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 Lab I

Tom Wilson, Department of Geology and Geography Objectives for the Day Gain familiarity with IX1D resistivity modeling activities (functionality is very similar to that used for the terrain conductivity model study) Get familiar with the geologic background related to the resistivity data we will model in lab to detect fresh-water aquifers resistivity differentiation of glacial drift and pre-glacial channel deposits resistivity changes associated with saline intrusion mask layer boundaries in many cases Qualitative methods used to estimate the number of layers and their resistivities. Well log ground truth

Today’s In-class problem Tom Wilson, Department of Geology and Geography 5 meters  2 =180  -m  1 =26  -m Current electrode Potential electrode P     What is the potential measured at P given current of 0.5 amps.

Pages 25 through 29 of today’s handout Tom Wilson, Department of Geology and Geography Copy over the folder IX1D-Res Bring up IX1D

Example data from a Wenner sounding? Tom Wilson, Department of Geology and Geography See interactive Excel file at Remember how to compute  a ?

How does the resistivity vary as a function of depth? Make a guess and see how you do. Tom Wilson, Department of Geology and Geography Change this to spacing instead of effective penetration

Also have a look at this xls file Tom Wilson, Department of Geology and Geography Linked on last Thursday’s pages at

Let’s take a look at some of the resistivity data from the Missouri area Bring up IX1D. Tom Wilson, Department of Geology and Geography Copy over IX1D-Res folder from the common drive and open sounding SS1. You should have something that looks like that below. Relabel the data display (y axis ) and model display (x axis) to show Apparent Resistivity (ohm-m) and Resistivity (ohm-m) respectively. Resistivity sounding SS1

Think about what you are seeing. How does the apparent resistivity vary as a function of depth? Tom Wilson, Department of Geology and Geography What can we infer from these variations in apparent resistivity about subsurface resistivity layering?

Don’t forget to skim through Frohlich’s paper (on last Thursday’s class page topic links) Tom Wilson, Department of Geology and Geography Some background information about the resistivity lab Refer to Frohlich and part 1 of the resistivity computer lab

Farmland – gently rolling topography Tom Wilson, Department of Geology and Geography

Note Drill Hole Locations along the profile line at left and below

Glacial outwash overlying pre-glacial channels that sit on dolomitic limestone bedrock Tom Wilson, Department of Geology and Geography Shallow and deeper aquifers

The bedrock is also a source of water, but has high dissolved ion concentration Tom Wilson, Department of Geology and Geography Bedrock aquifers, high dissolved solids and lower resistivity. This lowered resistivity makes it difficult to “see” the boundary in terms of resistivity contrast

Tom Wilson, Department of Geology and Geography Modeling results suggest this range may be a little more variable and extend from ~ 50 to 135  -m Limestone bedrock generally appears to have higher resistivity – up to about 60  -m

Tom Wilson, Department of Geology and Geography ss1ss5ss4ss3ss2 Look over discussion of this section on pages 347 & 348 of Frohlich’s paper 10m 100m L/2 where L is the current electrode spacing

Back to SS1 – developing models for more complicated soundings Tom Wilson, Department of Geology and Geography We will develop a qualitative interpretation of this sounding using “inflection point” and “extrapolation” rules. A. Where are the inflection points? This is a Schlumberger array and the x axis is usually labeled in terms of AB/2 or l OR L/2 Increasing depth

Tom Wilson, Department of Geology and Geography B. How many layers are there and C. What are their depths? Inflection points

Tom Wilson, Department of Geology and Geography This is a Schlumberger sounding. AB/2 is one-half the distance between the source and sink electrodes. A general rule of thumb to estimate resistivity boundary depth is that depth is about 1/2 the AB/2 distance. Note that AB/2 is l in some representations. ?

Tom Wilson, Department of Geology and Geography D. What are the resistivities of these layers?

The rising and falling apparent resistivity trends provide insights into relative differences of layer resistivity Tom Wilson, Department of Geology and Geography ?  1 = 23  -m  2 = 32  -m  3 = 23  -m  4 = 81  -m  5 = 49  -m  6 = 58  -m

Tom Wilson, Department of Geology and Geography A table of our estimates derived from inflection point and extrapolation approaches to interpretation.

Tom Wilson, Department of Geology and Geography From EDIT MODEL, input your starting guess for the resistivity and thickness for each layer.

How well did you do on your guess? Tom Wilson, Department of Geology and Geography The computed resistivity variations associated with our guess are shown as the solid line for comparison to the actual observations of apparent resistivity shown by the violet squares. The resistivity versus depth model is shown at right. Click the forward button

Tom Wilson, Department of Geology and Geography After multiple inversions you’ll obtain a resistivity model similar to that shown at right above. This particular model has about 4% error.

Shallow layer influences Tom Wilson, Department of Geology and Geography The influence of a shallow layer can have significant impact on several readings! Drag to higher 

The 30cm shallow layer distorts observations out to 10 meters Tom Wilson, Department of Geology and Geography Crank it back down

Model on data view Tom Wilson, Department of Geology and Geography We can also show the model on the data. Model on data

Tom Wilson, Department of Geology and Geography The derived model parameters can be viewed in the Edit Model window

Tom Wilson, Department of Geology and Geography If you click a data point on your data graph, its location and value will appear in a message box. Notice how the 5 th data point just doesn’t look right. It may be a bad data point and we should probably exclude it from the calculations.

Tom Wilson, Department of Geology and Geography We can do this through Edit Data. In the Edit Data window, turn on the mask option for point 5.

Tom Wilson, Department of Geology and Geography The masked data point is represented by an X.

Tom Wilson, Department of Geology and Geography With more iterations from the Edit Model window, our error can be further reduced. Note how the upper layers continue to shallow-out with increased numbers of iterations.

Tom Wilson, Department of Geology and Geography In this lab you’ll be carrying out the inversion process for all 5 soundings and putting together a geologic interpretation of the distribution of strata across this glacial channel.

Tom Wilson, Department of Geology and Geography Where are the shallow and the deep gravel aquifers?

Tom Wilson, Department of Geology and Geography Save your model under another name. You will want to retain derived model parameters to construct a cross section across this glacial channel. In the next lab, you may also want to experiment with equivalent models to help evaluate stratigraphic correlations between adjacent soundings.

Tom Wilson, Department of Geology and Geography What’s wrong with this model?

Tom Wilson, Department of Geology and Geography We haven’t incorporated what we know about the local geology (the borehole data) at a control point along the profile.

Tom Wilson, Department of Geology and Geography Start developing models for soundings SS2 through SS5. Look at the section described in Frohlich’s Figure 3 near sounding 7 (page 344). How could you test out Frohlich’s interpretation at SS1? Give this a try for Thursday. Make sure borehole control is incorporated in the models we develop using the computer Revisit Frohlich’s paper for additional context on the lab problem

Tom Wilson, Department of Geology and Geography Terrain Conductivity labs are due today Hand in the “in-class” problems today Problems due next Thursday. Don’t forget there’s a mid term exam on Thursday, October 2 nd (test review on the Sept. 30 th ). Looking ahead