1. Geology 5660/6660 Applied Geophysics 11 Apr 2014 © A.R. Lowry 2014 For Mon 14 Apr: Burger 302-340 (§5.5-5.11) Last Time: DC Electrical Resistivity Modeling One-Dimensional Sounding :  Layer-over-halfspace: Wenner  app ~   for a/z small; ~  2 for a/z large  Two layers over halfspace: May or may not pick up sandwiched layer depending on thickness, contrast  Resist does multiple layers, 1D, log a -spacing steps Profiling : Simple variations (e.g. vertical contact) have complicated profiles that depend on where resistivity contrasts occur relative to electrodes Sensitivity kernels indicate positive resistivity anomalies increase  app in most regions but decrease  app if between a current and voltage electrode!

2. (This is probably an erroneous measurement…) Saturday’s measurements of gravity on the West Cache fault:

3. “Sensitivity kernels” for apparent resistivity (assumes a homogeneous Earth model). This is a measure of how much a perturbation of resistivity at a given point in the Earth can be expected to change the measurement of apparent resistivity of the array (reds positive, blue-greens negative, yellow = zero).

4. Resistivity “ Pseudosection ”: Measurements are a combination of profiling (different x ) & sounding (different a -spacings) Plot / contour  app versus distance x and a -spacing increment n, where n = a/a 0 and a 0 is smallest a -spacing

5. (The image I showed on Wednesday is an example of an apparent resistivity pseudo-section):

6. Clear from the earlier modeling of Wenner apparent resistivity across a vertical contact that this approach will give a VERY approximate image with potential for a lot of “false” anomalies…

7. True inversion of the data will yield a much more reliable image (for much the same reasons that depth migration does in the seismic case!)

8. Resistivity pseudosection from the western trench location for the East Cache Valley fault zone near Paradise…

9. Resistivity pseudosection from the western trench location for the East Cache Valley fault zone near Paradise…

10. Used resistivity sounding & two layer model: Vertical sounding at 25.7 m. Maroon line is measured; blue line modeled using Excel spreadsheet based on Burger’s Table 5-4. Modeling with Resist (with interpolated a -spacing) differs in resulting estimates of thickness, but the differences are small.  1 = 101  m, h = 3.22 m  2 = 16.7  m RMS Misfit 1.897  m

11. Can model equally well by using constant thickness and varying the resistivities, or using constant resistivities & varying the thickness… Because these data did not go to a small enough a -spacing to nail down  in the upper layer.  1 = 101  m, h = 1.72 m  2 = 16.7  m RMS Misfit 0.847  m

12.  1 = 100  2 = 17 Layer-over-halfspace modeling suggested a variable- thickness upper layer with  ~ 100  m (Bonneville deposits) over  = 17 (Great Salt Lake fm). The data imply the trenched geomorphology represents a beach burm deposit.

13. Little Mountain DC Resistivity Profile…

14. The inverted model of resistivity structure…

15. Unsmoothed (top) and smoothed (bottom) inversion images are shown left. The differences in data misfit are relatively small despite the differences in resulting resistivity structure, so these are about equally reasonable. The near-surface, small-scale variations in the first model are likely more realistic than the smoothed 2 nd image, but either is possible at depth… Which tells us that the deep resistivity structure is poorly determined by our data. Grey line approximates a 15  -m contour used to define a polygon for gravity modeling in GravMag.

16. The best-fit gravity model corresponding to the polygon from the first (more variable) resistivity model is promising: It reduces the RMS gravity residual from 0.4855 (no model) to 0.4188 mGal and hints at an explanation for the hillside gravity variations that were not matched by earlier modeling of end- member polygons. However the polygon has an unlikely high  = 1600 kg/m3, and the center-of-mass of the polygon is offset to the east of the center-of-mass implied by the gravity anomaly. RMS = 0.4188 mGal  = 1600 kg m -3

17. The best-fit gravity model for the polygon from the second (smoother) resistivity model has slightly lower RMS gravity residual (0.4046 mGal) and a more believable  = 900 kg/m 3 (near the range that might be seen, e.g., for a sulfide-rich ore body in high-porosity limestone). But center-of-mass of the polygon remains offset to the east of the center-of-mass implied by the gravity anomaly. RMS = 0.4046 mGal  = 900 kg m -3

18. Just for fun, here’s a gravity model assuming just such a steeply west-dipping polygonal body, with surface contact corresponding to the low- resistivity body from our DC resistivity data inversion. RMS = 0.2053 mGal  = 760 kg m -3

19. East Cache fault in Green Canyon Pseudosection and inversion can be very similar though…

21. Lithology application: Sand/clay in a study of agricultural groundwater contaminant plumes.

22. Mapping an intrusive dike feature…

23. Mapping cave locations in karst terrain.