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Data Set ID:
NSIDC-0255

Ice Fabric Characteristics: Siple Dome, A Core, Version 1

This data set contains measurements of the orientation and deformation of the crystal c-axes (fabric) of ice core samples collected from the Siple Dome Ice Core A, Antarctica (81°S, 148°W) between 22.764 m and 992.385 m in depth. The instrument used for the measurements consists of a fiber-optic white light source, one fixed black-and-white video camera, and four rotation stages. The data are in ASCII tab-delimited text format and are available via FTP.

Geographic Coverage

Parameter(s):
  • Ice Core Records
  • Snow/Ice > Ice Fabric
Spatial Coverage:
  • N: 81, S: 81, E: 148, W: 148

Spatial Resolution: Not Specified
Temporal Coverage: Not specified
Temporal Resolution: Not specified
Data Format(s):
  • ASCII Text
Platform(s) GROUND-BASED OBSERVATIONS
Sensor(s): Not specified
Version: V1
Data Contributor(s): Larry Wilen
Data Citation

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Wilen, L. 2005. Ice Fabric Characteristics: Siple Dome, A Core, Version 1. [Indicate subset used]. Boulder, Colorado USA. NSIDC: National Snow and Ice Data Center. doi: http://dx.doi.org/10.7265/N54B2Z7V. [Date Accessed].

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Detailed Data Description

Format

Data are available as ASCII tab-delimited text files.

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File and Directory Structure

Data for each thin section of ice are listed in separate files, labeled by the section's depth and cut orientation (V or H).

The first line of a data file is the depth from which the sample was taken and whether the thin section was cut horizontally (H) or vertically (V) to the core axis. Each file contains three columns below the first line of data, titled "Grain #," "theta," and "phi." The "theta" and "phi" columns represent the polar angle and azimuthal angle (c-axis directions, both in decimal degrees), respectively.

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File Naming Convention

Each file name includes the depth of the particular sample in meters (m), followed by a v (vertical cut) or an h (horizontal cut), for example, "804.595v.txt."

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File Size

The file sizes range from approximately 1 to 50 KB.

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Volume

The data volume is 750 KB.

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Spatial Coverage

Siple Dome, Antarctica (81°S, 148°W)

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Parameter or Variable

Parameter Description

Ice fabric refers to the direction of the c-axes of an assemblage of ice crystals. The c-axis for one grain is specified by a polar and azimuthal angle or, equivalently, a point on a hemisphere of unit radius.

Sample Data Record

Following are the first several lines of "804.595v.txt." The first line of the data file is the depth from which the sample was taken and whether the thin section was cut horizontally (H) or vertically (V) to the core axis. Each file contains three columns below the first line of data, titled "Grain #," "theta," and "phi." The "theta" and "phi" columns represent the polar angle and azimuthal angle (c-axis directions, both in decimal degrees), respectively.

SDMA-804.595V
Grain # theta phi
0 -80.730153 77.069408
1 -81.266742 56.295845
2 -85.713908 23.790421
3 -77.642212 25.070803
4 -78.217221 76.893278
5 62.733258 23.870249
6 61.101735 34.381769
 
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Software and Tools

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Data Acquisition and Processing

Theory of Measurements

Ice has a hexagonal crystal structure consisting of stacks of basal planes of molecules arranged in hexagons. The crystal orientation is specified by the direction of a c-axis (perpendicular to the basal planes) and a 6-fold degenerate a-axis (in the basal plane). Ice is optically and acoustically uniaxial, so optical and acoustic techniques can only determine the direction of the c-axis. Hence, for most purposes, the fabric refers to the direction of the c-axes of an assemblage. The c-axis for one grain is specified by a polar and azimuthal angle or, equivalently, a point on a hemisphere of unit radius. The fabric is typically represented on a Schmidt plot, which maps each crystal c-axis direction from a point on the hemisphere to a circle, using an equal-area projection. The distribution of crystal axes is also often characterized using other average measures that indicate how random or concentrated the fabric is, or how the axes are distributed about the vertical axis.

