Aplis / Sedna 2007
Helicopter EM Data Aquisition
Report
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven,
Germany
May 18, 2007
Stefan Hendricks
Tel.: +49(0)471/4831-1874
Contact: stefan.hendricks@awi.de
Torge Martin
Tel.: +49(0)471/4831-1875
Contact: torge.martin@awi.de
Contents
1. Introduction
1
2. Data Processing
3
3. Delivery
3.1. List of Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. File Naming Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Data Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5
6
6
A. Profile Summary
7
B. Ice Observations Summary
19
List of Figures
29
List of Tables
30
1. Introduction
Based on the Applied Physics Laboratory Ice Station (APLIS) in the Beaufort Sea in April
2007, sea ice thickness has been surveyed by an helicoper electromagnetic (HEM) system
on several profiles which covers the local and regional thickness distribution as well as
their change in a time range over two weeks. This work has been funded by the NSF
respective the Sea Ice Experiment - Dynamic Nature of the Arctic (SEDNA) project.
The aim of this work is to delivery sea ice thickness information to quantify effects of
dynamics processes of sea ice at different scales. In total 11 flights have been performed
between the April, 4 and April, 13 (see figure 1.1). The dynamics of the sea ice has been
covered by two hexagonal GPS buoy arrays with a baselength of 20 km and 140 km
respectively. These buoys have been used to determine waypoints for the helicopter waypoints to compensate for ice drift. Most of the EM flights were dedicated to map sea ice
thickness within these arrays repeatedly.
Figure 1.1.: Flighttracks of Helicopter-EM measurements during SEDNA 2007
For the larger scale situtaion two long transects have been performed to gather sea ice
1
1. Introduction
2
thickness information from the coast of Barrow (71.4 ◦ N) up to 75 ◦ N. Local thickness
data can be compared to other datasets like drill hole information, ground EM thickness,
ULS ice draft as well as laser scanner mapping of sea ice freeboard. In addition to the
sea ice thickness information, geolocated ice observation with a digital camera have been
performed during each flight.
2. Data Processing
EM sea ice thickness sounding is based on the conductivity contrast between snow and
sea ice on the one side which neglegible electrical conductivity and the saline ocean water
on the other side. In the Arctic Ocean the conductivity of the sea water is about 2 to 3
magnitudes larger than the conductivity of the sea ice. The value of the ocean water conductivity was obtained by CTD measurements prior to the processing of the data (2400
mS/m, Rob Chadwell).
The EM-Bird contains a system of transmitter and receiver coils as well as a laser altimeter which is sampling the altitude of the coils above the sea ice. The transmitter coils
emitts a harmonic (primary) electromagnetic field which induces eletrical eddy currents
in the conductive sea water. These currents are the source of a weaker (secondary) electromagnetic field, which is detected together with the primary field in the receiver coil. The
basic quantity of the EM technique is the strength of the secondary field relative to the
primary field. The ratio is represented by the Inphase and Quadrature component, the
real and imaginary part of the complex signal. The response of the Inphase and Quadrature channels can be modeled by a 1D layered earth forward model dependent on the
height of the instrument over the water level. (see figure 2.1).
Figure 2.1.: Derivation of ice thickness from EM data. Both curve represent 1D forward model
results for Inphase and Quadrature of a homgenous halfspace model with sea water conductivity of 2400 mS/m. Black dots on top curve show Inphase component of measurements over
open water. Other dots represents measurements records with sea ice present. Horizontal bar
marks total thickness.
Since the conductivity of snow and sea ice is small compared to the sea water, the model
3
2. Data Processing
4
consists of homogenous halfspace with 0 mS/m for air, snow and water and 2400 mS/m
representing the ocean. The Inphase channel gives a stronger signal than Quadrature
throughout the typical range of altitude variation of the helicopter yielding better SNR,
therefore it is used for the final data product. The basic idea of EM sea ice thickness
sounding is to estimate the distance of the instrument to the water-ice interface by using the inverted model function and substract the laser range to get ice plus snow (total)
thickness. Sea ice thickness can be obtained by this 1D approach for each channel independently.
