Geology Reference
In-Depth Information
surfaces scatter energy, reducing reflection amplitudes. Similarly, small tar-
gets generate only weak reflections. Success with GPR generally requires
reflection of at least 1% of the incident wave (i.e. RC > 0.01) and the smallest
lateral dimension of the target to be not less than one-tenth of its depth.
To resolve the thickness of any layer, the travel time through it must be
greater than the pulse duration; that is, the thickness must be greater than the
signal wavelength. If the layer is significantly less than one wavelength thick,
interference effects (between reflections from its top and bottom) become
important and thickness becomes difficult to measure. The layer can still
be detected, as long as the net reflection amplitude exceeds the background
noise level.
10.1.3 Attenuation
Radar waves are attenuated in the ground because they cause currents to
flow, converting electrical energy to heat ( ohmic dissipation ), and signal
amplitudes eventually fall below detectable levels. The exponential decay is
governed by an attenuation constant that in most materials is approximately
proportional to both frequency and conductivity and also, where this varies
significantly from the free-space value, to magnetic permeability. The noto-
rious failure of GPR to penetrate clay soils is due to the greater attenuation
in more conductive media. The roughly linear variation of the attenuation
constant with frequency over the frequency range of a typical pulse implies
that pulse shapes will change with travel time, as the higher frequencies are
preferentially attenuated.
GPR penetration is also limited by scattering, which can become more
important than ohmic losses at frequencies above 1 GHz. Signals are scat-
tered from localised objects if their dimensions are comparable with the
GPR wavelength, and Rayleigh scattering, proportional to the fourth power
of frequency, occurs for objects smaller than the wavelength. This is an
important limitation on signal penetration in heterogeneous material.
In polarisable materials at radar frequencies, displacement currents that
depend on permittivity and the rate of change of the EM field are domi-
nant, because the displacements of charges within atoms and molecules, as
opposed to their transport by movement of ions and electrons, become im-
portant. This is not the whole story, because even vacuum has a permittivity,
but does at least explain the role of physical media.
The water molecule is highly polarisable, i.e. its electric dipole moment is
easily aligned by an applied electric field, and water has a remarkably high
relative permittivity ( ε = 80; Table 10.1). It absorbs energy more strongly
as frequency increases, at least up to the relaxation frequency of several
GHz (depending on temperature and the degree of binding to soils). Even at
500 MHz, water losses can be seen in otherwise low loss materials.
Search WWH ::




Custom Search