A ``complete'' SEPlib90 data set is made of 6 files,
three ASCII files and three binary files.
With the exception of the History File, the existence of all
the other files is optional.
The six files are connected to each other through
pointers contained in the ASCII files.
These pointers can be either simple UNIX file paths or more general URLs
(the URL option is not implemented yet!).
The path to the Header Format File (HFF) is specified by the
value of the hff parameter in the History File.
The path to the Grid Format File (GFF) is specified by the
value of the gff parameter in the History File.
Following is a brief ``graphical'' description of the connectivity
among the 6 files,
with the arrows 
representing the ASCII pointers.
In addition to the links to the other files, the History File contains the processing history of the data set. The Data Values File (DVF) is defined as collection of fixed length records (data records) that contain the data values. Typically the data records are seismic traces.
The header values are stored in the Header Values File (HVF), that is defined as a collection of fixed length records (header records) describing the geometry and properties of the associated data records. The header parameters are described in the Header Format File by a table of header keys. A header keys specifies the name of the header parameter (key name), its data type (key type), and its position in the header record (key index). The association between the header records and the data records is described below.
If the data set has been binned on a regularly sampled grid, the Grid Format File contains the description of the grid. The Grid Value File contains the mapping information between the grid cells and the corresponding header records. The Grid Value File does not exist if the gridding is regular; that is, there is a one-to-one correspondence between grid cells and header records.