Short shovel Fiskars: 5m
Handgun: 1.6 m
Knife: 1 m
Posted on: April 04, 2011, 05:24:17 PM
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.
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