Geoscience Reference
In-Depth Information
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GPR
TDR
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fIGURe 24.3
Soil water content profile from grass (0 to 6 m) onto cornfield (6 to 16 m).
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fIGURe 24.4
Example soil water content profile on grass field collected with vehicle-mounted ground-
penetrating radar (GPR) system shown in Figure 24.2.
observed in the surface reflectivity data. At the first site, a profile was collected starting on a rela-
tively flat grass area and running onto a corn field perpendicular to the corn rows. The corn had been
harvested at this time, so only the short stubble remained. The data, at 0.25 m sampling intervals,
were collected with a Sensors & Software pulseEKKO 1000 system using antennas with a center
frequency of 450 MHz, separated by 0.75 m and elevated 1.3 m above the surface. A TDR profile of
water content was measured along the same profile using 20 cm long probes. The soil water content
measured with the surface reflectivity method (Figure 24.3) is clearly more variable than the TDR
results and over the corn field is lower, on average, than the TDR.
In another example data set, a profile was acquired on a grass field using a Sensors & Soft-
ware, Noggin 500 GPR. This GPR has a center frequency 500 MHz, and an antenna separation of
0.15 m. Data were collected continuously with a vehicle-mounted system. GPS provided positioning
information resulting in a spatial sampling interval of ~0.15 m. Water contents ranging from 14 to
18 percent were measured at five locations on this field. The variabilities in water content observed
in the GPR-based measurements (Figure 24.4) are higher than suggested by the limited number of
TDR data.
These example data sets and others that have been acquired demonstrate that there is significant
apparent spatial variability in the measured soil water content and, in some cases, significantly dif-
ferent water contents from those measured with TDR. Redman et al. (2002, 2003) attributed these
