Preliminary release - Beta test
Version: 2.4 Contents
Migration velocities work flow
VelPrep Projects
Every Velprep project has a file file .vpr which stores parameter information about units, display parameters etc., together an associated database file containing velocity and horizon data. The database file is in the open source Firebird format, derived from Borland's Interbase.
Velprep has all the necessary methods for relatively painlessly inserting, editing, sorting, querying of tabular velocity data. Externally, the data can be dumped to other databases, for storage as a project, or imported/exported as an ASCII text comma delimited file. Lynx formats TAX and VEL can also be imported, so that data from old projects can be utilised.
There are several methods of putting velocity data into the database
Import from CSV
Create a spreadsheet with column headings PROFILE,'TRACE or SP,TIME2,VRMS. Note that column 2 will be TRACE if the VCPs are refenced by trace number, or SP if referenced by shotpoint.
Velocity Preparation and Conversion /VcnvForm
VcnvForm/PFSelLabel1
Select Profile Invokes the profile selection panel, with a list box showing names of profiles currently in the database table
Click the "tick" button to select highlighted profile for editing etc.
Click the "cross" button to close the profile selection panel
&File File Menu for opening and saving database tables
Open Table Opens an existing database table
New Table Creates a new, empty table
Import shows sub menu of file import options
csv and txt imports tabular data as text or csv files
Lynx 3S Import from "old-style" Lynx 3-S velocity files.
Promax Import from Promax export velocity file. Currently, this is "hardwired" to the following characteristics:-
StartProfileFlag = 'Migration Vels' - the start of each profile's data has this string
LineID = 'LINE ' - the prefix for the profile line name
EndProfileFlag = '' - (null) searches for a line with this string to signal end of profile
LinesToskip = 2 - skip this no. of lines after start of line, before inputting data
SPCol = 1, SPWid = 10 - Shotpoint/CDP/trace column and width
TCol = 20, TWid = 10 - Time column and width
VCol = 40; VWid = 10 - Velocity column and field width
Standard import from a blank delimited ASCII table with lines in the form
Linename Shotpoint time velocity
Linename is the profile name
string
Shotpoint, time, V are numeric values (with or without decimal
point) separated by 1 or more blanks. Velocities will usually be in feet or
metres per second and will be read into the VRMS filed of the database table,
times in milliseconds.
Export shows sub menu of file export options
Lynx 3S Outputs a selected profile as an old-style Lynx 3-S velocity file
ASCII Table Writes all data in the table as a space delimited ASCII file in the form
col 1-20 Linename
col 21-30 Shotpoint F10.1
col 31-40 time F10.1
col 41-50 velocity F10.1
&Profiles Main menu option for profile editing, selection and viewing
Delete Delete selected profile from the table
Edit Edit a profile as velocity "points", as on a seismic section
Insert XYs Insert XY values into the table, using a .LOC format location file
New Profile Creates a new profilein the table, for manual data input using Edit option.
Rename Rename the selected profile
SP<-->Trace Calculate trace numbers from shotpoint numbers, or vice versa. Particularly useful when data are entered "by shotpoint" and you wish to use the data by trace number, by exporting to a Lynx 3-S velocity file
View Invokes the velocity profile viewer, to show a contoured cross section of the selected profile
Table Main menu option for velocity table operations
Clear clear the velocity table - this will remove all data!
Edit Edit the velocity table as a database grid
About shows standard about box with version information
page index
page : Recalculate SPs / Traces
page : RMS Velocity Control Point
page : Enter new velocity profile name
Parameter setup
VelPrep Project Options
Velocity Data File - When creating a new project, you will be asked to enter the name of the velocity database table to use for storing project and Velocity data. You can select an existing one, or create a new one.
Use Location Data - If the VCPs are to be referenced to actual XY locations on the ground (rather than just using trace or shotpoint numbers) enter the name of a .loc file or shape file containing the locations for the profiles and their VCPs.
Cross section scales - specify the default horizontal and vertical scales for velocity cross section display
Direction :
(type option, SPfromTR,
TRfromSP )
The SPfromTR option will calculate shotpoint numbers for all
velocity records in the database table, for the chosen profile, using each
record's trace number and SP number at trace 1 and increment entered below.
The TRfromSP option will calculate trace numbers for all velocity records in the database table, for the chosen profile, using each record's shotpoint number and the SP number at trace 1 and increment entered below.
SP number at trace 1 :
(type single,
limits -1000000.0 to 1000000.0) Enter the shotpoint number corresponding to
trace 1 on the profile
ShotPoint Increment :
(type single,
limits -1000000.0 to 1000000.0) Enter the shotpoint increment per trace
This point active :
(type option,
Yes, No ) Currently ignored. This has been retained for backward compatibility
with older Lynx 3-S migration programs.
