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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.
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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).
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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 drawin
Closed contours indicate
Magnetic anomalies
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