EM31 field how-to

Equipment needed in the field:

General instructions:

  1. Remove the EM31 from its box and before attaching the transmitter and receiver, check that the batteries are good by turning scale knob to BATT and then mode knob to OPER. Values displayed will be +/- 6.0 more or less for new batteries and the manual suggests replacing if below +/- 4.3. Turn mode to Off. The instrument uses 8 C cell alkaline batteries.
  2. Many times, you can now attach the transmitter and receiver and hold the instrument about 1m off the ground, meter facing up. Turn the mode knob to OPER. The range knob will be at either 100 mS/m or 10 mS/m for most of our uses. The display has two numbers. The top, labelled conductivity, is the quadrature phase measurement and is usually the number we are interested in, as this should reflect the ground conductivity. The in-phase number is generally used for calibration but also can be used when looking for buried metal.

Calibrations

  1. The general calibration is first done with only the transmitter attached. With the instrument on resistive ground, set the range knob to 1000 mS/m. The instrument should read 0 in the resistivity window. If not, the zero control knob can be adjusted: it is accessed from the battery compartment and is the adjustment screw closest to the "bottom" end of the box (part farthest from the meter displays). Note to be sure that the nut on this screw is tight when done so that the instrument doesn't drift while being used.
  2. With the instrument off, attach the receiver coil. Set the range to 100 mS/m and the mode switch to COMP. The display should read 0; if not, use the coarse and fine compensation knobs to get it to 0.
  3. The phase check is more awkward. Turn the mode switch to PHASE. Note the value in the conductivity window. Turn the coarse compensation knob one click clockwise. If the value displayed is the same (within +/-0.2 mS/m), all is well and you can click the coarse knob back to its original position.
  4. If there was a difference, then after returning the coarse knob to its original position, turn the phase screw 1/4 turn clockwise and compare the reading you now have and that with the coarse knob turned one step clockwise. If the difference in values has decreased, continue rotating the phase screw clockwise until the two readings match within 0.2 mS/m. If the value has increased, turn the phase screw counterclockwise and repeat until the two readings with two settings of the coarse knob match. When done, make sure the nut is tightened to prevent the phase screw from turning during operation and return the coarse knob to its original orientation.

Absolute calibration

  1. We have measured the resistivity ~10m west of the Bummers Rock trail about 50-75m from the edge of the parking lot. Our 1-D model suggests the EM31 should get measurements of about 1.25 mS/m; wandering about the area shows variations from about 0.7 to 1.5 mS/m. If the EM31 yields these values, all is good and no further calibration is needed. Note that the values in the parking lot will be much higher and there is a pipe under part of the lot.
  2. If necessary to calibrate, place the instrument on the ground in the location chosen as the reference location. Adjust the QF knob (under the battery panel, farther one from the box side) until a value 1.12 times the correct conductivity is obtained. Resistivity of bedrock near Bummers Rock trail is about 1300 ohm-m, so a value of 0.86 mS/m would be desired but note that given the variability in the area a somewhat higher value might be appropriate (one approach is to first find the lowest conductivity values and then do this calibration in that spot).

Storage

Remove the C cells from the battery board to prevent corrosion.

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