Gravimeter Glossary: Key Gravity Terms for Geophysics, Navigation, and Subsurface Detection

Gravimeter Glossary

As gravity-based technologies expand from mineral exploration to navigation and infrastructure monitoring, understanding the core terminology behind gravimetry is essential. Take a look at our glossary to help your understanding of this often complex field.

Core Concepts

Gravimeter

An instrument that measures the local gravitational acceleration, often with microgal (µGal) precision. Gravimeters reveal subsurface features like voids, ore bodies, geological features, or structural weaknesses.

Gravity (g)

The acceleration due to Earth’s gravity field. While the global average is ~9.80665 m/s², local deviations carry valuable geological information.

Gravity Anomaly

The difference between measured gravity and a theoretical model. Positive anomalies often indicate denser material; negative ones suggest voids or low-density rock.

Gravity Units: Gal, Milligal, and Microgal

Gal (short for galileo)

The base unit of gravitational acceleration in gravimetry, named after Galileo Galilei.

1 Gal = 1 cm/s² (or 0.01 m/s²).

This unit is rarely used in field data but is important as the foundational measure.

Milligal (mGal)

One thousandth of a gal.

1 mGal = 0.001 Gal = 10⁻⁵ m/s².

Used in regional gravity surveys (e.g. mineral exploration) where anomalies may range from a few to hundreds of milligals.

Microgal (µGal)

One millionth of a gal.

1 µGal = 0.000001 Gal = 10⁻⁸ m/s².

Used in high-resolution applications such as:

• Detecting small voids or tunnel features
• Gravity-aided navigation
• More detailed mineral exploration studies
• Monitoring groundwater or volcanic movement

Gravimetry Instruments & Methodologies

Relative Gravimeter

Drift is a limiting factor for many gravity instruments. Relative gravimeters accept this fact and operate by measuring differences in gravity between locations. A base or absolute station is used as the reference from which other readings are related. Lower short term drift on an instrument is still important as it reduces the frequency that users must return to the base station.

Absolute Gravimeter

Measure gravity directly using known physical constants, often via laser interferometry and free-fall masses. Used for calibration or absolute station characterisation.

MEMS Gravimeter

Uses Micro-Electro-Mechanical Systems to create ultra-compact, robust sensors. VBA MEMS is a unique patented technology used by SMG Gravity. Lower size, weight and power use of MEMS technologies offer huge potential for the development of a new generation of gravity instruments for use in traditional field application, as lightweight instruments in mobile platforms (land, air and submersibles) and for miniaturised solutions in borehole / wireline tools.

Strapdown Gravimetry

A method where the gravimeter is rigidly mounted to a moving platform (e.g., drone, ship or aircraft) and motion compensation is applied via Inertial Measurement Units (IMUs). Essential for mobile or airborne surveys.

Gimbal / Stabilised Gravimeter

A system that isolates the gravity sensor from platform motion by mechanically or actively stabilising it.

Data Processing & Interpretation

Gravity Gradient

The rate of change in gravity over distance — used to detect small, localised features such as cavities or thin ore veins.

Gradiometer

An instrument that measures the difference in gravity between two sensors separated by a known distance. Enhances spatial resolution and is less sensitive to platform motion — often used in airborne or marine surveys.

Base Station

A stable location used to monitor drift and provide a reference for mobile or time-dependent surveys.

Absolute Station

A stable location that has been measured with a low uncertainty absolute gravimeter to give an absolute value. Typically used to characterise or calibrate instruments or to use as a reliable base station.

Drift

A slow, non-geological change in instrument output due to thermal, mechanical, or electronic factors. Mitigated through calibration or reference stations.

Forward Modelling / Inversion

• Forward Modelling: Simulating gravity data from a known subsurface model.
• Inversion: Reconstructing subsurface structure based on measured gravity anomalies.

Free-Air Correction

Adjusts for gravity change due to elevation, assuming no intervening mass. Helps standardise data across varying terrain heights.

Bouguer Correction

Accounts for the mass between the measurement point and sea level. Essential for removing terrain effects in mountainous regions.

Terrain Correction

A refinement to Bouguer correction that factors in the precise shape of nearby hills, valleys, or structures.

Isostatic Correction

Adjusts for large-scale gravitational effects caused by Earth's crust being in balance (isostasy). Often used in regional surveys.

Applications and Markets

Gravity-Aided Navigation (GRAVNAV)

Uses gravity signatures and gravity maps to estimate position in environments where GNSS (eg GPS) is unavailable, such as underwater. Also known as Alternative Navigation as it does not require the use of GNSS.

Terrain Gravity Database

A pre-surveyed regional gravity map used in navigation or anomaly interpretation. Acts as a reference layer for real-time matching in GRAVNAV systems.

Void Detection

Identifying underground cavities or low-density zones (e.g. tunnels, sinkholes, old mine workings) by detecting negative gravity anomalies.

Density Contrast

The difference in density between subsurface materials, which causes measurable variations in the gravity field. Used in mineral or geological exploration.