Refraction field how-to

Equipment needed in the field:

• field notebook
• map
• refraction recording unit
• 2 12-channel cables
• 24 geophones
• deep-cycle 12 volt battery (charged)
• grounding cable and stake
• power cable
• sledgehammer with trigger
• trigger cable extender
• measuring tape
• at least 26 surveying flags
• compass
• GPS or equivalent
• total station(optional)
• spare 1.4 Mb (high density) floppy disks
• extra roll of seismograph paper
• seismograph manual
• voltmeter (optional)
• camera(optional)

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 (usually 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. 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). (Consult summary manual handed out in class)
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 (you can only use numbers in the file name).