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'.
Ø The magnetic intensity of rocks of earth at a given location is measured directly by very sensitive instruments called magnetometers.
Ø specially designed magnetometers are used for measuring horizontal intensity or vertical intensity.
Ø These 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.
Ø This gives the isoanomalies map.
Ø From such maps the location and extension of materials of highly magnetic or less magnetic bodies can be determined.
Ø Another instrument for measuring the magnetic intensity over wider areas in the shortest possible time is Airborne Magnetometer.
Ø It consists of a highly sensitive detector element that is made to hang down from a plane or helicopter.
Ø This 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 magnetic surveys.
In the 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.
Applications. The magnetic methods have been successfully used in prospecting for magnetic ore bodies and for oil exploration.
Ø This 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.
Ø The 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.