THREE DIMENSIONAL ESTIMATION OF VOLUMETRIC WATER CONTENT USING MULTI-FREQUENCY GPR GROUNDWAVE DATA

Characterizing the near-surface soil water content distribution is important for precision agriculture and groundwater remediation applications, but conventional point measurements and remote sensing techniques are often insufficient to characterize water content heterogeneity at the field scale. Ground penetrating radar (GPR) groundwaves are a geophysical technique that can be used to estimate water content quickly over large areas, and recent studies have indicated that the groundwave sampling depth is a function of antenna frequency. The objective of this research is to explore the potential of multi-frequency GPR groundwave data for characterizing the three dimensional soil water content distribution at the field scale.

GPR groundwave data were acquired using four pairs of antennas with central frequencies of 100-, 250-, 500-, and 1000-MHz over a 7-acre field site. Variable-offset surveys were acquired to determine an appropriate offset for common-offset data and to aid with data interpretation. Common-offset data were then acquired using a sled system and multi-channel adapter that allowed simultaneous data acquisition with four antenna pairs. In addition to the GPR data, vertical gravimetric water content and soil texture measurements were acquired in shallow boreholes.

Variable-offset GPR data were used to identify the airwave and groundwave wavelets in the common-offset surveys for each frequency, and the arrival times for each of these waves were noted. The groundwave velocity was calculated using the travel time of the groundwave and the antenna offset, and a petrophysical relationship was used to convert the velocity to water content.

Preliminary results from this experiment indicate that multi-frequency groundwave data can be used to characterize the three-dimensional volumetric water content distribution. The pattern of water content across the field site is similar for all GPR frequencies, but the absolute values of water content vary somewhat between frequencies. The vertical distribution of water content estimated from the GPR data is similar to that observed in most of the gravimetric water content profiles.