Fallon, NV Canal Survey Resistivity: MiniREs and VLF

1. Fallon, NV Canal SurveyResistivity:MiniREs and VLF 2009 George Lightwood Scott Craig Danny Lazzareschi

2. Overview • Location • Purpose • Methods • Results • Interpretations • Conclusions

3. Location • Dry canal bank west of Fallon, NV • Local mapped units are Badlands soil and Hawsley Sand. • Typical resistivities for soils consisting of predominantly of clay minerals containing brackish water: 1 to 100 Ω-m. • Soils consisting of sand and silt typically have a resistivity on the order of several hundred Ohm-meters

4. Purpose • Use electrical methods to look for locations of seepage and internal erosion of canal walls • Previous studies found these methods effective Asch, T. H., M. Deszcx-Pan, B.L. Burton, and L.B. Ball, 2008, Geophysical Characterization of the American River Levees, Sacramento, California, using Electromagnetics, Capacitively Coupled Resistivity, and DC Resistivity, Open-File Report 2008-1109, U.S. Department of Interior, U.S. Geological Survey, Washington, D.C. Panthulu, T.V., C. Krishnaiah, J.M. Shirke, 2001, Detection of seepage paths in earth dams using self-potential and electrical resistivity methods in Engineering Geology 59, pp 281-295.

5. Methods:MiniRes • Wenner Array • A-spacings of 1, 1.47, 2.15, • 3.16, 4.64, 6.81 and 10 meters • ρa=2πaΔV/I • Thanks to L and R Instruments

6. Survey • Soundings 1-9 in canal bank profile • Sounding 10 on playa • Sounding 11 at toe of canal bank • Soundings 12, 13 on ReMi Line • 8 4.64 A-Spacing

7. Methods: VLF • Very Low Frequency(15-25Hz) • Geonics EM16R • Distant high power sources • Finds EM coupling with conductors in the earth • Anomalies perpendicular to direction toward source • Source in Jim Creek, WA • Measurements made at locations of first 9 MiniRes

8. Results: Resistivity Soundings

9. Results- Resistivity profile

10. Interpretation: Resistivity Sounding • Resix to calculate resistivitites of layers • Resist to graph • Typical sounding at Station 8

11. Interpretation: Resistivity Sounding • Sounding 10 on Playa • Thinner resistive layer • Sounding 11 at toe of canal bank • Opposite layering

12. Interpretation: Resistivity Cross Section

13. Results:VLF • Apparent resistivity from 8 to 11 Ohm-m • Phase angles from 34 to 38 degrees • Tried to construct a 2-layer model from the data • Resistivity of the top layer was assumed to be on the order of 10 to 30 Ohm-m • not successful at identifying contact • method is poorly suited to defining the boundary between a highly conductive layer overlying a layer of higher resistivity (Podder and Rathor, 1983) • Soil was at upper limit of conductivity senitivity Podder, M. and B.S. Rathor, 1983, VLF Survey of the Weathered Layer in Southern India, Geophysical Prospecting, vol. 31, pp. 524-537.

14. Conclusions • Canal Bank consists of 3 layers- high resistivity in between two low resistivity lavers • Top layer likely compacted clay • Middle layer likely sands excavated from the canal • Bottom layer likely clays • Middle sand layer a likely conduit for fluid flow • MiniRes much more effective than VLF for detailed site study

15. Thanks • Questions?