Total field magnetometers like the optically pumped cesium magnetometer are passive devices, they do not send out waves or pulses. They measure distortions in the earth’s normally homogenous magnetic field and can sense distortions due to ferrous objects at great distances.
The basic rule of thumb is that one ton (1000 Kg) of steel or iron will give us a 1nT anomaly at 100 ft. or 30m. Since the amount of distortion falls off as the cube with distance (compare a metal detector which falls off as the inverse 6th power!) and is linear with mass, every time we cut the distance in half, we can see 1/8th the mass. Therefore, we can sense 250 lbs. (100kg) at 50 feet (15m), or 30lbs (15kg) at 25 feet (8m), or 4lbs (2kg) at 12 feet (4m).
However this is not the whole story. The factors given above are for induced magnetic fields only. Many targets also have remnant or permanent magnetic effects (meaning they have become magnetized either in production or by the earth’s field) and can therefore have larger anomalies by a factor of 3 or 5 or more. Also many hollow objects like barrels or other tubular structures appear as though they are solid due to self-shielding from the earth’s field, and thus have much larger anomalies than their mass would predict alone. Pipes fall off as the inverse square and are thus detectable at even greater distances. Please see our Applications Manual for Portable Magnetometers for more information.