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Magnetometry

Magnetometry measures perturbations in the ambient magnetic field caused by contrasts in magnetic susceptibility – the ability of a substance to take on an induced magnetism caused by its immersion in the Earth’s magnetic field. The magnetic susceptibility of a rock or soil is directly proportional to its iron content, usually in the form of the minerals Hematite (Fe2O3) or Magnetite (Fe3O4). Hence it is only sensitive to ferrous metals. However, iron is often present in accumulations of non-ferrous ores, making magnetometry a staple in mining exploration.

The physics of magnetometry is complex, for three main reasons:

  1. magnetic fields are vectors, having a direction and a magnitude;
  2. the direction and magnitude of the Earth’s field changes with location, latitude in particular; and
  3. there is often a certain amount of permanent magnetism (independent of the induced magnetism), especially when magnetite is present.

As such, the amplitude and shape of the magnetic anomaly for any given object depends not only on the shape of the object, but on its location and (in the case of magnetite) the orientation of the permanent magnetic moment of the object relative to the Earth’s field.

On the other side of the ledger, the Earth’s magnetic field is very well understood, and distortions of the anomaly related to latitude and longitude can be corrected. Also, Cesium vapor magnetometers such as the G-858 and G-859 are sensitive to changes in the parts-per-million range, making magnetometry one of the most sensitive of all geophysical techniques. It is used in land, marine, and airborne platforms.

Common Applications

  • Mining exploration(iron, gold, copper, tin, diamonds (kimberlites)
  • Unexploded ordnance (UXO) detection
  • Archaeology
  • Utility location
  • Regional geology

Considerations

  • The Earth’s field changes diurnally, requiring a base station if the diurnal changes are to be corrected for. Whether or not this is necessary depends on the specific application. A diurnal correction may be absolutely necessary in an archaeological application, and superfluous if searching for buried well casings.
  • Cesium magnetometers inherently have “dead zones” where if the Earth’s magnetic field lines pass through the sensor along the polar and equatorial axis the sensor will not be able to make an accurate measurement. Our CSAZ (Cesium Sensor Active Zones) program can help you determine the optimal sensor orientation for your survey area.

Benefits/Limitations

  • Magnetometry detects only ferrous materials.
  • The amplitude of the magnetic anomaly for an object decreases as the inverse cube with distance. In other words, as we double the distance to the object, we only have 1/8th of field intensity. For example, if an object one meter deep has an 8 nT anomaly, that same object two meters deep would have a 1 nT anomaly. In magnetics, depth of investigation is driven more by the object’s depth and magnetic moment rather than field methods or technology used.
  • Cultural noise, such as fences, passing cars, buildings, and other ferrous metals can interfere with the signals from the object you are attempting to detect.

Deliverables

  • The final product of a magnetometer survey is typically a magnetic anomaly map like the one shown below:
Pearl Harbor UXO Survey--Diurnally Corrected

Pearl Harbor UXO Survey -- Diurnally Corrected

Best Tool

G859

Best Practices

  • The cesium and proton magnetometers are both very sensitive instruments. So sensitive that they will be able to measure the small magnetic parts often found on clothing (steel buttons, eyelets in shoes, zippers, etc.). For best surveying practices check any article of clothing that may contain ferrous material before surveying as it could impact the quality of the data.
  • A general understanding of typical size and shapes of the magnetic signature of your target is needed to prepare your own survey. In general, a small target like an UXO, will require a line separation of 2-5 meters depending on the smallest target of interest. However, for a geological survey a line spacing of 50m may be sufficient to properly map out the features of interest. If you are surveying with 2 meter line spacing for a geological application you will be wasting a lot of time by oversampling your survey area; and wasting time means you are wasting money! Check out our video series about Marine Magnetometry for a general discussion of magnetometry and about survey spacing for small target detection.

Further Reading

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