Geology Reference
In-Depth Information
passive imaging radars. Relatively detailed aspects of EM
waves relevant to remote sensing systems are presented in
Chapter 7.
The wavelengths involved in the visible range are
extremely small, from about 400 to 800 nm, whereas the
wavelengths in the C‐band of microwaves used in
European remote sensing satellites (ERS) and Radarsat
SAR systems (section 7.2) are much longer (in the range
of a few centimeters). Whatever the wavelengths or fre-
quencies used in the above-mentioned active imaging
radar systems, the forwarding waves are invariably polar-
ized. Of course, this does not apply to passive systems
that receive radiations emitted by the objects. Nevertheless,
even in those systems the analysis involves polarized
components of the signals received. It is, therefore, real-
ized that a simple, rather very basic, description of polar-
ization of light will be beneficial simply because polarized
light is commonly used for examining the structure of
ice. In principle, these descriptions of the polarized light
also apply to electromagnetic waves in general, including
radiations in the microwave frequencies. This is followed
by a narration of relevant optical properties of ordinary
ice (Ih), which are utilized in the techniques for revealing
the crystallographic structure of ice. Most important
of these properties is the birefringence of ice (Ih), which
is responsible for developing different shades and/or
colors, depending on light sources, in thin sections of ice.
A simple analysis is presented for ascertaining the opti-
mum range of thickness (0.4-0.8 mm) for the best colors
when thin sections are observed through cross‐polarized
white light.
The text then proceeds with a section on the prepara-
tion of thin sections of ice and sea ice in particular. Since
snow is an integral part of any ice cover and since the
microstructure of snow on ice covers are never discussed,
a small section is added for bringing attention to methods
recently developed for revealing the microstructural
aspects of snow using polarized light and sections with
thicknesses around 0.1-0.2 mm. The core of this section
is the solid‐state DMT technique, which preserves the
integrity of the ice and snow samples. A section on view-
ing thin sections follows. This includes a description of
polariscopes with large field of views for the accommoda-
tion of thin sections, up to 300 mm in diameter contain-
ing a large number of grains. The chapter concludes with
a brief description of a thermal etching technique that is
extremely powerful yet very simple and suitable for field
applications. Another powerful technique that can also
be used readily in the field is the use of the dual process
of chemical etching and replicating. The replicas can be
preserved and transported easily for further examinations
after returning from field trips. Both thermal etching and
chemical/replicating technique require polishing of only
the top surface.
Field techniques for obtaining samples of full‐thickness
sea ice cores or ice blocks or to conduct field measure-
ments of certain properties such as insitu temperature
profiles and vertical salinity distributions are not covered
here. Detailed descriptions of these methods carried out
on sea ice within a small area at a fixed location (Eclipse
Sound, near Pond Inlet, Baffin Island) for every week and
through the entire year for two consecutive years are
given in
Sinha and Nakawo
[1981] and
Nakawo and Sinha
[1981]. It suffices to mention here that when sampling ice
in the field precautions must be taken to preserve its
integrity as it exists in nature. It is impossible to remove a
full‐thickness core, for example, from the ice sheet with-
out any loss of brine at the bottom of the core. However,
the most important precaution is to ensure that a mini-
mum of brine drainage occurs during the extraction
process and prior to making the thin sections. For this
reason, ice cores and blocks should be extracted at ambi-
ent temperatures as low as possible (< −20 °C). This may
sound very restrictive, but not impossible in polar region
of the Arctic and the Antarctic. Of course, the ambient
working conditions could be far from pleasant, certainly
not summer adventure tour type. In any case, the freshly
sampled specimens should be packed immediately in
insulated bags and transported (if necessary for shipping
to distant laboratories) in insulated boxes filled with dry
ice, making sure that the samples do not come in contact
with dry ice in order to avoid any thermal shocks. The
samples should then be stored in deep freezers at tem-
peratures less than −30 °C, which is below the eutectic
points of most of the predominant salts in seawater. All
salinity measurements should be conducted in the field
laboratories as soon as possible after sampling. In case
the samples have to be shipped, preliminary measure-
ments must be made in the field and again in the labora-
tory at the final destination to make sure that no brine
drainage had occurred in the intervening period.
It must be emphasized that nothing substitutes the
work that has been carried out in the field if performed
with utmost care and precautions. In addition to the on‐
site measurements that are traditionally carried out, all
the necessary microstructural investigations on sea ice
must and should also be carried out in the field soon after
recovering the samples from the site. Ice researchers may
think that this is easier said than done, but the present
authors actually adhered strictly to this principle. Instead
of taking ice to distant laboratories, they took the labora-
tories to the ice. One such well‐known example is the
studies of Pond Inlet for ice in Eclipse Sound. Other
examples are the long‐term investigations in Mould
Bay and the Hobsons Choice ice island, presented in
Chapter 5. The ice island studies on MY sea ice was car-
ried out primarily because that site provided access to old
sea ice year after year. Nonetheless, various laboratory
