# Geophysics

My Blogs (olelog) are mainly based on my daily reading of earth science news.

Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

Geophysics is the study of the Earth by quantitative physical methods, especially by seismic, electromagnetic, and radioactivity methods.

## Gravity

The variation in density between diferent rock bodies in the underground affects the Earth's gravitational field, expressed by the acceleartion due to gravity (g). The acceleration due to gravity varies according to the mass of a body and its distance from the point of measurement. Large rock bodies of different density tend to sink or rise within th crust to get into isostatic equilibrium, but this takes tme. Masses out of isostatic equilibrium produce gravity anomalies.

Anomalies produced by bodies with a lower than average density in the crust, but which are prevented from floating upwards by the strength of the crust, result in a lower g value than normal - a negative anomaly. Like wise, denser bodies that cannot sink produce positive anomalies.

 Bouguer gravity anomaly map of New Jersey from United States Geological Survey

Gravity is usually measured in units of acceleration. The standard unit of acceleration is 1 metre per second squared (abbreviated as m/s2). Other units include the gal (sometimes known as a Galileo, in either case with symbol Gal), which equals 1 centimetre per second squared. 1 mGal (milligal) is one thousandth of 1cm/s2.

An instrument used to measure gravity is known as a gravimeter, or gravitometer. Gravimeters may be regarded as special purpose accelerometers.
When measuring the Earth's gravitational field, measurements are made to the precision of microgals to find density variations in the rocks making up the Earth.

A Bouguer anomaly map in principle shows the deviation from the Bouguer gradient of 0.1967 milligals/m which considers the mean rock density (2.67 g/cm3) below the measure point. 2.67 g/cm3 is approximately the density of granite. A shallow gabbro body with a density of around 3 g/cm3 will show up as a positive anomaly.