Terrain- Permittivity Mapping: Gpr Measurements Of Near-Surface Soil Moisture

GPR can be used to rapidly and non-intrusively measure soil moisture content<br>over spatial scales that are intermediate between the point measurement techniques (e.g.<br>TDR and neutron probes) and remotely sensed soil moisture measurements which are<br>made at the regional scale (e.g. airborne SAR measurements). In bi-static GPR surveys<br>the direct ground wave passes from the transmitter to the receiver through the nearsurface<br>soil. The GPR ground wave velocity can therefore be used to determine the<br>permittivity (or dielectric constant) of the near-surface soil. Estimates of soil moisture<br>content can be obtained using dielectric mixing equations. Variance in the velocity of the<br>ground wave can largely be attributed to variance in the level of soil moisture. We are<br>using GPR ground wave velocity measurements, with a constant antenna off-set, to map<br>lateral variations in terrain permittivity and to estimate near-surface soil moisture content.<br>Experiments have been conducted to determine the effects of frequency, antenna spacing,<br>and soil type on the soil moisture estimates and the effective depth of penetration.<br>Furthermore, we have used this methodology to monitor changes in soil moisture during<br>controlled spill experiments. Ongoing experiments include the monitoring of seasonal<br>fluctuations in soil moisture over a grass covered field at the Weston Geophysical<br>Observatory in Weston, Massachusetts.