1. Module No. 18 Electrical Methods
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

2. Overview of Soil Electrical Conductivity (Resistivity)
Correlated to
Water saturation, fluid conductivity, porosity, permeability, presence of metal
Can be used for
Geologic feather with distinctive electrical properties
Locate contaminant plume
Salt water intrusion
Stratigraphic unit
Sinkholes, fractures, buried drums and tanks
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

3. Methods of Measurement
Directly
Galvanic resistivity method
Better vertical resolution
Less sensitive to cultural noises
Indirectly
Electromagnetic Induction
Requires no direct contact with ground surface
Data can be acquired quickly
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

4. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Mode of Investigation
Profiling
Detect lateral variation across a site by a series of readings along a line using a fixed configuration of coils or electrodes
EM is typically used in profile mode
Sounding
Estimate vertical variations in electrical conductivity or resistivity
A resistivity sounding is made by taking many readings with increasing electrode separation at a single location
A EM sounding is made by taking readings at a single location with several coil spacings and coil orientations
Data are inverted to produce a model of conductivity variations with depth
Resistivity sounding typically provides better vertical resolution than EM soundings
Profiling and sounding can be used together to obtain 3D model
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

5. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
EM and Resistivity
Both measure apparent ground conductivity
EM very sensitive to highly conductive media
Thin, conductive layer may dominate over much thicker, low conductive layers
For very high conductivity, measurement becomes non-linear
Resistivity method less sensitive to thin, high conductive layers and can measure the lowest and highest apparent conductivities
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

6. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Applications
Environmental, groundwater, geotechnical, and archaeological work
Example applications include
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

7. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Resistivity Method
DC current survey P p n N
I positive for current into ground, negative for current out of ground
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

8. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Resistivity Method
Geometry factor of electrode arrays
Dipole (at least 6 times spacing)
Infinity ( at least 10 or 30 times spacing)
Wenner array
Schlumberger array
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

9. Issues with Array Configuration
Penetration depth and resolution
(plot in Handout)
Noise
Variation in current source typically small
Most uncertainty due to voltage measurement
Noise due to induction of cable and natural voltages
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

10. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

11. Depth of Investigation
Although the array geometry determines depth of investigation practical limits of depth of are determined by ground resistivity.
Signal attenuated by 1/r3
By Ohm’s Law V=IR therefore high R gives high signal V. Receiver can detect transmitter at long Tx/Rx separation in resistive earth. Low R gives small V so transmitter must be near receiver in conductive earth.
Can get more separation and therefore greater depth in resistive earth.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

12. Resistivity Profiling
Detect lateral changes
Array parameters are kept constant and depth of penetration changes only with subsurface layering
Array needs to be portable
Depth information can be obtained if layer information is available (two layer of know and constant resistivity, for example)
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

13. Resistivity Profiling
Target
Steeply dipping contact between two rock types of different resistivity, concealed under thin and uniform overburden
Usually exist in man-made environment
Gravel lenses in clays, ice lenses in Arctic tundra and caves in limestone are more resistive than their surroundings but tends to be small and difficult to detect
Small bodies that are good conductors such as oil drums and sulphide ore bodies are more easily detected using EM methods.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

14. Resistivity Depth-sounding
Investigate layering, using arrays in which the distances between some or all electrodes are increased systematically
Portability is less important than profiling
Wenner and Schlumberger array are popular. Schelumberger array is more portable
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

15. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Capacitance Coupling
Dipole aerials
Insulated electrode
ground
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

16. Principles of Operation
Similar to Galvanic (Direct Contact) Resistivity
Geometric ‘K’ Factor used to Calculate ra, s.t.
ra = KDV/I
Contact is made CAPACITIVELY at frequency of approximately 16 kHz.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

17. Capacitively Coupled Resistivity
Traditional resistivity uses probes hammered into the ground
CCR uses antenna dragged along the ground
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

18. How are the dipole cables coupled to ground?
Dipole electrodes are coaxial cables
Coaxial shield acts as one plate of capacitor and is driven by 16.5 kHz AC signal.
The earth acts as other plate of capacitor.
Insulator acts as dielectric of capacitor
AC signal passes from cable to earth via capacitance. DC signal is blocked.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

19. How Capacitive Coupling Works
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

20. Depth section using capacitively-coupled resistivity measurement
A series of measurements are made along a profile by towing the array with a constant transmitter-receiver separation. Then the transmitter-receiver distance is changed and CCR is again pulled over the same profile giving another series of readings, but corresponding to a greater depth.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

21. Detection of Cavity in Karst
The following slides show a test in which an OhmMapper was dragged over a known cavity. The position of the cavity matches well with the high-resistivity target in the depth section.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

22. WREDCO OhmMapper Survey, Line 0 East Cavity detection study in West Texas Photo courtesy of Jay Hanson
Orange flag marks 30 meter position
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

23. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Cavity? Uncovered!
This 1 meter wide cavity was located at the 31 m position on the transect. Its roof thickness is about 1 meter. The cavity’s height is 2.5 meters and its length is 6-8 meters.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

24. OhmMapper image over Line O East
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

25. Litigation survey for cavity under private house
The next slide shows the results of a survey done to determine the cause of damage to a home in Florida. The results was evidence that proved the damage was the result of a karst cavity under the house.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

26. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Results of OhmMapper resistivity survey over suspected karst cavity. The highly resistive area near surface is taken as proof of cavity. Study done by R.C. Kannan Assoc. of Largo, FL
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

27. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Bedrock mapping
The following slide shows the results of survey to map bedrock. The conductive (blue) top layer is taken to correspond to the sedimentary layer. This was confirmed by the observation that the areas on the depth section showing no sediments generally corresponded to rock outcropping.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

28. Bedrock Mapping. Courtesy of Wredco Geophysical, Spooner, WI
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

29. Advantages of Capacitively-Coupled Resistivity
Fast - data can be collected at a walking pace
Portable - one man operation
Automatic - can be vehicle towed
Flexible - can be used for profiling and sounding
Versatile - used an accessory for G-858 cesium magnetometer
Low power - works in very high resistivity environments without supplemental power
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

30. Self Potential (SP) and Induced Polarization (IP)
unidirectional current flow in the ground and produce voltage (SP) anomalies that can amount to several hundreds of millivolts on the ground surface.
Can be applied in exploration for mass sulphides and other applications
Refer to plots in handouts
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

31. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008IP
Artificial current flowing in the ground cause part of rock to be electrically polarized (like charging a capacitor). If current suddenly ceases, the polarization cells discharge field that can be detected at the surface.
Disseminated minerals produces large polarization effects and IP are widely used in exploring for base materials.
Refer to plot in handout
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

32. Electromagnetic Induction
Noise for other survey (such as resistivity survey)
Originally used for search for conductive sulphide mineralization
Increasingly used for area mapping and depth sounding
Small conductive mass within a poorly conductive environment has a greater effect on induction than on DC resistivity.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

33. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Principle
EM field induced by flow of sinusoidal alternating current in a wire or coil (Continuous Wave EM)
Transient electromagnetic (TEM) methods, changes are produced by abrupt termination of current flow
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

34. Different Dipole Configurations
Maximum Coupled
Minimum Coupled
Horizontal coplanar
Vertical coplanar
Vertical coaxial
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008

35. ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
EM methods
Frequency typically below 1000 Hz
Response Parameter
Similar to frequency response function (incorporated frequency and coil properties)
Spacing and penetration
Refer to plots in handouts
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008