Difference betwen seismic file format

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riccardo
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Difference betwen seismic file format

Post by riccardo »

good evening

can someone help me to understand the different seismic file format?

SU Big Endian - little endian
SEGY
SEG-2

admin
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Re: Difference betwen seismic file format

Post by admin »

SU is a subset of SEGY headers. It is used by Seismic Unix.
Big or Little endian has to see with the order of bytes. For more information, please read https://en.wikipedia.org/wiki/Endianness
SEG-2 is a binary format with little link to SEGY familly. It is mainly used by active source recording devices.

None of these format are really well designed for long time series. They mostly used for short time recordings of active sources.

jassaf2
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Joined: Mon Aug 03, 2020 6:22 pm

Re: Difference betwen seismic file format

Post by jassaf2 »

Hello Marc,
I have been trying to prepare segy MASW files using Obspy python package and load them into geopsy newest version (3.2.2). However, events and X location of the receivers are not being automatically loaded into geopsy. I am defining the coordinates in obspy under segy dictionary of headers using "x_coordinate_of_ensemble_position_of_this_trace" for X position. However, it seems geopsy does not read this information. Do you know what geopsy looks for as an X position in the segy files?
I tried to attach the segy file for reference, but i was not able to.
Thank you.

admin
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Re: Difference betwen seismic file format

Post by admin »

For receiver coordinates, receiverXCoordinate and receiverYCoordinate from trace header are read (offsets 81-84 and 85-88, respectively). For source coordinates, sourceXCoordinate and sourceYCoordinate, also from trace header (offsets 73-76 and 77-80, respectively).

The event is systematically created for SEGY files. Is it correct? What may be missing is the coordinates of the source if you are not writing it at same place.

Put your SEGY file into a zip or tar,gz file before uploading. The forum accept a limited number of file extensions. Did you try 3.3.2 instead of 3.2.2?

jassaf2
Posts: 2
Joined: Mon Aug 03, 2020 6:22 pm

Re: Difference betwen seismic file format

Post by jassaf2 »

Thanks Marc for your reply.
I managed to successfully write the receiver coordinates and source location by defining segy header in obspy as following "trace.stats.segy.trace_header.group_coordinate_x" and "trace.stats.segy.trace_header.source_coordinate_x". Geopsy successfully imports these attributes. The only problem is that writing segy files in obspy only allows integers in the header and thus issues can rise if the source distance is not an integer or for 0.5 m spacings. These can be modified easily using headers in geopsy.

admin
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Re: Difference betwen seismic file format

Post by admin »

-For XY coordinates, there is factor and another one for the Z that you can define in the SEGY header.
It is a bit tricky to use these factors. Here is the code in geopsy to get the true factor value from the header value.

Code: Select all

    if(field.coordinateFactor>0) {
      return field.coordinateFactor;
    } else if(field.coordinateFactor<0) {
      return -1.0/field.coordinateFactor;
    } else {
      return 1.0;
    }
This is the same for elevationFacor (offset 69-70]. coordinateFacor it at offset 71-72]. Both are integers.

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