Resistivity field how-to

Equipment needed in the field:

General instructions:

  1. First a caution: NEVER use this equipment in a thunderstorm. If you can hear thunder, you are probably out of luck. If lightning is striking within a few miles, it can destroy the equipment.
  2. Determine the location of your profile(s), laying out in such a manner that you can put out cable to your farthest expected distance (100m from the unit is about as far as you can go)
  3. Place the resistivity unit at the center of the spread
  4. For Wenner array, you will be spacing the electrodes out evenly (electrode separation of a/2 out from the resistivity unit to the potential electrodes in either direction, and 3a/2 from the unit to the current electrodes). Usually you measure the current electrodes out from the potential electrodes. The spacing a will increase evenly on a logarithmic scale:

    5. measurement electrode spacing (a) a/2 3a/2
    1
    1 m
    0.5 m
    1.5m
    2
    1.47 m
    0.735 m
    2.205 m
    3
    2.15 m
    1.075 m
    3.225 m
    4
    3.16 m
    1.58 m
    4.74 m
    5
    4.64 m
    2.32 m
    6.96 m
    6
    6.81 m
    3.40 m
    10.21 m
    7
    10 m
    5 m
    15 m
    8
    14.7 m
    7.35 m
    22.05 m
    9
    21.5 m
    10.75 m
    32.25 m
    10
    31.6 m
    15.8 m
    47.4 m
    11
    46.4 m
    23.2 m
    69.6 m
    12
    68.1 m
    34.0 m
    102.1 m

     

  5. Connect the far (current) electrodes to the "C" terminals on the unit; the near (potential) electrodes to the "P" terminals, with P1 and C1 for the electrodes on the same side of the unit.
  6. Select your current. You can go as high as 20 mA; lower values are usually fine.
  7. Select what you want to measure. Checking the battery can be done by selecting "BATT" and pushing the button. Usually you will want to view the resistivity in ohms (1 or 100 scale is usually a good start) (this is actually just V/I, voltage measured over the current). As a check, you might want to measure a voltage to see that the voltage isn't too low.
  8. Select the number of cycles to average over. In a noisy environment, a large number might be needed, but usually 4 is adequate.
  9. After checking that nobody is touching the electrodes, push the measure button. Each cycle will produce a beep; wait until all the cycles are done. The unit averages the values for you.
  10. Record the electrode separation, current setting, and resistivity in the field book. You might reserve a column for the apparent resistivity, which is simply the measured resistivity times the electrode spacing times 2 p.
  11. Errors: error 1 means to reduce the current; error 2 means to increase the resistivity range, error 3 means that the voltage range must be increased, error 5 means the battery is dying (try a lower current); error 6 try and reduce the current and, if that fails, increase the range. Other errors: consult the manual, pp. 3-10 ff.
  12. If ground currents are suspected, reverse C1 with C2 and P1 with P2 and remeasure. Differences are due to ground currents (averaging the resistances should then be done later). Write each measurement in the notes.
Please send mail if you encounter any problems or have suggestions.

GEOL4714/5715 home | C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder

Last modified at Tuesday, September 12, 2000 12:27 AM

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