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. Ideally examine survey mode (want Refraction), layout, shot point. Determine where files will be stored and what the starting number will be. This is also the time to decide on sample interval and duration (adding a little pre-shot time can help make it clear when the trigger was initially missed).
  10. Turn on the noise monitor to check for bad channels. (Under menu 7, Conduct Survey, 4, Bring Noise Monitor to Front) (some gain adjustment might be needed--pushing up and down arrows on keypad will adjust).
  11. Walk the line to verify where channels 1, 12, 13, 24 are. Mark these in the field notes along with location information, if necessary.
  12. 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). When the noise levels are low, hammer the strike plate. Key "5" to bring the gather window to the front.
  13. View traces; save if good, delete is bad. Usually the first time through you have to adjust trace amplitudes. Can autoscale with 6 key within survey mode, or can 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 as necessary to stack up good energy.
  14. 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 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).

Once per field collection (usually):

  1. Acquisition parameters (in 4-Acquisition>1-Sample Interval/Record Length). Choose a sample interval and record length (note it is a length so if you have a negative delay, the end time will be that much earlier). So for a 100m layout with 1500m/s material, you would need at least 100ms duration and probably quite a bit more; if the frequencies of interest are around 70-100Hz (typical range of hammer blow), if wanting 40 samples/cycle, then would be looking at 0.25 ms sample interval. (The maximum number of points is over 16,000). Reflection gathers would go longer.
  2. Acquisition Filters (in 4-Acquisition>2-Acquisition Filter). This is a fixed filter that will not be affected by changes in the display later on. Often turning on the 60 Hz notch filter is useful, but can go without any depending on the environment.
  3. Stacking rules. (in 4-Acquisition>4-Stack Options). Want Auto Stack (alternative of Replace means you are not stacking at all). If you want to have a chance to unstack latest addition, have a non-zero Unstack Delay.
  4. Other options under Acquisition are usually unchanged (high gain on the preamp, stack polarity positive).
  5. Storage location (in 5-File>1-Storage Parameters). Set the initial file name number. Usually SEG-2 has been used. You can set the directory to store data in.

Starting a new shotpoint:

  1. Set the geophone interval (2 Geom>2-Geophone interval) if value seen is wrong.
  2. Set the shot location (7-Do Survey>3-Shot Parameters or 2-Geom>3-Group/Shot locations). You can get confirmation that you've entered the right value from the little diagram of the layout.
  3. Clear the screen if needed (7 DoSurvey>2-Clear or just press 2).

For each shot:

  1. Enable the noise monitor (4 or DoSurvey>4-Maximize Noise Monitor Window). Decide when noise is low enough to justify a shot; be sure to arm the trigger ("1" or DoSurvey>1 Arm/Disarm
  2. Strike the strike block with the hammer with the trigger switch. Switch to the Shot Window by pressing "5" , ideally before the hammer hits (or can go through the DoSurvey menu)
  3. If using a non-zero unstack delay, decide if you want to keep that hammer blow. Disarm the trigger to await decision on saving/continuing to stack.

Adjusting display:

  1. Usually we need to adjust the amplitude of the traces. "6" (Do Survey>6 Auto Scale Traces) will first auto scale. With the shot window active, the right arrow key will allow alltraces to be rescalled up or down. "CLR" will end this. The left arrow key allows individual traces to be scaled. (An alternative, including the auto gain control option, is in 6 Display>1 Shot Parameters>2 Gain Style)
  2. The display filters and scale can all be adjusted (this is usually just once a field experiment but can be changed if desired).6 Display>1 Shot Parameters>1 Display Boundary allows for the length of time displayed to be changed (if the vallue is less than the length being recorded, scrolling will be enabled in the shot window). The filters are 6 Display>1 Shot Parameters>5 Display Filters. A notch filter is here too, if you decided against one in the Acquisition Filters option.

Ending a shotpoint:

  1. Save the file (you can also save intermediate shots while gathering if desired). This is "7" or DoSurvey>7 Save.
  2. Print out a hard copy (doing this after saving will include the file name on the printout). This is "8" or DoSurvey>8 Print Shot Record. If the printout is too short or long, go to 0 Print>1 Shot Printer Parameters and adjust acordingly (for some reason, these changes sometimes only take effect after one more print).

End of survey: . System>0 Close Controller and Shutdown Windows. Wait for the message you can turn off the computer before turning off power!

If wanting to access Windows, look for the task bar at the bottom of the screen when scrolling to the edge.

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 then select #1 - 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. And 1 is to toggle the arm/disarm on the hammer. 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 Friday, September 16, 2022 4:45 PM