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

Outline Introduction Field Methods Data Analysis Results Conclusions

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

Equipment L&R MiniRes Resolution - .1 milliOhm Frequency - 2.5 Hertz Voltage - 640 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.

Field Methods 15 Vertical Electrical Soundings (VES) Wenner Array 10 A-spacings from 1.0 - 31.6 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

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

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)

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

Data Synthesis Combine multiple profiles Cross sections show spatial patterns

Yosemite Valley VES Stations

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

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

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

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

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.

Questions?