Picking First Arrivals

BSU is distributed with a Octave procedure, segpic.m, which can be used to pick first arrivals. Copy the procedure into your working directory that has the files you wish to pick. Start Octave, and then run the procedure from within the Octave text window

segpic;

The procedure will prompt you for a file name and a maximum time to display. For example, you might answer twave.seg for the file, and .20 for the maximum time to pick the first arrival SH-waves. Then, a graphic window will be displayed showing the first trace. Position your mouse at the point where you feel the first arrival is, and click with your left mouse button. Only the lateral position is read from the mouse click, so you don't have to be concerned about the vertical placement of the mouse, other than it be somewhere inside the plot window. The procedure moves on to the next trace after the click. This is a rather simple routine, and you can't go backwards. After all the traces have been picked, the procedure writes the picks to a file, (trace #, pick time) pairs.

The picks can be written to the file trace headers by running bpic in your X-term window. For demonstration, the I have picked file twave.seg, and in the interest of better signal to noise, picked not the first motion, but the zero crossing from the first peak to trough of the first arrival SH-wave. This occurs about .005 seconds after the first motion. The picks are inserted into headers with the command, 

bpic twave.seg 1 twave.pic -.005 

The last argument, -.005, shifts the value in the twave.pic file to a point earlier in time, where the first motion is located. The output file is called, bpictwav.seg. You can then move this to replace the original file, twave.seg, thus completing the operation. Now twave.seg will have the picks in the headers.