Base station magnetometers for ground mag surveys.

While any kind of magnetometer can be used as a base station magnetometer to record temporal variations in the geomagnetic field, some are more accurate and convenient than others. It is convenient to use a base station from the same manufacturer since the software that comes free with those magnetometers can diurnally correct that data. For example the Geometrics MagMap software can diurnally correct G856 and G859 data, and the GEM systems GEMlink software can diurnally correct GSMP-35 and GEM-19 data. However the GEM software cannot diurnally correct Geometrics magnetometer data and vice versa.

A GSM-19 Overhauser base station magnetometer sensor with a view.

A GSM-19 Overhauser base station sensor with a view.

It is also convenient to synchronise the time on GEM systems magnetometers of the same type, since a “synch” cable can be used to do this automatically and accurately. Be sure to have the correct time (preferably GPS time) on the rover magnetometer first.

The Geometrics G856 and GEM-19 proton magnetometers are commonly used as base station magnetometer. These magnetometers have accuracies of around 0.15 nT. This is good, but not great. Better to use an overhauser magnetometer as the base station with an accuracy of 0.015nT. This is 10 times more sensitive than the plain vanilla proton magnetometers most surveyors use.

We have sometimes used our GEM k-mag as a base station, and the slowest this magnetometer can sample is 1Hz. This is quite fast for a base station. Suffice it to say the diurnal correction for these surveys was most accurate!

More commonly, Modern Mag uses GEM overhauser magnetometers as magnetic base stations since they are more accurate and reliable than proton magnetometers. They also time “synch” with our GEM k-mags via a cable making the diurnal correction even more accurate. They don’t require an external battery an never run out of power if the internal battery is fully charged. Thus you avoid the awful feeling of getting back to the base station and seeing that your proton magnetometer has stopped working! You wonder how much of the days work was lost, and you can’t find out until you get back to your computer and download the data.

Feel free to contact Modern Mag about your own magnetometer needs, and for specialist advice about the right magnetometers, software and equipment for your survey.


  1. Justin Ward
    Apr 22, 2013

    Hello Andrew,
    Modern Mag uses GSM-19W Overhauser magnetometers for our base stations. These ground magnetometers do not require an external battery and are 10 time more accurate than proton magnetometers that most surveyors use. The battery easily lasts all day on a full charge.
    In dense brush, I would also recommend Overhauser magnetometers. They have the fewest external cables and are the lightest of ALL manufacturers. Modern Mag uses GSMP-35s “k-mags” in dense terrain but we have developed a aluminium/titanium backpack with custom machined components that allow us to keep the GPS and magnetic sensor below head height. This reduces the height of the equipment and means it does not hook on trees as easily. In future we will offering this backpack for sale. Watch this space….
    Not sure where you are located, but for reference Modern Mag is a GEM reseller in Australia.

  2. James
    Jul 16, 2013

    I manage the geophysical program for a small exploation company in Queensland. We have a g859 and a g856 by Geometrics for our mag program. I am looking to replace some equipment, possibly the entire set up and I have tried to reach Gem for a quote on a new or used rover and base with similar capacity as our current set up. I have not had a reply yet and was wondering what the cost difference is. Depending on the survey area some surveys require substantial heading correction which is a key factor in shifting to gem. Any insight would be much appreciated. Cheers

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