Example data of measurements over open water and with sea ice present is shown in figure 2.1. The Inphase data in ppm is plotted versus the range of the laser altimeter. Over
open water the datapoints have to coincide with the 1D model function. An overpass
over a large lead with open water on April 13th shows a very good agreement with the
model, confirming the chosen conductivity value. Sea ice lead to a reduction of the laser
range for at a given ppm value, which leads to a shift of the datapoints to left of the model
curve by the total thickness (horizontal bar in figure 2.1).
Negative ice thickness values can occur when the laser range is larger than the model
result for a given Inphase value. Roll events of the EM bird and surface waves can result
in such negative ice thickness values up to -5 till -10 cm which lies within the accuracy of
the system.
Drill hole measurements have shown that the general accuracy over level ice lies in the
range of ± 10 centimeters. Over deformed ice in particular over ridges the EM can underestimate the real ice thicknes by about 50 %. One reason is possible saline water intrusion
in unconsolidated ridges, which yields in significant conductivity values of the sea ice. A
second effect is the footprint of the EM system, caused by the diffuse nature of the electromagnetic field. The footprint is defined as a box in which 90% of the induction process
take place and has a size of about 4 time the height of the instrument. For normal measurement operations the footprint varies between 40 and 60 meters. Sea ice topography
features smaller than the footprint are smoothed out and underestimated in thickness.
EM data is recorded with a frequency of 10 Hz yielding a datapoint spacing of 3 to 4
meters for typical speed of the helicopters. This datapoints are oversampled by the laser
altimeter by a factor of 10 (100 Hz). For the data processing the laser datapoints are
averaged to match the EM footprint.
3. Delivery
3.1. List of Profiles
One EM data file is delivered for each flight. A more detailed description of the EM data
is given in the appendix.
HEM_SDN07_20070404T225737_20070405T004126
Northern outer buoy array ( WP: 60, 53 )
HEM_SDN07_20070405T012315_20070405T031037
South-western outer buoy array ( WP: 62, 55 )
HEM_SDN07_20070405T165801_20070405T185631
Eastern outer buoy array ( WP: 57, 61 )
HEM_SDN07_20070405T222347_20070405T232202
Inner buoy array ( WP: all buoys )
HEM_SDN07_20070407T000557_20070407T005843
Validation line and multibeam site
HEM_SDN07_20070409T182741_20070409T192855
Inner buoy array ( WP: all buoys )
HEM_SDN07_20070410T214156_20070411T001540
North transect
HEM_SDN07_20070411T191302_20070411T205103
South-western outer boy array ( WP: 62, 55 )
HEM_SDN07_20070412T002326_20070412T020215
Northern outer buoy array ( WP: 60, 53 ) and ridge site
HEM_SDN07_20070413T001433_20070413T022352
Eastern outer buoy array ( WP: 57, 61 )
HEM_SDN07_20070413T220316_20070414T003129
Transect to Point Barrow
Table 3.1.: List of Profiles
5
3.2. File Naming Convention
6
3.2. File Naming Convention
The filename contains a shortcut for the campaign and the start and stop time of the data
file. The id for the APLIS/Sedna field campaign is SDN07.
HEM_CMPID_SSSSSSSSSSSSSSS_PPPPPPPPPPPPPPP.dat
Token
Description
CMPID
Contains campaign name ( 3 letters + 2 digits of year )
SSSSSSSSSSSSSSS
PPPPPPPPPPPPPPP
YYYYMMDDTHHMMSS : Start and Stop time
Table 3.2.: File naming convention of EM data files
3.3. Data Format
The EM data is delivered in blank separated ASCII data format described in table 3.3. All
time tags are standard GPS time.
