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The FM-36 fluxgate magnetic gradiometer, by Geoscan Research, has been the instrument most used for magnetic surveys in archaeology. The following graphically illustrates the method of tuning the instrument advocated by Dr. Lewis Somers of Geoscan Research, which differs from the procedure illustrated in the FM-36 manual.

Sensor alignment of the FM-36 is essential for obtaining high quality results, especially when using the 0.1 nT scale (almost always the case). Ideally, with sensors perfectly aligned, the same measurement will be obtained regardless of the direction of facing or instrument tilt. Although this state is impossible to achieve, one may approach it for moderate periods of time with proper tuning.

Tuning Conditions, Setup, & Precautions
As with all magnetometers the operator should be free of ferrous metals and individuals or machinery should be kept distant from the tuning site while under use. The trimming tool (a plastic screwdriver with a small steel tip), used for balancing the instrument, should be kept away from the sensor heads at all times. An assistant may hold it or it may be placed in a pocket on the opposite side of the body since the amount of metal is small.
The instrument should be powered on and warmed up at least 20 minutes prior to initiating the tuning process.
The instrument must be held in a vertical position in each of the following tuning steps. Using a second person as a spotter can help meeting this requirement.
Never apply pressure to the instrument tube as it can distort the sensor alignment. Hold it away from your body.
The instrument should always be held at the same height above the ground surface during the tuning procedures.
The tuning process requires facing each of the cardinal directions, north (N), south (S), east (E), west (W), but aligned magnetically (i.e., magnetic north not true north). These facings should be relatively accurate, within 2-3^o of the correct direction. It is advantageous to employ a spotting compass to identify landmarks or to place flags in each direction.

Tuning Location
The location selected should be free of any large magnetic gradients or anomalies. Use the instrument to check for relatively uniform measurements taking care that it is kept vertical and facing the same direction while doing so.
Raising oneself off the ground through use of a stool or other pedestal will help to reduce the effects of near-surface magnetic gradients.
A location central to the survey area is convenient for return visits. It can be marked with a peg, paint, or by leaving the raised platform in place.

Digital Averaging & Zeroing the Instrument
Place the instrument in digital averaging mode with it set to display the mean of every 16 readings. Averaging will lend stability to the apparent measurements facilitating the tuning process.
The tuning process requires frequent zeroing of the instrument. This is accomplished with the "Zero" key which must be depressed for four "beeps."

1. Adjusting Balance

1.a. Zero the instrument with it facing magnetic north, held vertically, and up-side-down. Hold it as high as possible to maximize distance from magnetic gradients caused by soil anomalies.

1.b. Turn the instrument over so that the top and bottom sensors "trade places" (if the bottom sensor was previously at chin level the top sensor should be placed at that position). With the instrument right-side-up and still facing north observe the measurement and then use the trimming tool to adjust the balance control to exactly one-half that measurement.

Repeat steps 1.a. and 1.b. until the instrument reads zero in the vertical position.

2. Set North-South Alignment

2.a. Since the instrument has been zeroed facing north (in step 1) while facing south note the measurement. Use the N-S alignment control to adjust the reading to exactly one-half the initial value.

2.b. Face north and zero the instrument.

Repeat steps 2.a. and 2.b. until the instrument reads approximately zero while facing south. How close to zero depends on the accuracy needed. I usually try for at least +/- 0.4 nT.

3. Set the East-West Alignment

3.a. Face east and zero the instrument.

3.b. Face west and note the measurement. Use the E-W alignment control to adjust the reading to exactly one-half the initial value.

Repeat steps 3.a. and 3.b. until the instrument reads approximately zero while facing west. How close to zero depends on the accuracy needed. I usually try for at least +/- 0.4 nT.

4. Repeat Steps 1-3

Steps 1-3 are repeated, with the final step being a check of instrument balance (step 1), until the desired tuning accuracy (e.g., +/- 0.4 nT) is achieved. At this point the instrument should read approximately zero regardless of facing. Be sure to take the instrument out of digital averaging mode if that setting is not desired in the subsequent survey.

5. Subsequent Tune-ups

The FM-36's measurements tend to drift through time. One may assess this drift by noting a trend away from zero (the modal value) during the course of survey or, more precisely, by comparing measurements at the same location before and after some lapse of time (e.g., survey of a grid). Drift tends to be more pronounced early in the day while the instrument's electronics are still warming up. Hot, sunny days tend to cause greater drift than cloudy cool days, probably due to thermal expansion and warping of the main tube from sunshine. The tuning process should be repeated periodically to insure high quality measurements.

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