Writing SEG-Y files with more than 32767 samples per trace

Users Mailing List Archive
1

Hi

I'm trying to use ObsPy to convert some GCF files into SEG-Y, however I'm running into problems when the number of samples in a trace is larger than 32767. It seems the problem occurs while trying to write the header.

The error output is:
>>> print stream
1 Trace(s) in Stream:
Seq. No. in line: 1 | 2013-01-24T11:15:00.000000Z - 2013-01-24T11:19:59.995000Z | 200.0 Hz, 60000 samples

>>> stream.write(out, format='SEGY')
File "/usr/lib/python2.7/dist-packages/obspy/core/stream.py", line 1418, in write
   writeFormat(self, filename, **kwargs)
File "/usr/lib/python2.7/dist-packages/obspy/segy/core.py", line 432, in writeSEGY
   segy_file.write(filename, data_encoding=data_encoding, endian=byteorder)
File "/usr/lib/python2.7/dist-packages/obspy/segy/segy.py", line 228, in write
   self._write(file, data_encoding=data_encoding, endian=endian)
File "/usr/lib/python2.7/dist-packages/obspy/segy/segy.py", line 268, in _write
   self.binary_file_header.write(file, endian=endian)
File "/usr/lib/python2.7/dist-packages/obspy/segy/segy.py", line 411, in write
   file.write(pack(format, getattr(self, name)))
struct.error: 'h' format requires -32768 <= number <= 32767

So the question is, can I specify the header to be written with some other format than 'h' and how do I do this

regards Peter

2

Ok, so forget about this question

Reading up on the SEG-Y rev1 format I find that this is a limitation of the format as the number of samples are encoded using 2-byte two's complement (signed) integers.

regP

3

Hi Peter,

there has been some discussion on this in the past (cannot find it right
now). Some SEG-Y readers/writers also work with unsigned integers for
the number of samples which doubles the maximum allowed samples. In the
interest of maximum compatibility we did not do this back then and ObsPy
treats all integer values in SEG-Y as signed.

In the SEG-Y spec there is (if I remember correctly) no mention if the
numbers should be signed/unsigned but some fields clearly require signed
integers so for ObsPy they now are all signed values.

We should probably improve the SEG-Y error messages though...

Cheers!

Lion

4

Hi Lion,

Well, the SEG Y rev 1 (http://www.seg.org/Portals/0/SEG/News%20and%20Resources/Technical%20Standards/seg_y_rev1.pdf) states in section 3.3. (Number Formats) that:

"All values in the Binary File Header and the
Trace Header are two's complement
integers, either two bytes or four bytes long."

So signed it is

regP

5

Hi Peter,

thanks for the clarification - this settles it then!

Cheers!

Lion