Geophysics

Geophysics

1. The value of gravity itself cannot be used to identify a particular rock type because too many variables are involved

   • It is more useful to plot the variation in values from place to place. These values which are different from the average are called gravitational anomalies

2. Corrections must be made to allow for non-geological effects

   • Latitude - Due to the spin, the earth is slightly flattened at the poles and it bulges slightly at the Equator
   • Altitude; the free air anomaly - if a location is at a height of 100m above sea level it is 100m further away from the centre of the earth. The value is adjusted to sea level by adding 0.31 milligals per meter.
   • Bouguer Anomaly - the final value must account for the extra 100m of mass which is above sea levels. 0.1 milligal per meter is subtracted from the free air anomaly to remove the effect of the mass

3. Gravity anomalies are used to investigate the interior of the earth and also in the search for ores and geological structures which may hold hydrocarbons

   • Typical variations in the value of gravity 20 milligals, although over the deep ocean trenches they exceed 300 milligals

4. Electromagnetic Surveys

   • Electromagnetic (EM) surveys work in much the same way as transformers,
     • An alternating current is passed through the primary transmitting coil
     • A magnetic field is induced which spreads into the ground
     • The magnetic field induces a secondary current in any underground material which can act as a conductor, such as water or iron-bearing minerals
     • This current sets up an alternating secondary magnetic field
     • This field is detected by the secondary receiving coil and the information is sent to the data logger
   • On a smaller scale this would be the same as a metal detector on the surface
   • The depth penetration into the ground can be increased by setting the two coils further apart
   • The instruments can be used on the ground, in ships or even in aircraft - for example to locate sulfide ore deposits
   • EM surveys are used to detect the presence of fluids in sub-surface rocks e.g. partial melting in the upper mantle and below mid-ocean ridges means there is an increase in conductivity of the rock
   • Partial melting of only 1-2% can be detected, especially if the magma is hydrated
   • Surveys show where the melting takes place under the mid-ocean ridge