Geology Reference
In-Depth Information
undesirable effects at atomic scale such as modification
of the density of dislocations. If thin sections are to be
used later for further examination or processing by repli-
cating or etching the surface (thermally or chemically),
then they have to be stored in small containers at tempera-
tures below −20 °C, surrounded by crushed ice or snow in
order to minimize the rate of sublimation. Thin sections of
ice can be viewed and photographed using a polariscope
and/or optical microscope, as described in section 6.3.
The pioneering work of the Canadian researcher
Perla
[1982] led to the introduction of a number of possibilities
for preparing plane sections for snow. One of the most
useful techniques he suggested was the use of supercooled
liquid‐state dimethyl phthalate to fill the pore spaces in
snow and then allowing the liquid to solidify and preserve
the structure of the snow mass. Since then, his guidelines
have been useful for many investigations.
Good
[1987] has
explored it further and constructed a three‐dimensional
image of faceted grain snow (depth hoar) by serial cut-
ting of snow at different spacings.
Brzoska et al.
[1999]
have performed serial cut on snow, but the technique is
time‐consuming and the ratio of the sample size to reso-
lution is not satisfactory.
Schneebeli
[2000] developed a
system that consisted of slicing, segmentation, and recon-
struction of snow samples filled with dimethyl phthalate.
The 3D structure of snow could be reconstructed and
visualized on the computer within 2 h. The method is use-
ful for structural information of snow sample but fails to
get the textural information of snow.
Coléouet al.
[2001]
used X‐ray absorption tomography to build 3D high‐res-
olution image of snow (10
μ
m
3
) and obtained the porosity
and discrete local curvature of snow. No doubt, the X‐ray
tomography is good for analysis of single or bi‐crystals
but not good for polycrystalline mass. Moreover, it
requires very special and expensive equipment that is not
really portable for field use. Also, it requires special
health‐related precautions.
Classification of snow samples based on thick section
method is difficult in recognizing the type of snow having
similar structure. By the application of DMT together
with thermal etching (presented in section 6.4.3.), tex-
tural details of both vertical and horizontal thin sections
of different snow samples can be obtained. It allows one
to examine the texture of different types of snow, which
was not possible by the traditional thick section tech-
nique. The DMT in conjunction with thermal etching is
capable of bringing out the grain or subgrain boundaries
and clearly show the type of bonds between crystals (geo-
metric or crystalline) both by polarized light for relatively
large‐angle boundaries and small‐angle boundaries by
thermal etching. This way, one could characterize the tex-
ture of various snow types. The texture of snow, which
has not been studied so far, is added information that can
further be used to classify a snow sample in addition to
the common snow class.
Practically very little is known about morphological
changes in snow on sea ice at the grain‐scale level,
although these changes are known to affect not only the
emission, absorption, and scattering of electromagnetic
waves in the visible and the microwave range but also
thermomechanical properties of the top layers. To this
extent, considerable progress has been made by
Satyawali
et al.
[2003] in the application of DMT to snow on
6.2.3. Double‐Microtoming Technique
for Thin Sectioning of Snow
Almost always, ice covers have blankets of snow. Snow
is also present in the environment on the ground and the
mountains. Snow cover on floating ice of lakes, rivers,
and oceans, as pointed out earlier, has a profound influ-
ence on the growth and structure of ice, and hence on
remotely sensed images acquired by using electromag-
netic waves at optical or microwave frequencies. Moreover,
the top surfaces of SY and older MY ice covers on oceans
also become very porous and often reach densities not far
from those of consolidated snow covers. Since very little
attention has been paid to the structural aspects of snow
covers in earlier sections of this topic, it is important to
describe, albeit briefly, the characteristics of snow as a
material and the extension of DMT for examining the
structure of snow.
Physical and mechanical properties of snow can be
well understood from its microstructure and texture.
Microstructure properties refer to geometry of snow
crystals and pores in a sample. Texture properties refer to
crystallographic orientations of snow crystals. A proper
characterization of snow microstructure and texture is
essential for adequate classifications of snow samples
and to understand a possible relationship between tex-
ture and material properties of snow. However, as snow is
subjected to the changes in temperature, vapor pressure,
etc., structural changes continue before, during, and after
snowfall. Therefore, snow is thermodynamically unstable.
To investigate such a material it is important to slow
down the natural snow metamorphism and minimize
mechanical and thermal stresses during sample handling,
transport, and analysis.
Snow has been traditionally classified based partly on
examining thick sections. Classification of snow was
proposed by
Bader
[1954] as well as by a committee in
Canada [
ICSI
, 1954]. These authors have developed a
section preparation technique to examine snow by pho-
tomicrography and image processing. This, however,
could not reach to the satisfaction level. International
Commission on Snow and Ice then suggested a new clas-
sification [
Colbec et al.,
1990] based on the knowledge
gained a few decades before 1990.
