These include some of the oldest geophysical methods of
exploration and take into account the fact the earth is a gigantic magnet with
definite magnetic field, the intensity of which can be calculated for any part
of the earth.
But, as the earth is not ideally homogenous, the theoretical
values of magnetic intensity may be quite different (higher or lower) than the
values observed at given locations.
The difference between these two values is a magnetic anomaly
(compare gravity anomaly) and forms the basis for interpreting bodies of
magnetic ores and also such structures that can cause magnetic 'highs' or 'lows'.
magnetic intensity of rocks of earth at a given location is measured directly
by very sensitive instruments called magnetometers.
designed magnetometers are used for measuring horizontal intensity or vertical
observations are repeated in different directions over the selected area, and
from the values so obtained, the magnetic anomalies are calculated.
Ø Lines are
then drawn through the points of similar anomalies.
gives the isoanomalies map.
Ø From such
maps the location and extension of materials of highly magnetic or less
magnetic bodies can be determined.
instrument for measuring the magnetic intensity over wider areas in the
shortest possible time is Airborne Magnetometer.
consists of a highly sensitive detector element that is made to hang down from
a plane or helicopter.
helps in recording total magnetic intensity along the line of flight
Ø The lines
of flight, the height of flight and speed of the plane are controlling
parameters which require very careful planning and execution of the aerial
simplest cases, an unusually high magnetic intensity as shown by the isoanomaly
map can be taken as indicative of a magnetic ore body in that area.
Ø In actual practice, however, a number of factors
have to be considered and corrections applied for arriving at reliable results.
Of these, corrections due to magnetic storms are quite important.
magnetic methods have been successfully used in prospecting for magnetic
ore bodies and for oil exploration.
method has been specially useful in locating weathered zones, caves and major
and minor cavities in rocks of susceptible compositions in the foundations.
Ø In this
method these are the sound waves which are transmitted in a controlled manner
from the boreholes through the bodies of rocks and
their transmission velocities recorded.
acoustic wave velocity sketches are prepared for the site from which the size
and position of caves can be interpreted.
Ø The acoustic surveys are reliable when the site is made up of broadly homogenous rocks and the wave transmission velocities are recorded very carefully.