A gradiometer measures the
gradient
In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gradi ...
(numerical rate of change) of a
physical quantity
A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For examp ...
, such as a magnetic field or gravity.
Types of gradiometer
There are at least two types of gradiometer to measure
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s:
# ''Axial gradiometer''. This device consists of two
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, o ...
s placed in series (i.e., one above the other). The result coming from the device is the difference in
magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ( ...
at that point in space, in other words, the result is the difference between what each of the magnetometers detects.
# ''Biaxial gradiometer''. This device consists of three magnetometers measuring the gradient of the magnetic field in two directions.
# ''Triaxial gradiometer''. This device consists of four magnetometers measuring the gradient of the magnetic field in three directions.
# ''Planar gradiometer''. This device consists of two magnetometers placed next to each other. The result coming from the device is the difference in flux between the two loops.
Each sensor type responds differently to certain spatial signals.
Axial gradiometers are good for measuring depth, while planar gradiometers can measure weak signals even under a lot of noise.
See also
*
Gravity gradiometry
Gravity gradiometry is the study and measurement of variations ( anomalies) in the Earth's gravity field. The gravity gradient tensor is the spatial rate of change of gravitational acceleration; as acceleration is a vector quantity, with magnit ...
*
Magnetoencephalography
Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs (s ...
*
Geophysical survey (archaeology)
In archaeology, geophysical survey is ground-based physical sensing techniques used for archaeological imaging or mapping. Remote sensing and marine surveys are also used in archaeology, but are generally considered separate disciplines. Other ter ...
References
Measuring instruments
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