Environmental Engineering Reference
In-Depth Information
essential that quantitative data are interpreted based on the specific properties of the
soil at the site. In interpreting the data, it is also important that the three CPT
components − namely friction ratio, cone resistance and fluorescence intensity − be
considered when developing a so-called “signature” for the polluted areas, thus
allowing better identification of contaminant type and soil [LAM 95].
12.3.2.5.
Cone penetrometer-based on Raman probes
A fiber optic probing system based on Raman spectroscopy was developed over
a decade ago [BRA 95, KYL 94, KYL 97]. [SIE 99] indicated that the Raman effect
differs fundamentally from that of the fluorescence because the photons are not
completely absorbed by the molecules − they just collide with the molecule and
scatter, thus inducing vibrational or rotational transitions in the structure of the
molecule. The collisions occurring between the incident light of the photons and a
molecule can be elastic or inelastic. When an elastic collision occurs, the energy of
the incident light is preserved, thus the scattered light has the same frequency as the
incident light [SIE 99]. This phenomenon is known as Rayleigh scattering and
induces what is called the Rayleigh line (scattered light). When an inelastic collision
occurs, there are two possible outcomes [SIE 99]. First, the incident photon can
make the molecule undergo a quantum transition to a higher energy level. In this
case, the incident photon gives up energy and the scattered light will have a lower
frequency than the incident light. In the second situation, the molecule may already
exist at an elevated vibrational or rotational energy. In this case, an inelastic photon-
molecule collision will normally lead the molecule to transition to a lower quantum
level, releasing energy that is gained by the incident photon. The scattered light will
thus have a higher frequency than the incident light [COL 90]. The change in
frequency between the incident light and scattered light is known as the Raman
shift. The set of all Raman frequencies for given species of molecule is called a
Raman spectrum (see Figure 12.12). [SIE 99] indicated that the shape and position
of Raman bands of a particular chemical species are unique and are not a function of
radiation frequency. However, the intensity of Raman scattered light is proportional
to one over the fourth power of the incident light frequency. Therefore Raman lines
are most easily observed with visible and infrared laser sources [SIE 99]. [SIE 99]
pointed out that it was difficult to detect the phenomenon since the Raman scattering
occurs within a time frame of approximately 10-12 seconds.
The system configuration is very similar to CPT-LIF except that the excitation
wavelength is 514 nm and the data collected are the Raman spectrum [KYL 97,
ROS 98]. As fluorescent probes, the laser energy of Raman probes is transmitted by
a given length of fiber in a cone penetrometer and passes through a sapphire
window to target contaminants in the subsurface [SIE 99]. The light scattered by the
target is then collected through the optical system behind the sapphire window and
run by a fiber to a detector.
