Geol 492/692- Setting Survey Geometry

For your first-arrival picks to export easily to SeisOpt@2D, and to easily make time-distance plots of your picks, you must have correct geometry information loaded from your SEG-Y file into Viewmat. Check this by selecting setHeader from the On Each Vector sub-menu of the Methods menu, and you will find essentially no geometry information included with our ``dixie-refr.sgy'' file:
- The dialog pops up showing geometry info for the first trace in the data, at Header Index 0. You can enter any other index within the range given, and press the Check Header button to see the geometry for that trace. The traces and their headers are indexed in the order they are displayed - first to last record, left trace to right.
- Sample a few headers at scattered indices and observe the consistency of your geometries. The record numbers and source point numbers should be correct as shown. Receiver point numbers are not well-determined from SEG-Y files, and it is all right if they are not correct.
- Source and receiver coordinates must be correct. They must be in meters, and they must make sense according to your understanding of the survey's geometry. They may be two-dimensional: the X-coordinates or the Y-coordinates may be zero for every point. Elevations may be zero, but must be consistent.
  Our ``dixie-refr.sgy'' example was recorded on a 48-channel cable with 50 feet between takeouts. The low-index channels, on the left, are on the east end of the receiver spread. The high-index channels on the right are on the west end. The receivers remained stationary while the location of each record's source changed. The first record is from a shot 150 feet off the east end of the spread. The last record is from a shot at the west end. The other records are from shots in between.
- Offsets should be in meters. They are not critical for exporting picks, and can be recomputed as long as the coordinates are correct.
- Some SEG-Y files scale coordinates and/or elevations incorrectly, often by a power of ten. If this is the case, you can select fixCoords from the On Each Vector sub-menu of the Methods menu. The dialog that pops up will report the maximum and minimum offsets currently present in the trace headers, in meters. (These have nothing to do with any offset labeling you may have entered into the display Properties window.) The ``dixie-refr.sgy'' file has no offset or geometry information at all, so fixCoords is not needed. If the offsets are incorrect, you may have to scale the coordinates. Enter your factor to Multiply Coordinates By and click the fixCoords button to quickly scale all the headers and recompute offsets. Then bring up the fixCoords dialog again and check that the min. and max. offsets make sense.
Our ``dixie-refr.sgy'' file does not have enough geometry information included, and you must enter survey geometry information in the form of observer's reports and surveyor's reports.
- Select Apply Geometry from the Edit menu of the data display.
- The Geometry tool pops up. Basically, you create observer's and surveyor's reports in plain text in your favorite spreadsheet, Wordpad, or SimpleText, and copy and paste the text into the text area of the Geometry tool. After selecting the type of input geometry and unit system above the text area, you click the Apply button below.
- To see how to create the text of the observer's and surveyor's reports, select a Type of Input Geometry above the text field, and click the Show Format button below. The text area will then give you a detailed explanation of the text the Geometry tool will need.
- First, create an observer's report according to the format given in the tool. You will need one line of observer's report text per record, in the order the records were read from the SEG-Y file. Here is the observer's report for our ``dixie-refr.sgy'' file. It has five lines, in the order of the five 48-channel shot records:
```
0106	193	1	196	243
0078	207	1	196	243
0084	219	1	196	243
0090	231	1	196	243
0095	243	1	196	243
```
  Copy it to the clipboard from your spreadsheet or from the web browser window here (using Control-C, Alt-C, or Command-C), then select the Geometry tool's text area and paste it in (using Control-V or Command-V). Make sure the Type of Input Geometry is set to Observer's Report, and click the Apply button below. Note that these five records were from a stationary array of 48 receiver channels extending from station 196-243, east to west. The first shot is from station 193, 3 stations east off the east end of the receiver line. The last is from station 243, on the west end; the other records are from shots within the receiver line. The shot at 219 is a center shot.
- Second, create a surveyor's report or station coordinates list according to the format given in the tool. You will need one line of station coordinate text per record, in any order. Note that there are two orders for the coordinate text accepted (X-Y-Z or Z-Y-X). Our ``dixie-med-vp.txt'' file is in Z-Y-X order. Copy it to the Windows clipboard from your spreadsheet (using Control-C, Alt-C, or Command-C), then select the Geometry tool's text area and paste it in (using Control-V or Command-V). Make sure the Type of Input Geometry is set to the appropriate Station Coordinates option, that the Unit System is set according to your input units, and click the Apply button below. Note the ``dixie-med-vp.txt'' file is in feet.
- Use the fixCoords or setHeader tool from the On Each Vector sub-menu of the Methods menu to re-check your new geometry.
- You can save your file, with its revised geometry, back into the SEG-Y file, or write a new SEG-Y or RG file. Use the File->Write Binary File... selection in the menu bar of the data plot.
  - From the Edit menu of the Plot Parameters window, also save a parameter file, to preserve your axis labeling.
  - When you read a SEG-Y file written by Viewmat, it will always have the 3600-byte reel header, so leave that option checked.
  - If you save a raw binary float file, you will lose all the geometry and other information in the trace headers.