Gradiometry

Gradiometry is the measurement of spatial gradients of a physical field. In geophysics and geodesy, gravity gradiometry refers to techniques that quantify variation in Earth’s gravitational field over space by measuring differences in gravity acceleration between closely spaced sensors. The gravity gradient is described by the components of the gravity gradient tensor, which are second derivatives of the gravitational potential with respect to spatial coordinates. By obtaining several gradient measurements, one can map subsurface density contrasts that would not be apparent from absolute gravity measurements alone.

Principle and instruments: A gravity gradiometer typically consists of multiple accelerometers or pendulums mounted in a fixed geometry so that differential accelerations between sensors are recorded. Sensor technologies include classical mechanical gradiometers, superconducting gradiometers, and modern quantum devices such as atom interferometers. The latter offer high sensitivity to accelerations and thus gravity gradients, but require sophisticated infrastructure and precise environmental control.

Applications: Gravity gradient data are used in mineral and hydrocarbon exploration, groundwater and geothermal studies, volcanology and tectonic monitoring, and civil engineering for subsurface characterization and infrastructure projects. In navigation, gravity gradient measurements can complement inertial sensors to improve drift performance in airborne, marine, and space applications.

Limitations and challenges: Measurements are sensitive to platform motion, tides, ocean loading, and environmental noise; data processing requires accurate calibration and corrections for tidal effects. Interpreting gradient data involves solving inverse problems that relate gradients to subsurface density variations, which are non-unique without supplementary information.