Magnetic Surveying Instruments
The instruments used in magnetic surveying are magnetometers, which are complex instrument that measures both the orientation and strength of the magnetic field. . Instruments used in modern surveys are highly accurate, allowing the local magnetic field to be measured to accuracies of 0.002%. When the magnetic field of a rock unit is measured, the result is actually a measure of the field as it is being effected by the earth’s magnetic field, as well as any other large bodies of magnetic rock which are present in the vicinity. There are several types of instruments available in the market.
Magnetometer systems used for commercial applications include fluxgate, proton precession, caesium vapour (optically pumped) magnetometers and gradiometer magnetometers. The systems operate on broadly similar principles utilising proton rich fluids surrounded by an electric coil. A current is applied through the coil, which generates a magnetic field that temporarily polarises the protons. When the current is removed, the protons realign or precess along the line of the Earth's magnetic field. The proton precession produces a small but measurable electric current in the coil, at a frequency proportional to the magnetic field intensity.
Fluxgate magnetometers are based on the principle of saturation of magnetic materials. A typical electromagnet has an iron core around which the current-carrying coil is wound. The coil's magnetic field is greatly strengthened by the iron, because the iron atoms (or arrays of such atoms arranged in crystals) are magnetic. In ordinary iron, the magnetic axes of its atoms point in random directions, and the sum of their magnetic fields is close to zero. When current flows in the coil, however, its magnetic field lines up the magnetic axes of atoms in the core, and they add their magnetism to the one created by the electric current alone, making it much stronger.
But there exists an obvious limit to the process – when all atoms are lined up, a condition known as the ‘saturation magnetization’ of the iron, the iron core can provide no further help. If one further increases the current in the coil, the magnetic field only increases by the amount due to the electric current itself, with no contribution from the core. There are certain materials (ferrites) where saturation occurs abruptly and completely, at a stable defined level. If a large enough alternating current is driven through a coil wrapped around a core of such material, the core's magnetic polarity flip-flops back and forth, and saturation occurs in each half of the cycle, in symmetric fashion.
A fluxgate magnetometer, which consists of a small, magnetically susceptible, core wrapped by two coils of wire. An alternating electrical current is passed through one coil, driving the core through an alternating cycle of magnetic saturation (ie magnetised - unmagnetised - inversely magnetised - unmagnetised - magnetised). This constantly changing field induces an electrical current in the second coil, and this output current is measured by a detector. In a magnetically neutral background, the input and output currents will match. However, when the core is exposed to a background field (the earth’s magnetic field), it will be more easily magnetised in alignment with that field and less easily magnetised in the direction opposite to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field.
Fluxgate magnetometers, paired in a gradiometer configuration, are commonly used for archaeological surveys. Most instruments are capable of resolving magnetic variations as weak as 0.1nT (roughly equivalent to one half-millionth of the earth's magnetic field strength).
Proton Precession Magnetometer:
By far the most commonly used magnetometer in both stationary and mobile modes are proton magnetometers. In these instruments the sensor element is water (or some other liquid containing a large number of hydrogen nuclei eg kerosene or methanol) in a small bottle surrounded by a suitable coil. A strong magnetic field (polarizing field), oriented at a large angle to the earth’s field direction, is applied by sending a direct current in the coil to displace the protons out of the earth’s field. When the polarizing field is switched off, the protons, while returning to their original alignment, precess (to re-align themselves with the "normal" magnetic flux density) for a short time around the direction of the earth’s ambient field. The frequency of this precession is related to the absolute earth’s field through a well known constant, - the gyrometric ratio of the proton. The measured frequency divided by the gyrometric ratio gives the value of the earth’s total field.
An important advantage of instruments constructed on this principle is that orientation of the sensor is not critical. The only requirement is that the polarizing field should make a sufficiently great angle with the direction of earth’s field. Also, in contrast to the flaxgate magnetometers, which can measure the field continuously, the proton magnetometers give a series of discrete measurements at intervals of a few seconds because of the polarizing and relaxing time taken by the protons.
Optically pumped magnetometer:
Optically pumped magnetometers, also called alkali vapour magnetometers, have a significantly higher precision than other types of magnetometers. They consist of a glass cell containing the vapour of an alkali metal such as caesium, rubidium or potassium, which is energized by light of a particular wavelength. The valence electrons in the alkali atoms are partitioned into two energy levels 1 and 2. The wavelength of the energizing radiation is selected such that it excites electrons from level 2 to the higher level 3 (a process termed polarization). Electrons pushed up to level 3 are unstable and spontaneously fall back to levels 2 and 1. As this process is repeated, level 1 becomes fully populated at the expense of level 2 becoming under-populated. This process is known as optical pumping and leads to the stage in which the cell stops absorbing light and turns from opaque to transparent. The energy difference between levels 1 and 2 is proportional to the strength of the ambient magnetic field. Depolarization then takes place by the application of radiofrequency power. The wavelength corresponding to the energy difference between levels 1 and 2 depolarizes the cell and is a measure of the magnetic field strength. A photodetector is used to balance the cell between transparent and opaque states. The depolarization is extremely rapid so that readings are effectively instantaneous.
The sensitivity of optically pumped magnetometers can be as high as ±0.01 nT. This precision is not required for surveys involving total field measurements, where the level of background ‘noise’ is of the order of 1 nT. However, this kind of precision is required in magnetic gradiometers described below, which rely on measuring the small difference in signal from sensors only a small distance apart.
Gradiometers measure magnetic field gradient rather than total field strength. Magnetic gradient anomalies generally give a better definition of shallow buried features such as buried tanks and drums, but are less useful for geological tasks. The magnetic gradiometer consists of a pair of alkali-vapor magnetometers maintained at a fixed distance from each other. In ground-based surveying the instruments are mounted at opposite ends of a rigid vertical bar. In airborne usage two magnetometers are flown at a vertical spacing of about 30 m. The difference in outputs of the two instruments is recorded. If no anomalous body is present, both magnetometers register the Earth’s field equally strongly and the difference in output signals is zero. If a magnetic contrast is present in the subsurface rocks, the magnetometer closest to the structure will detect a stronger signal than the more remote instrument, and there will be a difference in the combined output signals.
The gradiometer emphasizes anomalies from local shallow sources at the expense of large-scale regional variation due to deep-seated sources. Moreover, because the gradiometer registers the difference in signals from the individual magnetometers, there is no need to compensate the measurements for diurnal variation, which affects each individual magnetometer equally. Proton-precession magnetometers are most commonly used in ground based magnetic gradiometers, while optically pumped magnetometers are favored in airborne gradiometers.
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