Tom Wilson, Department of Geology and Geography The computer will do a lot of this work for you, but you still have to model each sounding, one-by-one. You will learn how to use the computer to model terrain conductivity data next week

35. Tom Wilson, Department of Geology and Geography Back to the San Juan Basin

36. Tom Wilson, Department of Geology and Geography

37. 11m Massive Sand 1.2m Shale

38. Tom Wilson, Department of Geology and Geography Coal Mine Refuse Pile Preston County Coke and Coal Refuse area

39. Tom Wilson, Department of Geology and Geography Acidic drainage from the refuse area Acidic water drains out of the spoil

40. Tom Wilson, Department of Geology and Geography Settling/treatment pond

41. Tom Wilson, Department of Geology and Geography Topical lime treatment

42. Tom Wilson, Department of Geology and Geography Top of refuse pile EM31 magnetometer Magnetic anomaly Terrain conductivity and magnetic surveys

43. Tom Wilson, Department of Geology and Geography

44. Preston County Coal Refuse Area High Conductivity Coal Refuse

45. Tom Wilson, Department of Geology and Geography Sting and Swift resistivity meter

46. Tom Wilson, Department of Geology and Geography Low resistivity = high conductivity High Conductivity

47. Tom Wilson, Department of Geology and Geography A. Can the EM31 detect this AMD zone at more than double the “exploration depth” associated with this instrument when operated in the vertical dipole mode. How many Z’s do we need? Opportunities for Questions

48. Tom Wilson, Department of Geology and Geography How does the setup change for the case shown at left? Modification to the AMD problem High conductivity weighted by small area

49. Tom Wilson, Department of Geology and Geography ….. Review …. How many different conductivity layers will you actually have to consider? What does that equation look like that you’d need to solve? Modification to the AMD problem

50. Tom Wilson, Department of Geology and Geography Some additional perspectives 27.26 13.4 Also known as the Pitfloor

51. Tom Wilson, Department of Geology and Geography Solution of this problem requires a simple extension of the approach we’ve developed. We now have 4 conductivity layers & the equation you need to solve will look like this. In this problem we retain the conductivity of the contaminated regions as  AMD = 100 mmhos/m and add a bedrock with conductivity of  BR = 10 mmhos/m.  S is the conductivity of uncontaminated spoil (4 mmhos/m) Computing z’s for depths of 10, 20, 30, 40, and 50 feet using the EM31 vertical dipole configuration we can easily solve for the contribution of the contamination zone to the overall ground conductivity measured at the surface of the spoil. z R V (z) 0.83 0.53 1.67 0.29 2.5 0.196 3.33 0.149 4.17 0.119 5 0.1

52. Tom Wilson, Department of Geology and Geography Relative contribution of the AMD zone to the overall ground conductivity. EM31 vertical dipole mode In this particular case, the utility goes well beyond the rule of thumb “exploration depth” perhaps down to two times that depth in this particular example.

53. Tom Wilson, Department of Geology and Geography Recommended homework format - write down a sentence explaining what you are going to solve for List given variables associated with the problem Show calculations for additional variables needed to solve the problem. Show calculations used to calculate requested quantity State your result Consider 8.4.xls (see class web page) and errors in equations 8.22, 8.25, 8.32 and related equations. IC pbschap8.xls

54. Tom Wilson, Department of Geology and Geography Next week we’ll try our luck with the computers and begin some computer modeling work. Finish up AMD in-class problem Today and hand in Problems 8.5, 8.6 & 8.7 are due next Thursday. Begin reading the resistivity chapter (Chapter 5) in Berger, Sheehan and Jones The two terrain conductivity paper summaries will be due two weeks from today on Thursday September 15 th.