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```                  PROCESS T2D  (TIME TO DEPTH CONVERSION)

Parameters, alphabetically:
addwb     edepth    fno       lno       osi       sdepth
vtp       vtype

Document Date: 6 August 2008
modifications:
October 2006 - Change osi preset to 1.
- Change sdepth and edepth preset to not given.
- Print minimum and maximum depth when last trace.

T2D transforms time domain traces to depth domain traces using user
defined velocity functions.  The velocities may be varied temporally and
spatially.  The velocities may be average velocities or interval
velocities.  Interval velocities are converted to average velocities.
Average velocity depth conversion utilizes the formula d=t/2*v whereas
interval velocity depth conversion is d(n+1)=(t(n+1)-t(n))/2*v(n+1).

The velocity function may be varied spatially between velocity control
points.  Velocity control points are defined as either shot point numbers
or rp numbers. Each parameter list has a start and end control point with
the parameters being constant for all points between the first and last
control point of the list.  For example if the first control point=100
and the last control point=110, the points 101 to 109 will also have the
same parameters applied.
e.g.  FNO 1 vtp 2100 1.1 3100 2.1 end
FNO 3 vtp 2300 1.3 3300 2.3 end
FNO 4 vtp 2200 1.0 2500 1.5 3000 2.0 end

Results in FNO 2 vtp 2200 1.2 3200 2.2

The average velocity function is evaluated for every time sample for the
control points.  Traces between velocity control points receive a
velocity function that is the linear interpolation of the velocities of
the corresponding times on the adjacent control points.

Spatial variation is by shot if the data are sorted by shot and is varied
by rp if the data are sorted by rp.

The output sample interval is changed by PROCESS T2D to whatever parameter
OSI is set to.  OSI 1 means 1 millisecond (.001 second), which works very
for sub-bottom profilers using a time domain sample interval around 50
microseconds. Thus, in order to plot depth data, use plot parameters for 1
mil time data.

The output units of T2D are meters.  Whenever the SEG-Y header uses
milliseconds, T2D converts it to meters. Whenever the SEG-Y header uses
seconds, T2D converts it to kilometers.

Each parameter list must be terminated with the word END.  The entire set
of t2d parameters must be terminated by the word END.

Example 1 - sub-bottom profiler:
t2d
vtype ave vtp 1475 0 end
end

Example 2:
t2d
sdepth 0 edepth 20000 osi 10 vtype ave
FNO 1 vtp 1500 2.2 2200 4.5 3500 6.7 4500 7.0 end
FNO 223 vtp 1500 2.9 2100 4.2 3500 6.6 4500 7.0 end
FNO 253 vtp 1500 3.0 2100 4.9 3500 6.5 4500 7.0 end
end

THE PARAMETER DICTIONARY
--- --------- ----------

FNO    - The first shot (or rp) to apply the velocities to.  Shot (rp)
numbers must increase monotonically.
Preset=1

LNO    - The last shot (rp) number to apply the velocities to.  LNO must
be larger than FNO in each list and must increase list to list.
Default=fno
VTYPE  - The type of velocity function given in the VTP parameter.
= AVE,  average velocities.
= INT,  interval velocities.
Preset = int.  e.g.  vtype ave

VTP    - Velocity-Time-Pairs.  A list of velocity and two-way travel time
(in seconds) pairs.  VTP must be given in each T2D parameter
list.  A maximum of 25 pairs may be given.  Data times before
the first given time in vtp will receive the first given
velocity.  Likewise, data times exceeding the last given time in
VTP will receive the last velocity given in VTP.
Default=none. e.g.  vtp 1490 1.0 2000 2.0

SDEPTH - The start depth of the output depth section.  The SEG-Y header
is changed so that the deep water delay = sdepth.  All traces will
have the same delay when SDEPTH is used.  If SDEPTH is not given,
the delay will be converted from time to depth on every trace.
Preset = none    e.g.  sdepth 1000.

EDEPTH - The end depth of the output depth section.  All output traces
will have the same end depth when EDEPTH is given.  SDEPTH must
be used when EDEPTH is given.  If EDEPTH is not given, the output
trace will have the same number of samples as the input time trace.
Preset = none     e.g.  edepth 8000

OSI    - The output sample interval expressed as the distance between
samples.  The output sample interval should be similar to time
sample  intervals.  e.g.  osi 4 gets written in the SEG-Y as 4
meters whereas in time it would be 4 mils.
Preset = 1. (Meters).  e.g.  osi 10.

ADDWB  - When given a value of YES, the water bottom time is added to the
velocity function after spatial variation has been done.
Preset=no

END    - Terminates each parameter list.