Thread: Problem with merging large files

Hi Brian

Thank you for your reply. I was suspecting something like that. The shift shows up when I use the function CUT on the 'timed' signal blocks - I use a script to extract 10 minutes signal blocks based on a table with local earthquake times. In general the shift gets stronger the further you are from the timing event, which would suggest the float precision error accumulates over multiple blocks.

Gonna try to work around it by calculating the time shift and then using script to merge and split the files in shorter chunks.

Thanks

Lukas
________________________________
From: sac-help-bounce<at>iris.washington.edu [sac-help-bounce<at>iris.washington.edu] on behalf of Brian Savage [savage<at>uri.edu]
Sent: 09 February 2017 01:47
To: SAC Help
Subject: Re: [sac-help] Problem with merging large files

Lukas,

I would suspect that you are running up against the precision of the floating point value used for timing in SAC. See the attached plot. It shows the maximum time value (y-axis) without loss of precision for a variety of sampling rates (x-axis). Anything above the line with the triangles will not have sufficient precision.

Your data is samples at 100 Hz (10^-2), and the maximum time you can represent without precision loss is a couple of days, but not a month.

I guess I am not surprised you are seeing shifts at the end of the month. Does this shift occur within the plots or does it show up in another way ?

Brian

On Feb 7, 2017, at 2:38 PM, Lukas Janku <lukas.janku<at>pg.canterbury.ac.nz<lukas.janku<at>pg.canterbury.ac.nz>> wrote:

Hi everyone

I am having problem (probably) related to some maximum length of the seismic data within Sac.

I have been dealing with some really crappy seismic data with messed-up timing. 3 Traces from the same sensor. The data had frequent gaps in them and for some reason, the gaps are occasionally of different lengths for different components. I have about 1 month of continuous record (100 Hz sampling). My timing correction included:

1. Merging all the files from one component together (have a computer with large RAM).
2. Use some of the recorded earthquakes where I know the p-wave arrival for getting the timing of my 1-month block right.
3. Splitting the month-long file into day-long files using a script.

The problem I get is when I use an earthquake at the beginning of the 1-month block (consistent for different components), at the end of my correction process my 3 components are slightly shifted with respect to each other near the end of the 1-month block.

Does anyone has an experience with similar issue? The only reason I can think off is the different length of the gaps in the data, combined with huge amount of merged data, can make some internal variable 'overflow' at different time for different components... But that's just an impression.

Thanks a lot

Lukas Janku
University of Canterbury

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