Gradiometer

A gradiometer is a sensor designed to measure the spatial gradient of a physical field rather than its absolute value. It typically uses two or more sensing elements separated by a fixed baseline and compares their outputs to determine how the field changes over space. By focusing on differences, gradiometers reject uniform background fields and many forms of environmental noise.

Magnetic gradiometers are among the most common types in geophysics and archaeology. They employ magnetometer pairs or arrays, which can be based on fluxgate, optically pumped, or superconducting (SQUID) devices, to measure the gradient of the magnetic field. Gravity gradiometers measure the gradient of the gravitational potential, typically with two or more accelerometers arranged along a baseline; spaceborne versions have been developed to map the gravity gradient tensor. Other variants exist for electric, acoustic, or optical fields, but magnetic and gravitational examples are the most widely used.

The fundamental principle is differential measurement: the gradient components are proportional to the rate of change of the field with position. In gravity gradiometry, outputs relate to second derivatives of the gravitational potential and are commonly expressed in Eotvös, where 1 E = 10^-9 s^-2. In magnetic gradiometry, gradients are often reported in units such as nanotesla per meter.

Applications include mineral and oil exploration, archaeological prospecting, geological mapping, and satellite gravity field missions (for example, gravity-gradient measurements in space missions). Inertial navigation and defense contexts also employ gradiometers to improve drift performance and detect spatial anomalies. Advances focus on increasing sensitivity, enhancing common-mode rejection, and enabling operation in challenging environments.