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

    For the new Sting unit:

  5. Check that the initial setup is correct. This only needs to be done once in a survey.
    1. Power on the instrument. Note the battery voltage. We have found that sometimes if the voltage on the external battery is low enough that the unit will continue to function but will yield nonsense results.
    2. Press MEN to get to menu 1
      1. Press F1 to check that the array type is set to Wenner. If it is not, use +/- keys to set properly. MEN will return to higher menu
      2. Press F3 to set the maximum current. 10 or 20 mA should be fine for our purposes. Return to higher menu (MEN).
    3. Press 2 to get to menu 2, instrument setup.
      1. F1 gets you the maximum cycles. Four is usually fine unless environment is noisy. MEN to move back up.
      2. F2 gets you the percent accuracy to savea measurement. 2% is a good number here.
      3. F3 sets the units. From this screen, F1 is most significant: the linear units should be meters. MEN as needed to go back up to higher menus.
    4. Press 6 (after MEN, as needed) to check that the Swift mode is off (swift mode is used with a special cable for an automated survey). Type F1 if needed to toggle Swift mode to off.
  6. Connect electrodes to the unit. Current electrodes go in to the A and B terminals on the upper left, potential electrodes go to the M and N terminals on upper right.
  7. Repeat the following for each electrode spacing.
    1. Get to menu 1 (Survey Data). Press F2 to enter the electrode spacing, then F2 again to actually type in the value of the electrode spacing. F3 then accepts the spacing and returns to menu 1.
    2. Press the MEA button in the lower right of the keypad. You will be taken to a measurement display:

      ρa=
      apparent resistivity Ωm
      		   Wenner
      ΔV/I=
      resistance R Ω
      		   N= number of cycles
      I=
      current mA
      		    

      The italicized text will be the numbers you will need to record in the field notes. Note that the apparent resistivity is only correct if you entered the correct electrode spacing. The voltage over current value (resistance) is correct regardless of the value and should be recorded in the field notes, as should be the current.
  8. If there is a suspicion that there is a ground current interfering with the measurement, reverse the potential and current electrodes (swap A with B and M with N) and remeasure the resistivity. Put both measurements in the field notes.

Troubleshooting the Sting: Error code HVOVL means there is no current setting that is working. Usually the AB circuit is the problem--see if wires are continuous. Can also reset electrodes, especially the current electrodes, to see if they can make better contact. Error TXOVL means that the current into the ground varied during measurement. AB circuit again should be examined. Usually Sting will redo the measurement. Error INOVL is unlikely in our setup, but means the input amplifier was overloaded on a second measurement. Repeated beeping and a blank screen means the battery has too low a voltage to continue.

 

For the older ABEM Terrameter:

  1. 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.
  2. Select your current. You can go as high as 20 mA; lower values are usually fine.
  3. 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.
  4. Select the number of cycles to average over. In a noisy environment, a large number might be needed, but usually 4 is adequate.
  5. 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.
  6. 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 π.
  7. 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.
  8. 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.

Storage

Make sure unit is off. Probably charge the internal battery (don't leave on charge indefinitely). Review storage instructions if any in manual.

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, February 26, 2019 11:10 AM