1. Electrical Resistivity Survey of Yosemite Valley, CA
Christie Carey
Trevor Price
Spencer Whitman
Jade Bourdeau-Hernikl

2. Outline
Introduction
Field Methods
Data Analysis
Results
Conclusions

3. Study Details Location: Yosemite Valley, CA, USA Objective:
Assess stratigraphy of valley sediments using electrical resistivity
Timeframe:
March 19-21, 2017

4. Equipment L&R MiniRes Resolution - .1 milliOhm Frequency - 2.5 Hertz
Voltage Volts
Current: 2.0 and 20 milliAmps
Other Equipment
Electrodes, Shielded Copper Cable,
Tape measure
Resolution= smallest part of the scale that can be shown… so the resolution on the mini res can measure down to .1 milli ohms.

5. Field Methods 15 Vertical Electrical Soundings (VES) Wenner Array
10 A-spacings from m per sounding
Effective depth ~ 16 m for 31.6 m spacing
Current on outside, receivers on inside
Recordings read directly in the center of the array.
Effective depth – zone which constitutes ~ 50%
of electrical signal

6. Working Hypothesis
Geologic model from Newcomb and Fogg (2011).

7. Data Inversion
RESIST v 1.0.9*1
Determine most likely combination of model parameters to produce observed data
Layer Thickness
Layer Resistivity
# of Layers
Many models give similar fits between observed and model predicted apparent resistivity
*1(Burger et al., 2006)

8. Hypothesis Testing Assume bedrock underlying infinite halfspace layer
Increase thickness of original halfspace layer
Until RMS = 1.1 * Original RMS

9. Data Synthesis Combine multiple profiles
Cross sections show spatial patterns

10. Yosemite Valley VES Stations

11. Stations 2, 1, 3, 4 N S Upper layers may reflect:
Soil Zone
Water Table
Lower layers may reflect:
Minimum thickness of intermediate layers

12. Stations 9, 8, 7, 10, 11 N S General Pattern:
Higher Resistivity at ground surface
Lower Resistivity depth (water- saturated?)
Lowest layer assumed to be resistive

13. Stations 13, 3, 5, 6, 7 W E
Station 6:
Depressed water table near Yosemite Village?
No identified pumping wells nearby
Pumping wells are located in confined aquifer

14. Conclusions
Electrical resistivity successfully identified shallow (0-8 m) contrasts in electrical resistivity
Minimum thicknesses for intermediate depth layers have been constrained with electrical resistivity
Longer cable lengths would allow for more comparison with deeper geophysical methods
Resistivity data does not sense deep enough to reject/confirm Newcomb and Fogg model

15. Citations
Newcomb, N. J., & Fogg, G. E. (2011). HYDROGEOLOGY OF A PORTION OF YOSEMITE VALLEY: GROUNDWATER AND SURFACE WATER INTERACTION AND CONCEPTUAL GROUNDWATER MODEL (p. 55). National Park Service.

16. Questions?