Characterization of Layered Resistivity Models: Case Histories and Characteristic Curves

1. Environmental and Exploration Geophysics I Resistivity IV Characteristic Curves and Case Histories tom.h.wilson wilson@geo.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV

2. 2 Layer Characteristic Curves were developed for a variety of different models. 3 Layer 4 Layer

3. Characteristic curves and survey responses

4. The effect of variations in depth to a layer of higher resistivity L=AB/2 (Schlumberger Array)

5. The effect of resistivity variations - positive resistivity contrast

6. Three layer curves- varied depth to top of middle layer. Three layer curves illustrate the effect of varied depth to base of middle layer.

7. The Method of Characteristic Curves • The method of Characteristic Curves (Two layer case) • Summary of steps • Set 1=a1 • Construct the ratios a/1 for each spacing. • Guess a depth Z ….

8. In the graph at right, we have the variations in a/1 plotted for three different guesses of Z – the depth to the interface. Plot a/Z ( the Wenner spacing divided by your guess of the depth) versus a/1

9. Recall, that once you have determined k, it is straightforward to compute 2 ? 1 = a (shortest a-spacing)

10. Data acquired with constant a-spacing on a 2D grid. High resistivity stream channel gravels are delineated by the 250 ohm-meter contour

11. Melted areas in permafrost Constant a-spacing

12. Profiling Depth Exploration depth remains constant and the measured variations in ground conductivity provide a view of relative variations in conductivity at the exploration depth Constant Spread Traverse

13. Location of gravel deposits in a clay alluvium Constant a-spacing

14. Tri-potential resistivity method 2a Can you compute the geometrical factors for these various electrode configurations? 3a -6a

15. Questions about the technique? Increase in apparent resistivity measured by the CCPP electrode array is actually an indicator of a low resistivity – perhaps water filled - fracture zone.

16. Case History Resistivity Profiling Surveys on the Hopemont Farm in Terra Alta, WV Survey performed by Eb Werner for Dr. Rauch

17. Survey was conducted for the City of Terra Alta to locate a water well. From Werner and Rauch

18. The tri-potential resistivity response over an air-filled fracture zone. Model data CPCP CPPC CCPP From Werner and Rauch

19. The Terra Alta surveys conducted by Werner and Rauch employed measurements at three different a-spacings - 10ft, 20 ft and 40 ft. • Lines were positioned to cross a photolineament and were from 250 to 500 feet in length. • Readings were made at 10 foot intervals. Things to avoid- Conductive materials buried or in contact with the ground. Buried telephone cables and metallic pipelines, fences, metallic posts and overhead power lines

20. Interpretation approach The apparent resistivity measurements made by Werner and Rauch were interpreted within the context of the tri-potential response predicted by the Carpenter model (see earlier figure). “.. The present problem involves only the confirmation of the existence and exact location of a fracture zone mapped from other information. ” “ … it is only necessary to locate anomalies characteristic of vertical discontinuities …” “ … the graphic plots were inspected visually for those anomaly responses ..” Anomalous areas were plotted on location maps. “Alignments of such anomalies at or near the location of the postulated fracture zone were accepted as confirmation of the existence of the fracture zone.

21. Sounding Coil spacing Midpoint Expanding Spread 40m 20m 10m 3.7m EM34 EM34 EM34 EM31 Surface 5.5m depth Exploration 15m depth Depth 30m depth 60mdepth

22. Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below From Werner and Rauch

23. High resistivity would normally indicate what kind of fracture zone. N S The anomaly around 100 feet is considered to be “data noise” The feature at 320 feet is interpreted to be the “fracture zone” response. Note that this feature is not marked by highs in the CPPC and CPCP measurements Line 1 The 20 foot a-spacing profile reveals a more pronounced fracture zone anomaly at about 320 feet along the profile. The response suggests high reistivity From Werner and Rauch

24. Red dots locate prominent “fracture zone” anomalies observed on all three a-spacings Line 1 The blue line indicates the probable location of a major fracture zone. Given the 10 foot station spacing location of the zone is accurate to no more than ±5 feet Line 6 From Werner and Rauch

25. N S The fracture zone anomaly appears consistently on Line 6 at approximately 125 feet along the profile Line 6 The anomaly broadens as the a-spacing increases because electrodes in the array extend over the anomalous region at greater and greater distances from the array center point. From Werner and Rauch

26. Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below From Werner and Rauch

27. Due October 11th Resistivity Lab Your report should be a minimum of 3-4 pages double spaced (12pnt with 1 inch margins and the length does not include figures). AGAIN - Number your figures and make specific reference to them in the text of your report. Be sure to label important features on those figures when they are mentioned in your text. Use of captions is recommended. Questions?