Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 115

similar to the grid shown in Figure 5.3. In this case, however, the grid lines are typically
1 meter apart in both directions. The data obtained are displayed using computer
programs. In this way a three-dimensional picture of subsurface features can be
visualized. Computer programs can also be used to filter the raw radar data to give a
clearer picture of subsurface structures.
Table 5.1 gives the RDP for a number of common environmental components, but does
not give a figure for saltwater, which is an electrical conductor and thus does not have
any appreciable RDP. Ground-penetrating radar thus cannot be used in saltwater or in
salty soil conditions. In this case salts will include all the common fertilizer elements and
soil amendments, such as lime or calcium carbonate. Also, highly conductive clays are
not amenable to GPR methods. While GPR is useful in dry soils, it thus cannot be used
on many dry-region soils because of their high salt content. Note that this can also
include natural deposits of calcium carbonate, which are quite common in arid and
semiarid soils.
A number of other common environmental conditions interfere with GPR. The
radiation used in radar has a magnetic component, as does all electromagnetic radiation.
Thus soils or other media that are magnetic or high in iron or other magnetic materials
such as magnetite cannot be investigated using this method. Other common soil and
regolith components such as cobbles will also cause anonymities in the GPR signal.
Some subsurface features may be hard or impossible to detect by GPR. If the angle of
incidence refracts the waves away from the receiver the object will not be seen. Also, in
some instances the speed of the instrument over the surface may affect the clarity of the
readings [6].
5.4. REMOTE SENSING
The term remote sensing can be applied whenever data obtained from the environment
are transmitted from that location back to some central receiving station, processing
center, or office. This definition might include the case in which photographs are taken
from an airplane and the film physically delivered to the processing center. The most
common interpretation of remote sensing, however, is data about the Earth gathered by
satellites and transmitted back to Earth [7].
In all likelihood, remote sensing of the field being investigated has already been done,
thus one should look at various archives of remote sensed locations to determine if the
location of interest is there. These data archives can be accessed on the Web through the
Distributed Active Archive Centers (DAAC) and the Earth Observatory System (EOS)
data gateway (both of these are National Aeronautics and Space Administration, NASA,
sites) [5, 6]. Both these sites are extremely complicated and hard to navigate. It will take
some time to find the information you are interested in; however, if the desired data are
available, obtaining a copy of the images is relatively simple.
One potential problem with available data is that the images may not include the
portion of the spectrum that is most important to the sampling program. If the needed
data are not available, a request can be made to have an image taken of the area. It is
relatively simple and not very expensive to have satellite pictures taken of specific areas