INVERSION ALGORITHMS FOR THE MICROWAVE REMOTE SENSING OF SOIL MOISTURE.

摘要

Two experiments were performed employing swept frequency microwaves for the purpose of investigating the reflectivity from soil volumes containing both discontinuous and continuous changes in subsurface soil moisture content. Discontinuous moisture profiles were artificially created in the laboratory while continuous moisture profiles were induced into the soil of test plots by the environment of an agricultural field. The reflectivity for both the laboratory and field experiments was measured using bi-static reflectometers operated over the frequency ranges of 1.0 to 2.0 GHz and 4.0 to 8.0 GHz.; Reflectivity models that considered the discontinuous and continuous moisture profiles within the soil volume were developed and compared with the results of the experiments. This comparison shows good agreement between the smooth surface models and the measurements. In particular the comparison of the smooth surface multi-layer model for continuous moisture profiles and the field experiment measurements points out the sensitivity of the specular component of the scattered electromagnetic energy to the movement of moisture in the soil.; Although the agreement of the smooth surface reflectivity and the measurement is good, the smooth surface models do not adequately explain the frequency dependence exhibited by the measured reflectivity of both experiments. In order to account for the frequency dependence of the measurement, a specular transmission coefficient for a rough surface is derived and incorporated into the reflectivity models. The roughness corrected models gave improved agreement with the measurements indicating that swept frequency microwave measurement techniques can be used to account for both moisture gradients within the soil and structures at the soil surface. Most notable about the roughness corrected models and the measurements is that coherent phase effects due to the interference of a subsurface reflection from the moisture gradients and a surface reflection can occur in the presence of surface roughness.