Module No. 18 Electrical Methods

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

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

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

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

Applications

• Environmental, groundwater, geotechnical, and archaeological work

• Example applications include

Resistivity Method• DC current survey

P Np n

aIV 2/ I positive for current into ground, negative for current out of ground

Resistivity Method

NnpNNpPp

IV

1111

2

Geometry factor of electrode arrays

Dipole (at least 6 times spacing)

Infinity ( at least 10 or 30 times spacing)

Wenner array

Schlumberger array

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

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.

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)

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.

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

Capacitance Coupling

• Dipole aerials

Insulated electrode

ground

Principles of Operation

• Similar to Galvanic (Direct Contact) Resistivity

• Geometric ‘’ Factor used to Calculate , s.t.

a = V/I Contact is made CAPACITIVELY at frequency of

approximately 16 kHz.

Capacitively Coupled Resistivity

Traditional resistivity uses probes hammered into the ground

CCR uses antenna dragged along the ground

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.

How Capacitive Coupling Works

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.

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.

WREDCO OhmMapper Survey, Line 0 EastCavity detection study in West Texas

Photo courtesy of Jay Hanson

Orange flag marks 30 meter positionOrange flag marks 30 meter position

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.

OhmMapper image over Line O East

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.

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

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.

Bedrock Mapping. Courtesy of Wredco Geophysical, Spooner, WI

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

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

• 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

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.

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

Different Dipole Configurations

Horizontal coplanar

Vertical coplanar

Vertical coaxial

Maximum Coupled Minimum Coupled

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