CEAREX Sea Ice Data

The CEAREX sea ice data include ice floe acceleration data, ice floe deformation data, and sea ice stress data.

Information in this document has been derived from documentation files on NSIDC's CD-ROM titled Eastern Arctic Ice, Ocean and Atmosphere Data, Volume 1: CEAREX-1, version 1.0, 8/91.

Contents

Data Description
Data Acquisition - Processing - Organization - Format
Investigators | References | Acronyms
CD-ROM Documentation Files

Data Description:

Ice Accelerometer Data

The CEAREX ice accelerometer data were collected during Leg II of the POLARBJORN Drift from October through November 1988. The data consists of 76 sampled data hours that occurred between Julian Day (JD) 308 and JD 330, and include most hours from the four periods during which significant accelerations above the noise level were observed. The data include all of the hours analyzed in Martin and Drucker (1991). Other hours are included for completeness and to provide several hours in which nothing happened as a control.

Buoy activity consisted of a noise floor and at least four periods of enhanced activity. Most of the observed peaks occur in these periods:

• JD 12 through JD 314
  
• JD 17 through JD 321
  
• JD 22 through JD 324
  
• JD 27 through JD 329

The investigators found that accelerations during the first period resulted from shearing of the adjacent ice; those during the second period from a mixture of shearing and ridging, and those during the third period from ridging. Most activity occurred during the fourth period when the floes broke up and experienced large accelerations. Ice acceleration measurements stopped on JD 329 when the last coherent signal was received before the buoys were apparently destroyed by ice forces.

Ice Floe Deformation Data

Ice floe deformation data files hold the time series of the deformation of a single line defined by a master and a remote transponder pair.

Sea Ice Stress Data

Because ice stresses could be correlated with other environmental parameters, CEAREX provided an ideal setting for measuring the stresses. Pack ice geophysical stresses in the eastern Arctic were monitored for the two month drift period by investigators from BDM International, Inc. (BDM) the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL).

BDM investigators measured in-plane compressive stresses in a multiyear floe in the eastern Arctic during October and November 1988. In addition to stress data, ice mechanical properties of flexural strength, in-plane compressive strength, and elastic modules of ice samples were determined in a shipboard laboratory. Stress invariants, principal stresses and direction of the largest compressive stress are included on the CEAREX CD-ROM.

CRREL investigators obtained high quality pack ice stress data. Stresses were monitored during a floe disintegration when a large deformation occurred at the end of the experiment. Data from different vertical levels and different horizontal locations were obtained.

Data Acquisition:

Ice Accelerometer Data

The CEAREX ice accelerometer data were collected from the ship POLARBJORN, which was frozen into the sea ice north and east of Svalbard. The ship was moored between two multiyear ice floes, a kilometer-scale floe (Alpha) and a 100 meter-scale floe (Beta). These floes in turn were surrounded primarily by multiyear ice floes. During the drift, three ice accelerometer buoys were deployed at three sites (Alpha-1, Alpha-2, and Beta), where the buoys were located on their named floes. The buoys were used to measure the ice accelerations associated with deformation and breakup. The investigators also measured the relative compass heading of each buoy. Details of the buoy deployment and of their locations relative to the ship and the floes are given in Martin and Drucker (1991).

Each buoy contained a compass and a set of three orthogonally mounted accelerometers, with two accelerometers in the horizontal plane and one in the vertical. For each buoy, the accelerometers are referred to as X, Y and Z, where Z is vertical, and X and Y lie in the horizontal plane, but have no particular orientation. At the beginning of each hour, each buoy transmitted 840 seconds of acceleration data, followed by 60 seconds of calibration and compass data. At the ship, the data were received, digitally sampled at 10 Hz, and recorded. The compass heading was sampled at hourly intervals and transmitted with the calibration data. The standard deviation of the quiescent acceleration was about 0.5 mm/s2, except for buoy Alpha-2, channel Y, which was noisier than the others and had a standard deviation of about 1 mm/s2 . The bandpass of the accelerometers was 0.04 to 5 Hz or 0.2 to 25 s.

