The earth's gravity field varies from 983. 221 gals at the Poles to 978. 049 gals at the Equator. As anomalies of the order of 0. 001 gal can be significant in oil exploration the unit in gravity surveys is one thousandth of a gal, the "milligal". The modern gravimeter is a very sensitive instrument that can measure changes in gravity of 0.01 milligal, that is 1 part in 100 million of the earth's gravity field.