Propagation of a ground‐penetrating radar (GPR) pulse in a thin‐surface waveguide

Abstract

Field observations are tested against modal propagation theory to find the practical limitations upon derivation of layer permittivities and signal attenuation rates from a radar moveout profile over two-layer ground. A 65-MHz GPR pulse was transmitted into a 30-60-cmthick surface waveguide of wet, organic silty to gravelly soil overlying a drier refracting layer of sand and gravel. Reflection profiles, trench stratigraphy, resistivity measurements, and sediment analysis were used to quantify the propagation medium and possible attenuation mechanisms.Highly dispersive modal propagation occurred within the waveguide through 35 m of observation. The fastest phase velocity occurred at the waveguide cutoff frequency of 30 MHz, which was well received by 100-MHz antennas. This speed provides the refractive index of the lower layer, so the near-cutoff frequencies must match a lower layer refraction. A slower, lower frequency phase of the dispersed pulse occurred at about 60-70 MHz, with an average attenuation rate of about 0.4 dB/m. Similar events appear to have reflected back and forth along the waveguide. Modal theory for the average layer thickness shows all primary events to be different aspects of a TE 1 mode, predicts the correct 30-70-MHz phase speeds and low-frequency cutoff phenomenon, but also predicts that the 60-70-MHz group speed should be slightly lower than observed. An Airy phase was apparently out of the bandwidth. Two-dimensional finite-difference timedomain modeling qualitatively simulates the main field results.After accounting for an inverse dependency of amplitude on the square of the range, the high resistivity of the surface layer accounts for the 0.4-dB/m attenuation rate for the 60-70-MHz phase of the pulse. However, erratic amplitudes, interface roughness, and the reflected packets indicate scattering. We conclude that permittivities can be well estimated from dispersive moveout profiles given an average surface layer thickness, and the wide bandwidth of GPR antennas allows the full dispersion to be seen. Attenuation rates appear to be derivable from the higher frequency part of our dispersive event, for which attenuation might be least affected by the waveguide dispersion.

Response of bottom sediment stability after carp removal in a small lake

“…Three parallel lines are observed, and the first line from the water surface is the lake bottom (see the GPR pulse box in Figs. 2(b) and 3(b)) with the water depth less than 2 m. The distance between each line is approximately 0.25 m (based on V water EM ) close to the GPR resolution (Res water ¼ 0:17 m), and these lines are probably not actual layers but from the side lobes of the reflected GPR wavelet (Arcone et al, 2003). EM waves attenuated fast so that lake bottom in deep water (depth > 2 m) was undetected on GPR results.…”

Propagation of Radar Pulses from a Horizontal Dipole in Variable Dielectric Ground: A Numerical Approach

“…Several of these events may be highly dispersive, which has been treated [12][13][14], but only for horizontal layers. The dispersion is generated because the wedge is initially much less than an in situ wavelength in thickness, and it is complicated by thickness changes.…”

“…13) is taken from a 70-MHz moveout survey over an electrically thin 0.34 m (average thickness) layer of moist silt (ε r = 23) above drier sand and gravel (ε r = 10.6). At an initial dominant frequency of 70 MHz, the wavelength in this thin layer is 0.9 m. We have analyzed the ground waves in this profile extensively [13], but not the air wave characteristics. The layer thickness is variable [13].…”

“…At an initial dominant frequency of 70 MHz, the wavelength in this thin layer is 0.9 m. We have analyzed the ground waves in this profile extensively [13], but not the air wave characteristics. The layer thickness is variable [13]. In the south side profile there appear many events between a relatively Figure 13.…”

“…These interfacial solutions substantially differ from those for free space, as discussed by many authors [9][10][11]. Arcone [12] Arcone et al [13] and Liu and Arcone [14] treated the dispersive propagation caused by a thin layer with a 2-dimensional (2-D) finite difference time domain (FDTD) numerical model. Liu and Arcone [14] demonstrated how the near-surface stratigraphic structure plays an important role for different antenna polarization modes.…”