Subsurface Imaging for Archaeology - Spring 2009
Midterm Take Home Exam - Due 3/24/2009, 2:00 PM

You can email me color reports when you are done.

Part I. The file ‘arch_exam_Wenner.DAT’ contains electrical resistivity data simulated for an archaeological investigation. I want you to use RES2DINV to interpret, via inversion, those data.

Steps for RES2DINV

• Start program (install demo at home from: www.geoelectrical.com), or work in CHC 11 when no class is present. The entry code is 82497.
• File/read data file - note details
• Inversion/Least squares inversion yields a result.
  
  

Figure 1. Display of the exam data in RES2DINV.

Part II. Use RES2DMOD to create a subsurface model (Figure 2) whose forward response closely matches the measured results (arch_exam_Wenner.DAT). This step will take some fiddling, start with init_block.mod (unzip it - UM server won't provide .mod files) and modify it slowly until you get a satisfactory response. This should not take 'forever', if you are really spinning your wheels, let me know. What you are doing is letting your inversion results guide your development of a subsurface model which yields results similar to the data. You arrive at a cross section image, like the bottom half of Figure 2, of the archaeological target.

Steps for RES2DMOD

• Start program (install demo at home from (www.geoelectrical.com)), or work in CHC 11 when no class is present. The entry code is 82497.
• File/read file with forward model
• Model Computation/calculate the apparent resistivity values
• Edit/edit model – log contours
• left click in the model, cells turn white, click on a different resistivity,
  • Model Computation/calculate apparent resistivities
  • Edit/Edit model - shows the current result, compare to data
  • Continue in this loop until your model response is similar to ‘arch_exam_Wenner.DAT’; you’ll never get it exact and remember there might be some experimental or ground noise to explain.
    
    
    
    Figure 2. RES2DMOD of a foundation wall system. The upper window
    shows the forward calculation from the resistivity model in the lower
    window. The lower image is a 2D cross section.
    

When you have a satisfactory match of forward model results and observations, write a professional report which includes any figures you think are necessary. Remember, you can screen capture images from the programs with ALT-Print_Screen and then paste them into a word.doc. Provide a short, accurate explanation of the steps you took and why, along with the results. I want to see that you understand what you are doing, not just clicking. Write it such that your answer is understandable to an archaeologist who has not taken any geophysics. Remember, I am going to assess your thinking and explanations. Ultimately, provide an archaeologically reasonable explanation for the result. About two (certainly less than 5) carefully crafted pages, not including figures, should cover it.


Part III. Assume that the resistivity model from Part II is located in the Missoula area. Use PBLOCK.EXE to calculate a forward magnetic anomaly over one of the sources in your electrical model. Write this up as above, but focus on experimental design, like you did for the gravity anomaly earlier in the semester. Make sure you explain what assumptions you made and what parameters (declination, inclination, intensity, susceptibility, etc.) you set and why. I want to see your reasoning and understanding. Our magnetometers easily have a resolution of +/- 1 nT so use that in your discussion of experimental design and expected results. State and defend whether or not magnetics would be a good discovery technique for this case.

As an alternative to PBLOCK, if you like to tinker with software, there is a DOS-based magnetic anomaly program on my web site. MAGCAD will provide much more control than PBLOCK but is more cantankerous and works best in Windows XP or earlier.

Figure 3. Forward calculation, using PBLOCK, of a subsurface
prism with positive susceptibility contrast, north is to the right.

Part IV. Find and read a paper from the professional literature that is an application of magnetic exploration in something (archaeology, environmental geology, geological structure, etc.) you are interested in. Write a one-page (+/-) synopsis of the paper using this as a rough outline:

• Problem
• Method
• Experimental design
• Results
• Conclusions
• Your thoughts.