Geoscience Reference
In-Depth Information
air. Measurements of the wave propagation velocity can be taken to determine soil water contents
using various functional relationships between the two quantities (e.g., Topp et al., 1980). The GPR
technique is a nondestructive measurement, which is an advantage in comparison to other electro-
magnetic wave-based methodologies, such as the TDR technique.
In principle, all kinds of GPR measurements require one transmitting and one receiving antenna.
In this study, the fixed offset (FO) method in which the distance between the two antennas is held
constant was used. Although a 1 GHz antenna (MALÅ Geoscience, Malå, Sweden) was found to be
suitable for the investigated lysimeters, we preferred a 500 MHz antenna (SIR-10A from Geophysi-
cal Survey Systems Inc., Salem, NH) for the large sand tank. The measurements were recorded at
twelve times during the vegetation period from March to September in the lysimeter study in order
to identify absolute soil water content changes. In the sand tank study, where we were aiming at
depicting the proceeding water front following single irrigation events, GPR data were acquired at
15 to 20 min intervals.
25.2.1.1
Migration of Radargrams
The processing steps applied to the acquired GPR data were first counting back the applied field
gain curve, then normalizing in space to 1 cm trace interval, and afterward reapplying a realistic
gain curve taking the attenuation of electromagnetic waves in a midelectroconductive environment
into account. No filtering was applied to the data beneath a low-cut filter during acquisition for
signal stability.
In order to obtain depth profiles, the GPR signal originally acquired in the time domain has to
be converted to a depth section. This procedure called “migration” is a common process in seismic
data processing. During the migration, the data are not simply rescaled, but, for example, effects
of the acquisition in the time domain such as diffraction hyperbolas at edges or isolated bodies are
taken into analysis. In this case, a classic Stolt migration algorithm was used to convert the time
section into a depth section (Stolt, 1978). The procedure is based on a basic velocity model that
could not be computed directly from the acquired database. However, from the independent TDR
measurements in eight depths, it was possible to set up a bedded velocity model neglecting strong
variations in the horizontal plane. Some artifacts may be introduced using such a simplified model,
but these are not critical compared with analysis on those arising from time-domain data.
25.2.1.2
normalized Maximum Reflection Amplitude Analysis (nMRA)
The energy transmitted into the soil will be, in part, reflected when contrasts in soil permittivity
(e.g., soil water contents) are encountered. Figure 25.1 shows a theoretical derived GPR radargram
with a fixed antenna separation (single offset) for a layered soil profile with soil water content
differences. The first reflection from the top is the direct wave, and the second indicates the different
reflection amplitudes with changing water content.
The maximum reflection amplitude is computed by (a) selecting an interval of interest and
(b) calculating the absolute value of the maximal amplitude. The values can be presented in nor-
malized form by dividing the selected value with the maximum value of the considered interval
(NMRA). In this study, NMRA values derived from one soil depth (from migrated radargrams)
were statistically analyzed and compared to soil water content distributions.
25.2.2 e
x P e R i M e n t a l
s
e t u P
Two setups were used to generate the experimental database. In both cases, a 2 m thick sand body was
investigated. Although a lysimeter stand with a natural vegetation cover subjected to atmospheric con-
ditions served for absolute water content investigation on a seasonal scale, we used a sheltered large
physical sand tank (Schmalz et al., 2003) for temporal and spatial high-resolution measurements.
