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.
Buoy activity consisted of a noise floor and at least four periods of enhanced activity. Most of the observed peaks occur in these periods:
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.
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.
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.
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.
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.
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.
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 ( _ ).
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.
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:
Data fields are:
Each CRREL stress sensor data file consists of a header line followed by data, with each data line containing:
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
For a complete list of all CEAREX investigators, please refer to the CEAREX Investigator Address List.
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.
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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.