The investigators arrived at the ship on October 11 (JD 285) and deployed the three buoys by October 22 (JD 296). Data collection occurred for 15 minutes each hour beginning at six minutes past the hour. The hourly transmitting and recording process, which was interrupted at approximately weekly intervals to download data from the computer, continued until November 25 (JD 330). Between JD 296 and JD 330, the ship and buoys drifted southwest in the broad passage between Svalbard and Franz Josef Land toward Kvitoya Island, a small island east of Svalbard lying between the Arctic Ocean and the Barents Sea. Pritchard and twenty-eight others (1990) shows the drift track of the ship for this period. Before JD 310, the ice drifted as a solid body, and no activity from the buoys was observed. Between JD 310 and JD 329, accelerations were observed in the ice cover during discrete periods. On JD 327, floes Alpha and Beta broke up, and the ice experienced its largest accelerations and rotations. This continued until JD 329, when the buoys were apparently destroyed by compressive forces.

Martin and Drucker (1991) concentrate on the ice behavior between JD 310 and JD 330. Martin and Becker (1987, 1988) discuss instrument design.

Ice Floe Deformation Data

Ice floe deformation data were collected during the drift phase of CEAREX by a master and a remote transponder pair. One master transponder (B) was located over the bridge of the POLARBJORN while the second master transponder (A) was located on Alpha floe perpendicular to the ship at the bridge location of the first transponder. The master transponders defined a base line115 meters long, perpendicular to the ship. The three remote transponders were located in the vicinity of the ship. Remote 1 was about 1 kilometer dead ahead of the ship. Remote 2 was on Beta floe about 500 meters from the ship. Remote 3 was beyond Beta floe, separated from Beta floe by an active lead. Raw data were gathered at two minute intervals from JD 282 through JD 320.5. At JD 320.5, the POLARBJORN moved and the baseline was lost.

BDM Stress Data

Stresses were measured using a hydraulic fluid-filled, flatjack-type stress sensor, 20 cm in diameter. The sensors were manufactured by GEOTECH. Coon (1988) describes the sensor. The range of the sensors was 0 to 689 kPa with an accuracy of +/-1.7 kPa.

The sensors were installed approximately 230 meters from the ship. At the stress site, ice thickness averaged 1.60 meters, with thickness variations of less than 20 cm within a 15 m region. Coon et al. (1989) discuss the details of implantation methods and data collection. The sensors were installed at roughly the neutral surface of the floe. Three sensors were installed in a rosette pattern to allow calculation of principal stresses. A thermistor was installed at the stress sensor depth to monitor ice temperature. Sensor and thermistor data were recorded on a Campbell Scientific data logger. Data samples were taken once per second, averaged over a two minute interval, and the two minute average values stored in a Campbell Scientific SM107 storage module. Stress data were subsequently downloaded into a Macintosh SE computer. Coon et al. (1989) discuss the data processing. There is a continuous record from JD 279 through JD 327, except for data dropouts on JD 283 and JD 284. Missing data on day 284 are due to a test of the stress sensors to check their coupling to the ice.

In addition to stress data, ice mechanical properties of flexural strength, in-plane compressive strength, and elastic modulus of ice samples in the vicinity of the stress gauges were determined in a shipboard laboratory. Coon et al. (1989) and Lau and Browne (1989) discuss test methods and present selected data. The ice samples tested were predominantly first year lead ice, however, a limited number of flexural tests were run on multiyear ice. All tests were run in a controlled temperature laboratory at -10 degrees C. Compression tests were run on samples where the load was applied in the horizontal or "C" plane. Bending tests were loaded in the plane of crystal growth. Tests were run at a strain rate of approximately 1.0 x 10**-4.

Stress invariants, principal stresses and direction of the largest compressive stress are included on the CEAREX CD-ROM. While stress invariants are a useful parameter for comparison with other scalar quantities such as ambient noise, they provide no information about the directionality of the stress. To compare stress with vector quantities such as wind, current or deformation, the user must have information about the stress direction. For a discussion of the stress state in a body (i.e. principal stresses, stress invariants, relationships between stress and strain, etc.), please refer to the CD-ROM documentation file.