Trace number : (type long integer, limits 1 to 32000) The trace
number for this velocity point
ShotPoint number :
(type single, limits -1000000.0 to
1000000.0) The shotpoint number for this velocity point
X : (type
single, limits -1.e+7 to 1.e+7) The X-coordinate for this velocity point.
Y : (type single, limits -1.e+7 to 1.e+7) The Y-coordinate for this
velocity point
Note that these coordinates will apply to all the time-velocity pairs
for a given velocity point. If the time-velocity pairs apply at varying X-Y
coordinates, as when velocity data are depth migrated, or for deviated well
data, use Velprep's table editor to achieve this manually, or create a
different SP value for each TV pair.
(type long
integer, limits 0 to 20000)
Velocity points are entered as time velocity
pairs
Profile Name :
(type string)
Enter the name (up to 20 characters) for a velocity profile
clear all database table records continuing with this action will delete all records from the database table
could not find the data file the specified file could not be found
profile exists already the specified profile name already exists
Create Datum
Loading data into Velprep
VelPrep program description
The program has three elements
The velocity data are stored as a database table, which allows the usual inserting, editing, sorting and querying of single or multiple velocity profiles. Other file formats, Lynx VEL and TAX, or comma delimited files, can be imported to, or exported from, the database table. The table may be processed in various ways, to generate interpolated grids of velocity points or functions, for use in migration, time-to-depth or depth-to-time conversion, ray tracing or even attribute analysis. Depending on the application, the table fields may change their meanings slightly, e.g. VINT may be used for Dix interval velocity, or instantaneous (interval) velocity.
When working in VelPrep, the table data that you see are in a temporary copy of the database. Changes are only commited to the project's database file when you "Save Table as..." or close the project
Note on Firebird / Interbase Databases
The database file used in this implementation of Velprep uses the open source "Firebird" format (also known as Borland Interbase). Firebird is a fully fledged client-server SQL database, which uses a single database file with extension .gdb. Velprep has work-arounds for two potential problems with this appoach
A Velocity control point (VCP) is a set of velocity data which are referred to a given point on the surface - the surface point may be specified as XY or Lat/Lon coordinates, or as a line name and shotpoint or trace. The velocity tables, "velocity boxes", seen along the top strip of many seismic sections, each represent a VCP.
A velocity profile is a series of regularly or irregularly spaced velocity control points (VCPs) along a polyline on the surface of the Earth, usually taken from the velocity analyses along the top strip of a seismic section, or generated from a model.
Wherever velocities are required between VCPs, or for complete profiles having no VCPs of their own, some kind of velocity model is required to do the necessary interpolation.
Bi-linear interpolation between profileVCPs. This method can be
used for a profile which intersects one or more VCPs (i.e. for a normal seismic
profile with stacking velocities at known positions). At each VCP, a time-VRMS
curve is piecewise linearly interpolated from the time velocity pairs. To
obtain V(x,t), at a given point x,t between in between VCPs, these curves give
V = (x-x1) * (V(x1,t) - V(x2,t)) / (x2-x1).
This method preserves the exact input values at the VCPs, which may show considerable scatter if derived from stacking velocities. It takes no account of layering, and so becomes less applicable as dips increase. If interpolated to a regular grid, the velocity points can be smoothed by applying an n-point running average or other low pass filter.
Velocity profiles are displayed as an overlay of control point values, with line or colour contours.
Running the application
In deep water, seismic velocity tables become less reliable with increasing depth, because the travel times to any subwaterbottom event are dominated by the (slow) travel time to the water bottom. In a gently subsiding basin where strata are parallel or sub-parallel beneath the water layer, assuming constant layer interval velocities, the rms velocities will be largely determined by the water depth and by the interval velocities within the layers. An estimate of the rms velocities at any point can be calculated from the water depth and a velocity function rpresenting the interval velocities of the layers.
Use this option to generate velocities, suitable for migration, on the basis of a "prototype" velocity function (PVF), which represents the average RMS velocity control point found at a given depth. To come up with the PVF you need to get an idea of what the regional velocities look like for a given set of lines.
creating a PVF
Important note. All operations currently use trace number for horizontal location. You can enter velocity points by SP, but remember to create the correct trace numbers using the SP<-->Trace utility.
To use the PVF
Water layer synth
reference by trace (SP not tested)
Profile name for PVF - whatever you called it
trace in PVF for
required velocity control point
Tax file auto search Yes will generate velocities for all TAX
files in current direcory
Tax file auto search No will generate only
for selected TAX file
N.B. line name(s) extracted from TAX file. Generated
Velocities will appear in master table
Create 1st function at trace - the trace or SP no for 1st generated
function. Dont forget that the mute pattern at the beginning/end of the profile
will cause dreadful things to happen if you choose a trace there!
Interval
between function, traces and no. of functions will determine how many functions
you end up with. About 5, distributed along a line is usually enough
When you press OK the new functions will be generated. Check that they are there and print one out to check that looks reasonable.
Export to TAX option to load them into the TAX file, to be used in TracePro migration.