Last modified 14 years ago Last modified on 01/23/2009 11:40:41 AM

Menu entry Calibration

Advanced option. Slowness and back-azimuth values determined by an array record are usually not perfect, since the waveforms arriving at the array are distorted by heterogeneities on the receiver side. This command allows to correct the slowness/azimuth measurement for such systematic errors. It uses a lookup table with slowness correction vectors. These correction vectors are found by investigating representative events in the past, comparing the bearing of the array with the theoretical slowness and back-azimuth from the array center to the location given by international data centers (like ISC or NEIC). These difference vectors are used to correct the measured slowness/azimuth values. The simple algorithm implemented here takes the next correction vector and shifts it to the location of the current measurement in slowness-azimuth domain. The end of the shifted arrow points to a corrected (calibrated) result. This method is applicable only if the data set of teleseismic records is big enough to provide a dense coverage of correction vectors over the whole range of possible slowness/azimuth pairs. Also the correction table is usually made with records of the complete array. If an actual record misses one or even more stations the correction vectors may not be suitable any more. For the GRF array such a correction table is provided in the distribution.

There is a calibration lookup file $SH_INPUTS/calib_lookup.txt which tells SHM which correction table is to be used for which stations. It contains one line per entry of the format <stationlist> <calibfile>. Please note that <stationlist> must not contain blanks. Lines starting with ! are comments and ignored by the program. The only entry in this file is the GRF array pointing to the file of slowness correction vectors $SH_INPUTS/calib_grf.dat. This file is formatted as follows:

  • One line per entry
  • The first 26 characters of each line are comment
  • The following four numers are beam slowness, beam azimuth, corrected (theoretical) slowness and corrected azimuth
  • The remaing part of the line is again comment

The way to proceed is as follows: after determining the slowness/azimuth (with Plane Wave or FK) select the menu entry Calibration. A new window pops up showing an area in slowness/azimuth domain around the measured values. Black arrows indicate available correction vectors within this area. The red arrow is the suggestion of the program to correct the beam measurement. The top of the red arrow may be moved with the left mouse button to adjust the suggested correction manually. The resulting location and region name is displayed in the top left corner of the window. The window has a small drop down menu with the following entries:

  • Accept: accept the current correction values, close window and return to main program.
  • Quit: do not accept corrections, close window and return to main program
  • Screendump: make hardcopy of the window
  • Set Calib Flag: mark this event as a candidate for an entry in the calibration file
  • Clear Calib Flag: clear a previously set calibration flag.

Leaving the window with Accept adds the corrected slowness/azimuth values to the analysis parameters and uses those for following teleseismic locations. Leaving with Quit does not change anything in the parameter set.

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