GCFXtract Shot-files

GCFXtract is a multi-platform tool for reading disks which were written in the DFD format, as used by Guralp digitizers, including CMG-6TD digital instruments. GCFXtract can convert GCF input data into (non-Passcal) SEG-y format. Because this format limits the number of samples that can be recorded into a single file, a "shot-file" is used to specify a starting time for each individual output file. This document explains how to prepare and use shot-files.

The shot-file consists of lines, each of which specifies a single "shot time" which corresponds to a single output file. Each such line consists of three fields, separated by commas:

  • The shot number, expressed as an ASCII integer
  • The human-readable date and time, in the format dd/mm/yyyy HH:MM:SS This field is ignored by GCFXtract.
  • The date and time in unix format, specified to six decimal places (micro-second accuracy).
Lines which are not in this format are silently ignored so comments, column headers and other information can be freely incorporated.

An example shot-file might look like this:

Sample shot-file for GCFXtract

Shot Number,Human Time,Unix Time

7,09/08/2014 09:17,1407575844.129800
6,09/08/2014 09:17,1407575852.697800
5,09/08/2014 09:17,1407575861.061800
4,09/08/2014 09:17,1407575869.347800
3,09/08/2014 09:17,1407575877.913800
2,09/08/2014 09:18,1407575886.355810
1,09/08/2014 09:18,1407575894.751810

The shot-file can be specified using the GUI interface or from the command line. If specified on the command line, the pre-shot data duration and the post-shot data duration must also be specified. The following arguments are relevant to SEG-y files:

/d:code : Specifies the output format. Set code to segys for SEG-y format with shot-file processing.

/s:filename : Specifies the path to the shot-file.

/pre:value : Specifies the pre-shot data duration in seconds.

/post:value : Specifies the post-shot data duration in seconds.

/offs : Turns on removal of DC offsets during conversion.

/p190:filename : Specifies the path to the (optional) P1/90 file. Only 'S' records are processed.

A typical invocation might look like this:

gcfxtract /d:segys /s:C:\Data\ShotFile.csv /pre:1 /post:7 /offs
This would direct GCFXtract to record files with eight-second durations - one second pre-shot and seven seconds post-shot - using shot times from C:\Data\ShotFile.csv and with DC-offset-removal active.

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