Magnetic Methods

    The geophysicist measures the strength of the earth’s magnetic field when he is surveying magnetic areas, of course, this will vary locally depending on the quantity of magnetic material which is in the underlying rocks, to find buried metal objects like underground fuel tanks, magnetic surveying has also been used with considerable success.

    The local magnetic field will be strong in the areas where the rocks have high magnetic susceptibility; as is natural, the local magnetic field will be weaker where the rocks have low magnetic susceptibility. This has two applications. Firstly, deposits with magnetic minerals – for example, skarns, nickel deposits bearing pyrrhotite and iron deposits – can be directly detected using magnetic surveying. Secondly, magnetic surveying can be used as a help to geological mapping. Units which have higher susceptibility will show up as areas of high strength in the magnetic field.

    Magnetic surveys do not necessarily have to be done on the ground. An aerial magnetometer is a highly sensitive instrument which can either be trailed below a helicopter or an airplane, or it can be mounted on an aircraft in a “stringer” as it can be called. If you combine readings from this instrument with repeated aerial video photography, a magnetic map of a large area can be plotted. The geological agencies which are controlled by the government frequently contract for aerial surveys, they publish the results with the intention of encouraging exploration.

    An electric current is generated and forced into the ground from widely spaced electrodes in this method. The current flows through the earth to complete the circuit, and the quantity of current that flows depends entirely on the resistance that the rock offers. This can be measured by probing the grounds with pairs of electrodes which should be connected to sensitive voltmeters.

    A conductive orebody which contains economic metallic sulphides is very capable of causing an anomalously low resistance. The same will happen to a fault plane which is lined with graphitic material, a fracture containing a brackish solution or a barren sulphide. Geological evidence must be used to interpret the results from this method.