Magnetics Field How-to

Equipment needed:

Assembling the magnetometer

For the EG&G (Geometrics) magnetometers say that the coil should be installed vertically and then the sensor should be oriented with the north arrow pointing toward magnetic north (this does not have to be very precise).This is a bit harder with the GEM instrument but presumably should be the same.

Taking a reading:

1) Find a spot; these are usually spaced more or less evenly along a profile. Recorder should stand some distance away. Sensor should be "pointed" approximately north (what you point depends on the sensor a small bit). Avoid obvious metal. For us, the sensor is usually vertical and the long axis should point north (there is a north arrow on some, but can point either north or south).

2) Press proper button(s) to get a reading. On the gradiometer, you hit the button on the little external switch, then recover the measurements with the buttons on the front (RECL/ENTER) that allow you to go through stored measurements. The fancy GEM unit has screwy things, like pressing the O and F buttons to turn it off, but measurements are usually straightforward through the menu provided you are not in base mode. The basic unit you just press READ.


Often we will have auto tuning on. There have been times though when that hasn't worked well. The ambient field should be set to around 53,000 nT or so.

G856 magnetometer: Manual tuning. If you press TUNE (twice?), you should first see a value from the last reading in the field/time area and a two digit number in the station/day area. That second number is the signal strength; it ranges from 0 to 99. This display is then replaced by tunE in the field/time area and the tuning value (in 100s of nT) in the station/day area. If the signal strength is high and the tuning value is near the field reading, all is good. If not, hit SHIFT and then the first three digits of the desired tuning value and then enter. To check if this took, hit READ and then TUNE after the reading to see the signal strength as above. Auto tuning: If auto tuning is on, you cannot manually tune the instrument. Hit AUTO and TUNE and the display should say A tunE on or A tunE off. If it is on, you can turn it off by hitting AUTO TUNE CLEAR. AUTO TUNE ENTER should turn it on. If you somehow turn on auto measuring (used for when the equipment is used as a base station), AUTO CLEAR will stop it. In AUTO mode, you will get an error pressing the READ or STORE keys. As a base station: Press AUTO; the number is then the repeat time in seconds for measurements. You can accept that by hitting ENTER; otherwise press SHIFT and enter the number of seconds and then press ENTER.

Log entries:

The beginning of the log should include the names and duties of the field party (e.g., "John Jones scribe, Bob Smith reader, Jim Brown GPS"), the id of the instrument being used (including any GPS unit, if applicable), the time zone being used, and the date. A brief summary of the day's agenda doesn't hurt. Which kind of instrument, note sensor height(s).

For each profile, indicate the starting point both with an absolute and relative (locally descriptive) location. Note how you will be spacing measurements out (both your distance spacing goal and means you are using to diot --e.g., pacing, whose pace). Ideally indicate profile azimuth and goal. Might note recognizable points along the way (e.g., summits, road crossings). Should include occasional GPS locations as appropriate. Also not changes in profile orientation.

For each measurement, normally try for 3 measurements in each spot. If things are crazy, perhaps shift off a bit (this can be caused by locally high gradients). Also note measurement reliability if available (varies from device to device--one had a light flash in corner during measurement--5 was good, 1 was terrible)

Survey Overview:

Ideally we have a base station operating the whole time so we can remove changes caused by solar effects. At minimum, we measure at the start and end of the survey at the same spot (mark the spot carefully at the start so you get the same spot at the end).

Base station: (usually would use the GEM GSM-19)

Set up Base: Navigate to the "survey" menu by pressing the two menu buttons at the same time, and then pressing A for "survey." To create a new file, push "n" (6) and then "f" (3). Choose "B" for base station mode. Then go through set-up (F) to set sample frequency, etc. (A couple years ago it seemed that holding down A for several seconds from main menu worked). Many of the other functions make sense more or less from the manual (currently the device is running v5.0). As a check, the Boulder Magnetic Observatory data is online at <> (Choose BOU from the station list on the left).


For magnetometers with internal D cells, remove the batteries

For GEM GSM-19, charge the battery and then shut down for storage as recommended in that unit's manual (more than just turning off)

For unit with internal battery, check 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 11:09 AM