Estimation of Surface Soil Moisture by a Multi‐Elevation UAV‐Based Ground Penetrating Radar

Abstract

The measurement of soil moisture is important for a wide range of applications, including ecosystem conservation and agricultural management. However, most traditional measurement methods, e.g., time-domain reflectometry (TDR), are unsuitable for mapping field scale variability. In this study, we propose a method that uses an unmanned aerial vehicle (UAV) to support a ground penetrating radar (GPR) system for spatial scanning investigation at different elevations above ground level. This method measures the surface reflectivity to estimate the soil moisture, exploiting the linear relationship between the ratio of the reflected and the direct wave amplitudes along with the reciprocal of GPR antenna height. This relationship is deduced in this study based on the point source assumptions of a transmitter antenna and ground reflections, which is confirmed by numerical simulation results using the gprMax software. Unlike previous air-launched GPR methods, the UAV-GPR method presented here removes the limitations of a steady transmitter power and a fixed GPR survey height and the need for calibration of antenna transfer functions and geophysical inversion calculations, and thus is simpler and more convenient for field applications.We test the method at field sites within the riparian zone and a river-island grassland adjacent to the Yangtze River. The results from the field study illustrate comparable measured soil moisture to those obtained invasively using TDR. The root mean square error (RMSE) of surface reflectivity and soil moisture values between UAV-GPR with 2 8 antenna height investigations and TDR in the grassland are 0.03 and 0.05 cm 3 /cm 3 , respectively.

Advancements in Drone-Based (UAV) Ground Penetrating Radar for Accurate Boundary Mapping between Disturbed Clayey Soil and Natural Rock

“…The drone's flight altitude was optimized to maintain a consistent ground clearance for data accuracy. Figure 5 shows the typical calculation scheme [28,29] for estimating the radius of the maximum exposure area under the drone at the surface (R1) and underground (R2) as a function of desirable drone altitude (H) and requirable target depth (D). Based on Eqs.…”

“…Based on Eqs. 1-3, R1 is a function of the drone's altitude (H), speed of the electromagnetic wave in material (v), and radar center frequency (f) [29][30][31].…”

“…Overcoming these challenges requires innovative techniques like ground-penetrating radar (GPR) to understand subsurface structures comprehensively. Beyond foundation engineering, in agriculture, traditional land GPR has already found applications in monitoring soil water content during irrigation in clay-disturbed soils [5] and mapping nonlinear boundaries between covering clayey soil and natural soil for agricultural purposes [6]. For example, Guo et al [7] utilized landbased GPR to study the 3D sedimentary architecture of recent sediments in the Yungang braided river study area in Datong (China), emphasizing the need to characterize unstable, disturbed ground quantitatively.…”

“…However, the challenges of using GPR in clayey soil, where electromagnetic waves rapidly decay, are acknowledged [23]. This difficulty is manageable, as Cheng et al [6] demonstrated when using land GPR for mapping disturbed clayey layers in agricultural fields.…”

“…Despite the challenges, Edemsky et al [23] highlighted penetrating clayey soil with aerial GPR due to fast wave decay. Experiences in land GPR applications, such as [6] mapping of disturbed clayey layers in agricultural fields, suggest potential avenues for success.…”