Depth Conversion

Owned by Erlend Kvinnesland
Last updated: Aug 11, 2015 by Anders Moe

Seismic data is used to visualize the Earth's sub surface. This is achieved by measuring travel times of reflections from pressure waves propagating through the ground. Seismic data and interpretations of seismic data will therefore often have z values in travel time, not depth. In order to get a depth value for a travel time value, we need to do a depth conversion. A depth conversion combines the velocity of the pressure wave with the travel time to get a depth value. 

Data sets may be depth converted in Geocap using the command Velocity Model Depth Convert. This command uses a Velocity Model as input in order to determine the velocity of a the pressure wave. 

 

Velocity Model

A velocity model is stored as an item in your project. It represents the seismic velocities in an area and is used as input to the  Velocity Model Depth Convert command.

To create a new velocity model

  1. Locate or create a folder of type Generic.
  2. Right-click the folder and click Set Schema. Click Velocity models.
  3. Right click the folder and click Generate Velocity Model
  4. Adjust the model parameters.
  5. Click Execute.

A new velocity model will be created in the folder. 

To view or edit an existing velocity model

  1. Right click the velocity model
  2. Select Edit
  3. The model editor dialog will appear.
  4. View or edit the model parameters.

Supported velocity models

Geocap currently supports these types of velocity  models:

Constant Ground Velocity

The constant ground velocity model is a single layer velocity model. It uses one constant ground velocity. Input to the velocity model is water velocity, the seabed in time, and the ground velocity. The Seabed data may be interpreted lines from 2D seismic (IHorizon or PolyData) or grids (StructuredPoints).

Layer Velocity model

In the Layer Velocity model you may use seismic horizons  as interval velocity boundaries. The horizons may be interpreted lines from 2D seismic (IHorizon or PolyData) or grids (StructuredPoints). You may set the interval velocity of each interval using a table. In the table you also need to set the top horizon of each interval. In addition you need to set the water velocity as input. 

When going from top to bottom the layer velocity model will initially use the water velocity as interval velocity until it meets the top horizon. From the top horizon it will use the top horizon velocity as interval velocity until it meets the next horizon, continuing this pairwise matching of successive horizons for the rest of the seismic data.

During depth conversion the interval velocities are used to calculate an average velocity.

RMS Velocity (Stacking velocities)

The RMS velocitiy model uses RMS velocity data sets as input. It is possible to select a single data set in the project as input, but you may also select a folder containing multiple RMS velocity data sets as input. In some cases Stacking velocities may be used as RMS velocities. 

When the velocity model computes the velocity of a point, it will use the RMS velocity points within a search radius as input. The RMS velocity data set, the interpolation method, and the search radius are input to the RMS velocity model.

In order to generate an average velocity, the velocity model will run Dix formula on the RMS velocity to generate interval velocities. An average velocity is computed from the interval velocities.

Note that the search radius should be large enough so that all points where you need a velocity will have RMS points within the search radius. However if the search radius is set too big, the velocity computation for a point will include too many RMS points, and the calculation may be slow. The recommended radius will depend on the density of the RMS velocity dataset.

Average velocity cube

An average velocity cube is a 3D volume which holds the average velocity at the cube nodes. The cube may be generated in Geocap, or it may be imported from other software. Such a cube is used as input for the Average Velocity cube velocity model. You may  also specify a seabed surface and a water velocity which will be used above the seabed surface. The cube will be used to look up velocities below the seabed surface.

Specifying a seabed surface is optional. If a seabed is not specified, the velocity model will only look up velocities in the cube. 

The velocity model will clamp the last valid velocity below the bottom velocity in the velocity cube. It will also clamp the first valid value above the first valid value of the cube. 

 

Depth Conversion

The command Velocity Model Depth Convert can be used to depth convert data in Geocap. The command is available on a velocity model.


To depth convert data using a velocity model

  1. Right click the velocity model
  2. Select Velocity Model Depth Convert, and the command editor will appear.
  3. Browse in the data you want to depth convert into the Input field.
  4. Select a folder for the output
  5. Click Execute.

 

The command can be used to depth convert many different datatypes in Geocap, including

  • Seismic 2D lines
  • IHorizon (seismic interpretations)
  • Poly Data (points, lines, polygons)
  • Structured Points (Grids)

In the input field it is possible to browse in multiple data sets. You may also specify folders as input. If this is case, all the data sets in the folder will be depth converted. When the input field has folders, the same folders will be created in the output folder. 

The depth conversion command has options to store the average velocity used to do the depth conversion of each point in the result. You may also choose to store the Instantaneous Velocity. The Instantaneous Velocity is computed by looking at the change of average velocity from the velocity model around a point. For seismic lines the  Instantaneous Velocity is the interval velocity of a single sample in the seismic. Storing and visualizing these velocities will make you able to quality control the velocity model used for the depth conversion.

If you store velocities when depth converting seismic lines the velocity values are stored as seismic attributes to the resulting seismic lines. You may visualize the velocities with the regular Seismic Display command. Note that storing velocities will make the depth conversion of seismic lines slower.