Geology Reference
In-Depth Information
6
Laboratory Techniques for Revealing the Structure
of Polycrystalline Ice
Unlike most natural materials, snow and ice exist in
nature at extremely high thermal states. This is even truer
in case of ice sheets floating on their own melt. Their
structure and physical properties, and hence structure‐
property relationships, are subjected to continuous
changes with time and environmental conditions. The
word “structure” encompasses macro‐ and microstruc-
ture of the material at the surface, near surface, and the
bulk of snow and ice bodies. In the context of this topic,
the structure‐property relationship is narrowed down to
the structure‐electromagnetic wave relationship and in
particular structure‐microwave interactions. Structure‐
property relationships related to engineering problems of
ice, although extremely important for one of the prime
uses of remotely sensed images, are outside the scope of
this topic.
Interpretation of remotely sensed images of ice require
a broad understanding of the structural aspects of the
observed material that affect the emission, transmission,
and scattering of electromagnetic waves. It is imperative
that insitu structural properties be obtained under field
conditions. However, there are limitations in that area of
activities as presented in previous chapters. As a conse-
quence, samples are collected from the field and trans-
ported to laboratories for the evaluation of a number of
physical properties. Often the laboratories are far away
and samples are stored and shipped. This is not desirable
in many situations and particularly for sea ice subjected
to desalination and thereby structural changes. It is,
therefore, preferable to conduct the required laboratory
investigations as soon as possible after sampling. Field
laboratories can be placed either on top of the ice covers
or nearby shore close to the sampling site or on ships,
including icebreakers. Such facilities don't have to be
more than a shelter protected from the elements (see
Chapters 4 and 5 for examples of state‐of‐art investiga-
tions in field laboratories).
Laboratory work on natural snow and ice involves
measurements of a few parameters such as salinity,
density, grain structure and texture, thermal and electri-
cal conductivity, chemical composition, and dielectric
constant. Chemical analysis may also include tests for
identification of the oxygen isotopes in sea ice samples to
discriminate snow ice from frazil ice. But perhaps the
most commonly performed and satisfactorily revealing
laboratory work is the examination of microstructural
features of crystallographic composition, such as fabric
diagrams. Traditionally, the central process in this exami-
nation has been the preparation of thin sections (<1 mm
thick) from ice cores/blocks and viewing them under
polarized light in order to examine grain structure and
texture necessary for “crystallographic classification of
natural ice,” presented in section 4.3. The usual methods
of thin sectioning, commonly used by the glaciologists
for many years for such structure and texture based
classifications, are covered in this chapter. However,
significant refinements have been made in many of these
investigation techniques over the last three decades.
Moreover, analysis of microstructure of materials, ice
included, are required not just for texture‐related struc-
ture but also for understanding wide‐ranging aspects of
material response. The knowledge is scattered and not
necessarily recognized by the sea ice remote sensing
community. An effort has also been made here in bring-
ing out a number of methods in the forefront that are not
generally known or used.
The chapter starts with a section on rudimentary
aspects of electromagnetic (EM) waves that are applica-
ble to the entire spectrum, from light waves in the visible
and infrared range to microwaves used in active and
