Lynx Exploration Archivist

Specification for Velocity Database

Version 3.20

Data are stored internally in two tables VELDATA and PARAMDATA

VELDATA table

Each record in the VELDATA table basically represents a row in a seismic velocity control point, which with additional fields locates a point in the subsurface. It has the following record structure :-

PROFILE,TRACE,SP,TIME2,VRMS,VINT,X,Y,DATUM,DEPTH,DIP,DIPAZ,LABEL,STATUS

  1. PROFILE (text) - the name of the seismic profile (line name)
  2. SP (floating point) - the seismic shotpoint number, defining a position along the profile
  3. TRACE (integer) - the trace number or index of the velocity point in the profile
  4. X (floating point) - the X-coordinate of the velocity point
  5. Y (floating point) - the Y-coordinate of the velocity point
  6. DATUM(floating point) - height of reference plane above geoid / sea level
  7. TIME2 (floating point) - the two-way time, from datum.
  8. DEPTH (floating point) - the Z-coordinate, or depth from datum
  9. VRMS (floating point) - the Root Mean Square (or average) velocity down to the point
  10. VINT(floating point) - the interval velocity of the layer above the point X,Y (Z,time)
  11. DIP (floating point) - the dip of the interface at the point (X,Y,Z) measured in decimal degrees below horizontal along the azimuth.
  12. DIPAZ (floating point) the azimuth, in decimal degrees clockwise from North, of the direction of maximum dip at the point (X,Y,Z)
  13. LABEL
  14. STATUS

Some of these fields are optional - for example data will often be entered from the velocity tables on seismic sections as Velocity Contol Point records PROFILE,SP,TRACE,TIME2

the TRACE and SP fields allow velocity control points to be referenced either to trace number, or to shotpoint number. Historically, Lynx .VEL files, used by 3S and early windows versions of our migration software could only use trace-referenced control points. VEL files are therefore in practice always referenced to trace number. This caused problems when using velocity data with lines that had been mergerd or truncated, or traces interpolated. The introduction of .TAX (trace auxiliary) format enabled velocities to be more easily referenced to shotpoint numbers and this is generally the preferred method.

Internally, the velocity data table is maintained in a sorted state, with a sort key based on profile,shotpoint,trace,time. This means that if shotpoint referencing is used, all trace numbers should be set to the same constant, e.g. 0 or 1, to ensure that sorting works correctly.

PARAMDATA table

PARAMDATA stores a list of parameters used to define the units for the fields in VELDATA, plus other auxiliary parameters.

  1. VUNITS Velocity distance unit, i.e. metres or feet (per second)
  2. ZTUNITS verical Z time units
  3. ZDUNITS vertical Z depth units
  4. XCOORDTYPE Profile coordinate type, i.e. trace or shotpoint (for displaying velocity data)
  5. LOCATEBYSP Locate Velocity Control Points by shotpoint (true), or by trace (false) (for locating velocity data)
  6. XYUNITS horizontal XY coord units as feet, metres, decimal degrees ets.