Environmental and Exploration Geophysics I

1. Environmental and Exploration Geophysics I Resistivity III tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Tom Wilson, Department of Geology and Geography

2. Mid Term Mid term grades are due on October 9th – end of next week. Mid term seems a little early! We will have a test next Tuesday The test will include questions on materials covered in class through this coming Thursday. We’ll spend about half the class on review this Thursday, so bring your questions! Tom Wilson, Department of Geology and Geography

3. Computing apparent resistivity for the two-layer problem Bring up the Excel file linked to Reflection Coefficient on class web page. Tom Wilson, Department of Geology and Geography

4. What will the response look like if 1 is 10 -m and 2 is 30 -m and the interface is located at a depth of 10m New rule of thumb? For the Wenner array use inflection point location divided by 2 ≈ depth Tom Wilson, Department of Geology and Geography

5. Review of general ideas about potential field and current distributions as discussed in the text We handled things similarly in class but with a little less algebra Tom Wilson, Department of Geology and Geography

6. The calculations yield a pattern of semicircular to elliptically shaped lines of equipotential Tom Wilson, Department of Geology and Geography

7. Bring up the Excel file linked to Potential Differences and calculate vertical potential profile at x =-2 and x=2 along the surface Tom Wilson, Department of Geology and Geography

8. From the text we are given the fraction of the total current flowing above depth z - z is depth and d is current electrode spacing Tom Wilson, Department of Geology and Geography

9. Look at the worksheet Current in the Excel file Tom Wilson, Department of Geology and Geography

10. Subsurface current flow distribution d=3a Tom Wilson, Department of Geology and Geography

11. Wenner Array Current Fraction depth/a or z/a 3a In the preceding diagram z was depth. In this diagram z is depth divided by a which is 1/3 rd the current electrode separation. Remember the a-spacing refers to the electrode spacing in the Wenner Array. Tom Wilson, Department of Geology and Geography

12. Change in potential as a function of depth Remember the terrain conductivity relative response functions? 12.5 % of the change in potential occurs between a depth of z=2/3rs d to d. d=a For the Wenner, array d is the source-to-potential electrode distance - or a. Note the similarity of this “sensitivity” curve to the relative response function (V(z)) used with terrain conductivity data. You can also think of this curve as indicating the contribution of intervals at various depths to the potential between one current and one potential electrode. Tom Wilson, Department of Geology and Geography

13. Measured potential is a weighted responses At each electrode (M and N we have a weighted response to the subsurface resistivity distribution. The combined potential is the potential difference between M and N z/a or multiples of a We see that in general for the Wenner array the peak sensitivity of the array to subsurface resistivity distributions occurs at depths approximately equal to the a-spacing. Tom Wilson, Department of Geology and Geography

14. By comparison to the characterization of instrument response as a function of depth and intercoil spacing for the terrain conductivity method, the resistivity relationships are defined much more qualitatively. Mostly we have general rules of thumb. Tom Wilson, Department of Geology and Geography

15. Qualitative interpretation of a resistivity sounding - How many layers have been sensed in this resistivity sounding? The observations consist of apparent resistivities recorded at various a-spacings (Wenner array) or l-spacings (Schlumberger array). See empirical methods on pages 95 and 296 of Berger. Tom Wilson, Department of Geology and Geography

16. Let’s examine the utility of the interpolation approach using the in-class data set you worked up last week. Recall how to determine apparent resistivity? Tom Wilson, Department of Geology and Geography

17. Name: ________________________ Also in the potential differences Excel file linked to the class page… You’ve already worked through these, but the calculation sheet is here for your reference Tom Wilson, Department of Geology and Geography

18. Back to the Resistivity Lab > Recall the site context & background What information do we need to incorporate into modeling of the first sounding? Tom Wilson, Department of Geology and Geography

19. Well bore observations are integrated into a geologic cross section along this profile Tom Wilson, Department of Geology and Geography

20. Incorporating wellbore observations as a constraint on the model developed from the resistivity sounding SS1 Let’s enter this model (edit data) and see what happens There is well control on the southern end of the profile of soundings. Examination of the depths yields the following model. Tom Wilson, Department of Geology and Geography

21. Let’s invert a few times and see what happens Result of inversion Frohlich’s Model Try and adjust the inverse model to match the well based interpretation. Tom Wilson, Department of Geology and Geography

22. Assume well depths are correct and fix them, then iterate What else could we do? ft Tom Wilson, Department of Geology and Geography

23. compare Tom Wilson, Department of Geology and Geography

24. 7 meter thick gravel Sand with some clay Limestone bedrock ft Tom Wilson, Department of Geology and Geography

25. Use the resistivities provided by Frohlich as a guide to your interpretation of models derived from other soundings. Tom Wilson, Department of Geology and Geography

26. Combine into single lower resistivity layer – the gravel aquifer? Remember Frohlich could not pick bedrock on SS2. Limestone Bedrock? On SS2 place an additional layer in the model (6 or 7 instead of 5) SS1 An alternative inversion of SS2 SS2 Tom Wilson, Department of Geology and Geography

27. Examine equivalence Clay zones and perhaps 2 fresh water gravels, but the equivalent models show what we might anticipate for a deep boundary with low resistivity contrast. Tom Wilson, Department of Geology and Geography

28. Limestone Bedrock? Could bedrock be inferred from the models? ~ 70 meters to bedrock ? 80m SS2 to SS3 Tom Wilson, Department of Geology and Geography

29. We will discuss the details of the lab write up next Thursday (the 8th), but obviously you will want to discuss how your models compare with those derived by Frohlich. ss1 ss2 ss3 ss4 ss5 Tom Wilson, Department of Geology and Geography

30. Display your results on linear scale graph paper ft Manual adjustments with iteration yields an equivalent model (dashed line). The high resistivity zone between 10 and 48 meter depths has a resistivity similar to that of the basal stream gravels. Tom Wilson, Department of Geology and Geography

31. In the remainder of the class, continue to develop your resistivity models Tom Wilson, Department of Geology and Geography

32. Next Class • Keep working on the resistivity lab. We will take plenty of time to work through this and address questions as we go. • Bring your questions about the test to class on Thursday for a short review session. • Target date for turning in the resistivity lab is Oct. 13th Tom Wilson, Department of Geology and Geography