4 - types of magnetometers explained

Magnetometers are devices which consist of sensors that measure Magnetic field intensity and direction. There are different types of these devices for such purpose, However when it comes to magnetic data acquisition in geophysics they can be categorized into two groups depending on type of the field (direction) measured, which are Vertical field such as Fluxgates and Total field such as Proton Precessing and Alkali vapor magnetometers.

Here down I have written to you this post about the basics of these devices, so that you can understand their operation principles.

Flux - Gate Magnetometer

It applies Electromagnetic induction concepts.

In the Fluxgate method a high susceptibility alloy rod (fluxgate) is used which responds parabolically to an applied magnetic field strength. The fluxgate is excited with a sinusoidally varying magnetic field using a coil around it.

The resultant field within the fluxgate becomes
H = H0 + psinωt
A secondary coil is used to measure the induction voltage due to the varying magnetic field inside the fluxgate.

The amplitude of the voltage is both a function of the applied field and Earth’s ambient field. The measured magnetic field is the component of the Earth’s magnetic field along the direction of the fluxgate.

Fig: Simple Coils used in Fluxgate Magnetometer

Advantages

It can make vector (directional) measurements

It can record continuously

Disadvantages

It is sensitive to temperature

It measures the field only in the direction of coil.

N.B: Accuracy with good insulation (5 - 10nT).

Proton - Precession Magnetometer

Since, the Earth’s magnetic field is not strong enough to align the protons. In order to align the protons in one direction, a strong magnetic field is initially applied and protons are precessed in a certain direction.

When the applied magnetic field is shut off, the protons still have their motion, but now they precess due to the Earth’s magnetic field. Then, the precession frequency gives the magnitude of the magnetic field which is the Earth’s magnetic field, itself.

Remember that Single proton nuclei exhibit an angular momentum, which yields a Net dipole - Moment. Movement of protons will tend to Align with an external field.

Proton will "Precess" to align with new field, if external magnetic field changes

Fig: Simple Mode of working for Precession Magnetometer. (Reynolds, 1991)

Precession occurs due to the finite angular momentum of the protons.

The frequency of the precession is a function of the applied magnetic field which is given by

ω = γB

where γ is a constant number (gyromagnetic ratio of the proton).

Advantages

It does not need to align bottle with the field

It is fairly lightweight yet rugged.

The magnetic field is measured using the frequency of the electric signal.

Disadvantages

It can't record continuously

It can't measure vector field

N:B: Measurements accuracy (0.1 - 1nT), with sampling time of 0.5 - 2 Sec.

Alkali - Vapor Magnetometer

The working principle of these Magnetometers is the same as that of Proton Precession Magnetometer, but the only difference is that here the excitation of the alkali Vapor is done by means of optical methods so as to have the precession frequency.

These devices are sometimes known as "Optically Pumped Magnetometers"

What does it consist of?

It consist a Bottle filled with caesium or Rubidium Vapor

It have Polarized light source of the same element

There is a Coil to generate radio frequency magnetic field

There is Light detector.

Advantages

There is no need to align bottle with field

It is very rapid and almost continuous.

Disadvantages

It can't measure Vector field

N.B: Accuracy (0.01 - 1 nT).

Magnetic Gradiometer

It records the difference between two measurements that are made close together (magnetic gradient). In simple words we can say it consists of two identical magnetometers (sensors) such as Fluxgates separated at a fixed distance.The difference value is divided by distance between sensors.

Depending on the type of survey, such as for Airborne Surveys the separation distance is at range 2 - 5 m while for ground based surveys it is 0.5m as standard. The magnetic gradient is given in nanotesla per unit meter (nT/m)

Advantages

There is no need for diurnal variation correction

It provides high resolution of near surface features.

Disadvantages

It can't measure large scale features.

Essentially automatic removal of regional.

It have lower signal to noise ratios

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