Geology Reference
In-Depth Information
Table 14.2.1
Generalized emissivity values of some common materials at ambient earth temperature.
Material
Emissivity value
Asphalt
0.95
Brick (red)
0.93
Brick (lime clay)
0.43
Coal
0.95
Grass
0.98
Limestone
0.95
-
1.00
Soil, dry
0.92
Soil, wet
0.95
Sand
0.90
Sandstone
0.67
Snow
0.80
-
0.90
Water
0.90
-
0.95
Wood
0.80
-
0.95
Note: Emissivity values are compiled from the following sources
-
http://www.instrumart.com, http://www.infrared-thermography.com, http://www.
omega.com, and http://www.optotherm.com
compute the radiant temperature
, as this is the only remaining unknown in equation 14.2.3. It is important to
note that this radiant temperature (also denoted as
T
T
R
), derived from remotely sensed data, is almost always lower
than the actual
T
K
, of the target that we can measure directly in the
field. Ignoring the contribution from the atmosphere between the target and the sensor, this difference is due to the
target
in situ
temperature, or the kinetic temperature,
'
s spectral emissivity and is governed by the equation
1
=
λ
T
K
T
R
¼ ε
4
ð
14
:
2
:
4
Þ
Quattrochi et al. (2009) provide additional information about TIR remote sensing.
Microwave Region
O
ne of the more popular ways of data acquisition in the microwave region (0.75 cm to 1m) of the electromagnetic
spectrum is by active remote sensing, a technique of remote sensing where the sensor itself sends out a pulse of
energy at a discrete wavelength instead of relying on the sun, or the thermal state of the target as the source of
energy. This pulse comes from the sensor to the target at a fixed angle, rather than straight down and is
backscattered (bounced back) from the target. The sensor then measures the time delay between sending the signal
and receiving a backscatter response from the target to compute sensor-target distance, which is used to recreate the
geometry of the target (Earth
s surface). There are several factors that control backscatter: type, sizes, shapes, and
orientations of the scatterers in the target area; dielectric properties of the target area; frequency and polarization of
the pulses; and the incidence angle of the pulse. The data acquired by the senor in the microwave region therefore
provides information about the target (such as surface roughness, soil moisture content) that is complimentary to
the information extracted from data in the visible and infrared regions.
'
The information about target geometry is particularly useful as it helps to extract digital elevation information about the
area. If more than one set of ideally located and timed microwave data pairs over the study area are available, advanced
processing techniques (such as differential Interferometric Synthetic Aperture Radar (InSAR)) can be used to find
subtle deformations and map mining-induced subsidence or elevation increases caused by heaping of coal dumps.
Reviewing the principles, concepts, and advanced developments in the field of microwave, remote sensing data
acquisition and processing are well beyond the scope of this chapter. For additional information about this topic,
see Ketelaar (2009) and Gens and van Genderen (1996). Specific applications of the techniques discussed above, in
the context of coal-fire research, are discussed in section 14.4.4 of this chapter.
