NOAA / NESDIS / National Geophysical Data Center / WDC for SEG, Boulder, Colorado USA

Chapter 3: PALEOMAGNETIC PROCEDURES

Example 3.1

Use the program sundec to calculate azimuth of the direction of drill. You are located at 35 N and 33 E. The local time is three hours ahead of Universal Time. The shadow angle for the drilling direction was 68 measured at 16:09 on May 23, 1994.

Solution

The program sundec reads the time offset from the command line as an argument to a switch [-u]. It reads site latitude, longitude, year, month, day, hours, minutes and shadow angle from standard input.

Type the following:

% cat > ex3.1
35 33 1994 5 23 16 09 68
<control-D>
% sundec -u 3 < ex3.1

The computer responds:
154.2

Example 3.2

Use the programs di_geo and di_tilt to convert the data in ex3.2 into geographic and tilt adjusted coordinates. The orientation of laboratory arrow on the specimen was: azimuth = 347; plunge = 27. The strike was 135 and the dip was 21. (NB: the convention is that the dip direction is to the ``right'' of the strike).

Solution % di_geo< ex3.2

The computer responds:

5.3 71.6

which are the D and I in geographic coordinates.

Now type:

% cat > ex3.2a 5.3 71.6 135 21 < control-D>% di_tilt< ex3.2a

The computer responds:

which are the D and I in tilt adjusted coordinates.

Example 3.3 

1) Use the program pca to calculate the best-fit direction from the data in ex3.3. Use only the 20 to 90 mT demagnetization steps. 2) Use the program gtcirc to calculate the best-fit plane through the NRM to 20 mT steps. 3) Plot the data (and the best-fit line) using the program plotdmag. 4) Plot the data with D=300 on the horizontal axis.

Solution

Because each laboratory has its own file formatting convention, it is necessary to extract the desired data from a given file format using the grep and awk commands. If the above data are in a file named yourdata along data from other samples, then the data for the particular sample eba24a can be extracted using the grep command. Type:

% grep ``eba24a'' yourdata > tmp

Three data formats are supported by pca . If the data format of your file is different, the columns needed by pca can be stripped out using awk. For example, if columns 1, 3, 5, 6, and 7 are required, then type the following:

% awk '{print $1,$3,$5,$6,$7}' tmp > ex3.3

These two commands can be combined:

% grep ``eba24a'' yourdata | awk '{print $1, $3,$5,$6,$7}' > ex3.3

Now pca can be made to analyze the data from the 6th (20 mT) to the 13th (90 mT) treatment steps as follows.

Type:

% pca -p 6 13 < ex3.3

The computer responds:

eba24a p 8 20.00 90.00 4.5 307.9 39.8

To find out what these numbers mean, check pca or type:

% pca -h
 

The "p" indicates that this is a principal component direction (as opposed to a pole to a great circle), n is the number of data points used, [beg] and [end] are the bounding treatment steps used, mad is the MAD angle, and the declination and inclination are for the principal component. The output can be saved into a file using the UNIX redirect capability. To append to the end of a file called pca.out, type:

% pca -p 6 13 < ex3.3 >> pca.out

To calculate the pole to the best-fit plane and a corresponding MADplane, type the following:

% gtcirc -g 1 6 < ex3.3

The computer responds:

eba24 g 6 0.00 20.00 7.4 193.1 29.3

This output is similar to that from pca except that the g stands for great circle and the direction is that of the pole to the best-fitting plane. Finally, to plot the data with the principal component along an axis with a declination of 300and to view it with ghostview, type the following:

% plotdmag -rp 300 6 13 < ex3.3 | plotxy ; ghostview mypost

This causes plotxy to create a postscript file mypost

The program plotdmag can be used iteratively, first with no options for a quick look at the data and to display the index numbers for the treatment steps, then again to calculate best-fit lines and planes. To view a whole data file (e.g., yourfile), one can make a shell script as in the following example.

First list all the samples in the file (presuming that the sample name is in the first column.

% awk '{print $1}' yourfile | sort | uniq > yourshell

which makes a list of unique sample names (e.g., s1, s2, etc.). Now edit yourshell to look something like this:

grep s1 yourfile | awk '{print $1,$3,$5,$6,$7}' | plotdmag | plotxy;cat mypost > your.ps
grep s2 yourfile | awk '{print $1,$3,$5,$6,$7}' | plotdmag | plotxy;cat mypost >> your.ps
grep s3 yourfile | awk '{print $1,$3,$5,$6,$7}' | plotdmag | plotxy;cat mypost >> your.ps
grep s4 yourfile | awk '{print $1,$3,$5,$6,$7}' | plotdmag | plotxy;cat mypost >> your.ps
grep s5 yourfile | awk '{print $1,$3,$5,$6,$7}' | plotdmag | plotxy;cat mypost >> your.ps

Now you must make yourshell executable, by e.g.:

% chmod u+x yourshell

Execute it by:

% yourshell

and view the results by:

% ghostview your.ps

By pressing the spacebar, successive plots can be viewed. Finally, the plotdmag part of yourshell can be edited to put in the appropriate switches (e.g., -p 6 13 to put a line through the 6 - 13 data points, as illustrated above).

Example 3. 4
The data listed in ex3.4 are thermal demagnetization data for a specimen that had a 2 T field exposed along X1 a 0.4 T field exposed along X2 and a 0.12 T field exposed along X3. Convert the data to x1, x2, x3 components using awk and dir_cart. Put the temperature, and xi data in a file called cart using the UNIX utility paste. Now plot the 3-component IRM demagnetization data using plotxy.

Solution

The first task is to peel off the declination, inclination and intensity data, and pipe them to the program dir_cart (see also Example 1.1). Put them in a file called cart. This is done with the command:

% awk '{print $2,$3,$4}' ex3.4 | dir_cart -m > cart

Now we need to peel off the temperature steps from ex3.4 using awk and paste them with the cartesian component data into a file ex3.4a.

% awk '{print $1}' ex3.4 > tmp; paste tmp cart > ex3.4a

Finally we can plot the data using plotxy using the  command file ex3.4.co. Notes as to the meaning of the commands are given in the documentation plotxy.

Now type:

% plotxy<ex3.4.co

for  the output mypost file.

Example 3. 5

1) Use the program arai to convert the data from a paleointensity experiment contained in file ex3.5 to an Arai plot. The laboratory field applied during the ``in field steps'' was 50 micro T. At which temperature step do the pTRM checks (triangles) fail? 2) Calculate a best-fit line between the steps 175  and 350 degrees.

Solution

First check the documentation for arai. You will see that to make an Arai plot, calculating a best fit line between the 175 and the 350 temperature steps using a laboratory field of 50 micro T, type:

% arai -sf 175 350 50e-6 < ex3.5 | plotxy

The result is this mypost file.

Example 3. 6

Use the program pseudot to plot an Arai diagram for the pseudo-Thellier experiment whose data are contained in file ex3.6.

Solution

Type the following:

pseudot < ex3.6 | plotxy

and see the mypost file.