# Observed gravity

**Keywords:** observed gravity

**Description:** Shape of the Earth (Geodesy) Basin geometry Bedrock depths Fault locations Subsurface voids Location of local density changes A zero length spring is a spring which

- Shape of the Earth (Geodesy)
- Basin geometry
- Bedrock depths
- Fault locations
- Subsurface voids
- Location of local density changes

A zero length spring is a spring which when all turns are touching has a given tension still present which is equal to the tension required to extend the spring a distance equal to its original length,i.e. tension is proportional to actual length of the spring

This condition is met by pre-stressing the spring in winding so an initial force is required to separate the coils.

The difference in gravity between two stations is in part due to other factors in addition to the attraction of unknown anomalous masses, i.e.:

The process of correcting for these factors is known as gravity reduction and most often we reduce our data as if taken on the geoidal surface, this is the most convenient equipotential surface.

The difference between observed gravity (g_{obs} ) and theoretical gravity (g_{th} ) at any point on the Earth's surface after reducing the gravity readings to the geoidal surface (i.e. making the Free Air, Bouguer slab, and terrain corrections) is known as the Bouguer gravity anomaly or Bouguer gravity and results due to lateral variations in density in the subsurface.

3.) Convert difference to units of gravity ( g) by multiplying Rdg values by the gravimeter scale constant

5.) Calculate the Free Air (C_{FA} ) correction (below reference surface h is negative, above h is positive) and add to g_{obs} of the station

8.) Calculate g_{th} for the station and subtract from g_{obs} + C_{FA} - C_{BS} + C_{TC}. This is the complete Bouguer anomaly.

- g
_{1}is increased by the downward attraction due to sheet, h - g
_{2}is decreased by upward attraction of sheet, h - g
_{2}is also increased by Free-Air gradient