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Data Acquisition Methods

For each orientation of the section, the fabric instrument acquires a sequence of 20 images as the polarizers rotate by 5º increments. Each image is an average of 10 video frames. The image resolution is 576 by 576 pixels. The field of view is 100 by 100 mm. The image sequences are acquired for nine different angular settings of the table/sample given by: 0/0, 45/0, 45/45, 45/90, 45/135, 45/180, 45/225, 45/270, and 45/315. A computer takes 16 minutes to image and rotate all of the stages. The nine image sequences (60 MB) are stored on mass-storage media and analyzed whenever convenient, by specifying the grains or grain regions.

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Derivation Techniques and Algorithms

A simple mapping algorithm allows each grain region to be tracked through all the sequences. For each grain, a fit to the extinction curve yields an extinction angle for every sequence. These are denoted by αiexp in the equation below, where i indexes the sequence number. The technique for finding the unique c-axis orientation from the extinction angles is as follows.

Every c-axis direction (or point on a Schmidt plot) corresponds to a unique set of extinction angles for the nine sequences. The extinction angles are found by rotating the c-axis in the appropriate way (for each of the nine sequences), and then calculating the azimuthal angle of the rotated c-axis. See Wilen et al. (2003) for an example. Denote these theoretical extinction angles as αithcc), where θc and φc specify the c-axis direction, and i indexes the sequence number. Define the function:

formula.jpg

R2 is close to zero whenever the theoretical and experimental extinction angles are in close agreement. The factor of 2 in the argument of the sine accounts for the 90° degeneracy (adding multiples of 90° to αiexp does not change the value of R2). A minimization of R2 with respect to θc and φc determines the unique c-axis direction.

Processing Steps

The grains to be analyzed are specified in a number of different ways.

1. The grains may be chosen manually from the 0/0 sequence of images by clicking on a region in each grain.

2. A grid may be defined and each region on the grid analyzed. Grid points that fall on grain boundaries or other anomalous regions prove impossible to analyze and are automatically discarded.

3. Images may be analyzed to find grain boundaries, and the grain boundary outlines may be used to automatically pick regions in the center of grains. This works well on very clean sections, but in general, the researcher may need to correct some fraction of the outlines by hand. Either way, the grain boundary outlines may also be used to find textural characteristics of grains.

Error Sources

Many of the vertical sections were not mounted vertically in the instrument for various technical reasons; therefore, the pattern of c-axes is actually rotated from the vertical. The mount orientation of the samples is given in a separate file titled, "sample orientation.doc." This file contains a list of all the samples specified by depth, followed by the tipping angle.

To rotate the c-axes to "vertical," convert the two Euler angles to unit vectors, and rotate the vectors by the tipping angle about the z-axis.

To rotate the c-axes to "horizontal," rotate the vectors about the x-axis by 90°.

Data were taken primarily from the Siple Dome A core. Source cores for each thin section are listed in the "sample orientation.doc" file.

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Sensor or Instrument Description

The automated fabric instrument employs a fiber-optic white light source, one fixed black-and-white video camera, and four rotations stages. Two of the rotation stages contain the polarizers, and the other two (table and sample stages) are used to rotate the thin section of ice to specified orientations.

Setup.jpg
Ice fabric analyzer. Image courtesy of Wilen et al. 2003.
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References and Related Publications

Contacts and Acknowledgments

Larry Wilen 
Ohio University 
Dept. of Physics and Astronomy 
Athens, OH 45701 
USA

Acknowledgments: 

This research was supported by NSF OPP award #0135989, Collaborative Research: Fabric and Texture Characteristics of Micro-Physical Processes in Ice.

Document Information

Document Creation Date

March 2005

Document Revision Date

March 2005

Document Review Date

March 2005

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