3D SEG-Y Coordinate Mapping

Use the 3D SEG-Y Coordinate Mapping dialog to specify the Inline and Crossline fields in the trace header and how those values will be mapped to real-world X,Y coordinates. The volume is georeferenced using three orthogonal tie-points where both (Inline, Crossline) and (X, Y) coordinates are known.

By default, the tie points are calculated from trace-header values based on the (Inline, Crossline) and (X, Y) trace header fields specified in this dialog. However, not all datasets contain (X, Y) coordinates in the trace headers, so you may instead supply tie points to map the (Inline, Crossline) values to known (X, Y) coordinates. Regardless of the source of the (X, Y) coordinates, you should also specify the coordinate system that the (X, Y) values represent, so that the survey is properly georeferenced.

For SEG-Y files that do not strictly conform to the standards, you have the option to specify custom data types and non-standard trace-header locations; use the Trace Viewer to explore the trace headers and to determine the correct byte position and format for each of these fields.

3D SEG-Y Coordinate Mapping dialog options

Allow non-standard offsets

To properly interpret and georeference your seismic volume, you might need to specify non-standard locations and data formats for key data fields in the trace headers. Select "Yes" to enable selection of any odd byte position in the trace header. You may then specify the format of the 4-, 2-, or 1-byte value to be read, starting at that byte.

This selection is only necessary if both the position and data format of the trace header differ from the standard.

Select this option to view the trace-header values in standard locations; the file is scanned in this case using a standard set of formats and offsets.

Yes

Select this option if you need to select a non-standard position and data format for a trace header. Byte selection is now available from 1 to 239 – all the intermediate /odd offsets between the standard trace header locations are accessible.

If a non-standard position or format is specified for any of the trace headers, the Minimum and Maximum fields will display "N/A". See the Application Notes below for further details.

Inline coordinate

Specify the trace-header location of the inline coordinate. This is required to import 3D data.

Crossline coordinate

Specify the trace-header location of the crossline coordinate. This is required to import 3D data.

Source of tie points

Specify the source of the tie points to georeference your 3D SEG-Y volume. By default, the tie points are calculated from the trace headers. If the X- and Y-coordinates are not present in the trace headers, or if you have more precise values (i.e., from the Text Header or a survey file), you have the option to supply this information.

Calculate from trace headers	Keep the default option if the tie points are calculated from the trace headers. This option requires valid X- and Y- values in trace headers.
User supplied	Select this option if the tie points are supplied. In this case, the X- and Y- values are not required, and the X coordinate, Y coordinate, and Coordinate scalar fields are disabled.

X coordinate

Specify the trace-header location of the X-coordinate of the trace location. If “User supplied” tie points is selected, this field will be disabled.

Y coordinate

Specify the trace-header location of the Y-coordinate of the trace location. If “User supplied” tie points is selected, this field will be disabled.

Coordinate scalar

This read-only field shows the coordinate scalar (scale factor) to be used for the selected X- and Y-coordinate fields, which allows the integer values in the trace headers to be scaled by factors of 10. You may choose whether to apply this scalar to the selected X- and Y-coordinate fields. If you have elected to specify tie-points, this field will be disabled and ignored.

Coordinate system

Specify the coordinate system that the X- and Y-coordinate values represent. Output files will inherit this coordinate system.

To change the coordinate system information, use the Modify button located to the right of the field.

Coordinate system units

Read-only field that shows the horizontal units of the selected coordinate system.

If the vertical distance units differ from the horizontal units, they will be scaled to match the horizontal units before the output files are generated (grid, database, or voxel formats).

[More]

Click this button to display all of the non-zero trace headers that were detected in the file, including byte #, minimum value, maximum value, range, and step. This can be useful when trying to determine the trace headers that store the coordinate fields specified in this dialog.

When using custom data types and/or non-standard trace-header locations, use the Trace Viewer to explore the trace headers and to determine the correct byte position and format.

[Text Header]

Click this button to view the Text Header in a separate, read-only window.

[Binary Header]

Click this button to view the Binary Header in a separate, read-only window.

[Trace Viewer]

Click this button to show the Trace Viewer in a separate, read-only window, which allows you to view the trace headers and trace data for any selected trace.

Application Notes

According to the SEG-Y standard(s), the Inline, Crossline, X-, and Y-coordinate fields are to be stored as 4-byte integers at predefined locations within the trace header. In non-compliant files, they may be stored at the correct location, but using formats such as 4-byte floating point values, or even 2-byte integers. You have the option to specify the format used to read the data at the selected position. In cases where both the position and data format of the trace header are non-standard, you may need to “Allow non-standard offsets” to allow selection of “in-between” (i.e., any odd byte index) positions.

The Trace Viewer has the same “Allow non-standard offsets” option. Use it to investigate the trace headers of any trace and find the correct fields; it interprets the bytes in each trace-header location in all supported formats.

Providing a new offset and format combination that is non-standard will set the Minimum and Maximum parameters to "N/A" (not available). To force recalculation of the minimum and maximum values, the trace headers will be rescanned, and the X ,Y coordinates will be recalculated based on your offset and format selection.

Once you are comfortable with the selection made, click on [Next] and a bar indicating the progress will appear. The next wizard dialog, 3D Survey Mapping and Georeferencing, will open and display the calculated X- and Y-coordinate values. Pressing the [Back] button will return you to the current dialog (3D SEG-Y Coordinate Mapping), which will display the recalculated minimum and maximum values.

Georeferencing a 3D SEG-Y Volume

There are two principal methods (in practice) for georeferencing a 3D SEG-Y volume - corresponding to the Source of tie points options:

The X, Y, Inline, & Crossline coordinates are stored in trace headers for every trace, from which three tie points are calculated.
The Inline & Crossline coordinates are stored in trace headers for every trace, and three tie points (locations where both Inline, Crossline and X, Y are specified) are provided to locate and scale the volume.
For example:

C14 IL 1, XL 1, (332800.5, 4260778.0 )

C15 IL 170, XL 1, (322874.25, 4265002.25)

C16 IL 170, XL 213, (338173.44, 4264910.0 )