**pg 11**

# Gravity: Notes

## Mass and Spring Measurements

The most common type of gravimeter* used in exploration surveys is based on a simple mass-spring system.
If we hang a mass on a spring, the force of gravity will stretch the spring by an amount that is
proportional to the gravitational force. It can be
shown that the proportionality between the stretch of the spring and the
gravitational acceleration is the magnitude of
the mass hung on the spring divided by a constant, *k*, which describes the stiffness of the
spring. The larger *k* is, the stiffer the spring is, and the less the spring will stretch for a
given value of gravitational acceleration.

Like
pendulum measurements, we can not
determine *k* accurately enough to estimate the absolute value of the gravitational acceleration to
1 part in 40 million. We can, however, estimate variations in the gravitational acceleration from place
to place to within this precision. To be able to do this, however, a sophisticated mass-spring system is
used that places the mass on a beam and employs a special type of spring known as a *zero-length*
spring.

# Gravity: Note Outline

- Overview
*pg 12* *-Temporal Based Variations-*- Instrument Drift
*pg 13* - Tides
*pg 14* - A Correction Strategy for Instrument Drift and Tides
*pg 15* - Tidal and Drift Corrections: A Field Procedure
*pg 16* - Tidal and Drift Corrections: Data Reduction
*pg 17* *-Spatial Based Variations-*- Latitude Dependent Changes in Gravitational Acceleration
*pg 18* - Correcting for Latitude Dependent Changes
*pg 19* - Vari. in Gravitational Acceleration Due to Changes in Elevation
*pg 20* - Accounting for Elevation Vari.: The Free-Air Correction
*pg 21* - Variations in Gravity Due to Excess Mass
*pg 22* - Correcting for Excess Mass: The Bouguer Slab Correction
*pg 23* - Vari. in Gravity Due to Nearby Topography
*pg 24* - Terrain Corrections
*pg 25* - Summary of Gravity Types
*pg 26*