Steven A. Arcone

The detection of water leakage along its transportation network has important societal impacts, such as avoiding a large volume of water wasted along the waterways or preventing water-related chemical or physical surrounding media deterioration. Among the vast domain of destructive techniques, Ground-Penetrating Radar (GPR) is a common and efficient tool used for detection in many near-surface contexts, and it is particularly efficient in civil engineering cases, such as utility detection, due to its fine resolution and the ease of data acquisition. A peculiar form of signal enhancement appears in GPR profiles recorded over spheres and cylinders where velocity contrasts exist between the body’s material and the surrounding medium. We used this enhancement to detect potential water leakages in water pipes. After exhibiting the signal enhancement effect in a laboratory sandbox experiment using a spherical glass ball, we verified the results with numerical experiments with varied sphere and cylinder sizes and dielectric properties. We then investigated field and numerical experiments of GPR transects above a “real life” water-leaking PVC pipe. Our results show that the water cylinder and water infiltration bulb produced a characteristic signal that could be used for detecting water leakages along water pipes. The largest amplitude in the GPR signal is caused by a bottom pipe reflection enhanced by the water bulb and not by the top of the pipe. We stress the risk of miscalculating the pipe’s depth during velocity estimation when amplitude enhancement conditions are met. Beyond civil-engineering impacts, knowledge on signal amplification phenomena can help GPR data interpretations in sedimentology and hydrogeology studies.

We measured the broadband (40 Hz-20 GHz) relative dielectric permittivity of sieved surficial samples of an arid-zone soil. The soil samples were equilibrated at 25 °C with relative humidities of 43, 54, 84, and 97 %. Debye-like Maxwell-Wagner-Sillars (MWS) polarization relaxation times of 309, 227, and 73 ns were calculated for the soil samples equilibrated at relative humidities of 43, 54, and 84 %, respectively. Due to electrode polarization, MWS polarization relaxations could be not be discerned on the soil sample equilibrated at 97 % relative humidity. The measured differences between the static and high-frequency relative permittivity (ε _s - ε _∞ ) due to MWS polarization were between 25 and 45 times that of the oven dry soil. This would cause the MWS relaxation to increase radar-wave attenuation rates at frequencies as high as 1 GHz. We found Debye-type relaxation times associated with adsorbed water of 13, 15, 17, 19, and 22 ps for an oven-dry sample and ones equilibrated at at relative humidities of 43, 54, 84, and 97 %, respectively. These frequencies are higher than previously thought. This progression with water contents may be due to the increase in soil solution salinity with increasing water contents. They were not likely caused by free water since a weakly resolved relaxation could be discerned corresponding to a relaxation time of approximately 8 ps, as expected for free-water.

Stimulated by the inquiry of using high frequency electromagnetic ground waves to communicate information among unattended ground sensors, numerical simulations of GPR performance in different near-surface geological settings were conducted. Two sets of 400 MHz GPR field data, one from Fort Richardson, Alaska, and the other one from Hanover, New Hampshire, were used to be the 'ground truth' to compare with numerical simulations. The numerical simulation algorithm we used adapts the finite difference time domain method, with a perfectly matched layer as the absorption boundary condition to truncate outbound waves. The signal impulse has a central frequency of 400 MHz, and the time step is 0.067 ns. We have simulated four cases: a combination of two radiation polarizations (TM and TE), and two geological settings, i.e., a sandy/gravelly half-space overlain by a silty/clayey layer (the case of Fort Richardson, AK), and a silty/clayey half-space overlain by sandy/gravelly layer (the case of Hanover, NH). The results depict the following implication. (1) More EM energy is radiated into the air as an air wave for the TM mode, and more EM energy will be sent into the ground when the TE mode is used, regardless of the geological setting. (2) Where a gravelly sandy half-space overlain by a silty/clayey layer, more EM energy will be trapped in the silty/clayey layer as a ground wave guide in the TE mode with almost no air radiation, when compared with the same radiation mode in the case of silty/clayey halfspace overlain by a layer of gravelly sandy. (3) For the geological setting of a sandy/gravelly half-space overlain by a silty/clayey layer, the TE mode only contains ground wave and the TM mode only contains air wave energy. This implies that for this case a far more complete separation of the air wave and the ground wave can be reached. These simulation results imply that transmission mode should consider the on-site geological setting when attempt to use the ground wave as a communication carrier.