A method for retrieving soil moisture (SM) profiles using multifrequency, ground-reflected electromagnetic (EM) waves is proposed. In a previous publication, a retrieval technique was developed, which used variations in the angular dependence of the reflectivity to infer the SM profile. In this follow-on work a more practical approach based on the frequency dependence of the modulus of the reflection coefficient for a single incidence angle is extended and its feasibility demonstrated. Two possible approaches are considered: the direct retrieval of the SM profile and the retrieval of the dielectric constant (DC) profile. The former immediately yields the parameter of interest, however, it requires a soil dielectric model linking the DC of the soil to its water content. Such a model, which depends on the type of soil, may not be immediately available. The latter does not require a linking model, but by comparing measured profiles of the DC under dry and wet conditions, the SM profile can be estimated. Both approaches are considered in this article and their feasibility investigated with the help of numerical simulations in the presence of multiplicative noise in the data. For the case of a direct retrieval of the SM profile, a representative soil dielectric model was used. The retrieval procedure was simplified and made more robust and the frequency independence assumption of the DC in the earlier work was removed.

Ready access to small, inexpensive, unmanned aerial vehicles (UAVs) allows for small-scale electromagnetic propagation and scattering experiments using airborne antennas. We consider a low-power, copter-mounted transmit antenna radiating at frequencies on the order of a hundred megahertz, corresponding to wavelength on the order of meters, which at some horizontal distance generates a vertical interference pattern due to the interaction of a direct and ground-reflected wave. The depth of the interference fringes, which is directly related to the modulus of the soil reflection coefficient, can be measured by a second copter-mounted receiving antenna. By varying the geometry of the copter pair, and/or the transmitted signal frequency, the soil moisture profile up to a depth of a few meters can, in principle, be retrieved. In this article, we present the results of numerical experiments designed to evaluate the sensitivity of the angle- and frequency-dependence of the measured reflection coefficient to the soil moisture profile. Our simulations indicate that the retrieval errors are small suggesting that the technique is feasible.

One proxy for global climate change is the change in the total mass of the Greenland and Antarctic ice sheets. Several complementary techniques have been used to estimate these changes with varying degrees of success. In this article, we describe a new approach that relies on the resonant behavior of ice masses. For very low electromagnetic (EM) frequencies (i.e., ≤2 kHz), pure ice acts like a strong dielectric resonator. Resonances can be excited in ice sheets by ambient EM noise from, for example, distant thunderstorms. The EM frequency spectrum measured in the vicinity of the ice mass should exhibit corresponding resonant frequencies. The evolution of the resonant modes over time can be used to monitor changes in ice mass and shape. The same approach can be, in principle, applied to monitor changes in the volume of fresh water reservoirs.

Pure ice at low frequencies up to 2 kHz and fresh water up to rather high frequencies up to 1 GHz have high dielectric constants. For this reason masses of ice and fresh water could potentially appear as natural dielectric resonators. In practice such natural resonators often have a form of planar waveguides. Theoretical estimates of the resonant frequencies of such waveguides are made. In the case of ice, the low-frequency threshold requires that the ice sheet should have a large depth exceeding roughly 500 m. Corresponding estimates were applied to the Greenland ice sheet. The case of fresh water reservoirs was also considered and an experimental attempt to measure such resonances at a small water reservoir was made.

Over the last 25 years, ground-based remote profiling has found increasing applications in atmospheric studies where its high vertical and temporal resolution and extended vertical coverage are giving scientists a clearer understanding of atmospheric processes in the troposphere. The authors describe the current status of techniques for vertical profiling of mean wind speeds, temperature, and humidity, and discuss options for combining them.>