May 3, 2004 1 Ground Penetrating Radar (GPR) Frequency range: 10–1000 MHz. Depths: 4–50 m Cost: $50K and more Applications GPR is used to map geologic conditions that include depth to bedrock, depth to the water table, depth and thickness of soil and sediment strata on land and under fresh water bodies, and the location of subsurface cavities and fractures in bedrock. Other applications include the location of objects such as pipes, drums, tanks, cables, and boulders, mapping landfill and trench boundaries mapping contaminants, and conducting archeological investigations. Speed of EM Wave rrrrrcuccεεεµεµεµ100∝=== Material Relative Permittivity, εr Pulse Velocities, m/ns Conductivity, mS/m Air 1 0.3 0 Fresh water 81 0.033 0.10-30 Sand (dry) 4-6 0.15-0.12 0.0001-1 Sand (saturated) 25 0.0055 0.1-1 Silt (saturated) 10 0.095 1-10 Clay (saturated) 8-12 0.106-0.087 100-1000 Granite (dry) 5 0 .134 0.00001 Concrete 5-10 0.134-0.095 Asphalt 3-5 0.173-0.134May 3, 2004 2Attenuation and Penetration xreEEα−=0 Er and E0 are the received and transmitted magnitude of the Electric field of the EM pulse, x is the propagation distance, and α is the attenuation in nepers/m. Attenuation in nepers/m of some materials Example. For dry sand the attenuation is about 0.5 nepers/m. Thus, after one meter the magnitude of the EM field has fallen to e-0.5×1 = 0.61. In other words, after one meter the amplitude is 20log(0.6) = 4.34 dB. Lowering frequency improves depth of exploration because attenuation primarily increases with frequency. As frequency decreases, however, two other fundamental aspects of the GPR measurement come into play. First, reducing frequency results in a loss of resolution. Second, if frequency is too low, electromagnetic fields no longer travel as waves but diffuse which is the realm of inductive EM or eddy current measurements.May 3, 2004 3Time-Bandwidth., Spatial Resolution )sinc()(ττffX = Bandwidth ~ 1/τMay 3, 2004 4Sample MeasurementsMay 3, 2004 5Sample GPR Systems 400 MHz 40 MHz 500 MHz SPECIFICATIONS: Transmit center frequency..............500MHz Average radiated power ................16mW Receiver type................Equivalent time sampling Input noise............................<10uV RMS Dynamic Range input....................120db Dynamic Range digitized ...............96db Digital resolution.................... 16 bits Antenna type.................Proprietary tapered ............................ impedance travelling wave Antenna impedance..................... 50 ohms Antenna VSWR...........................<1.6:1 System weight......................... 12.5kg Websites http://fate.clu-in.org/GPR_main.html http://www.du.edu/~lconyer/what_is_gpr.htm Ground Penetrating Radar Fundamentals. http://www.geology.ohio-state.edu/~jeff/Library/basics.pdf, and