Refraction field how-to

Equipment needed in the field:

General instructions:

  1. Determine the location of your profile(s), laying out in such a manner that expected refractions can be observed
  2. Place the recording unit, battery, grounding cable and stake at the center of the spread
  3. Roll out the geophone cables from the middle towards the ends, placing takeouts at desired distances (uaully measured with a tape).
  4. Locate the ends of your refraction spread with GPS or other suitable method. Also make a sketch map of the layout.
  5. Install geophones at measured locations (red clip to red/orange takeout, black clip to black takeout on black cable, orange to black and black to orange on the orange cable).
  6. Connect cables in this order:
    1. grounding cable to stake and recorder
    2. geophone cables to recorder
    3. power cable to battery
    4. power cable to recorder
  7. New unit: Attach mouse, keyboard if desired. Old unit: Remove LCD screen/keypad and make sure it is connected to the recorder
  8. Turn on power to the recorder.
  9. Turn on the noise monitor to check for bad channels. (some gain adjustment might be needed--pushing up and down arrows on keypad will adjust).
  10. Walk the line to verify where channels 1, 12, 13, 24 are. Mark these in the field notes along with location information, if necessary.
  11. Choose first shot point. Connect trigger to recorder, using extension cord if needed. Best if upward motion is smooth and slow (to avoid triggering on the upward swing and also to minimize the pre-event noise).
  12. View traces; save if good, delete is bad. Usually the first time through you have to adjust trace amplitudes--choose trace size and then auto adjust. From there, you can hand tune individual traces by choosing trace size and individual. If noise has a particular frequency content, consider turning on a filter to isolate the signal from the noise. Repeat 9-11 as necessary to stack up good energy.
  13. Once a happy stack is made, print it out (usually you want to adjust the time scale to be expanded) and save to a floppy/thumbdrive (you can only use numbers in the file name on old unit). Note that saving to seg2 allows the file to be used in the Geometrics SeisImager software (PC). Segy is a more universal standard.

Using the new Stratavision system:

This is a menu-based system. Can do everything from the built-in keypad, but mouse handy sometimes. You can input as much info as desired; many options controlled from multiple places (the DoSurvey menu has many of the key commands). Setting the directory for saving data should be done at the start; the system will autoincrement file names if a number. Some things to remember: The system must be "armed" for the trigger on the hammer to work (bar at the bottom of the screen must be green; if not, choose DoSurvey from the menu and the select Arm/Disarm and hit enter). You can quickly toggle between the noise and the shot views with 4 key (noise) and 5 key (shot) when menu isn't active. 8 will produce a print (this seems to use the last settings made from the Print Shot Window command in the Print menu, though sometimes it is slow to pick up on gain changes), 7 will save data, 2 should clear the memory. Autoscale is 6. When menus not active but the shot window is active, hitting a left key turns on ability to rescale individual traces manually while right key turns on ability to rescale all the traces at once. Up and down arrows will allow for scrolling up and down. The time window that is visible is set in the Display menu in the Shot Window parameters.

For reflection, the field gear and layouts are similar. Usually you collect common-offset stacks by first determining the presence of a clean reflection and a good offset where it is clearly visible, then by hammering at that distance from each seismometer, freezing that seismometer's trace after each appropriate gather.

Please send mail if you encounter any problems or have suggestions.

GEOL4714/5714 home | C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder

Last modified at Tuesday, September 20, 2011 10:54 AM