Transcript Topic 18 Electrical Methods

Module No. 18 Electrical Methods
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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
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
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
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
Applications
• Environmental, groundwater, geotechnical, and
archaeological work
• Example applications include
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Resistivity Method
• DC current survey
P
V  I / 2a
p
n
I positive for current into ground,
negative for current out of ground
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
N
Resistivity Method
V
I  1
1
1
1 





2  Pp Np pN Nn 
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
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
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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
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
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
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
Capacitance Coupling
• Dipole aerials
Insulated electrode
ground
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
Principles of Operation
• Similar to Galvanic (Direct Contact) Resistivity
• Geometric ‘K’ Factor used to Calculate a, s.t.
a = KDV/I
Contact is made CAPACITIVELY at frequency of
approximately 16 kHz.
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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
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
How Capacitive Coupling Works
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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 transmitterreceiver 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
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
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
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
OhmMapper image over Line O East
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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
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
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
Bedrock Mapping. Courtesy of Wredco
Geophysical, Spooner, WI
ECIV 431 CWRU Dr. Xiong (Bill) Yu, Spring 2008
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
Self Potential (SP) and Induced Polarization
(IP)
• SP:
– 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
• IP
– 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
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
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
Different Dipole Configurations
Maximum Coupled
Minimum Coupled
Horizontal
coplanar
Vertical coplanar
Vertical coaxial
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