Ground borne EM
ice
thickness data
from GreenICE field campaign 2003
Acquired during the Polarstern cruise ARK
19-1b
29th March - 24th
April 2003
GreenICE deliverable D7
Andreas Pfaffling & Jan Lieser
December 2004
Executive summary
This report summarizes results obtained during
the first GreenICE campaign in 2003. It was performed in Fram Strait on
board
RV Polarstern between March 29 and April 24, 2003. Ice thickness was
obtained
by means of ground-borne electromagnetic induction (EM31) mainly on the
big
multiyear floe Polarstern was anchored to from the 9th until
the 17th
April during the eleven day long ice drift station. The level ice
thickness of
the floe was found to be 3 meter with a small 2 meter thick refrozen
lead
in-between. All data and this report can be downloaded from
http://www.awi-bremerhaven.de/Modelling/SEAICE/GreenICE/2003/EM31Profiles.
Table of content
Executive
summary...........................................................................................................................
2
The method.......................................................................................................................................
2
Description of the used
instrument...................................................................................................
2
Results:.............................................................................................................................................
3
Floe wide EM profiles:.................................................................................................................
3
7th April,...................................................................................................................................
3
12. April,...................................................................................................................................
6
High resolution EM
profiles.........................................................................................................
7
1. April,.....................................................................................................................................
7
13. April,...................................................................................................................................
8
The method
With EM sounding, a low-frequency EM field is generated by a transmitter coil. This field induces eddy currents in the water, which in turn result in a secondary EM field. The strength of this field is measured by a receiver coil. As the strength of any EM field decreases with distance to the source, the secondary field strength decreases with increasing distance between the EM instrument and the water underneath the ice. Thus, the thicker the ice is, the weaker the secondary field becomes. Ground-based measurements have proven to provide very accurate data. Their calibration is evaluated by means of accompanying drill-hole measurements. However, ground-based measurements are only possible on single, thick floes, and the profile lengths are very limited
Description of the used instrument
The EM31 operates at a frequency of 9.8 kHz with a coil spacing of 3.66 m. The EM31 provides high accuracy data and the procedures are well established. The EM31 was placed into a Prijon kayak serving as amphibic sledge, to enable measurements over melt ponds and to shelter the instrument.
Results:
Table 1 gives an overview of GreenICE related EM31 groundwork during the Polarstern Arctic cruise ARK 19/1b in northern Fram Strait done in early April 2003. Work was carried out in two different approachs: (1) Long, GPS positioned EM tracks were performed to yield regional scaled ice thickness information, done on the drift floe Polarstern was anchored to. (2) Short profiles of 200m length with high resolution measurements of snow depth as well as ice thickness obtained by EM and auger where done both on a ship station on the way north as well as on the drift floe.
|
date |
UTC time |
Lat [ÁN] |
Lon [ÁE] |
length |
|||
|
20030407 |
10:09-12:28 |
81.9-81.92 |
9.55-9.65 |
6.5 km |
|||
|
20030412 |
12:45-13:07 |
81.85 |
9.91-9.99 |
1.3 km |
|||
|
20030401 |
9:30 |
80.427 |
12.817 |
200 m |
|||
|
20030413 |
Subset of 20030412 |
200 m |
|||||
|
|
samples |
TTEM |
drillings |
snowdepth |
TTdrill |
||
|
|
1777 |
3 (3.6) |
- |
- |
|
||
|
|
1313 |
2.9 (3.6) |
- |
- |
|
||
|
|
40 |
2 (2.1) |
1 |
0.1 (0.17) |
|
||
|
|
71 |
2.8 (3.4) |
11 |
0.05 (0.11) |
2.6 (3.1) |
||
Table 1: Summary of ground based EM sea ice thickness profiling. TTEM: Total thickness measured by EM, TTdrill: Auger thickness. All thickness and snowdepth is in meter and corresponds to the mode and (median) of the thickness distribution calculated from the profile data.