CRREL Stress Data

The POLARBJORN was moored to a multiyear floe and allowed to drift with the pack while the behavior of the ice, ocean and atmosphere were monitored. From September 18 to November 25, the ship drifted from 82 degrees 40 minutes north, 32 degrees 26 minutes east to 80 degrees 10 minutes north, 31 degrees 13 minutes east.

The stress measurements were obtained with biaxial vibrating wire stress sensors similar to those described by Cox and Johnson (1983). These instruments were chosen because of their relative ease of installation and maintenance, the extensive calibrations that had been previously performed (Cox and Johnson 1983), and their success in earlier field programs (Johnson et al. 1985). The sensor consists of a stiff steel cylinder which is 0.25 m long, 57 mm in diameter and has a wall thickness of 16 mm. The gauge is much stiffer that the ice, having a modulus of about 200 GPa, and thus should not be affected by spatial variations or temporal changes of the ice modulus. Six tensioned wires are set at 30ø intervals across the inside diameter of the gauge. Only three wires are necessary for the determination of principal stress components; the remaining wires are for redundancy.

The POLARBJORN was moored to a multiyear floe on September 16, 1988 at 82 degrees 40 minutes north, 32 degrees 26 minutes east. Over the next two weeks, four stress sensor sites were established on each of two adjacent multiyear floes. At site 1, on floe Alpha, three sensors were installed at vertical levels near the top, mid-depth and bottom of the ice sheet. At site 2, which was located on the edge of the multiyear floe Beta adjacent to a freezing lead, one sensor was placed in the first year ice while the remaining two were located at shallow depths in the multiyear ice. Because of sensor and data logger difficulties, the data from sites 3 and 4 were generally unreliable.

The sensors were installed in holes approximately 0.1 m in diameter made with a standard ice coring auger. Each sensor is attached to a length of PVC pipe suspended to the proper depth from the surface by a cross bar through the PVC extension. Once the sensor was placed in the hole at the proper depth and leveled in the horizontal plane, the hole was filled with fresh water. Under normal conditions, the hole completely froze within a day or two, depending upon the air and ice temperature. Stresses associated with the "freeze in" can be from 200 to 300 kPa, and usually take several days to dissipate.

At site 1, the multiyear ice was 1.60 m thick. Sensors were installed at depths of 0.25, 0.70 and 1.20 m. At site 2, one sensor was located in the first year ice at a depth of 0.20 m. The thickness of this frozen lead increased from 0.38 to 0.54 m during the stress monitoring period. The sensor was located about 7.0 m from the edge of the multiyear floe in the thin ice which was about 200 m wide. The two remaining sensors were located in 2.0 m thick multiyear ice, at distances of 2 and 15 m from the edge of the floe. The three sensors were configured in a straight line normal to the edge of the floe and the frozen lead.

Data from each site were recorded on Campbell Scientific, Inc. data loggers. The sensors were sampled and recorded at two minute intervals, and the period (frequency) of each wire in each sensor was stored. The data loggers cached data in memory modules that required replacement with empty modules at five to seven day intervals. Normally, no interruption in sampling occurred during this exchange, though there were occasional breaks in the data if the memory modules were filled to capacity.

Data Processing:

Ice Accelerometer Data

The acceleration data were minimally processed in order to allow the greatest flexibility of use. The only alterations of raw digital data were the removal of means, the filtering out of certain large errors due to radio noise, and the reformatting of binary data into ASCII.

The buoy acceleration signals were FM-telemetered to a ship receiver on three audio frequency FM subcarriers, individually discriminated, digitally sampled, and stored in binary files in a 2-byte format. The sampling resolution was approximately six bits per mm/s2 and varied slightly per channel. Calibration was provided by pre-deployment measurement and by the one-minute calibration cycle following each collection period.

