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 IV

Tom Wilson, Department of Geology and Geography Tabulating data always helpful Structuring your presentation helps organize your thoughts and also makes it very easy to follow Brief discussion of Terrain Conductivity Lab report

Tom Wilson, Department of Geology and Geography Cross section view

Tom Wilson, Department of Geology and Geography Inverse Model

Tom Wilson, Department of Geology and Geography Inverse model

Tom Wilson, Department of Geology and Geography Comparison

Tom Wilson, Department of Geology and Geography Equivalent Solutions

Tom Wilson, Department of Geology and Geography Data used to create the problem

Tom Wilson, Department of Geology and Geography Not used as often because of recent computer and hardware developments AGI’s Sting and Swift

Tom Wilson, Department of Geology and Geography High or low resistivity zones depending on the concentration of dissolved electrolytes

Tom Wilson, Department of Geology and Geography A simple 4-electrode system offers an alternative approach to fracture zone location – the Tri-potential resistivity method Switching current electrode positions in the Wenner array

Tom Wilson, Department of Geology and Geography Normal Wenner array configuration CPPC A conductive fracture zone would likely be one that was water filled High conductivity = low resistivity What is the geometrical factor?

Tom Wilson, Department of Geology and Geography CPCP The CPPC and CPCP electrode configurations both reveal the presence of a low resistivity zone What is the geometrical factor?

Tom Wilson, Department of Geology and Geography CCPP The CCPP electrode arrangement reveals the opposite response What is the geometrical factor?

Tom Wilson, Department of Geology and Geography Tripotential resistivity measurements help establish the association of a topographic lineament with a possible fracture zone The work of Dr. Rauch and some of his students

Tom Wilson, Department of Geology and Geography Good Devonian shale wells are located near fracture zones Dr. Rauch and students

Tom Wilson, Department of Geology and Geography CCPP The fracture zone response Is this a wet or dry fracture zone? Dr. Rauch and students

Tom Wilson, Department of Geology and Geography

Wet or dry?

Tom Wilson, Department of Geology and Geography Once the approach is validated, 3D coverage helps resolve the vertical and horizontal extents of contamination 2 meter a-spacing reveals the upper tip of the conta- minant plume 16 meter a-spacing reveals the base of the contaminant plume

Tom Wilson, Department of Geology and Geography Recall apparent resistivities (  a ) you computed in last week’s exercise. Where G = 2  a for the Wenner array

Tom Wilson, Department of Geology and Geography The procedures for doing this are fairly straight-forward 1.Set  1 =  a1 2.Construct the ratios  a /  1 for each spacing. 3.Guess a depth Z …. We’ve talked a little about the analytical method referred to as the method of characteristic curves

Tom Wilson, Department of Geology and Geography Summary of steps Set  1=  a1 Calculate the ratios  a/  1 for each spacing. Guess a depth Z (depth to second layer). Make three guesses. Compute the ratio a/Z Plot  a/  1 vs. a/Z on the characteristic curves (right) Select best guess based on the goodness of fit to the characteristic curves. Select k (the reflection coefficient) based on the “best fit” line. Compute  2, using relationship between k and  ‘s Characteristic curves

Tom Wilson, Department of Geology and Geography Plot a/Z ( the Wenner spacing divided by your guess of the depth) versus  a /  1 In the graph at right, we have the variations in  a /  1 plotted for three different guesses of Z – the depth to the interface. Estimating the depth and the resistivity of layer 2

Tom Wilson, Department of Geology and Geography Recall, that once you have determined k, it is straightforward to compute  2  1 =  a (shortest a-spacing) Hand in next Thursday

Tom Wilson, Department of Geology and Geography The Inflection Point Depth Estimation Procedure This technique suggests that the depths to various boundaries are related to inflection points in the apparent resistivity measurements. Again, the In-Class data set illustrates the utility of this approach. Apparent resistivities plotted below are shown over the model for both the Schlumberger and Wenner arrays. The inflection points are located, and dropped to the spacing-axis. The technique is suggested too be most applicable for use with the Schlumberger array. The inflection point rule varies with array type. For the Wenner array, the approximate depth to the interface (for the example below) is estimated to be 1/2 the inflection point value of a at which it occurs. For the Schlumberger array the AB/2 location of the inflection point is often divided by 3 to estimate the depth. In the case of the Wenner array we would get a depth of about 9 meters. In the case of the Schlumberger array – a depth of about 8 feet. Review Slide

Tom Wilson, Department of Geology and Geography Resistivity determination through extrapolation This technique suggests that the actual resistivity of a layer can be estimated by extrapolating the trend of apparent resistivity variations toward some asymptote, as shown in the figure below. The problem with this is being able to correctly guess where the plateau or asymptote actually is. Spacings in the In-Class data set only go out to 50 meters. The model data set (below) used for the inflection point discussion reveals that this asymptote is reached only gradually, in this case at distances of 500 meters and greater. Since most of the layers affecting the apparent resistivity in our surveys will be associated with thin layers, we are unlikely to be able to do this very accurately. The apparent resistivity will vary considerably over that distance rather than rise gradually to resistivities of individual layers. At best the technique offers only a crude estimate. Review Slide

Tom Wilson, Department of Geology and Geography Here are some plots of our synthetic or “test” data set. The model from which it is derived is shown at lower right. In this inverse model, the base of the layer appears to be located at a depth of about 7.5 meters

Tom Wilson, Department of Geology and Geography Equivalence - non-uniqueness... I know – you’re tired of hearing about equivalent solutions! BUT…

Tom Wilson, Department of Geology and Geography We used this simple in-class data set to introduce you to the resistivity modeling tools in IX1D

Tom Wilson, Department of Geology and Geography Plot a/Z ( the Wenner spacing divided by your guess of the depth) versus  a /  1 Given the equivalence of solutions illustrated in class an in the preceding slides you already know about what the answer to this question should be. Make your guesses for Z, plot your values of  a/  1 versus a/Z and see what you get. Turn in next Thursday.

Tom Wilson, Department of Geology and Geography Equivalent solutions can often be limited using information about the local geology as constraints!

Tom Wilson, Department of Geology and Geography

How well did Frohlich do? Let’s put his models to the test. 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.

Tom Wilson, Department of Geology and Geography Frohlich’s SS2 solution

Tom Wilson, Department of Geology and Geography Not so good – perhaps there is a typo on his figure. Maybe the 75 should be a 15

Tom Wilson, Department of Geology and Geography

Test next Tuesday, October 6th Work through the characteristic curve exercise and hand that in on the 8 th We will transition into Gravity during the week of the 13 th and 15 th so begin reading Chapter 6. We will conclude the resistivity lab exercise next Thursday October 8 th Resistivity Lab report will be due October 15 th Resistivity paper summaries will be due October 20 th

Tom Wilson, Department of Geology and Geography