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4.2.2  Field Information

Orient.txt formatted files

Paleomagnetists collect samples in the field and record orientation, location and lithology, etc. in field notebooks. This information is necessary for placing the data into a well characterized context and should be included in the MagIC contribution. Importing field data into the MagIC format using PmagPy requires you to fill in a tab delimited text file with a particular format, here called the "orient.txt" format. These should be placed in the MyFiles directory in the Project directory. When importing the data, you must figure out how your orientation and name schemes relate to what MagIC expects. The process is difficult because there are a multitude of possible naming conventions which relate specimen to sample to site, orientation conventions which convert specimen measurements in geographic and stratigraphic coordinates, etc. PmagPy supports a number of conventions and more can be added by Request. Here we go through each of these problems, starting with the orient.txt file format, and then covering sample orientation and naming schemes.

First of all, separate your sampling information into the locations that you plan to designate in the data base. Location name (er_location_name) in MagIC is a rather loosely defined concept which groups together collections of related sites. A location could be for example a region or a stratigraphic section. Location names are useful for retrieving data out of the MagIC database, so choose your location names wisely. Each orient.txt format file contains information for a single location, so fill one out for each of your "locations".

The First Line of the orient.txt file contains two tab delimited fields. The first is the word 'tab' and the second is the location name, in this example it is North Shore Volcanics. Use the same location name EVERY TIME you are asked for it for data related to this collection of samples.

The Second Line of the orient.txt file has the column names. The order of the columns doesn't matter, but the names of the columns do. Some of these are required and others are optional. The example above shows all the REQUIRED fields. Note that latitude and longitude are specified in decimal degrees. mag_azimuth and field_dip are the notebook entries of the sample orientation. Sample class, Lithology and Type are Controlled MagIC Vocabularies, so enter colon delimited lists as appropriate. Also, notice how some fields are only entered once. The PmagPy program (orientation_magic.py) assumes that the last encountered value pertains to all subsequent blank entries.

Optional Fields in orient.txt formatted files are: [date, shadow_angle, hhmm], date, stratigraphic_height, [bedding_dip_direction, bedding_dip], [image_name, image_look, image_photographer], participants, method_codes, site_name, and site_description, GPS_Az

Note   Column names in brackets must be supplied together and the data for stratigraphic_height are in meters.

For Sun Compass measurements, supply the shadow_angle, date and time. The date must be in mm/dd/yy format. Be sure you know the offset to Universal Time as you will have to supply that later. Also, only put data from one time zone in a single file. The shadow angle should follow the convention shown in the figure:

All images, for example outcrop photos are supplied as a separate zip file. image_name is the name of the picture you will import, image_look is the "look direction" and image_photographer is the person who took the picture. This information will be put in a file named er_images.txt and will ultimately be read into the er_image table in the console where addiional information must be entered (keywords, etc.).

Often, paleomagnetists note when a sample orientation is suspect in the field. To indicate that a particular sample may have an uncertainty in its orientation that is greater than about 5o, enter SO-GT5 in the method_codes column and any other special codes pertaining to a particular sample from the method codes table. Other general method codes can be entered later. Note that unlike date and sample_class, the method codes entered in orient.txt pertain only to the sample on the same line.

If there is not a supported relationship between the sample_name and the site_name (see sample naming schemes below), you can enter the site name under site_name for each sample. For example, you could group samples together that should ultimately be averaged together (multiple "sites" that sampled the same field could be grouped under a single "site name" here.

orientation conventions

Supported sample orientation schemes:

Samples are oriented in the field with a "field arrow" and measured in the laboratory with a "lab arrow". The lab arrow is the positive X direction of the right handed coordinate system of the specimen measurements. The lab and field arrows may not be the same. In the MagIC database, we require the orientation (azimuth and plunge) of the X direction of the measurements (lab arrow). Here are some popular conventions that convert the field arrow azimuth (mag_azimuth in the orient.txt file) and dip (field_dip in orient.txt) to the azimuth and plunge of the laboratory arrow (sample_azimuth and sample_dip in er_samples.txt). The two angles, mag_azimuth and field_dip are explained below.

[1] Standard Pomeroy convention of azimuth and hade (degrees from vertical down) of the drill direction (field arrow). sample_azimuth = mag_azimuth; sample_dip =-field_dip.

[2] Field arrow is the strike of the plane orthogonal to the drill direction, Field dip is the hade of the drill direction. Lab arrow azimuth = mag_azimuth-90o; Lab arrow dip = -field_dip

[3] Lab arrow is the same as the drill direction; hade was measured in the field. Lab arrow azimuth = mag_azimuth; Lab arrow dip = 90o-field_dip.

[4] Lab arrow orientation same as mag_azimuth and field_dip.

[5] Same as AZDIP convention explained below - azimuth and inclination of the drill direction are mag_azimuth and field_dip; lab arrow is as in [1] above. field arrow are lab arrow azimuth is same as mag_azimuth, Lab arrow dip = field_dip-90o

[6] Lab arrow azimuth = mag_azimuth-90o, Lab arrow dip = 90o-field_dip, i.e., field arrow was strike and dip of orthogonal face:

naming conventions

Supported sample naming conventions:

Structural corrections:

Because of the ambiguity of strike and dip, the MagIC database uses the dip direction and dip where dip is positive from 0 => 180. Dips > 90 are overturned beds. Plunging folds and multiple rotations are handled with the pmag_rotations table and are not implemented within PmagPy.

AzDip formatted files

  • AzDip format: This is a very simple file format with the sample name Azimuth Plunge Strike Dip where the Azimuth and Plunge are of the drill direction (Specimen's Z direction) or orientation convention #3 to convert to the MagIC standard. To convert strike to bedding dip direction, we would just add 90o. Here is an example AzDip file: