Environmental Engineering Reference
In-Depth Information
rings. Using this device, the probe is pushed into the soil to the desired depth, then
the central pin and outer ring are driven into the ground a short distance further.
The central pin acts as an antenna and the outer ring as the receptor and soil
container. A high frequency pulse (10-500 MHz) is then transmitted to the antenna
via a coaxial cable and propagated in the soil filling the area between the antenna
and the outer ring. The signal received by the outer ring is used to determine the
dielectric constant and conductivity of the soil. After measurement, the central
annular region of the probe is advanced to expel the soil sample from the
penetrometer and ready the probe for another push to a new depth. The probe is
particularly sensitive to dense and light non-aqueous organic liquids that have a low
dielectric constant and low conductivity relative to the typical background signal
produced by water and soil. Full-scale tests have indicated that the device could
determine the dielectric constants and conductivity values with an accuracy of 5%,
although collateral contamination between samples may constitute a potential
problem [BRA 97].
Coaxial
cable
Isolator
Retractable rim
receiver
Central antenna
Figure 12.9.
Probe impedance at high frequency [BRA 97]
12.3.2.3.
Fiber optic cone penetrometer
Recent developments in lasers and fiber optics have led to the creation of several
devices that perform
in situ
investigations of chemical analysis through optical
spectroscopy. In most cases the laser is used to induce a slight relative phenomenon
which, once observed and analyzed, provides information that can be used to detect,
identify or measure contaminants in the subsurface. The primary optical