Because of its greater machine independence, the ASCII format was used for the CEAREX CD-ROM. To convert to ASCII, the binary data were first read into memory arrays. Then, the mean of each time series was computed and removed. This was justified because the buoy instruments were AC-coupled and any remaining mean in the signal would be due to offsets in the discriminators.

After removing the means, the signals were passed through software that identified large single-point errors probably due to radio noise. A threshold of 5 mm/s2 above or below adjacent samples was used. Samples lying outside the thresholds were replaced by the mean of their immediate neighbors. These are not flagged.

The two byte samples represented signed integer data with a range of -32768 to +32767. These were multiplied by gain constants that had been established for each channel. These constants varied slightly by channel but were approximately 0.16 mm/s2 per bit. Thus, although the resolution of the five digit data representation is 0.01 mm/s2, its true resolution is only about 0.16 mm/s2 because of the analog to digital conversion (quantization).

The resulting data in units of acceleration have a possible range of about +/- 5000 mm/s2 . However, no actual accelerations above a few hundred mm/s2 were experienced. To reduce data size for the CD-ROM, the range was limited to +/- 999.99 mm/s2; each sample was then multiplied by 100 and rounded to integers to provide a five digit integer with no decimal point. Thus the samples on the CD-ROM can be put directly into acceleration units of mm/s2 by dividing by 100.

Ice Floe Deformation Data

Gaps in the raw data were linearly interpolated. The data were smoothed by low passing frequencies at a cutoff frequency of eight per day. The smoothed data were then resampled at 15 minute intervals. The accuracy of the ranges is about +/- 0.1 meter.

Data Organization:

The sea ice data are presented in 103 data files in three subdirectories on the CEAREX CD-ROM, one each for ice accelerometer data, ice floe deformation data, and stress data. The stress subdirectory contains two subdirectories, one for BDM stress data and another for CRREL stress data.

Ice Accelerometer Data

The ice acceleration data are organized into hourly files. Each file is named according to its collection day and hour in the format "Addd.hh" where "ddd" is the day of the year and "hh" is the hour in Universal Time (UT). Data collection occurred for 15 minutes each hour beginning at six minutes past the hour. Thus, for example, the file A328.01 was collected from 01:06 UT to 01:21 UT on JD 328 (November 23, 1988).

Each file consists of nine time series corresponding to the nine channels (three buoys; three channels per buoy). There are 9000 samples per channel, corresponding to a sampling rate of ten samples per second for 900 seconds. Approximately the first 8400 samples are acceleration data; the last minute of each collection period consists of calibration signals and compass readings. This period is easily distinguished visually from the data period by its large pulse-like excursions.

Ice Floe Deformation Data

The ice floe deformation data are contained in six files on the CEAREX CD-ROM. Each file holds the time series of the deformation of a single line defined by a master and a remote transponder pair. The correspondence between the data files and the master remote pairs is defined as follows:

• POS07.DFM master A/remote 2
  
• POS08.DFM master B/remote 2
  
• POS09.DFM master A/remote 3
  
• POS10.DFM master B/remote 3
  
• POS11.DFM master A/remote 1
  
• POS12.DFM master B/remote 1

Stress Data

The CRREL stress sensor data consists of six files, each containing the entire time series of data from one sensor.

Data Format:

Ice Accelerometer Data

The files are written as ASCII flat files of columnar number tables. There are nine columns of space-delimited numbers corresponding to the nine time series, and 9000 rows corresponding to the 9000 samples per channel. The order of the channels is as follows:

BETA X / Y / Z 1ALPHA X / Y / Z 2ALPHA X / Y / Z

The sample numbers are written as five digit decimal integers in a seven character field. This provides for one sign character (minus sign or space) and a minimum of one additional space between entries.

The decimal integers represent accelerations in units of 0.01 mm/s2. That is, the data may be read as mm/s2 by dividing by 100. The dynamic range is thus -999.99 to 999.99 mm/s2. Sample values in excess of this range are set to the limits (+/- 99999), but any such data should be considered spurious.

