Approved for Public Release; Distribution Unlimited Electrical Resistivity Imaging David Hull US Army Research Lab hull@arl.army.mil 17 Jun 2009 ARL Workshop on Personnel, Vehicle, and Tunnel Detection 16-17 Jun 2009 17 June 2009 1/14
Outline Background: state of the art Similar sensors and sensor technologies Results: where we are today Successes and technical hurdles 17 19 June August 2009 20082/14 2/26
Electrical Resistivity Resistance: R = V/I (Ω) Resistivity: = Rk (Ω-m) Material 17 June 2009 3/14 (Ω-m) Salt (sea) water 0.1 1.0 Fresh water 3 100 Pellegrino (bottled water) 6.9 Ultra-pure water @ 25º C 182k Drill mud, hydraul-ez 4.5 Clay (>10% moisture) 1 100 Loam (>10% moisture) 1 200 Alluvium 1 1000 Sand and gravel (dry) 50 10k Sandstone 10 1000 Limestone 100 10k Igneous rock (competent) 100 1M (from www.agiusa.com, www.aemc.com, others) V wh A R I l l V wh E l I J 1 (from www.wikipedia.org) MC Miller Co. soil test box (from www.mcmiller.com or www.farwestcorrosion.com)
Apparent Resistivity Like sample resistivity Inject electrical current Measure voltage V 2 a Rk I 1 1 1 1 r1 r2 r3 r 4 If the ground is homogeneous, then a Otherwise Take many measurements Perform an inversion from Colorado School of Mines, http://galitzin.mines.edu/introgp/ 17 June 2009 4/14
17 June 2009 5/14 Ground resistivity imaging
Ground resistivity imaging Rules of thumb for resistivity imaging: DC resistivity and Induced Polarization (IP) are complementary 1-D, 2-D, 3-D, time-lapse imaging possible (2-D shown) Useful sensing depth ~ 20% of array length (dipole-dipole) Useful resolution ~ 50% of electrode spacing (dipole-dipole) Trade-offs: object size, resistivity difference, depth 17 June 2009 6/14
Marine resistivity imaging 2-D vertical resistivity profile shows competent rock, underwater spring, lake bottom Modified SuperSting R8 with extra software for continuous resistivity profiling Graphite electrodes are towed behind a boat (typically 8x dipole-dipole array) 17 June 2009 7/14
Capacitively-coupled resistivity Replace fixed electrodes (stakes) with mobile electrodes (plates) AC operation required (capacitive coupling) Signal ~ plate area, 1/distance, voltage, operating frequency Noise ~ 1/speed, changes in capacitance (e.g., vibration) Geometrics Ohm-mapper system: Fixed dipole-dipole array is drug over ground Operation at 16.5 khz Multi-receiver variants Plastic-pipe culvert detected at 2 m: 17 June 2009 8/14
Impedivity Spectroscopy Impedivity = complex resistivity Real part: conduction currents (dominate at low frequency) Imaginary part: displacement currents (dielectric effects) Spectroscopy: ~1 khz to ~1 MHz Impedivity DC resistivity for f < 1 khz Impedivity often dominated by dielectric properties for f > 1 MHz Magnitude (Ohm-m) Phase (degrees) 17 June 2009 9/14
Static electrodes @ 0-50 khz Wenner array (COTS source and sensor) uses four equally-spaced stakes COTS source (current) electrodes with stand-off sensor (voltage) electrodes Stand-off source (current) electrodes with COTS sensor (voltage) electrodes Stand-off source (current) electrodes with stand-off sensor (voltage) electrodes 17 June 2009 10/14
Electromagnetic Induction Active EM induction: Transmitter coil generates pulsed magnetic fields Receiver coil measures induced magnetic fields (ringdown) between pulses Detects metals via secondary field caused by eddy currents (from www.geoviewinc.com) Towed EM-61 Geonics EM-61, etc.: Time-domain operation Widely used commercially Arrays in development (from www.echotech.com) 17 June 2009 11/14
Electromagnetic Induction (cont.) Active EM induction: Transmitter coil generates continuous magnetic fields Receiver coil measures induced magnetic fields Detects ferromagnetic objects as distortion of primary field Detects metals via secondary field caused by eddy currents GEM-5 Array Mounted on a Sled Geophex GEM-2, GEM-3, GEM-5: Frequency-domain operation Operates between 90 Hz 96 khz Uses bucking coil to null primary field Gradiometer mitigates motion clutter GEM-5 array uses seven gradiometers in a line array (from www.geophex.com) 17 June 2009 12/14
Electromagnetic Induction (cont.) Semi-active EM induction: Remote RF transmitter (e.g., AM radio tower) excites targets in far zone Receiver coil measures induced magnetic fields in the near zone Standoff permits use in lowflying airborne systems (resolution ~ standoff) DeltaEM gradiometer: Gradiometer on UAV nulls primary field and transmits data in realtime 17 June 2009 13/14 Magnitude (mv) Distance from Start (ft) Research Corporation 1600 1500 1200 1000 500 800 0 400 Mag -500 Phase -1000 0-1500 0.00 18.45 36.90 55.34 73.79 92.24 Phase (Degrees)
Questions/discussion David Hull US Army Research Lab 301-394-3140 david.hull@us.army.mil 17 June 2009 14/14