Geophysical Techniques

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Author: Mustafa AbdulSattar

Gravitational, Magnetic, Radioactive and Thermal are depend on the rock inherent properties Like (Density, Magnetism, temperature and radioactive properties) —-Passive

Electric–natural Currents or Artificial Electrical Source—passive

Seismic-Artificial Source of Energy —Active


The Gravity Method:

The weight of any body depends on the force of gravity at the spot.

The force of gravity various with elevation, rock density, latitude, and topography.

A mass suspended from spring, the amount of spring stretching is proportional to the force of gravity.

F=m . g

g: Acceleration of gravity.

since mass is constant, then stretch variations determine the variations in the acceleration of gravity (g).

The GRAVIMETER instrument used to measure (g) at stations. The readings are corrected for elevation, latitude and topography. The normal value of (g) subtracted from the corrected readings to compute the residual gravity. the values of residual gravity are plotted to the measuring stations to produce contour map of equal residuals.

The case in the middle is normal. In the left the basement rocks (denser) are deeper and sedimentary rocks (low density) thicker, the stretching of spring is less than normal. In the left the basement rocks near surface and the spring stretching is greater than normal.
 The value of g at sea level= 980 cm/s2
Closed contours represent the gravity anomalies may be denote a subsurface geological structure. The residual gravity are measured in Gal (Galileo) =1cm/s2
Gal= 1000 milliGal (mGal).
Depict the gravity anomalies map in the figure? And explain the type and the size of subsurface structure?
Depict the gravity anomalies map in the figure? And explain the type and the size of subsurface structure?





The Magnetic Method:
The local magnetic variation in the magnetic fields when the basement complexes near the surface and where concentrations of ferromagnetic minerals exist.
Magnetometers are the instruments used to measure the earth’s magnetic field. This instruments measure the intensity or field strength
Of the Earth’s field. This measured in Tesla (T) and Nano Tesla (nT) or (ϒ)
1ϒ= 10-9 T
Nowadays, most magnetic survey are made from airplanes
the magnetic intensity are recorded
the readings are corrected (from annual magnetic and other variations)
Residual field= corrected magnetometer reading- theoretical values
the residual are plotted on a map and contours of equal gammas are drawing Closed contours indicate Magnetic anomalies.