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).
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 /offsThis 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.