Transcript Slide 1
2009
George Lightwood
Scott Craig
Danny Lazzareschi
Overview
Location
Purpose
Methods
Results
Interpretations
Conclusions
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 Ωm.
Soils consisting of sand and silt typically
have a resistivity on the order of several
hundred Ohm-meters
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.
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
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
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
Results: Resistivity Soundings
Results- Resistivity profile
Interpretation: Resistivity Sounding
Resix to calculate
resistivitites of layers
Resist to graph
Typical sounding at
Station 8
Interpretation: Resistivity Sounding
Sounding 10 on Playa
Thinner resistive layer
Sounding 11 at toe of canal bank
Opposite layering
Interpretation: Resistivity Cross
Section
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.
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
Thanks
Questions?