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

Recall the site context & background What information do we need to incorporate into modeling of the first sounding?

 thicknessdepthInterp There is well control on the southern end of the profile of soundings. Let’s enter this model (edit data) and see what happens

Let’s invert a few times and see what happens  thicknessdepthInterp  thicknessdepthInterp Result of inversion Frohlich’s Model Try and adjust the inverse model to match the well based interpretation.

What else could we do? Assume well depths are correct and fix them and iterate

 thicknessdepth 25 – – – – – – –

Limestone bedrock 7 meter thick gravel Sand with some clay  thicknessdepthInterp

ss1ss5ss4ss3ss2 Limestone bedrock 7 meter thick gravel Sand with some clay

SS1SS2 Limestone Bedrock? Model Based on drill hole 7 An alternative inversion of SS2

SS2 to SS3 Limestone Bedrock?

SS4

SS5 6 layer SS5 3 layer

Soundings one through four: look for lateral consistency.

ss1ss5ss4ss3ss2

Plot your models of resistivity versus depth along the profile. Correlate resistivity horizons from left to right.

1.Develop your own interpretation of the subsurface using the cross section derived from your modeling of soundings SS1 through SS5. Are aquifers present across the area? Explain your results. (refer to your cross section(s)!) 2. Develop a hypothesis 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? Does the range of equivalent answers support the presence of a shallow and/or deep fresh-water aquifer – the precise placement of bedrock? Describe your hypothesis and the range of possible answers you obtained, and discuss how you used the equivalence computations to modify your model.

3. After you complete your cross section, compare your interpretation to Frohlich’s. Make direct comparisons to Frohlich’s result. 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 basement 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?

4. Number and label your figures for easy reference in your lab discussion. Use captions. They don’t have to be lengthy. The caption tells the ready quickly what they are looking at. 5. Organize your discussions in the requested abstract, introduction, results and conclusions format.

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.

You should be reading through the chapter on gravity – especially pages 32 – 72. We will meet again in lab this Thursday to introduce you to the gravity modeling software and basic conceptual ideas concerning the use of variations in the earth’s gravitational field as an aid to subsurface interpretation