Ground Penetrating Radar Kenton Mick

What is Ground Penetrating Radar? GPR is a technique used to locate objects or interfaces buried beneath the Earth’s surface. Typically operate with Frequency: 1-100 MHz Tx Waveform: Impulse Time Domain Rx: Sampling Receiver As component price decreases, Dynamic Range is improving through FM-CW and stepped frequency modulation.

Antenna Location Collocated Bi-Static Typical in most applications Uses the reflection from the buried object Bi-Static One antenna must be buried beneath the surface Uses forward scattering from the buried object Application: Borehole Tomographic Radar Imaging

Problems Clutter Efficiently coupling the energy into the ground Signals that are unrelated to the target scattering characteristics but occur in the same sample time window and have similar spectral characteristics. Can be from breakthrough between Tx and Rx or multiple reflections between the antenna and the ground. Typically the severity of the effects decreases with penetration depth Efficiently coupling the energy into the ground Reflections caused from the reflections from the air to the ground Penetration Depth

Typical Applications Archaeological Investigations Borehole Inspections Detection of Buried Mines Forensic Investigations Planetary Exploration Road Condition Survey

Archaeological Example Archaeological site in the city of Pompeii, Roman Empire Covered in volcanic deposit from eruption Used a Noggin Plus system with 250 MHz Time window of 100 ns Max depth of 5 meters of volcanic deposit There data only went 2 meters

Road Survey Example Difference in moisture levels between the asphalt and the ground below causes landslides to occur Experiment took place in Limiti di Greccio, Italy Had two analysis methods Amplitude Attenuation on return signal Change in frequency between transmit and received signal The frequency used varied between 3 frequencies 1 GHz 600 MHz 1.6 GHz

Analysis Center Frequency analysis in terms of Resolution and Depth Time window in terms of Depth Sampling Interval in terms of center frequency

Center Frequency with respect to Resolution Spatial Resolution places a lower bound on the center frequency Assuming a center frequency to bandwidth ratio of 1 So as frequency decreases, the spatial resolution increases

Center Frequency with respect to Depth The penetration depth places an upper bound on the frequency Assuming the target dimension is as close in size to a Fresnel zone in order for the return signal arrive coherently

Time Window The time window is the amount of time it take for the signal to return from the farthest penetration depth.

Selecting Sampling Interval Selecting the time interval at which samples will be taken on the return signal’s waveform must abide by the Nyquist Sampling Criteria. Typically an antenna for GPR will radiate a frequency (+/-) 50% the center frequency For this reason the sampling rate should consider the highest frequency component at least 1.5 times the center frequency.

Results Pompeii Max_frequency = 725 MHz to see 5 meters down Min_frequency = 50 MHz to be able to spatially resolve to 0.5 meters Time Window = 130.26 ns Sampling Interval of 3.33 ns Assuming K = 9.192 and velocity = 0.0998 [m/ns]

Results Limiti di Greccio, Italy 1 GHz Antenna 1,600 MHz Antenna Max_frequency = 3 GHz to get 1 meter of depth Min_frequency = 1 GHz to get 0.03 meter spatial resolution Sampling Interval = 0.167 ns for 1 GHz 1,600 MHz Antenna Min_frequency = 1,600 MHz to get 0.01875 meter spatial resolution Sampling Interval of 0.104 ns 600 MHz Antenna Min_frequency = 600 MHz to get 0.05 meter spatial resolution Sampling Interval = 0.278 ns Assuming K = 6.25 Velocity = 0.12 [m/ns]

Q25 GPR Available through US Radar Inc Center Frequency of 250 MHz or 270 MHz Sampling Interval of 10 ps to 6.4 ns Will it work for the two applications? Pompeii Yes, the center frequency of Q25 falls within the range and the sampling interval needed is met. Limiti di Greccio, Italy Yes but no, the center frequency of 250 MHz will give the required depth but not the resolution needed.

Resources D. J. Daniels, Ground Penetrating Radar, 2nd ed. Stevenage: IET, 2004. A. P. Annan, “Ground Penetrating Radar Workshop Notes,” Ground Penetrating Radar Workshop Notes. Sensors and Software Inc, Ontario, Canada, 1992. F. Benedetto and C. Schettini, “Evaluation of Geotechnical Stability of Road using GPR ,” IEEE, rep., 2011. P. M. Barone, E. Pettinelli, T. Bellomo, and C. Scarpati, “Applications of GPR to archaeology and geology: the example of the regio III in Pompeii (Naples, Italy),” rep.

Resources J. K. Russell and M. V. Stasiuk, “Characterization of volcanic deposits with ground-penetrating radar,” eoas.ubc.ca, 27-Dec-1996. [Online]. Available: eoas.ubc.ca/~krussell/epapers/bv_rs97.pdf. [Accessed: 28-Apr-2018]. R. Evans, M. Frost, and R. Morrow, “Assessing the influence of moisture on the dielectric properties of asphalt,” rep.