Floe wide EM profiles:
Large scale EM31 profiles were done measuring every 4 seconds while dragging the kayak continuously along a given or randomly chosen track. A handheld GPS was used to acquire the positions of the track. GPS positions and EM data were merged later in the lab using the time stamp of both devices.
7th
April,
first exploration of the chosen drift floe in
"random
walk".
As a ground borne sea ice thickness reconnaissance, the GreenICE field party explored the ice floe, which was chosen for the 11 day drift station by the shipboard scientists on Polarstern. The EM31 kayak was dragged across the whole floe from Polarsterns anchoring site to the opposite end where a small lead separated the adjacent ice floes. Then the profile turns left towards the northwestern margin of the floe turning back toward the ship due to decreasing visibility. The track and thickness is illustrated in figure 1, showing relative distance in meter to the starting point (rel. Northing, rel Easting) additionally to the geographical coordinates. On the way back not the full transect is displayed due to a GPS failure as a result of the low ambient temperature. On approximately half way to the far edge of the floe the line crosses the only explored area of the floe with thickness less than three meter (see also fig. 2). The localization of this thin area was very helpful for other work groups on the ship who depended on drilling or cutting through the ice which turned out to be a challenge on the three meter thick multiyear floe.
Figure 1: EM kayak track crossing the drift station ice floe along its full NE-SW extent.
EM kayak operators after 2.5 hours at Ð30ÁC
Figure 2: Ice thickness profile along track shown in fig. 4, showing the only less than 3m thick area of the floe, presumably a refrozen lead.
Figure 3: Ice thickness histogram computed from the thickness data obtain along the track shown in figure 1. Modal thickness is 3.0 meter, with a median of 3.6 m.
12.
April,
along coincident EM-Bird and Laser scanner
flight track:
On the 11th April the first of two coincident AWI - HEM and KMS - Laser scanner / Asiras flights was successfully carried out. The ground track of the airborne platforms was followed up on the floe during the flights. However, due to technical problems the data from the 11th were not convertible and therefore the profile was done again on the 12th April. Though the profiled line on the floe was a straight one, the track in figure 4 shows a bend, reflecting the ice drift during the kayak observations.
Figure 4: EM kayak track along the ground track of the HEM + Laser scanner flights on 11th April. Determined sea ice thickness is illustrated by the color of the line.
Figure 5: Ice thickness profile along track shown in fig. 4.
Figure 6: Ice thickness histogram computed from the thickness data obtained along the track shown in figure 4. The level ice thickness is 2.9 meter (median 3.6m).
High resolution EM profiles
1.
April,
mostly level ice
On the way north toward the final drift station in northern Fram Strait an ice station was held north of Svalbard on the 1. April. Along a 200m line EM31 and snow thickness measurements were conducted successfully. Following a drilling motor failure, however, only one auger thickness could be obtained. Snow thickness was measured by snow ruler in one meter spacing.
Figure 6: Sea ice + snow thickness profile from sledge EM (line) and one control drill (circle at profile starting position).
Figure 7: (a) Ice thickness histogram computed from the thickness data shown in figure 6. Mode: 2 meter. (b) Snow thickness distribution from 200 samples taken along the same line (mode: 10 cm).
13.
April,
subset of long, GPS based line on 12th
April:
As a follow up to the long, sledge EM transect on the 12th April, which transects the whole drifting floe, a subset of that line was repeatedly profiled with the EM31 accompanied by 11 drillings to obtain auger thickness for comparison one day later. The satisfying agreement between auger and EM thickness can be seen in figure 8. The small disagreement of the modal thickness of EM and Drill, of 2.8 m and 2.6 m respectively fits nice into the picture, taking into account that the point measurements of the auger miss out on some topography, which is sensed be the EM. Again, snow thickness was profiled in one meter spacing with ruler measure.
Figure 8: Sea ice + snow thickness profile from sledge EM (line) and auger (circles).
Figure 9: (a) Ice thickness histogram computed from the EM - thickness data shown in figure 8 (mode: 2.8 meter). (b) Snow thickness distribution from 200 samples taken along the same line (mode: 5 cm).