Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 19

Resistivity (resistance per unit of area), electromagnetic induction, and magnetic
susceptibility are some of the properties that have been used to investigate subsurface
features in soil. These methods all have severe limitations that restrict their applicability
and usefulness.
In spite of limitations there are occasions in which GPR and these other methods are
extremely useful. Buried explosives and explosive devices are likely to be found in any
field traversed by an army. These methods can detect both metal and nonmetal explosive
devices buried in soil. Detection of these devices is a safety issue, but their occurrence
will also alert the persons sampling to other possible field contaminants [17].
1.15.2. Remote Sensing
Remote sensing is another way to obtain information about a field. A sensor placed in the
soil and connected to a radio so that sensor readings are sent back to a central location is
remote sensing. Sensors held over the surface of a field on arms attached to a truck would
also be remote sensing. Often this type of sensing is used to calibrate sensors for specific
use in airplanes and satellites. Normally remote sensing is understood to refer to using
either airplanes or satellites to obtain information about the condition of the Earth's
surface and subsurface. The sensors may be cameras that obtain black and white or
colored pictures of the Earth's surface, or they may record electromagnetic radiation in
other parts of the spectrum.
More advanced remote sensing involves the use of spectrometers or special films.
These are sensors that record the electromagnetic radiation reflected in a narrow range of
wavelengths from the Earth's surface. Some sensors record wavelengths in the ultraviolet
part of the spectrum, some in part or all of the visible spectrum, and some in the infrared
region of the spectrum [18].
1.16. GLOBAL POSITIONING SYSTEM (GPS)
The GPS is useful in finding a position on the Earth's surface. Signals from two or more
satellites are combined by a receiving unit to provide a person or vehicle's location,
altitude, and speed. Airplanes and ships routinely use this type of information between
airports or at sea. It is also available on some automobiles, and there are also handheld
units that an individual can use. Receiving units, either on a vehicle or handheld, can be
used in sampling to identify sample sites. This capability can be extremely useful,
especially during first-time and subsequent sampling of the same field. It is common to
mark sampling spots using various types of physical markers. Unfortunately it is also
common for markers to be removed, lost, or destroyed between sampling times. The GPS
can guide samplers to the same sampling location even if the marker was forgotten or has
been moved or destroyed.
The GPS is also a surveying tool. It can make finding direction and elevation of points
far apart much easier and more accurate than can traditional surveying tools. It can make
surveying points separated by obstructions possible. The GPS can also be used to follow
the spread of pollution or a toxic compound. Not only can the direction of spread be