Abstract
At radar frequencies below 2 GHz, the mismatch between the 5 to 15 cm sensing depth of classical time domain reflectometry (TDR) probe soil moisture measurements and the radar penetration depth can easily lead to unreliable in situ data. Accurate quantitative measurements of soil water contents at various depths by classical methods are cumbersome and usually highly invasive. We propose an improved method for the estimation of vertical soil moisture profiles from multi-offset ground penetrating radar (GPR) data. A semi-automated data acquisition technique allows for very fast and robust measurements in the field. Advanced common mid-point (CMP) processing is applied to obtain quantitative estimates of the permittivity and depth of the reflecting soil layers. The method is validated against TDR measurements using data acquired in different environments. Depth and soil moisture contents of the reflecting layers were estimated with root mean square errors (RMSE) on the order of 5 cm and 1.9 Vol.-%, respectively. Application of the proposed technique for the validation of synthetic aperture radar (SAR) soil moisture estimates is demonstrated based on a case study using airborne L-band data and ground-based P-band data. For the L-band case we found good agreement between the near-surface GPR estimates and extended integral equation model (I2EM) based SAR retrievals, comparable to those obtained by TDR. At the P-band, the GPR based method significantly outperformed the TDR method when using soil moisture estimates at depths below 30 cm.
Highlights
In situ measurements of soil permittivity are a prerequisite for the calibration and validation of synthetic aperture radar (SAR) soil moisture retrieval algorithms.Despite the difficulties arising from different sensing depths of ground-based sensors and incomingRemote Sens. 2017, 9, 580; doi:10.3390/rs9060580 www.mdpi.com/journal/remotesensingRemote Sens. 2017, 9, 580SAR signals (Figure 1), the standard approach for in situ soil moisture measurements is the use of time domain (TDR)or frequency reflectometry (FDR) probes.Depending on and the 15 the sensorreflectometry type and length of the rods, the domain measurement depth usually varies between5 cm sensor length of the rods, the measurement depth usually varies between 5 cmthis andkind 15 cm. cm. type
In the second part a brief comparison between ground penetrating radar (GPR)/common mid-point (CMP) and SAR derived soil moisture estimates will be discussed to point out its virtues for calibration and validation purposes
We proposed an improved method for the estimation of vertical soil moisture profiles from multi-offset GPR data
Summary
In situ measurements of soil permittivity (i.e., soil moisture content) are a prerequisite for the calibration and validation of synthetic aperture radar (SAR) soil moisture retrieval algorithms.Despite the difficulties arising from different sensing depths of ground-based sensors and incomingRemote Sens. 2017, 9, 580; doi:10.3390/rs9060580 www.mdpi.com/journal/remotesensingRemote Sens. 2017, 9, 580SAR signals (Figure 1), the standard approach for in situ soil moisture measurements is the use of time domain (TDR)or frequency reflectometry (FDR) probes.Depending on and the 15 the sensorreflectometry type and length of the rods, the domain measurement depth usually varies between5 cm sensor length of the rods, the measurement depth usually varies between 5 cmthis andkind 15 cm. cm. type. In situ measurements of soil permittivity (i.e., soil moisture content) are a prerequisite for the calibration and validation of synthetic aperture radar (SAR) soil moisture retrieval algorithms. Despite the difficulties arising from different sensing depths of ground-based sensors and incoming. SAR signals (Figure 1), the standard approach for in situ soil moisture measurements is the use of time domain (TDR). Depending on and the 15 the sensorreflectometry type and length of the rods, the domain measurement depth usually varies between. 5 cm sensor length of the rods, the measurement depth usually varies between 5 cmthis andkind 15 cm.
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