Geology Reference
In-Depth Information
5
5
Top surface
Top surface
0
0
-
5
-
5
-
10
-
10
Bottom surface
Bottom surface
-
15
-
15
-
20
-
140
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20
-
120
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100
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80
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60
Distance (m)
-
40
-
20
0
20
0
20
40
60
80
100
120
Distance (m)
Figure 2.63
Top and bottom surface profiles of MY ice in the Nares Strait, North of Baffin Bay (left) and the Canadian Arctic
Archipelago (right). The two points with thickness more than 15 m represent the maximum length of the thickness auger. Therefore
the ice could be thicker. The missing points from the top surface correspond to locations of thick snow accumulation exceeded
1 m at some places so that the crew could not remove it completely [
Johnston and Timco
, 2008].
in Mould Bay, Prince Patrick Island in the High Arctic.
Borehole indentor (BHI) strength is three‐dimensionally
confined compressive strength of ice, measured using a
borehole indentor or a jack [
Sinha et al
., 1986;
Sinha
,
1987a;
Shkhinek et al
., 2010;
Sinha
, 2011;
Sinha et al
.,
2012]. The NRC-BHI test system, designed by the author
(N.K. Sinha) and can be seen in Figure 1.5, consists of a
high‐strength stainless steel hydraulic cylinder with two
laterally acting pistons and two indentor plates, curved to
match the wall of a borehole made in the ice. The new
Russian BHI [
Shkhinek et al
., 2010;
Sinha et al
., 2012]
consist of one piston with a curved indentor plate and a
large fixed back plate, operates in large diameter holes.
Once activated, the pistons inside the body of the jack
apply hydraulic pressure to the front and rear indentor
plates. The oil pressure and displacement of the indentor
plates in case of NRC-BHI are recorded by an external
digital acquisition system [
Sinha
, 2011;
Sinha et al
., 2012;
Johnston et al
., 2001].
Johnston et al
. [2003] used the
NRC-BHI and found that in August, SY ice has only 19%
of its mid‐winter strength. On the other hand, two MY
floes of about 5 and 6 m thick tested in June and August
showed that the strength was 56 and 47%, respectively of
their maximum mid‐winter strength of about 30 MPa.
For comparison purpose, FY ice in summer has about
10-20% of its mid‐winter strength by mid‐July. These
results clearly highlight the importance of distinction
between SY and older ice for the purpose of determining
the ice load on marine vessels and structures.
It is difficult to identify the age of MY ice visibly.
Trained ice observers of the Canadian Ice Service (CIS;
see Chapter 11) are able to distinguish SY from MY
ice with sufficient confidence but not the age of MY ice
beyond SY. However, this distinction is important
for marine operators because the mechanical load is
proportional to the age of the ice. In order to provide end
users with information and tools to identify old ice types,
the Canadian Hydraulics Centre of the National Research
Council of Canada completed a study to develop guide-
lines for the identification of these types [
Timco et al
.,
2008]. This was part of the Canadian Climate Change
Technology and Innovation Initiative (CCTII); an Arctic
Transportation Project.
The information used to distinguish between SY and
older ice includes geometrical and visual appearances of
ice floes and history tracing. Edges of SY ice floes exhibit
more angular geometry than the mostly rounded edges
of older ice, which are caused by the more frequent colli-
sion of floating floes. Melt ponds on SY ice are blue or
greenish‐blue and more elongated with preferred direc-
tion. On the other hand, ponds on older ice are usually
blue with no preferred shape or orientation. SY ice has
much less number of hummocks with less height than
older ice. Drainage patterns that interconnect puddles
and ponds at the surface are less extensive on SY ice.
Older ice is often covered with debris, which gives the ice
a characteristic dirty appearance. Finally, fragments of
SY ice overturned by a ship or any structure are less blu-
ish than fragments of MY ice [
Johnston and Timco
,
2008]. However, none of the above geometric or surface
features could be used to reliably distinguish between
SY and older ice in remote sensing data.
In a study by
Bjerkelund et al
. [1985], the authors con-
ducted detailed monitoring of FY ice throughout its
aging process into SY and third‐year ice in Mould Bay,
Prince Patrick Island in the Canadian western Arctic
(76°N, 119°W). The study covered the period from
October 1981 to May 1983 (i.e., two successive ice forma-
tion seasons). The experience of the Mould Bay experi-
ment is presented in details in section 5.1. The aging
