Scientifically correct answers may surprise you: the range of a magnetometer, or scanning depth, is ZERO. Remember the definition of a magnetometer: “A magnetometer is a device which measures the intensity of a magnetic field at a point where it is.” The principle of detection is that we are looking for variations in the magnetic field caused by our objects of interest, such as ferromagnetic material. So the right question is, “At what distance can our particular object, like a ferromagnetic object, be detected using a magnetometer?”, or, rephrasing - “At what distance magnetic field variations, caused by our object of interest, may be registered by a magnetometer?”.

Unfortunately, this question is very hard to answer in “general” - as mathematical estimation of detection range is extremely difficult and depends on dozens of factors, many of them may not be known at the time of survey planning.

Instead of making estimations, the standard approach is to use some reference data with a detection range for particular types of targets or surveys. These sources are based on the results of measurements in laboratory conditions and collected field data.

For example, the detection range for unexploded ordnance (UXO) can vary based on its size, and material - Discover more about this topic with our article.

Rule of 3rd power of the distance

In situations, when distance between target and magnetometer sensor is bigger than size of the target, amplitude of magnetic anomaly decreases as 3rd power of the distance between object and sensor. This applies to metallic objects and other targets made from magnetic materials detected by sensors.

In simpler words, increasing the distance by 2 times will reduce the anomaly by 8 times.

How does that rule work in practice? Below is the table of the magnetic anomaly measured above 105mm artillery shell at different distances, as well as calculated values using the rule of 3rd power of the distance. Anomaly here is the difference between the average level of the magnetic field in the surrounding area and the peak value above the target.

Field measurements were made at distances up to 2.5m, and you can see that estimated values correlate with real-life data very well. After 2.5m distance, only estimated values are presented.

Color coding in the rightmost values represents the detection probability of this target from some distance (for professional-grade airborne magnetometer system).

• Green - very high probability.
• Yellow - you can detect it in a magnetic clean area, the probability is average.
• Red (an anomaly is <1nT) - the probability of detection is very low.