Geoscience Reference
In-Depth Information
24
Soil Water Content
Measurement Using the
Ground-Penetrating Radar
Surface Relectivity Method
J. David Redman
ContentS
24.1 Introduction ......................................................................................................................... 317
24.2 Methodology ....................................................................................................................... 318
24.3 Sampling Volume................................................................................................................ 319
24.4 Field Measurements ............................................................................................................ 319
24.5 Scattering from Surface Roughness.................................................................................... 321
24.6 Conclusions ......................................................................................................................... 322
References ...................................................................................................................................... 322
24.1 IntRodUCtIon
Ground-penetrating radar (GPR) has been used extensively in the borehole, multi-offset subsurface
reflection and multi-offset ground wave configurations to measure soil water content (Davis and
Annan, 2002; Huisman et al., 2003). These GPR methods determine soil water content from a
measurement of the time required for an EM wave to travel a known distance within the subsurface.
This case study discusses the GPR surface reflectivity method used for the determination of soil
water content from measurements of the electromagnetic (EM) reflection coefficient at the air-
ground interface, using a GPR elevated ~1 m above the surface.
The time domain reflectometry (TDR) method is, at present, one of the most widely used meth-
ods for measuring soil water content. Unfortunately, the TDR method cannot be used to cover large
areas efficiently and provides only local point measurements covering a relatively small surface
area (~0.005 m
2
). For many agricultural applications, the ability to efficiently map soil water con-
tent over large areas at a resolution of ~1 to 10 m
2
would improve irrigation practices and reduce
water requirements. The surface reflectivity method could provide this capability by providing rapid
coverage of relatively large areas, because the GPR can be vehicle mounted and is not in contact
with the ground. The method also provides better surface coverage by giving average water content
measurements over typical surface areas of ~1 m
2
.
Previous field studies have demonstrated that the method has good potential (Chanzy et al.,
1996; Redman et al., 2002; Serbin and Or, 2002). It has been shown that the water content mea-
sured with the GPR surface reflectivity method was more variable than and, in some cases, was
substantially different from the TDR-determined water content (Redman et al., 2002). These effects
were attributed to surface scattering, vertical stratification of the water content, and other spatial
317
