Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Resistivity V

Lab and pretest review Tom Wilson, Department of Geology and Geography Post-test review/questions We’ll go through the development of a solution to one of the resistivity soundings from start to finish Pose a hypothesis and test it using equivalent solutions

Mid term point allocations Tom Wilson, Department of Geology and Geography Note that homework grades are posted along with current grade sheets listing percentages earned for different activities Recall that the regular section grade is based on 32% homework, 28% for the lab report, 5% for the paper summary, 5% class participation, and 10% for the midterm exam. At this point, your current grade is based on a total of 80%. Recall the final exam will count 20% of your total semester grade. In the writing section the percentages are the same accept that the mid term only counts 5% and the writing assignments 10%. The current grade is computed out of a total possible of 85%. Recall that the final exam in the writing section counts 15% of your total semester grade.

Lab report and paper summary Due Dates Tom Wilson, Department of Geology and Geography Final draft writing section essay 1 is due today October 7 th The resistivity paper summaries will be due this Thursday October 9 th The resistivity lab will be due next week, Thursday October 16 th.

Lab review opportunity and equivalence computations Tom Wilson, Department of Geology and Geography

The following slides are mainly a review of lab procedures discussed previously Tom Wilson, Department of Geology and Geography Inflection point location  l spacing Spacing/2  Depth Mark the inflection points

There’s some hint for the presence of that second high resistivity layer (we’ve seen it before). Tom Wilson, Department of Geology and Geography A small decrease in slope This little bump is suggestive of a high resistivity layer We’ll see what happens to it when we do an inversion

Enter your guess Tom Wilson, Department of Geology and Geography

Not a great fit, but we have the ups and downs in about the right locations Tom Wilson, Department of Geology and Geography

Mask the bad data point and iterate once Tom Wilson, Department of Geology and Geography

Now we’ve got something we can work with. Make a preliminary interpretation Tom Wilson, Department of Geology and Geography Shallow fresh- water gravel Shallow clays A much thicker fresh water gravel sand bedrock

Pose a hypothesis Tom Wilson, Department of Geology and Geography Shallow fresh- water gravel Shallow clays Another fresh water gravel sand bedrock

For example: could that high resistivity layer extending from 2 to 20 meters be a deeper, lower resistivity drift gravel? Tom Wilson, Department of Geology and Geography Shallow fresh- water gravel Shallow clays Near-surface fresh water gravel? Sand/silt bedrock

If you need to introduce some additional error to allow you one more iteration before doing the equivalence computations you can pull a couple layers off a bit Tom Wilson, Department of Geology and Geography

To test this interpretation we fix the parameters in the model as shown below. Tom Wilson, Department of Geology and Geography Run one more inversion. Then, calculate the equivalent solutions.

After an additional inversion you should still have 1.5 to 3% error Tom Wilson, Department of Geology and Geography Area of inquiry

Equivalent solutions in graphical and table form Tom Wilson, Department of Geology and Geography Variation in thickness Variation in resistivity

Crosscheck results with Frohlich’s histogram. Tom Wilson, Department of Geology and Geography

What do the model computations indicate about the possibility that this could be a deeper less resistive gravel aquifer? Tom Wilson, Department of Geology and Geography The resistivity can be considerably less, but it remains higher than that reported for the deeper gravel aquifers (40 to 50+  -m).

For SS3 Tom Wilson, Department of Geology and Geography You can make a reasonable argument that this interval is most likely one of the more resistive fresher water aquifers encountered in the region. It is unlikely to be a deeper less resistive aquifer Fresh, higher resistivity, gravel aquifer?

What other hypotheses could you form? Tom Wilson, Department of Geology and Geography What do the equivalent models suggest about the resistivity and depth of this bottom layer? Where is bedrock?

Getting comfortable plotting your data Tom Wilson, Department of Geology and Geography Recall that this can be modeled as a shallow high resistivity gravel aquifer. Consistent with well 7?

Cross section paper is available from the class web page Tom Wilson, Department of Geology and Geography Check the link on the class web page for more graph paper or go directly to Remember, the depth scale on the cross section you construct for this lab must be linear.

