Magnetometry Theory
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:
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Magnetic fields are vectors, having a direction and a magnitude.
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The direction and magnitude of the Earth�s field changes with
location, latitude in particular
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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
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Mineral Exploration (iron, gold, copper, tin, diamonds(kimberlites)
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Unexploded Ordinance (UXO) Detection
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Archaeology
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Land Remediation
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Regional Geology
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Civil Engineering
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Land Use Redevelopment
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
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Magnetometry detects only ferrous materials.
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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.
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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.
Equipment for the Job
We manufacture and sell magnetometers that can be used in marine, land,
and aerial environments.
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!
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