As the Earth is not a perfect sphere, its gravitational field strength fluctuates across its surface. Changes in topography, elevation, latitude, and subterranean density can affect the force of gravity. A gravimeter is a type of accelerometer that is used to precisely measure local gravitational field strength by measuring the constant downward acceleration of gravity.
Gravimeters are used when prospecting for subterranean deposits of valuable natural resources, including petroleum and minerals, as well as by geodesists to study the shape of the earth and its gravitational field. All modern gravimeters are designed with a highly stable inertial platform to balance the masking effects of motion and vibration.
Types of Gravimeters
Modern land gravimeters are broadly classified into two types – absolute and relative.
Absolute Free-Fall Gravimeters
Absolute gravimeters measure the rate at which a mass in free fall in a vacuum accelerates. A retroflector is used with a Michelson interferometer to count and time interference fringes and thus measure the acceleration of the mass.
The types of absolute free-fall gravimeters are:
- Falling corner-cube gravimeter – The motion of a free falling corner-cube retroreflector in vacuum is monitored by a laser interferometer, which detects optical interference to determine the rate of acceleration of gravity. Corner-cube absolute free-fall gravimeters are one of the most accurate types; however, their mechanical structure for repeated free-falling is not not suitable for mobile use and restricts their cycle time.
- Atom interferometer gravimeter – The acceleration of laser-cooled particles due to gravity is determined by calculating the phase difference of a wave packet of particles that is divided through a beamsplitter, with each beam travelling along two different paths, one slightly more elevated than the other and so experiencing slightly different gravity, before being recombined. This technique benefits from high sensitivity and accuracy, and fast cycle times. It is the most suitable gravimeter for mobile use.
Spring-Based Relative Gravimeters
Spring-based gravimeters measure only relative gravity by analysing the response of a spring to an attached mass acting in a gravitational field. They are often used in gravity surveys over large areas as they are compact, transportable systems with good sensitivity and drift rate.
The types of spring-based gravimeters are:
- Superconducting gravimeter – Also known as the ideal-spring gravimeter, superconducting gravimeters replace the spring with an a ultralow temperature supercondcuting sphere held in place by an induced magnetic field. The sphere is responsive to minute variations in gravity. This type of gravimeter can achieve sensitivities of one nanogal, one thousandth of one billionth (10-12) of the Earth surface gravity.
- LaCoste-Romberg gravimeter – A zero-length spring - one which has no extension at zero initial force - suspends a mass in a gravitational field, with the length of the spring directly proportional to the strength of the force. This type of gravimeter has good sensitivity and is suitable for mobile use, but it needs frequent recalibration.
Understanding the Results
While a basic accelerometer displays measurements in common units of acceleration, a gravimeter displays measurements in units known as gals, where one gal is defined as 1 cm/s2. The measure of Earth's gravity at surface level varies between 976 and 983 gals. These differences are due to mountains and other land features that possess varying densities typically ranging from tens to hundreds of milligals.
Some applications of gravimeters include:
- Petroleum and mineral prospecting
- Geophysical surveys and other geophysical research
- Precision inertial navigation