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

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

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

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

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

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

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

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 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

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 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

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

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

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

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) 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

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

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

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