Heritage Cases

1. Geophysical Exploration Techniques Geophysical prospecting and exploration is the geophysics applied to the location of mineral deposits or geological structures concealed beneath the surface of the earth. In general, a hidden ore body or geological structure associated with it must possess one (or more) physical property that is different from surroundings in order to cause a measurable effect or anomaly in a geophysical survey. The main physical properties exploited during geophysical prospecting are: • Electrical Properties • Magnetic Properties • Nuclear Properties • Gravity Properties The main instrument types used for geophysical exploration are discussed below: a) Magnetic Methods Certain types of ore, especially magnetite, ilmenite and pyrrhotite bearing sulphide deposits, produce distortions in the earth’s magnetic field. Some iron-rich manganese and chromium ore may also yield magnetic anomalies. The ferro-magnetic minerals have two distinct magnetic properties. One is that the earth’s magnetic field effectively turns the ore body into a large magnet, which in turn wraps the normal field, thus producing anomaly. The other is that the ferro-magnetic materials often have a residual magnetism due to their original formation and this residual magnetism may act at an angle to the earth’s magnetic field, thus strengthening or weakening the original field and thus forming anomalies. A magnetic survey may be established from the air or at ground level. b) Electro-Magnetic Methods When a transmitted electro-magnetic field is propagated through the ground it induces an electrical current in any conductor in its path. These secondary currents in turn produce their own alternating secondary electro-magnetic field, which opposes the primary field. The lower the resistance of the conductor, then the stronger the opposing current will be. Thus, if the induced field is passed through a good conductor such as ore body containing graphite, pyrrhotite, pyrite, chalcopyrite or magnetite, a strong secondary field is set up. c) Electrical Methods Three forms of electrical geophysical prospecting methods are used self-potential, resistivity and induced polarization. The self-potential method is useful as an indicator of near surface anomalies because it is cheap and simple to operate. If two non-polarizable electrodes are driven anywhere into the ground and connected to the terminals of a sensitive voltmeter, a small voltage is found to exist between terminals. In the resistivity method, an electric current is sent into the ground and a pair of electrodes and a sensitive voltmeter measures the resulting distribution of potentials. When an electrical current is passed into the earth, its theoretical paths through homogenous ground are known. d) Other Geophysical Techniques Several other techniques are available for geophysical prospecting such as seismic and gravity techniques, which are suitable for structural mapping although they have some application to specific types of ore body. Gravity techniques are based on small changes in the earth’s surface gravitational effect caused by a pool of rocks lying up to several thousand meters below surface. It is used to locate faults, anticlines and other structures and may also be used to detect high density ore bodies. Seismic methods are based upon physical characteristics by which large differences occur in the velocity of sound waves in geological strata.