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Measuring Soil Water Content with Ground Penetrating Radar

A Review

^a Center for Geo-Ecological Research (ICG), Institute for Biodiversity and Ecosystem Dynamics (IBED)– Physical Geography, Universiteit van Amsterdam, The Netherlands (currently Dep. of Landscape Ecology and Resources Management, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 26-32, 35372 Giessen, Germany)
^b Lawrence Berkeley National Laboratory and University of California, Berkeley, CA
^c Sensors and Software Inc., Mississauga, ON, Canada

View larger version (14K): [in a new window]   Fig. 1. Example of the Debye model for the real part (solid line) and imaginary part (dashed line) of the permittivity.

View larger version (10K): [in a new window]   Fig. 2. Wave fronts around a dipole source on the soil surface. A and B are spherical waves in the air and soil, respectively. Wave C is the lateral or head wave in the soil, and D is the ground wave in the air (after Annan, 1973).

View larger version (21K): [in a new window]   Fig. 3. Propagation paths of electromagnetic waves in a soil with two layers of contrasting dielectric permittivity (1 and 2) (after Sperl, 1999).

View larger version (13K): [in a new window]   Fig. 4. Idealized ground penetrating radar (GPR) transect measured with a fixed antenna separation over an anomalous wetter zone and a horizontal groundwater table (GWT). A marks the air wave, B marks the point reflector, and C marks the reflection from the groundwater table (after Davis and Annan, 1989).

View larger version (26K): [in a new window]   Fig. 5. Common-midpoint (CMP, top) and wide angle reflection and refraction (WARR, bottom) acquisition, where S denotes the transmitter location and R denotes the receiver locations.

View larger version (22K): [in a new window]   Fig. 6. Schematic wide angle reflection and refraction (WARR ) measurement. The ground wave can be identified as a wave with a linear move out starting from the origin of the x–t plot. In the slope equations, c is the electromagnetic velocity in air and x is the antenna separation (after Sperl, 1999).

View larger version (102K): [in a new window]   Fig. 7. Common-midpoint (CMP) measurement made with a 100-MHz antenna at the Cambridge Research Station, University of Guelph, ON, Canada.

View larger version (127K): [in a new window]   Fig. 8. Semblance plot of the common-midpoint (CMP) measurement shown in Fig. 7 to illustrate automatic extraction of velocity vs. time. Blue colors indicate a high semblance.

View larger version (52K): [in a new window]   Fig. 9. Wide angle reflection and refraction (WARR) measurement recorded on loamy sand with the 225-MHz antennas. The velocity of the ground wave is v and of the air wave is c, both in meters per nanosecond.

View larger version (13K): [in a new window]   Fig. 10. Calibration equation between gravimetrically determined soil water content (SWC) and refractive index (nWARR) determined from the ground wave velocity obtained with 225-MHz ground penetrating radar (GPR) antennas.

View larger version (19K): [in a new window]   Fig. 11. Comparison of nTDR and nWARR (225 MHz) for the same measurements shown in Fig. 10.

View larger version (54K): [in a new window]   Fig. 12. Maps illustrating the increase in soil water content (m3 m-3) due to irrigation obtained using ground penetrating radar (GPR) ground wave and time domain reflectometry (TDR) measurements. The maps were obtained by subtracting interpolated maps of soil water content obtained using GPR and TDR data collected before and after irrigation.

View larger version (19K): [in a new window]   Fig. 13. Schematic diagrams of borehole ground penetrating radar (GPR) wave paths between transmitter and receiver for ZOP (left) and MOP (right).

View larger version (43K): [in a new window]   Fig. 14. A comparison between estimates of soil water content (SWC) obtained from simple transformations of 200-MHz zero offset profiling (ZOP) and multi-offset profiling (MOP) borehole ground penetrating radar (GPR) as well as cone penetrometer (CPT) data at the DOE Hanford Site in Washington (modified from Majer et al., 2002).

View larger version (13K): [in a new window]   Fig. 15. Schematic raypaths (left) and common-offset data (right) for borehole ground penetrating radar (GPR) measurements made in a soil with a high velocity layer.

View larger version (92K): [in a new window]   Fig. 16. An elevated 500-MHz ground penetrating radar (GPR) system being used to measure surface reflection amplitude.

View larger version (30K): [in a new window]   Fig. 17. Water content profiles measured with air-launched 500-MHz ground penetrating radar (GPR) surface reflectivity method compared with measurements obtained using 20-cm-long time domain reflectometry (TDR) probes.

View larger version (14K): [in a new window]   Fig. 18. Reflection coefficient as a function of soil water content. Soil water content was calculated from soil permittivity with Topp's equation (Eq. [6]).