Well and Field Monitoring - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 242

Rayleigh scattering can be considered to be due to fluctuation in the density,
composition, and orientation of molecules. Hence it is dependent on the refractive
index in small volumes of matter (particularly in gases or liquids). However
Rayleigh scattering considers only random and incoherent thermal fluctuations,
whereas Brillouin scattering responds to correlated, periodic fluctuations. Rayleigh
scattering has no frequency shift associated with it. On the other hand, Brillouin and
Raman scattering have frequency shifts associated with them. The components of
these frequency shifts which are called Stokes and Anti - Stokes .
Brillouin Scattering
Brillouin scattering, named after Léon Brillouin, occurs when light in a medium
(such as air, water or a crystal) interacts with time-dependent optical density varia-
tions that change its energy (frequency) and path. The density variations may be due
to acoustic modes or temperature gradients. As described in classical physics when
the medium is compressed its index of refraction changes, and a fraction of the
traveling light wave, interacting with refraction index variations, is deflected so its
frequency changes.
Raman Scattering
Raman scattering 2 is another phenomenon involving inelastic scattering processes
of light (photons) with vibrational properties of matter. However, a small fraction of
the scattered photons (approximately one in ten million) is scattered by an excita-
tion, with the scattered photons having a frequency different from, and usually
lower than, that of the incident photons. In a gas, Raman scattering can occur with
a change in energy of a molecule due to a transition. Chemists are concerned pri-
marily with such transitional Raman Effect.
The detected frequency shift range and type of information extracted from a
sample, however, are very different. Brillouin scattering denominates the scattering
of photons from low-frequency phonons while for Raman scattering photons are
scattered by interaction with vibrational and rotational transitions in single mole-
cules. Therefore the two techniques provide very different information about a sam-
ple: Raman spectroscopy is used to determine the chemical composition and
molecular structure, while Brillouin scattering measures properties on a larger
scale—such as the elastic behavior. Experimentally, the frequency shifts in Brillouin
scattering are detected with an interferometer (Brillouin sensor system), while the
Raman setup can be based on either an interferometer or a dispersive grating spec-
trometer (fiber Bragg grating and spectrum analyzer).
2 It was discovered by C.V. Raman and K.S. Krishnan in liquids, and by G. Landsberg and
L.I. Mandelstam in crystals. The effect had been predicted theoretically by A. Smekal in 1923.