Velocity Re-Datum

Description Re-datum and shift velocity models.
Module(s) Explorationist, Image Gather Processing, Gather Attributes
Requirements Velocity Model
Related Velocity Conversion, Velocity Model (From Well Checkshots)
Works with Velocity

This process updates a velocity model with new starting times. It can be used to safely re-datum and shift velocity models.

Create a velocity re-datum process

Create a velocity re-datum process
  1. In the Control Panel, open the Process tab.
  2. At the tab header, click the Add icon and select New Process.
  3. Scroll to the bottom or type to search, and double-click Velocity Re-Datum.
  4. Type a name for the process and click OK.

Define the velocity re-datum settings

  1. At Velocity Type and Units, select from the drop-down list the type of velocity and units assigned to the velocity.
  2. At Mode, select to use a User-defined TWT shift or to Re-datum marine vels from recording datum to MSL.
  3. Velocity Volume: The input velocity volume to be re-datumed. The velocity model must start at TWT=0 or TVD=at 0.
  4. Velocity Type: The type of velocity.
    • Interval Velocity in Time
    • RMS Velocity
    • Depth values sampled in Time (ZT)
    • Average Velocity in Time
  5. Velocity Units: The units of the velocity (eg m/s).
  6. Mode: The process can be run in one of two modes.
    • User-defined TWT shift
    • Re-datum marine vels from recording datum to MSL

User-defined TWT shift

User-defined TWT shift

In User-defined TWT shift mode, two parameters are required:

  • TWT Shift (ms): The shift in two-way time required to re-datum the velocity model.
    • A constant TWT shift value will only allow for a positive shift 
    • A horizon based TWT shift  allows the horizon to be ghosted with a positive and negative number, thus allowing the velocity to be shifted both downward and upwards.Shifts of more than +/-500ms will give you a warning; more than +/-1000ms will be an error
      • Eg: If the TWT shift horizon of constant 500 ms is provided;
        • horizon + 0 ms: Will shift the interval velocity in time (VINTT) down by 500 ms
        • horizon + 200 ms: Will shift the interval velocity in time (VINTT) down by 700 ms
        • horizon  - 500 ms: Will NOT shift the interval velocity in time (VINTT)
        • horizon - 1000 ms: Will shift the interval velocity in time (VINTT) up by 500 ms.
  • Replacement Velocity (m/s): Only used when the TWT shift is positive (i.e. deeper). There are options to use a constant velocity or horizon based velocity.

Select from the drop-down list to either use a Horizon or a Constant. If a horizon is chosen, you can optionally input a constant to offset the horizon.

As a result of this process, a new velocity volume is available in the Volume tab.

Re-datum marine vels from recording datum to MSL

Re-datum marine vels from recording datum to MSL

This mode is typically used to re-datum picked NMO velocities at recording datum to MSL for use in migration. 

This mode requires two parameters:

  • Nominal Source Depth
  • Nominal Receiver Depth

The nominal source and receiver depths (along with a water velocity value) are used to calculate the required time shift for re-datuming. The water velocity for each trace is taken from the first sample of the input velocity model. This velocity value is also used as the replacement velocity in this mode.

As a result of this process, a new velocity volume is available in the Volume tab.

How it works

  1. Step 1: Conversion to Z-T or T-Z
    • If a velocity sampled in time is provided, Insight converts the input velocity to Z-T (depth values sampled in time)
    • If a velocity sampled in depth is provided, Insight converts the input velocity to T-Z (time values sampled in depth )
  2. Step 2: Calculation of output trace length
    • The smallest and largest shift in the entire TWT shift horizon will be calculated. Then the maximum output length will be determined by the direction and maximum amount of shift
      • If all shifts are positive and if shifts are a mix of positive and negative;
        • If velocity is sampled in time; output trace length = the original trace length + largest positive shift rounded up to the next sample
        • If velocity is sampled in depth; output trace length = the original trace length + largest positive shift * replacement velocity / 2000 rounded up to the next sample (/2000 because the shift is defined by TWT, not OWT).
      • if all shifts are negative
        •  then Insight output traces of the same length as they come in
      • Examples:
        •  if the input trace is 0-9000ms and the largest shift is 150 ms, the output trace is 0-9150ms (rounded up if necessary).
        •  if the input traces is 0-5000m, the replacement velocity is 1900m/s, and the largest shift is 150 ms, the output trace is 0-5142m (rounded up if necessary).
        • if the input trace is 0-9000ms and the largest shift is -50 ms, the output trace is 0-9000 ms
  3. Step 3  Shifting trace, prepending and appending velocity
    • For each trace, Insight:
      • If velocity is depth value sampled in time (Z-T)
        • shifts the time sampled trace by the appropriate amount at that location and recalculate the depth value at the shifted location preserving the interval velocity
          • At TWT+shift; Ds= D0 +(TS/2000 * V)
            • Ds: Depth value after shift  in m
            • D0: Depth value at TWT before shift in m
            • V: Replacement velocity in m/s
            • TS: TWT shift in ms
        • if the shift is positive, then additional samples are prepended from the first TWT shifted sample until TWT = 0.  The depth value of the appended time samples are calculated, preserving the replacement interval velocity
        • If the final sample is < the length of the output trace(shift is negative), Insight extrapolates the final interval velocity down to the end of the output trace. The depth values of the appended time samples are calculated, preserving the extrapolated interval velocity
      • If velocity is Time value sampled in depth (T-Z)
        • shifts the depth sampled trace by the appropriate amount at that location and recalculate the time value at the shifted location preserving the interval velocity
          • Because the shift is defined in TWT in the parameter, the replacement velocity is used to calculate the appropriate shift in depth sample.
            • Eg: A TWT shift of 50 ms is equivalent to 50 m if a replacement velocity of 2000 m/s is used 
          • At TVD+shift; Ts= T0 + TS
            • Ts: TWT value after shift  in ms
            • T0: TWT value at TVD before shift in ms
            • TS: TWT shift in ms
        • If the shift is positive, then additional samples are prepended from the first TVD shifted sample  until TVD = 0. The time value of the appended depth samples are calculated,  preserving the replacement interval velocity
        • if the final sample is < the length of the output trace (shift is negative), Insight extrapolates the final interval velocity down to the end of the output trace. The time value at the appended depth samples are calculated preserving the extrapolated interval velocity
  4. Step 4  Convert
    • Insight then converts/resamples back to the original velocity type, chopping the top off at 0ms or 0 m.

Note: If SRD is not 0, it is advisable to run velocity re-datum in using a time sampled velocity.