gravity

• Gravity surveys can be carried out along transects or grids. Gravity is measured in milligals using a gravimeter. The gravimeter measures variations in extension of a spring caused by variations caused by gravity. 
• Gravity reading have to be corrected to account for the influence of:

  • Height above sea level 
  • Latitude 
  • Mass of rocks above sea level 
• Differences in gravity are due to higher or lower than average levels. Negative anomalies are lower density rocks, positive anomalies are higher density rocks. 
• The bouguer anomaly, the deeper a body of higher or lower density rock, the more spread out the response is at the surface. A large deep body may produce a similar response to a smaller, shallower body. 
• Gravity readings alone do not tell you what is below the surface. Negative anomalies have a range of possibilities but does eliminate some possibilities, like denser rock below the surface. So gravity surveys are used alongside other information.