Geoscience Reference
In-Depth Information
fIGURe 23.1
View of the prototype surface barrier looking northeast. The surface is covered with sage-
brush planted in rows. The basalt riprap is seen on the left side of the photograph. In the background is an
irrigation system.
to determine their ability to minimize infiltration into buried wastes (Ward and Gee, 1997). The
surface of the barrier was constructed with a 2 percent slope and was revegetated with a mixture of
rabbit brush, sage brush, and native grasses (Figure 23.1). The barrier is above grade with a steep
(2:1) rip-rap protective side slope to the east and a more gentle (10:1) gravel side slope to the west.
Irrigation equipment was located at the north end of the site for the March and May surveys. The
barrier is engineered to store up to 600 mm of water in the winter and release this water by evapo-
transpiration over the rest of the year while limiting recharge to 0.5 mm/yr or less. The upper 2.45 m
of the surface barrier consists of a 1 m thick layer of silt loam with 15 percent pea gravel; a 1 m thick
layer of silt loam; a 0.15 m sand filter; and a 0.3 m thick gravel filter underlain by asphalt. Our task
was to evaluate the potential for using GPR to determine the temporal and spatial changes in the
soil moisture content and water storage in this upper ~2.5 m zone above the asphalt. Although the
research at Hanford does not involve a precision agriculture application, the goals of the project, to
map the changes in soil moisture, are similar to the goals for precision agriculture.
23.2 MethodS (GpR SURveyS)
GPR sends radar frequency EM energy into the ground through a transmitting antenna. This energy is
recorded at a receiving antenna placed near the transmitter. For the data used in this study, 100 MHz
antennas were used. The sample interval in time was 0.8 ns and 500 samples were acquired for each
trace for a recording window of 400 ns. We stacked (summed each time sample) the data 64 times
for the March data and 32 times for the rest of the data. The reduced number of stacks increased the
acquisition rate, yet did not deteriorate the data quality.
Two acquisition steps are necessary to conduct this type of survey. The first step is using the
common midpoint (CMP) method to determine the optimal antenna separation or offset. The CMP
method acquires data such that the midpoint between the antennas is the same. The two antennas
are started close to each other, then each antenna is moved away from the other at a set increment.
The antennas are stepped away until the data contain easily identified air wave, ground wave, and
reflection events. In our experiment, the optimal offset is chosen as the distance at which the air
wave and ground wave are sufficiently separated in time so that they do not interfere with each other
(Figure 23.2).
