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file-specification.md

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SeismicZFP File Specification

A SeismicZFP (.sgz) file consists of 3 parts:

  1. A header, containing:
    • Information needed to read the file
    • Values of invariant SEG-Y trace header fields
    • SEG-Y Textual and Binary file headers
  2. The compressed seismic data itself
  3. A footer, containing values of variant trace header fields from SEG-Y

The length of these component parts can be calculated as follows:

Size (bytes)
SeismicZFP Header 8192 (see below for details)
ZFP-compressed fp data padded(nIL x nXL x nS) x bitrate / 8
SeismicZFP Footer nHeaders x nIL x nXL x 4

The padding of the IL/XL/Samples dimensions is to the logical size of those dimensions stored in a single disk-block, which is dependent on the bit-rate. For example, at 2 bits-per-voxel the logical size of the data in a 4K disk block may be 4x4x1024, or 64x64x4, etc.

SeismicZFP Header (v0.1.7 onwards)

Byte encoding is little-endian.

Bytes Type Contents
0-3 uint32 Number of 4K blocks of header
4-7 uint32 Samples per trace
8-11 uint32 Number of crosslines
12-15 uint32 Number of inlines
16-19 int32 Minimum sample time/depth
20-23 int32 Minimum crossline number
24-27 int32 Minimum inline number
28-31 int32 Sample interval (μs/m)*****
32-35 int32 Crossline interval
36-39 int32 Inline interval
40-43 int32 Bits-per-voxel (negative signifying reciprocal)
44-47 uint32 Blockshape: IL-direction ****
48-51 uint32 Blockshape: XL-direction
52-55 uint32 Blockshape: Trace-direction
56-59 uint32 Number of 4K disk blocks for data
60-63 uint32 Number of bytes for each header array
64-67 uint32 Number of header arrays
68-71 uint32 Number of traces (unstructured files)
72-75 uint32 Encoded version number
76-79 uint32 Encoded source format 0=SEG-Y, 10=ZGY, 20=numpy
80-83 uint32 Encoded header-detection method ***
84-91 float64 Minimum sample time/depth*****
92-99 float64 Sample interval (μs/m)*****
100-959 --- --- Unused ---
960-979 bytes Hash of input data
980-2047 ** Default trace header values
2048-4095 --- --- Unused ---
4096-7295 SEG-Y SEG-Y First textual header
7296-7695 SEG-Y SEG-Y Binary header
7676-8191 --- --- Unused ---

** Invariant trace header values are stored in an 89x3 array of 4-byte entries, corresponding to the 89 fields in SEG-Y trace headers. For each row the enties are:

  1. Trace header start-byte
  2. Constant value
  3. Duplicated trace header start-byte

Storing whether trace header fields are duplicates of previous ones reduces the space needed to store the SGZ footer.

*** SeismicZFP supports multiple methods of determining which SEG-Y trace headers are present and/or duplicated. The file maintains a record of which was used (heuristic only until v0.1.10):

0=heuristic, 10=thorough, 20=exhaustive, 30=strip

**** Blockshape in IL direction is set to 1 for 2D files, also no bytes for 3D geometry between 4-40 are set.

***** This value may be overidden to provide higher precision by bytes 3273–3280 in the SEG-Y header, or equivalent in ZGY file These bytes were allocated in rev 2.0 for "Extended sample interval", as an IEEE double-precision float.