The Gravity Method

The force of gravity is not uniform over all the surface of the earth; it is actually slightly stronger where the underlying rocks are more dense and slightly weaker where they have less density. The difference is diminute, but nontheless it can be mapped and measured, this gives a geophysicist another tool which can be used for the appropriate purposes.

Gravity surveys use extremely sensitive balances to be able to detect the underlying rocks’ variations in density. They can be useful in conducting a quick reconnaissance survey of an area to delineate the most important rock types. This information can help to show areas which are favorable to exploration by other methods. They can also be used in exploration with more detail to detect deposits of minerals, which are most of the time denser than the rocks that surround them.

Seismic Methods
Mankind has had instruments that can measure the direction and the amplitude of shock waves which are produced by earthquakes for hundreds of years. These shock waves are acoustic waves which are very similar to sound waves. Like sound waves, these shock waves travel faster in rigid and dense bodies than they do in less rigid and less dense ones. They also reflect from the limits between different rock types, permitting the geophysicist to measure the time that they take to travel and determine the structure of the rocks under the surface of the earth.

Seismic prospecting is the method which is most widespread geophysically in the exploration of petroleum. Small artificial shock waves are generated at a selected point by either firing a charge of explosives in a drill hole that is not deep or dropping a heavy weight. The speed of the shock waves is meausred by timing their arrival at sensitive receivers which are known as geophones which are placed along the line of survey.