Reflecting Properties of the Seasonally Thawed Layer during Thawing and Freezing of the Light Loam Soil. Numeric-analytical Study

https://doi.org/10.1007/s11141-023-10250-2

We use numeric-analytical methods to study theoretically the reflective properties of the layered media of the seasonally thawed active layer (AL) in the process of their freezing and thawing in the ultrawide frequency band from 10 MHz to 10 GHz, as well as the process of interaction of 2.5-ns ultrawide band pulses in the megahertz frequency range with such media, for the light loam soil with the clay-fraction weight content equal to 20% as an example. The spectral amplitudes of the reflection coefficient, the time waveforms and delays, and the propagation velocity of the pulse are calculated in the case of a monostatic radar configuration with probing the AL to the nadir by an electromagnetic wave with a plane front. It is shown that the AL soil is a low-pass filter for the reflected waves, and the cutoff frequency of this filter decreases nonlinearly with increasing AL thickness. In this case, the average frequency in the spectrum of the probing pulse decreases from 527 MHz (before the pulse enters the medium) by 38% and 23% per meter during propagation through the thawed and frozen active layers, respectively. The velocity of the ultrawideband pulse and the group velocity of the pulse propagation, which is calculated at the average frequency of the probing-pulse spectrum before the pulse enters the medium, coincide in both the thawed and frozen ALs of the light loam soil. It is shown that the energy potential of the radar system should exceed 40 dB per 0.5 m of the AL thickness in the frequency range below 1 GHz to ensure probing of the lower boundary of the AL in the freezing or thawing light loam soil. The performed studies demonstrate the basic possibility and good prospects for developing pulsed ultrawideband radar systems in the megahertz frequency range for remote probing of the geophysical parameters of the layered structure of ALs in light loam soils.