Searching for metal with a magnetometer. Magnetic anomalies.

The physical principle of searching for metal with a magnetometer. As you know, our planet has its own magnetic field. It comes from the bowels of the molten nucleus and spreads far beyond the atmosphere. It means that the magnetic field is everywhere, its strength is relatively the same in any particular location and may only slightly vary. If you take a few measurements of the magnetic field strength in one location, the values will be approximately the same. Ferrous objects cause distortion in lines of force of the Earth's magnetic field and the values of its strength near ferrous objects differ substantially from the natural strength of the magnetic field. Thus, searching for metal with a magnetometer comes down to finding magnetic anomalies caused by ferrous objects. A magnetometer can also detect irregularities in the soil structure (a well, a mine, a cave). It cannot detect objects made of non-ferrous metal, but if it comes to archeology, non-ferrous metal is usually found near a ferrous object (a barrel, a wrought chest, a cast-iron pot, etc.) or is somehow related to the irregularities in the soil structure (a pit, a well, a cellar, etc.). You can see the full power of a magnetometer during a search for large objects (from the size of a handgun to that of a tank). And the larger the object is, the further the magnetometer will be ahead of other metal detectors. The interference of small objects (nails, shrapnel) is neutralized and they do not interfere with the search. The practical depth is 1.5-2 meters for a handgun, 15-20 meters for a car. Besides, the target is detected not only right under the sensor but also sideways, which makes the search much easier and reduces the possibility of missing something. The readings of the device will get to the maximum immediately near the object or the device will be overloaded and you will hear the corresponding sound signal so you will always be able to determine where exactly the object is under the ground.

Proton magnetometer

The operating principle is based on nuclear precession (a change in the orientation of the rotation axis) around the direction of the magnetic field.
Hydrogen nuclei (protons) present in a proton-rich fluid get polarized under the influence of the artificial magnetic field created by the solenoid oriented at the approximately right angle to the Earth’s magnetic field. After that the polarizing magnetic field is interrupted.
Protons start to freely precess around the direction of the Earth’s magnetic field until the nuclear spins reach the new state of equilibrium. The precession of the protons induces a small electromotive force in the receiving coil. The frequency of this signal f is the same as the frequency of the proton precession and correlates to the strength of the magnetic field F according to the formula 2pf = gF, where g is the gyromagnetic ratio of the proton, which is known with high accuracy. The change in the coil frequency makes it possible to determine the magnetic field strength.

The signal from the coil is increased by about 5 million times, noise is filtered out, the signal is digitized and sent to the microprocessor for further processing, measurement and visualization.

How the DEEPGEOTECH magnetometer works

You turn the device on and search an area. You can see the value of the magnetic field strength on the screen. If a magnetic anomaly (for example, caused by a metal object hidden in the ground) is detected, you hear a sound signal. The readings from the device are visualized on the screen in the form of a graph. It is possible to continuously save the data (field strength, time, GPS coordinates) to a file of the Excel format in order to process it further on a personal computer. We offer the device in two versions: a magnetometer and a gradiometer. Both devices have high sensitivity and resolution characteristics. The gradiometer is a more functional version, it has all the features of the magnetometer plus additional features. The advantages of the magnetometer include a lower price, smaller weight and lower power consumption.

Comparing a magnetometer to other metal detectors

Magnetometers are actually the long-range artillery of metal detection. No other physical principle of metal detection (VLF, PI, etc.) can compare with magnetometry concerning the depth of ferrous objects hidden under the ground and there are a few explanations for it. All metal detectors emit a signal first, it penetrates the ground, reaches the object, reflects off it and travels back through the ground. Unlike that, magnetometers do not emit anything, but only measure the existing megnetic field since it is already everywhere, including under the ground. Thus, you can theoretically detect objects located twice as deep using megnetometry. But the depth of detection also depends on the magnetometer parameters - its sensitivity and resolution characteritics. The smaller the geomagnetic disturbance it can detect is, the deeper the object it will detect can be. Different types of magnetometers have different resolutions characteristics. Proton magnetometers are unquestionable leaders and have much better resolution characterstics than any other magnetometers (fluxgate, resistance , Hall-effect, etc.). There is only one disadvantage in them - the measurement rate is not high due to the peculiarities of the method: about one measurement per second.