To facilitate machine reading of the data and simplify importation into spreadsheets, graphics software, or other software, the data files contain no headers. It is essential that the file name not be lost as it contains the only reference to the file identity. The file names are in the format "A308_09.ACC" where "308_09" represents "308.09," the beginning year-day of the data. Because the file names on a CD-ROM must have a 3-character extension, the decimal point in "308.09" was changed to an underscore ( _ ).

Compass Data

In addition to the acceleration data, a single ASCII file on the CD-ROM contains compass readings for selected hours between JD 300 and JD 330. The compass readings were manually derived from the information in the 60-second calibration cycle at the end of each hourly sample. Because of the laborious nature of extracting them, compass readings are given only as needed; i.e., hourly readings are only provided during periods of rapid rotation. This occurred primarily after JD 327, when floes Alpha and Beta broke up into small floes that rotated relative to one another. The compass readings are in units of degrees with arbitrary orientation. Because these numbers were derived manually, their accuracy is probably on the order of +/- 5 to 10 degrees. Missing values are flagged as 999 in the compass file.

Ice Floe Deformation Data

The FORTRAN format is (f8.4,2x,f8.1,1x):

• Time (JD) f8.4
  
• Blanks 2x
  
• Range (meters) f8.1
  
• Blank 1x

BDM Stress Data

IMPORTANT NOTE: Different sign conventions are used in stress measurements for the BDM and CRREL stress data. The BDM data employs standard solid mechanics sign conventions in which negative stress indicates compression and positive stress indicates tension. The CRREL data uses the rock mechanics notation in which compression is positive and tension is negative.

Two types of stress data files are presented on the CEAREX CD-ROM. File names with the extension ".INV" are stress invariants, while file names with the extension ".PRI" contain principal stress and direction of stress. Files are named with the Julian Day (JD) range of the data in each file.

Stress Invariant Files

Data fields are:

• time (JD)
  
• first invariant (kPa)
  
• second invariant (kPa)

The FORTRAN format of each field in the data record is 3(E9.6), with a comma following fields one and two. Column one of each field is the sign; positive is a blank, negative is "-". Missing data values are coded as "0.000000." As a data sample, the first three records from file 279_284.inv are:

• 2.795403E+02,-3.689658E+00, 2.707161E+00
  
• 2.795417E+02,-3.787702E+00, 2.753809E+00
  
• 2.795431E+02,-3.855021E+00, 2.694015E+00

Principal Stress Files

Data fields are:

• time (JD)
  
• first principal stress (Sigma 1) (kPa)
  
• second principal stress (Sigma 2) (kPa)
  
• direction of sigma 2 (degrees measured counterclockwise from east)

The FORTRAN format of each field in the data record is E9.6 with a comma following fields one, two and three. Column one of each field is the sign; positive is blank, negative is "-". Missing data values are coded as "0.000000". As a data sample, the first three records from file 279_284.PRI are:

• 2.795403E+02,-9.824974E-01,-6.396819E+00, 7.700866E+01
  
• 2.795417E+02,-1.033894E+00,-6.541511E+00, 7.591252E+01
  
• 2.795431E+02,-1.161006E+00,-6.549036E+00, 7.558513E+01

CRREL Stress Data

IMPORTANT NOTE: Different sign conventions are used in stress measurements for the BDM and CRREL stress data. The BDM data employs standard solid mechanics sign conventions in which negative stress indicates compression and positive stress indicates tension. The CRREL data uses the rock mechanics notation in which compression is positive and tension is negative.

Each CRREL stress sensor data file consists of a header line followed by data, with each data line containing:

• time (decimal JD)
  
• the first stress invariant
  
• the second stress invariant

Variables are separated by commas. Missing or bad data are coded as "******".

