GEOPHYSICAL APPLICATIONS FOR GENERAL EXPLORATION ETS 306 GEOPHYSICAL APPLICATIONS FOR GENERAL EXPLORATION

EXPLORATION??? What is exploration? Significant economic mineral deposits are more difficult to find than other natural resources such as forests and land suitable for agriculture. Deposits have to be discovered. They are fixed in location, which means that they are not necessarily found in convenient areas. They are also fixed in size and quality which determines when and how they can be developed.

EXPLORATION??? An investigation of unknown regions. A search for the unknown. An activity for purpose of discovery.

EXPLORATION??? ‘As exploration proceeds, and as it is followed up by detailed scientific study, wave after wave of knowledge flows over the earth’s surface, each forming, as it consolidates, the ground upon which the next will spread’. (Mill, McClure’s Magazine (Nov 1894) 3:540)

GEOPHYSICAL EXPLORATION Generally exploration programs are structured around three fundamental stages: Area selection Data gathering Data evaluation

GEOPHYSICAL EXPLORATION TECHNIQUES SOME BASIC PRINCIPLES: All geophysical methods remotely sense a material property of the earth (e.g. seismic velocity, rock density, electrical resistivity, magnetization etc). Knowledge of these material properties must be then interpreted to determine which rock type is present. Geophysical methods can be divided into active and passive techniques.

BASIC CONCEPTS In an active technique, it is necessary to generate a signal (e.g. in seismic reflection surveying, sound waves are generated with an explosion). In a passive technique a naturally occurring signal is detected (e.g. the pull of gravity of a buried object)

BASIC CONCEPTS Geophysical and geological studies complement one another.

GEOPHYSICAL EXPLORATION Mineral deposits usually possess physical properties that are different from those of the surrounding rocks. Explorers use scientific techniques called geophysical methods to measure these variations in physical properties of rocks (e.g. density, magnetism, electrical conductivity, natural radioactivity, heat capacity etc) as a guide to the possible locations of mineral deposits.

GEOPHYSICAL EXPLORATION Geophysical methods offers a means of looking into the earth. Geophysical exploration can be conducted on the surface of the earth, in the air (also called airborne geophysics or remote sensing) or in the sea (known as marine geophysics).

GEOPHYSICAL EXPLORATION METHODS Magnetic and Gravimetric methods: The Earth acts as a giant magnet, generating a field about itself that influences or captures other objects that are either magnetic or may be magnetised, particularly objects containing iron. Magnetometers, the instruments required for magnetic surveys, are simple but sophisticated. Magnetic surveys may be undertaken from the air or on the ground. The data are then taken back to the office to be processed and presented as a magnetic map.

MAGNETIC METHODS For detailed work to discover the presence of concealed deposits, magnetic surveys are usually conducted on the surface of the earth (on the ground). Many ore minerals have higher density than the surrounding rocks. By measuring variations in the Earth's gravity field over an area, an indication of the nature of the underlying geology and the likely presence of deposits can be obtained.

GRAVITY METHODS Surveys are often undertaken over large areas, and provide regional information on the nature of rocks often at considerable depth

ELECTRICAL METHODS Mineral deposits and geological structures display a wide variety of electrical properties, including electrical conductivity, and capacity to hold an electric charge. Electrical surveys are normally conducted along surveyed grid lines and require electrodes, usually in the form of porous ceramic pots filled with copper sulphate,(or short metal stakes) to be placed in the soil or rock, along a traverse. These are removed on completion of the survey.

ELECTROMAGNETIC METHODS Here, electric current is transmitted through a coil or loop of wire laid upon the ground. Rocks or mineral deposits that are electrical conductors deform the resultant magnetic field. If the area to be explored is large or remote, reconnaissance stage EM surveying may be achieved by remote sensing. This technique is known as airborne EM surveying.

SEISMIC METHODS… Seismic methods rely on studying ways in which sound or equivalent wave forms produced on the surface travel through the underlying rock. Different rock formation and geological structures affect these energy waves in specific ways and, by studying the results obtained, it is possible to predict the nature of the concealed geology. There are two basic types of seismic surveying - refraction and reflection

SEISMIC METHODS… The refraction method is used to study ground conditions, such as depth of weathering or faulting, within 50 metres of the surface. Therefore, it is extensively used in quarrying and construction foundation studies. It is not generally used in exploration for mineral deposits. The reflection method is normally used for deep penetration and the understanding of geological stratigraphy and structure.

SEISMIC METHODS… The energy source is critical in seismic surveying. It determines the depth of penetration. A vibrator or explosives are usually used in reflection surveys. Seismic methods are commonly used in exploration for oil and coal but are not common in exploration for metalliferous deposits in geologically complex mineralised areas. For more than 70 years, reflection seismic methods have been used with great success to explore sedimentary basins for hydrocarbons.

GEOPHYSICAL EXPLORATION FOR ENIRONMENTAL STUDIES Geophysical methods for environmental studies can be used for the following purposes: Identifying abandoned and concealed mine openings, Tracing toxic substances, including metals or radioative species released to air and (or) water, resulting from sulfide mineral oxidation, delineating geologic structures that control the flow of potentially toxic water.

GEOPHYSICAL EXPLORATION FOR ENIRONMENTAL STUDIES… Surface and borehole geophysical techniques can provide information on the physical and chemical properties of bedrock, unconsolidated sediments, and groundwater.

GEOPHYSICAL METHODS APPLICATIONS oil and gas exploration mineral exploration hydrogeology monitoring contamination and remediation tectonic studies earthquake hazard studies

MINING GEOPHYSICS Mineral explorers rely on a wide range of geophysical techniques including magnetic, gravity, electrical, electromagnetic and radiometric, and above all on strong knowledge of hard-rock geology and on results of local field mapping and exploration drilling.

MINING GEOPHYSICS By itself, no geophysical anomaly can simply be correlated with lithology (Lyatsky, 2004). Instead, anomalies arise due to variations in some specific physical properties of rocks. These physical properties are a function of the rocks’ entire history as well as their present state. By itself, an anomaly says nothing about the nature, lithology or age of its rock source.

MINING GEOPHYSICS Gravity and magnetic methods are extremely useful in both mineral and oil exploration. Seismic reflection methods have been used extensively in the oil and gas industry, even though gravity and magnetic methods are equally useful.

GRAVITY METHODS Gravity readings on earth are not the same everywhere. Gravity variations may be due the following and therefore must be corrected for due data interpretations in gravity measurements: the earth’s rotation, polar flattening, diurnal variations due to tidal forces of the sun and moon, density of local rocks.