Rotating the Horizontal Data into Alignment with Source (genbrot and brot )

Once the headers have been updated with the down-hole tool orientation, the next step is to rotate the horizontal data into a single orientation. This removes any spin that has occurred while dragging the tool up the hole. The genbrot program is run interactively to generate a script which can then be executed. The script automates running program brot on all the shot gathers. Program genbrot is set up to process data according to the channel definitions of section 6.7.8 above. Thus, only the data on traces 2 and 3 are rotated.

The program brot is quite flexible in how to rotate the data. Program genbrot, however, makes a single choice and would have to be modified if that did not suit a user's needs. The default choice imposed by genbrot is to align the data so that the T-component down-hole becomes aligned with the T-component of the reference phone. This particular rotation was based on the decision to align the T-component of the reference phone with the long axis of the source (see Figure 11).

The result of the above decisions is to align trace 3 with the long axis of the source, to the extent that the radiation from the source is also aligned with the long axis of the source. While one might think that the source shown in Figure 10 would radiate SH-motion polarized parallel to the long axis of the source (aligned with the hammer blows), it appears that variations in soil stiffness under the source can cause a couple, twisting the polarization axis of the radiation slightly out of alignment with the source axis (see Michaels (12) ). Thus, a more proper statement might be that the horizontal data are aligned with the major axis of the source radiation polarization ellipse, rather than with the long axis of the source itself.

As a final caution, if a user has not acquired the data with the geometry as shown in Figure 11, the blind use of the program genbrot may produce unwanted and unexpected results. There are many ways to set up the source and reference phone about the bore hole (both in location and component orientation). My choice was designed to permit SH-waves to be recorded from depth to the surface, with as little interference as possible from other wave fields.

The dialog for genbrot would be as follows:

 |----------------------------------------|
 | Copyright (C) 2017 P. Michaels         |
 |       All rights reserved              |
 | See GNU General Public License         |
 |----------------------------------------|
 gbrt:  TIME: 16:12:13 DATE: 27/Apr/2017

Enter alpha prefix (char) of *.seg data to be rotated

EXAMPLE: if enter 1, then files 1001.seg to 1010.seg
                     would be processed if sequence
                     numbers 1 and 10 entered next

L

 ...L
 Enter first file number to process

1
Enter last file number to process
146
Output in file===>gobrot

The next step would be to make the bash script, gobrot, executable:

chmod +x gobrot

After running gobrot, you will have files named brotL001.seg through brotL0146.seg. These data are rotated to the standard orientation described above. Not only are the data rotated, but the headers have been changed for traces 2 and 3 to reflect their new azimuths.