 File name       Sensor        Ice       Distance From      Ice
                 Depth      Thickness      Floe Edge        Type
---------------------------------------------------------------------
Group 1

 CRREL1A.INV      0.25 m       1.6 m         200 m         Multiyear
 CRREL1B.INV      0.70 m       1.6 m         200 m         Multiyear
 CRREL1C.INV      1.20 m       1.6 m         200 m         Multiyear

Group 2

 CRREL2A.INV      0.20 m       0.6 m           7 m         Young ice
 CRREL2B.INV      0.25 m       2.0 m           2 m         Multiyear
 CRREL2C.INV      0.25 m       2.0 m          15 m         Multiyear

Investigators:

Max D. Coon, BDM International, Inc. - BDM stress data
Robert Drucker, University of Washington - acceleration data
William D. Hibler III, Dartmouth College - deformation data
Mark A. Hopkins, Dartmouth College - deformation data
Paula A. Lau, BDM International, Inc. - compressive stress data
Seelye Martin, University of Washington - acceleration data
D. K. Perovich, U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) - stress data
W. B. Tucker III, U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) - stress data

For a complete list of all CEAREX investigators, please refer to the CEAREX Investigator Address List.

References:

Coon, G. F. N., and Johnson, J. B. 1983. Stress Measurements in Ice. U.S. Army CRREL Report 83-23, 31 pp.

Coon, M. D. 1988. Ice monitoring during CEAREX. In Workshop on Instrumentation and Measurements in the Polar Regions. Proceedings, 405-409. Sponsored by IEEE Oceanic Engineering Society, Marine Technology Society, Monterey Bay Aquarium, U.S. Navy and Science Applications International Corporation.

Coon, M. D., P. A. Lau, S. H. Bailey, and B. J. Taylor. 1989. Observations of ice floe stress in the eastern Arctic. In POAC 89. Port and Ocean Engineering Under Arctic Conditions. Proceedings, 44-53. Lule, Sweden: University of Technology.

Johnson, J. B., G. F. N. Cox, and W. B. Tucker III. 1985. Kadluk ice stress measurement program. In International Conference on Port and Ocean Engineering under Arctic Conditions (POAC), 8th, Narssarssuaq, Greenland. Proceedings, (1):88-100.

Lau, P. A., and C. M. Browne. 1989. Bending and compression properties of young sea ice. In OCEANS'89, Proceedings, (4):1292-1297. IEEE Publication no. 89CH2780-5.

Martin, S. and P. Becker. 1988. Ice floe collisions and their relation to ice deformation in the Bering Sea during February 1983. Journal of Geophysical Research 93(C2):1303-1315.

Martin, S. and R. Drucker. 1991. Observations of ice floe collisions during Leg-II of the POLARBJORN drift. Journal of Geophysical Research 96(C6):10567-10580.

Martin, S. and R. Drucker. 1991 Observations of short period ice floe accelerations during Leg II of the POLARBJORN drift. Journal of Geophysical Research 96(C6):10567-10580.

Martin, S. and P. Becker. 1987. High frequency ice floe collisions in the Greenland Sea during MIXEX-84. Journal of Geophysical Research 92(C7):7071-7084.

Popov, E. P. 1968. Introduction to Mechanics of Solids. Englewood Cliffs, NJ: Prentice Hall.

Pritchard, R., and twenty-eight others. 1990. CEAREX drift experiment. EOS, Transactions of the American Geophysical Union 71(40):1115-1118.

Timoshenko, S. P., and J. M. Gere. 1972. Mechanics of Materials. NY: Van Nostrand Reinhold Company.

Tucker, W. D. III, D. K. Perovich, M. A. Hopkins and W. D. Hibler III. 1991. On the relationship between local stress and strain in arctic pack ice. Annals of Glaciology 15:265-270.

Tucker, W. B. III, and D. K. Perovich. 1991. Stress measurements in drifting pack ice. Cold Regions Science and Technology 20(2)119-139.

Acronyms:

AC - alternating current
ASCII - American Standard for Computer Information Interchange
CD-ROM - Compact Disc-Read Only Memory
CEAREX - Coordinated Eastern Arctic Experiment
cm - centimeter
GPa - gigapascal
JD - Julian Day
kPa - kilopascal
NSIDC - National Snow and Ice Data Center
POAC - Port and Ocean Engineering under Arctic Conditions
PVC - polyvinyl chloride