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

This should take you up one level to the c:\apps\programs directory. This will execute the resistivity modeling software. Bring up the resistivity modeling program RESIXIP

From INPUT GET - Request the second sounding - SS2

Specify the main file name - reslab

Input your starting model. The important thing about this model is that you at least have the appropriate number of layers. Note that layer thickness is requested - not depth.

?  1 = 23  -m  2 = 32  -m  3 = 23  -m  4 = 81  -m  5 = 49  -m  6 = 58  -m

Look at the sounding and estimate the number of layers along with rough estimates of their resistivity.Note that in the interactive EDIT Model option available under INTERPRET you can easily pick depths and resistivities with your cursor Go To INTERPRET EDIT MODEL

Return to EDIT MODEL and input rough estimates of resistivity and thickness.

Make a forward computation - click on Forward

Invert and edit your model to obtain a good match between calculated and observed resistivity and also of the model to the known geology of the area.

Click on EDIT MODEL to obtained the dashed outline of the derived resistivity model.

select ANALYZE EQUIVALENCE To finish up the resistivity lab you will consider whether alternative - equivalent - models may be more representative of the subsurface geology.

Go ahead and take the defaults

The equivalence parameters are listed as they were in EMIX34. In this case you have resistivities of the 5 layers listed in rows 1-5 followed by the thickness of the 4 layers.

You can view the equivalent models.

… and with luck maybe you can PLOT them.

You can also view the data on the various equivalence models on the screen

Here the resistivities and thickness of all the equivalent models are tabulated.

… You should be able to print the equivalent models by following the printer set-up instructions given to you the other day.

Problem: After you have completed the inversion and achieved a relatively good fit between the calculated and observed apparent- resistivities (on SS1 and another sounding of your choosing) run the ANALYZE EQUIVALENCE operation on those two soundings. We will modify this to be any two soundings of your choosing, i.e. SS2 and one other.So please make a note to yourself. Also ….. Before running equivalence …. Use the FIX option under CALCULATE to FREE and FIX the resistivities and thicknesses or depths of specific zones of interest. Since you are trying to evaluate the reliability of fresh-water acquifer identification, choose a suspected aquifer interval and evaluate the range of possible depths and resistivities for the interval that could give you similar answers. Do the range of equivalent answers all support the presence of the fresh-water aquifer? What is the range of possible depths to the top of the aquifer? Analyze equivalence for one shallow and one deep interval. ……. Use PLOT EQUIVALENCE and PRINT EQUIVALENCE to obtain data sets for interpretation. On page 24 of your lab manual - note the following discussion of the problem for you to address

Pick a zone you feel may be associated with the fresh water gravels. Perhaps the zone highlighted at left…? This zone corresponds to the third resistivity interval. To evaluate the range of equivalent responses associated with this interval free its resistivity and its thickness along with the thickness of the layers above and below. SS2

Free and fix the appropriate parameters ……

INVERT … And then ANALYZE EQUIVALENCE

Accept the default parameters... Examine the resistivities and thickness of freed parameters to determine their effect.

View Equivalence model to see how the range of equivalent solutions might affect your geologic interpretation. Try re-running equivalence but, this time, free the resistivities of the intervals above and below the aquifer.

Discuss the following questions in your lab report Offer your own interpretation of the subsurface cross sections provided by the SMOOTH and individualized resistivity models. Are aquifers present across the area? Explain your results. Do you agree with Frohlich? Where and why, or why not? Discuss and compare the individualized and SMOOTH models. Number and label your figures for easy reference in your lab discussion. What do the equivalent models suggest about the limits of your interpretation? Do the range of equivalent answers all support the presence of shallow and/or deep fresh-water aquifer? What is the range of possible depths to the top of the aquifer? Organize your discussions in an abstract, introduction, results and conclusions format.

Although there is no required page limit for the write-up to the resistivity labs, I would expect that a minimum of 3-4 pages double spaced (12pnt with 1 inch margins) would be required to adequately discuss the background to this exercise along with a discussion of your results and general comparison to those obtained by Frohlich. You should number and caption your figures and make specific reference to them in the text of your report. Be sure to label important features on these figures where referred to in the text.

INTERPRET ESTIMATE SMOOTH MODEL

SHOW SMOOTH MODEL

0.1meters 1meter Locate the tops of the layers derived in the smooth model by points and assign the value of the resistivity in the underlying layer to that point. Then contour the resistivity values. Shallow Gravel? Deep Gravel? Sandy clay? Clay?