Use nearby borehole data Tom Wilson, Department of Geology and Geography Limestone bedrock at ~ depth of 32 meters ~7 meter thick gravel Sand with some clay ft There’s only one well that coincides with the soundings you are working with and that is Well 7 near sounding SS1

Remember our earlier test of Frohlich’s model for SS1 between two boreholes. Tom Wilson, Department of Geology and Geography  thicknessdepthInterp We went through an inversion of the data to get a better estimate of the resistivity of the last layer and then.. Above is the model Frohlich developed. The check at right shows that he was certainly in the ball park. However the match is a little off as we discussed earlier.

A result for SS4 Tom Wilson, Department of Geology and Geography

Work your way across the line Tom Wilson, Department of Geology and Geography

A look at SS5 Tom Wilson, Department of Geology and Geography How well did Frohlich do on this one? SS5  a depth

Tom Wilson, Department of Geology and Geography Computations based on Frohlich’s model don’t provide high enough apparent resistivities. Equivalent models suggest that there is little doubt that the high resistivity layer has all the earmarks of a shallow fresh water gravel. All the possible solutions here would point to the presence of a very thick and very fresh water aquifer

Tom Wilson, Department of Geology and Geography 1.Construct a cross section derived from your modeling of soundings SS1 through SS5. (5 points) 2.Develop and describe your interpretation of the subsurface. Use the cross section to illustrate your interpretation. Are aquifers present across the area? Explain your results. (6 points) 3.Develop two hypotheses to be tested using the equivalence computations. What do the equivalent models suggest about the viability of your hypothesis – about the limits of your interpretation? For example, does the range of equivalent answers support the presence of a shallow and/or deep fresh-water aquifer? What did you find about the possible presence of bedrock and the range in depth to the bedrock/channel interface? (2 points)

Incorporate discussion of equivalent solutions into discussion of your interpretation Tom Wilson, Department of Geology and Geography 4. Describe your hypotheses and the range of possible answers you obtained. Show equivalent models generated by IX1D and discuss how the results of equivalence analysis could give rise to more than one possible subsurface interpretation. Refer to labeled figures. Remember figures should have captions. (2 points).

How does your interpretation compare to Frohlichs? Tom Wilson, Department of Geology and Geography ? ss1ss2 ss3 ss4ss5 Bedrock?

Tom Wilson, Department of Geology and Geography 5. After you complete your cross section, compare your interpretation to Frohlich’s (i.e. discuss in your lab report). Make direct comparisons to Frohlich’s result (you do NOT need to show comparative models, just comment on how your results compare to those of Frohlich’s in his Figure 7). Have you been able to better define the extent of basal gravel aquifers than Frohlich did in his cross section (see Figure 7 of his paper) Have you been able to more accurately define the boundary between bedrock and overlying, less resistive, intervals? If the resistivity increases at the base of your model did you feel confident that you could tell the difference between gravel and limestone bedrock? (counts for about 2 points as part of general discussion which counts as 5 points)

Tom Wilson, Department of Geology and Geography 6. Number and caption your figures for easy reference in your lab discussion. The captions don’t have to be lengthy, but should succinctly inform the reader what they are looking at. 7. Organize your discussions in the requested abstract, introduction, results and conclusions format. (Roughly 8 points out of 30) Basic structure (Abstract, Intro & Background ….)3 Presentation of results, describe overall interpretation (basic model)6 Develop two hypotheses2 Discussion of equivalent tests2 Geologic cross section5 General discussion5 Improvements on Frohlich?2 Figures with captions5 30

Tom Wilson, Department of Geology and Geography 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.

Should we work through a sounding from scratch? Tom Wilson, Department of Geology and Geography

For next time Tom Wilson, Department of Geology and Geography Final questions regarding the resistivity lab … Remember Tuesday is fall break and there is no class This Thursday we will begin discussion of gravity methods (start reading chapter 6!)

Lab report and paper summary Due Dates Tom Wilson, Department of Geology and Geography Final draft writing section essay 1 is due today October 7 th The resistivity paper summaries will be due this Thursday October 9 th The resistivity lab will be due next week, Thursday October 16 th.