A gravimeter is an instrument used to measure gravitational acceleration. Every acceleration due to gravity lab report has an associated gravitational potential.
The gradient of this potential is an acceleration. A gravimeter measures this gravitational acceleration. The first gravimeters were vertical accelerometers, specialized for measuring the constant downward acceleration of gravity on the earth’s surface. The change from calling a device an «accelerometer» to calling it a «gravimeter» occurs at approximately the point where it has to make corrections for earth tides. Though similar in design to other accelerometers, gravimeters are typically designed to be much more sensitive. Their first uses were to measure the changes in gravity from the varying densities and distribution of masses inside the earth, from temporal «tidal» variations in the shape and distribution of mass in the oceans, atmosphere and earth. Gravimeters can detect vibrations and gravity changes from human activities.
Depending on the interests of the researcher or operator, this might be counteracted by integral vibration isolation and signal processing. The resolution of the gravimeters can be increased by averaging samples over longer periods. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology. Most current work is earth-based, with a few satellites around earth, but gravimeters are also applicable to the moon, sun, planets, asteroids, stars, galaxies and other bodies. Gravitational wave experiments monitor the changes with time in the gravitational potential itself, rather than the gradient of the potential which the gravimeter is tracking. The term «absolute gravimeter» has most often been used to label gravimeters which report the local vertical acceleration due to the earth.
Relative gravimeter» usually refer to differential comparisons of gravity from one place to another. They are designed to subtract the average vertical gravity automatically. They can be calibrated at a location where the gravity is known accurately, and then transported to the location where the gravity is to be measured. There are many methods for displaying acceleration fields, also called «gravity fields». This includes traditional 2D maps, but increasingly 3D video. Since gravity and acceleration are the same, «acceleration field» might be preferable, since «gravity» is an oft misused prefix.
Gravimeters for measuring the earth’s gravity as precisely as possible, are getting smaller and more portable. A common type measures the acceleration of small masses free falling in a vacuum, when the accelerometer is firmly attached to the ground. Atom interferometric and atomic fountain methods are used for precise measurement of the earth’s gravity. The term «absolute» does not convey the instrument’s stability, sensitivity, accuracy, ease of use, and bandwidth.
So it and «relative» should not be used when more specific characteristics can be given. The most common gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. They are basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured. The largest component of the signal recorded by the superconducting gravimeter is the tidal gravity of the sun and moon acting at the station.
Many broadband, three axis, seismometers in common use are sensitive enough to track the sun and moon. When operated to report acceleration, they are useful gravimeters. Because they have three axes, it is possible to solve for their position and orientation, by either tracking the arrival time and pattern of seismic waves from earthquakes, or by referencing them to the sun and moon tidal gravity. Recently, the SGs, and broadband three axis seismometers operated in gravimeter mode, have begun to detect and characterize the small gravity signals from earthquakes.
These signals arrive at the gravimeter at the speed of gravity, so have the potential to improve earthquake early warning methods. Newer MEMS gravimeters, atom gravimeters — including atom chip gravimeters offer the potential for low cost arrays of sensors. The term for the «force constant» changes if the restoring force is electrostatic, magnetostatic, electromagnetic, optical, microwave, acoustic, or any of dozens of different ways to keep the mass stationary. Precise GPS stations can be operated as gravimeters since they are increasingly measuring three axis positions over time, which, when differentiated twice, give an acceleration signal. The satellite borne gravimeters GOCE, GRACE, are mostly operating in gravity gradiometer mode. Wikimedia Commons has media related to Gravimeters.
Miniaturized Gravimeter May Greatly Improve Measurements». Eos, Transactions, American Geophysical Union, electronic supplement. Laughlin, Department of Physics, Stanford University». Academic Dissertation at the University of Helsinki, Geodetiska Institutet. Further Development of a High Precision Two-Frame Inertial Navigation System for Application in Airborne Gravimetry».