Versions Compared

Old Version 38

changes.mady.by.user Former user

Saved on

compared with

New Version 39

changes.mady.by.user Former user

Saved on

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.
Section
Column
width60%

Introduction

This documentation goes through the working steps and menus in Geocap for depth conversion of time surfaces applying a time-velocity cube. There are other methods for depth conversion than those described here, but this method is favourable when stacking velocities and well data are present. This depth conversion documentation deals with quality control and analysis of input data. Then gridding into a time-velocity cube and finally performing depth conversion of time grids in a Layer Model into depth grids.

The velocity cube reflects the information in the velocity data in an organized way. Working with velocity cubes will make depth conversion of time data to depth data more easy.

Column
width40%



On this page:

Table of Contents

Steps in depth conversion

Section
Column
width60%
Note
titleSteps in depth conversion of time data using time-velocity cube
  1. Reading stacking velocities into a folder as: x y time velocities
  2. Inspecting the velocity data
  3. Gridding into a velocity cube
  4. Displaying the velocity cube
  5. Checkshot wells can be used to trim the velocity cube through sophisticated updating techniques
  6. The velocity cube is then used for depth conversion of time grids into depth grids
  7. Calculation of cubes for RMS and Interval velocities using the generated velocity cube. (Option)

The folders used to save data in this project are of type Generic and renamed to indicated their contents. The various datasets will also be set to their proper schema type so that only relevant command objects are shown.

The documentation follows a specific dataset and is thus close to a case study. This implies that other dataset may apply slightly different display options due to size and attribute saving in scalar or field data. The general principles for depth conversion should however be the same in all cases when using the method of a velocity cube.

Column
width40%


Example of velocity cube holding stacking velocity data

Reading stacking velocities

Section
Column
width60%

Geocap provides import command objects (CO) for reading stacking velocities organized in ASCII column data.

If the file is large, keep the input as low as possible. To generate a velocity cube only x y time velocity are needed.

Example input: stacking velocities in ASCII columns, size ~ 400MB.

Info
titleReading stacking velocities into a project
  1. Create a Generic folder, give it a proper name
  2. Right click on the folder and select *Import->Stacking Velocities.
  3. Browse in all stacking velocity datasets
  4. The cell structure is kept by specifying 'Value change' and set up 'cell separator' for the actual column
  5. The schema type is set to 'Stacking velocities' to get the correct commands visible on the dataset
  6. In this case the time values are in the z coordinates and the velocity values are scalar values
Column
width40%


Geocap project for stacking velocity data and velocity cubes

The actual project was generated the standard way by opening a new project and gradually populate it with folders when needed. The stacking velocities was imported into a generic folder given the name All points in line cells, just to indicate that the stacking velocities are organized as vertical lines, called line cells in Geocap syntax. The schema setting of the dataset is Stacking Velocities.

Inspecting the velocity data

The stacking velocity file has nearly 7 million points and there are dedicated COs to make a fast display if necessary like Utilities->Map vertical cells in fast mode. This example applied Map Data and Display survey lines (will only display the top points as lines) to get a visual impression of the different surveys in this dataset.

...

To display a smaller part of the surveys one can use the CO under Utilities->Display stacking velocities in line mode. Set always the correct attribute (scalar or velocity if field data) and select for example Eliminate outside cursor: A small part of the dataset around the cursor is selected for display and also saved in workspace as elioutcursor. Similar for an area zoomed into when in 2d mode and part of dataset can be saved as workdata in workspace.

Cube gridding from stacking velocities
Anchor
cube_gridding
cube_gridding

The command menu Cube gridding from stacking velocities can be activated for data with schema Stacking Velocities. Stacking velocities have the format: x y two_way_time velocity. Two_way_time is used because time is recorded after having traveled down to the reflector and up again.

...

The Result cube page allows for saving the cube into a folder. A copy is also saved as in workspace as cubegrid. The cube dataset is given the schema Cube. Thus the cube will have many relevant commands visible for further actions.

Testing cube gridding

The Test page in the gridding menu allows for testing the gridding at various cube planes to check the gridding parameters and study the quality of the input. This is done as shown in the display below.

...

After testing the gridding at a few planes and securing all parameters are properly set, the overall gridding of the stacking velocities into a velocity cube can take place. The resulting cube can then be visualized with various display techniques.

Updating the cube with checkshot data

In many cases there are also control data for the velocity cube available in form of checkshot wells. These well data should have the format: x y depth one_way_time. They can be used to update and correlate the velocity cube so that the layers will match the checkshot wells. Use the command menu Utilities->Updating velocity cube to checkshots.

The Set update filter option is available for excluding wells that obviously has wrong starting velocities, namely the sound velocity in water.

Checkshot wells organized in a folder

Sometimes the checkshot wells must be generated by merging velocity and time from one dataset into the well position of another dataset. That is done by the command object Merge x y with two data parameters found under schema type Line folder utilities. Set your folder with well positions to that schema type, or copy the command object from the repository into Item Commands on that folder. Then one can generate the proper checkshot wells from separate well data information.

...

The above figure shows that the cube matches according to the color code of the checkshot wells by displaying the bottom layer before updating the cube (to the left) and after updating the cube (to the right). After smoothing and updating the velocity cube it can now be used for depth conversion.

Checking the cube visually

Section
Column
width50%

The velocity cube is displayed by dedicated commands available through the Cube schema.
Upper left viewport shows surface contours for the cube using the commands Cube contours.

Upper right shows planes in the cube colored and contoured with velocity using Cube views->Map X Y Z planes at cursor position. That command requires a cursor position inside the cube frame which can be set hitting the keyboard letter p or y while pointing at the screen.

Column
width50%


Displaying velocity cube with three planes and transparent surface boundary

Section
Column
width50%

Bottom left is a cube overview contoured as a boundary surface using Cube contours.

Bottom right is cube layer at level 4400 displayed as terrain using Display selected cube layers. This menu allows for displaying single cube layers or arranges multiple layers in viewports if this option is checked in on the Viewport display setup page.

Displaying individual layers in a cube is important for checking the cube contents and also for presentation purposes.

Column
width50%


Various ways to display cube information

Depth conversion using a dedicated command object

The velocity cube is useful for depth conversion of a set of time grids into depth grids. This formula is used: depth_grid = time_grid x velocity_of_time_grid.

...

The above picture shows an example of depth conversion a time grid into depth grid using a velocity cube and the same procedure as described above. All grids in the layer model were depth converted simultaneously.

Creating a RMS velocity cube and interval velocity cube

Having a stacking velocity cube, Geocap can do calculation on the cube and convert it into other cube types. Activate the commands Utilities->Perform calculation on the velocity cube.

...