SEISNC Standard for xarray

The xarray seismic specification termed seisnc can be used by segysak to output NETCDF4 files is more performant for Python operations than standard SEG-Y. Unlike SEG-Y, xarray compatable files fit neatly into the Python scientific stack providing operations like lazy loading, easy slicing, compatability with multi-core and multi-node operations using dask as well as important features such as labelled axes and coordinates.

This specification is not meant to be prescriptive but outlines some basic requirements for xarray datasets to work with SEGY-SAK functionality.

SEGY-SAK uses the convention .seisnc for the suffix on NETCDF4 files it creates. These files are datasets with specific 1D and 2D coordinates and have a single variable called data. The data variable contains the seismic cube volume or 2D line traces. Attributes can be used to provide further metadata about the cube.

3D and 3D Gathers

SEGY-SAK uses the convention labels of iline, xline and offset to describe the bins of 3D data. Vertical dimensions are twt and depth. A typical xarray dataset created by SEGY-SAK will return for example

>>> seisnc_3d = segysak.segy_loader('test3d.sgy', iline=189, xline=193)
>>> seisnc_3d.dims

Frozen(SortedKeysDict({'iline': 61, 'xline': 202, 'twt': 850}))

2D and 2D Gathers

For 2D data SEGY-SAK uses the dimension labels cdp and offset. This allows the package to distinguish between 2D and 3D data to allow automation on saving and convience wrappers. The same vertical dimensions apply as for 3D. A typical xarray in 2D format would return

>>> seisnc_2d = segysak.segy_loader('test2d.sgy', cdp=21)
>>> seisnc_2d.dims

Frozen(SortedKeysDict({'cdp': 61, 'twt': 850}))

Coordinates

If the cdpx and cdpy byte locations are specified during loading the SEG-Y the coordinates will be populated from the headers with the variable names cdp_x and cdp_y. These will have dimensions equivalent to the horizontal dimensions of the data (iline, xline for 3D and cdp for 2D).

Attributes

Any number of attributes can be added to a siesnc file. Currently the following attributes are extracted or reserved for use by SEGY-SAK.

ns number of samples per trace

ds sample interval

text ebcidc header as ascii text

measurement_sys vertical units of the data

d3_domain vertical domain of the data

epsg data epsg code

corner_points corner points of the dataset in grid coordinates

corner_points_xy corner points of the dataset in xy

source_file name of the file the dataset was created from

srd seismic reference datum of the data in vertical units measurement_sys and d3_domain

datatype the data type e.g. amplitude, velocity, attribute

percentiles this is an array of approximate percentile values created during scanning from SEG-Y. Primarily this is useful for plotting by limiting the dynamic range of the display. The percentiles are in percent 0, 0.1, 10, 50, 90, 99.9 & 100.