Gravity Flashcards
What are the applications of measuring gravity?
To detect lateral differences in the densities of subsurface rocks.
Useful for finding buried bodies and structures, such as igneous intrusions and some faults, on scales that range from a few metres to tens of kilometres across.
What does Newton’s law of gravitation say?
Every particle in the universe attracts every other particle with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the square of the distance between them.
F=Gm1m2/r^2
What is mass?
A measurement of how much material is in an object.
Constant value.
What is weight?
Measurement of the gravitational force exerted on that material in a gravitational field. Depends on the location of the object - a person at the equator will weigh less than one at the pole
What is the density difference of rocks at surface vs at depth?
The density of exposed rock is often less than that of the same rock at depth, because of weathering and open cracks and because often it is not fully saturated with water.
What is a gravity anomaly?
o The contrast in density and thereby the excess mass compared to surrounding rocks produces the anomaly.
Which different types of measuring gravity is there?
Absolute gravity: A classical method of measuring gravity value to determine absolute value in some certain points. Expensive and time consuming.
Relative measurement of gravity: A gravimeter measures the relative gravity based on Hook’s law. A gravimeter has a mass suspended in a spring. Hook’s law is F=mg=k(S-S0) where K is the elastic constant of spring
Why is conversion to mGal done for gravity data?
Gravimeters differ slightly from each other. A conversion table is provided by manufacturers.
Why is instrumental drift corrected for with gravity data?
The daily changes if gravimeter is kept at the same spot due to the slow creep of the spring and the tidal distortion, g can vary up to 0,3 mGal.
Measured by periodically returning to the base station.
Why is latitude correction done with gravity data?
Gravity increases towards the pole. Stations closer to the pole than the base station is subtracted from the measured gravity.
0,812sin(2latitude) mGal/km polewards
Why is the Eötvös correction done with gravity data?
It has to do with the rotation of the Earth.
Correction only needed if measurements were taken from a moving vehicle.
The correction is roughly 2.5 mGal for each km/h in an east to west direction
Which topography related corrections are done with gravity data?
Free-air correction: This correction is calculated to acount for the decrease of gravitational acceleration with distance from center of the earth.
Bouger plate (excessive mass) correction: The gravitational acceleration of the rock-mass is included in the measured gravity and its influence is removed from the observed gravity.
Terrain correction (complete Bouger correction): Is due to the gravitational attraction produced by the rock mass surrounding the station, for example hills.
What is the Bouger anomaly?
The combined data reductions give the Bouger anomaly
What are regional anomalies?
The larger features produce Bouguer anomalies that are smooth over considerable distances which is called regional anomalies. Useful for investigating deeper sources and understanding large-scale structure of the earth crust (mountain ranges, oceanic ridges, subduction zones.)
What are residual anomalies?
The local and smaller features produce more sharper anomalies. If the regional is removed, what remains is representing the residual anomaly. (For near surface sources and geological exploration applications)
To allow the residual anomaly to be separated from the regional, the survey should extend well beyond the gravitational influence of the target body.
