Topic:
Factors to consider when using BasicTD.

Description:
A tutorial on how to run BasicTD is available from the Bookshelf.

Solution:
Horizons, markers, and grids are selected by using the surface name.  When using 
markers and horizon/grid interpretation, the markers and horizons/grids must 
belong to the same surface.

There can be problems when more than one marker value is associated with a
surface.  If the results from using the "Limit Marker Input by Source" option
are unsatisfactory, disassociate the unwanted markers/sources from the surface
associated with the marker.  BasicTD may use the most recent marker loaded into
or created in the project as the value for the Marker Depth (TVD).  "Limit
Marker Input by Source" has been corrected in GF3.8.1.

There is a problem in the depth conversion for projects that are in ft. and
have a non zero seismic datum.  This has been corrected in GF3.8.1.


Final depth maps may not cover the same area as the input horizon used.
The model extents dictate the area over which conversion takes place.  

Fault segments converted to depth are available for posting in the depth domain.
Converting the same fault segment to depth using different models will result in
multiple copies of the fault segment.  The unwanted fault segments will be
posted with the correct ones if they are not deleted.


Sometimes, holes or gaps may occur on the deepest surface that is converted to
depth.  Currently, data that is below the deepest BTD time grid will not be 
converted to depth.  This will be changed in GF3.8.1 when the velocity from
the last surface will be used for all data that is deeper then the last time grid.


There are 2 methods to remove the gaps left in the depth converted horizon.

Method 1:
Bias the deepest BasicTD time grid so that the resulting grid lies below the
time grid for that surface. Save the new grid to the original BasicTD time
grid name so that it will be part of the VMO.  DO NOT RERUN THE MODEL WITH
THIS NEW TIME GRID. The BasicTD time and velocity grids can be edited in the
Model Editor to extend their coverage.

Method 2:
Create a constant time surface that is below the deepest horizon of interest.
Include this horizon in the "Velocity Model Input" stage.  This surface does
not have to be converted to depth. This will ensure that the depth map for
the deepest surface of interest does not have "blank" grid areas. 



EDITING VMO GRIDS
The extent of the BasicTD time and velocity grids are determined by the number
and location of the scatter set points (boreholes) used in the model.  These
grids can be edited in the Model Editor and extended to cover the area
interpreted in the seismic survey.  Some methods to edit or to complete the grid
coverage:

Post the scatter set data, add additional points, regrid the scatter set data.
Save the resulting grid to the BasicTD time or velocity grid.

Create pseudo boreholes for use in BasicTD.  Locate the boreholes at the edges
of the data area.  Make sure that interpretation exists for the surfaces used in
the model at these locations.  If you are not using a regional checkshot
associated with time grid or interpretation for depth information, you will have
to add marker names and values to each of the pseudo boreholes.  Add the pseudo
boreholes to the borehole set used in the model.  Continue with the BasicTD
workflow.

Post the BTD_velocity_model grid or the BTD_time_model grid on the basemap.
Contour the grid and save the contours.  Post only the newly saved contours; use
the Model Editor to edit the contours.  Extend the contours to the limits of 
the data to be converted to depth.  Save the contours.  Regrid the data, using
the edited contours as the data input.  Save the new grid to the BasicTD
velocity or time grid.





Last Modified on: 19-JAN-01