Magnetometers – Types & Applications like Metal detectors and Geographical Surveys

What are Magnetometers?

Magnetometers are extensively used in various applications like geographical surveys, archeological surveys, metal detector, space explorations, etc. to detect the mineralization and geological structures. In oil and gas industry these meters  play an important role for a directional drilling process. These meters are available based on the type of applications like land, airborne, marine and micro-fabricated atomic magnetometers.

Magnetometers are used to measure the strength of the magnetic field and in some cases direction of the field. These come under scientific instruments. A sensor which is attached to this device measures the flux density of the surrounded magnetic field around it. Since the magnetic flux density is proportional to the magnetic field strength so the output directly gives the intensity or strength of the magnetic lines. Earth is surrounded by the lines of flux which vibrate at the different frequencies depending on the locations. Any object or anomaly which distorts this magnetic field is detected by magnetometer.

These devices can detect two types of magnetism, permanent and temporary magnetisms. In temporary magnetism, the magnetically susceptible material acquires the magnetic field from the external field, so the higher the material magnetic susceptibility stronger is the induced magnetic field. This type of measurement is used in archaeological processes. Some of the sources of the permanent magnetism are (like iron, other metals) useful while measuring the magnetic field strength. However these devices are also making use of the magnetic properties of the nuclei of atoms.

2 Types of Magnetometers:

Magnetometers are divided into two basic types: scalar and vector manometers. Scalar manometer measures the scalar value of the magnetic flux intensity with very high accuracy. These are again differentiated as proton precession, overhauser effect and ionized gas magnetometers.  A vector manometer measures the magnitude and direction of the magnetic field. These are divided into various types as rotating coil, Hall Effect, magneto resistive, fluxgate, search coil, SQUID and SERF magnetometers. All these type of manometers are discussed briefly below.

1. Scalar Magnetometer

  • Proton Precession Magnetometer

It uses the nuclear magnetic resonance (NMR) to measure the resonance frequency of the protons in a magnetic field. A polarizing DC current is passed through a solenoid, that creates high magnetic flux around the hydrogen -rich fuel like kerosene. Some of these protons are aligned with this flux. When the polarizing flux is released, the frequency of precession of the protons to normal realignment can be used to measure the magnetic field.

Proton Precision Magnetometer
Proton Precision Magnetometer by engineersgarage
  • Overhauser Effect Magnetometer
Overhause Magnetometer
Overhause Magnetometer by whoi

This also works on the same principle of proton precession type but in place of the solenoid a low power radio frequency signal is used to align the protons. When an electron-rich liquid combines with hydrogen, it is subjected to a radio frequency (RF) signal. By overhauser effect protons are coupled to nuclei of the liquid. The precession frequency is linear with the magnetic flux density and thus can be used to measure the field strength. It requires less power consumption and has  faster sampling rates.

  • Ionized Gas Magnetometers

It is more accurate than the proton precession magnetometer. This comprises of photon emitter light and vapor chamber filled with the vapors like cesium, helium, and rubidium. When the atom of the cesium encounters the photon of the lamp, energy levels of the electrons are varied at the frequency corresponds to the external magnetic field. This frequency variation measures the intensity of the magnetic field.

2. Vector Magnetometers

  • Fluxgate Magnetometer
Fluxgate Magnetometer
Fluxgate Magnetometer by wikimedia

These are used for the high sensitive applications. A fluxgate sensor drive has an alternating drive current that runs a permeable core material. It consists of magnetically susceptible core wound by two coils of wire. One coil is excited by the AC supply and constantly changing field induces an electrical current in the second coil. This current change is based on the background field. 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.

  • SQUID Magnetometers

It consists of two superconductors separated by thin insulating layers to form two parallel junctions. These are very sensitive for the low range intensity fields and most commonly used to measure the magnetic fields produced by brain or heart in medical applications.

  • Search-coil Magnetometer
Search Coil Magnetometer
Search Coil Magnetometer by nasa

These are based on the principle of faradays laws of induction. It comprises of copper coils that are wrapped around a magnetic core. The core gets magnetized by the magnetic field lines produced inside the coils. The fluctuations in the magnetic field results to the flow of electrical currents and the changes in voltage due to this current are measured and recorded by the magnetometer.

  • Rotating Coil Magnetometer

While the coil is rotating magnetic field induces the sine wave signal in the coil. This signal amplitude is proportional to the strength of the magnetic field. But this type of method is outdated.

  • Magneto Resistive Magnetometer

These are of semiconductor devices in which the electrical resistance varies with the applied or ambient magnetic field.

Applications of Magnetometer:

  • Archaeology

To detect the archaeological sites, buried and submerged objects

  • Coal exploration

Used to locate the sills and other obstacles which results in explosion

  • Military applications

Used in defense and navy to perform the submarine activities.

  • Defense and aerospace

Used on land, in the air, at and under sea, and in space applications

  • Oil and gas exploration

Used while drilling the discovered wells

  • Drilling sensors

Used to detect the direction or path for the drilling processes

  • Plasma flows

Used while studying about solar wind and planetary body

  • Health care monitoring

Used to perform cardiac applications like diagnostic system capable of non-invasively measure of  heart function

  • Pipe line monitoring

Inspecting corrosion of the pipeline in the underground systems and also for monitoring purposes these are used

  • Surveyors

Used in the geophysics applications

  • Compasses
  • Space applications
  • Image processing of the magnetic data

I hope my article leaves you with the basic knowledge about magnetometers. Now that you know about Magnetometers, I leave a question for you- How can you differentiate Magnetometers based on their sensitivity. Furthermore any queries on this concept or on electrical and electronic project Kindly leave your question and  answer in the comment section below.

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