Column
Description
Format
Unit
1
Year
I4
–
2
Month
I2
–
3
Day
I2
–
4
Time
F8.2
Seconds of the day
5
Fiducial Number
I9
–
6
Latitude
F12.7
degree
7
Longitude
F12.7
degree
8
Distance
F12.3
m
9
Thickness
F8.3
m
10
Laser Range
F8.3
m
Table 3.3.: File format for EM data delivery
A. Profile Summary
7
A. Profile Summary
8
Figure A.1.: Profile Summary of first flight on April, 4
A. Profile Summary
9
Figure A.2.: Profile Summary of second flight on April, 4
A. Profile Summary
10
Figure A.3.: Profile Summary of first flight on April, 5
A. Profile Summary
11
Figure A.4.: Profile Summary of second flight on April, 5
A. Profile Summary
12
Figure A.5.: Profile Summary of flight on April, 6
A. Profile Summary
13
Figure A.6.: Profile Summary of flight on April, 9
A. Profile Summary
14
Figure A.7.: Profile Summary of flight on April, 10
A. Profile Summary
15
Figure A.8.: Profile Summary of first flight on April, 11
A. Profile Summary
16
Figure A.9.: Profile Summary of second flight on April, 11
A. Profile Summary
17
Figure A.10.: Profile Summary of flight on April, 12
A. Profile Summary
18
Figure A.11.: Profile Summary of flight on April, 13
B. Ice Observations Summary
19
B. Ice Observations Summary
Figure B.1.: Ice Observation points of first flight on April, 4
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B. Ice Observations Summary
Figure B.2.: Ice Observation points of second flight on April, 4
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B. Ice Observations Summary
Figure B.3.: Ice Observation points of first flight on April, 5
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B. Ice Observations Summary
Figure B.4.: Ice Observation points of flight on April, 9
23
B. Ice Observations Summary
Figure B.5.: Ice Observation points of flight (first leg) on April, 10
24
B. Ice Observations Summary
Figure B.6.: Ice Observation points of flight (second leg) on April, 10
25
B. Ice Observations Summary
Figure B.7.: Ice Observation points of first flight on April, 11
26
B. Ice Observations Summary
Figure B.8.: Ice Observation points of second flight on April, 11
27
B. Ice Observations Summary
Figure B.9.: Ice Observation points of flight on April, 12
28
List of Figures
1.1. HEM Flighttracks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2.1. Derivation of ice thickness from EM data . . . . . . . . . . . . . . . . . . .
3
A.1. Profile Summary of first flight on April, 4 . .
A.2. Profile Summary of second flight on April, 4
A.3. Profile Summary of first flight on April, 5 . .
A.4. Profile Summary of second flight on April, 5
A.5. Profile Summary of flight on April, 6 . . . . .
A.6. Profile Summary of flight on April, 9 . . . . .
A.7. Profile Summary of flight on April, 10 . . . .
A.8. Profile Summary of first flight on April, 11 .
A.9. Profile Summary of second flight on April, 11
A.10.Profile Summary of flight on April, 12 . . . .
A.11.Profile Summary of flight on April, 13 . . . .
B.1.
B.2.
B.3.
B.4.
B.5.
B.6.
B.7.
B.8.
B.9.
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Ice Observation points of first flight on April, 4 . . . . . .
Ice Observation points of second flight on April, 4 . . . .
Ice Observation points of first flight on April, 5 . . . . . .
Ice Observation points of flight on April, 9 . . . . . . . .
Ice Observation points of flight (first leg) on April, 10 . .
Ice Observation points of flight (second leg) on April, 10
Ice Observation points of first flight on April, 11 . . . . .
Ice Observation points of second flight on April, 11 . . .
Ice Observation points of flight on April, 12 . . . . . . . .
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List of Tables
3.1. List of Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. File naming convention of EM data files . . . . . . . . . . . . . . . . . . . .
3.3. File format for EM data delivery . . . . . . . . . . . . . . . . . . . . . . . .
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