# Gridding Velocity Horizons

This workflow will create an average velocity horizon from TWT and TVDSS information at each well marker location, using the formula: velocity  = 2000 * depth / two-way time.

In Map View, post the X, Y, TVDSS, and TWT values for each marker in a well using the Control panel Map settings for the selected wells. The TWT values are calculated using the active Time Depth table (checkshots) for the well. Knowing the TVDSS and TWT enables us to calculate the Velocity for each marker (X/Y) location. 1. In Map View operations, use the Create Horizon from Well Markers operation to generate a sparse horizon of TVDSS marker values at the marker location. The X/Y  location for the marker is extracted from the active well deviation, the well head position is only used for vertical wells. 1. Regrid the sparse TVDSS marker horizon to either a regular X/Y grid or 3D IL/CL survey. Horizon maths, used later, does not work with sparse horizons, only regular grids.

For the corresponding TWT value you have 2 options. Either:

3a.  TWT from TimeDepth table (Checkshot)

1. Repeat Create Horizon from Well Markers  for the same marker outputting TWT.

Note: This will calculate TWT from the TimeDepth (checkshot) table in the well. 1. Regrid the sparse TWT marker horizon to either a regular X/Y grid or 3D IL/CL survey as before. Horizon maths, used later, does not work with sparse horizons, only regular grids. OR

3b.  TWT from Interpreted TWT horizon

1. Use Horizon maths to sample the TWT horizon at the regridded TVDSS locations. The formula (h1*0) + h2 creates a value of 0 at the well marker locations, before adding the TWT from the horizon.
• Horizon "h1" is the regridded TVDSS Marker horizon created previously
• Horizon "h2" is the TWT horizon that intersects the marker location

The results is TWT at the marker location and Nulls everywhere else.

1. After calculating the results, rename the horizon to something more memorable. The formula will be captured in the notes for audit purposes. 1. Calculate the Vave using horizon maths. One horizon input is the regridded TVDSS marker locations. The second input is either:
• Regridded TWT calculated from TimeDepth tables (Checkshot) (3a); or
• TWT sampled from the horizon (3b).

In this example I have selected the TWT sampled from the TWT horizon (3b).

Velocity = 2000* depth / TWT

2000 * h1 / h2

Where h1 is the TVDSS marker and h2 is the corresponding TWT value.

Note: 2000* to convert to OWT and vel units m/s.

• Set the vertical dimension to <<none>>
• Set the output class to Velocity (Average)
• Rename the Final  horizon to something more memorable. The formula will be captured in the notes for audit purposes
1. Interpolate/extrapolate the marker location Vave across the region.