Magnetic survey is the oldest and most widely used geophysical tool in mineral exploration for investigation of iron ore, magnetite, ilmenite, pyrite- and pyrrhotite-rich sulfide, and NieCuePGE deposits. The magnetic method is a passive method since it only measures the existing magneticfield strength and does not amplify or modify it.
The magnetic methods are more popular in mineral exploration because the magnetic data can be quickly recorded from the air in conjunction with other geophysical
surveys. The airborne potential field surveys provide regional coverage of large areas at a comparatively low cost in the shortest time.
6.4.1 Concept
The investigation of subsurface geology based on anomalies in the geomagnetic field resulting from varying magnetic properties of underlying rocks and minerals is the basic principle of magnetic survey. The directional properties of magnetite-rich rock (lodestone) were discovered centuries before Christ. This was modified with knowledge of Earth’s magneticfield or geomagnetism and its directional behavior between the 12th and 16th centuries. The quantification of directional properties of geomagnetism and local anoma- lies with growing sophisticated instrumentation became increasingly significant for mineral prospecting during the 18th century onward.
Magnetic surveying for mineral investigation with high- precision instruments can be operable in air (airborne), sea (marine), and land (ground). Airborne survey is appropriate for scanning large areas during reconnaissance to delimit target areas for detailed ground survey during the pro- specting stage. The process is rapid and cost effective. A
“bird”is used as a magnetic sensorfixed to a string in the tail of an aircraft. A“fish”is used to tow a sensor behind a ship to remove the magnetic effect of the vessel. It is effective for investigation of ocean floor polymetallic nodules. The ground magnetic survey is suitable for pro- specting over relatively small areas previously defined as targets by airborne surveys.
6.4.2 Theory
A magnetic field or flux density develops around a bar magnet. It flows from one end of the magnet to the other.
The flux can be mapped by sprinkling iron filings over a thin transparent sheet set over a bar magnet or by a small compass needle suspended within it. The curve orientations of ironfilings or magnetic needle are calledlines of force that converge to points at both ends of the bar magnet.
These points are located inside the magnet, are referred as poles, and always occur in pairs. A freely suspended bar magnet similarly assumes a position in the flux of Earth’s magneticfield. The pole that aligns to point in the direction of the geomagnetic north pole is called thenorth-seeking or positive pole. It is balanced by a south-seeking or negative pole of identical strength at the opposite end of the magnet. The lines of force always diverge from north or positive pole and converge to south or negative pole (Fig. 6.11).
The magnetic field “B”or flux density due to a mag- netic pole of strength“m”at a distance“r”from the pole is
expressed as the force exerted on a unit positive pole at point “P.”It is defined as:
B¼(m0$m)/(4p$mR$r)
wherem0is the constant corresponding to magnetic perme- ability of a vacuum andmRis the relative magnetic perme- ability of the medium separating the poles.
The unit of measurement of magnetic intensity is gamma (g), which is equal to 109T or nT (nanotesla). The total magnetic intensity of Earth in the polar region is 60,000gor 60,000 nT, and at the equator it is 30,000g.
6.4.3 Earth’s Magnetic Field
Earth possesses the property of a huge magnet with north and south geomagnetic poles aligned 11.5 degrees away from the geographical North Pole (to the west) and South Pole (to the east). The orientation of a freely oscillating magnetic needle at any point on Earth’s surface depends on the direction of the geomagnetic field at that point. The geomagnetic field, “F,”at any point has few elements to represent its magnitude and direction. The components are a vertical (Z), horizontal (H), declination (D), and inclina- tion (I) as shown in Fig. 6.12. Declination is the angle between magnetic north and true or geographic north.
Inclination (I) is the angle of F with respect to the hori- zontal component H. Magnetic anomaly is caused by the superimposed presence of magnetic minerals and rocks on the normal geomagneticfield at that location.
FIGURE 6.11 Lines of forces caused by a bar magnet always diverge from north or positive pole and converge to south or negative pole.
6.4.4 Rock Magnetism
The magnetic susceptibility of rocks depends mainly on the proportion of rock-forming minerals. The most common rock types are either nonmagnetic or very feebly magnetic.
Rocks develop a susceptibility to magnetism with a higher proportion of magnetic minerals like magnetite, ilmenite, and pyrrhotite. Mafic/ultramafic rocks are usually more magnetic due to higher content of magnetite than acidic igneous rocks. Metamorphic rocks vary in magnetic prop- erty. Sedimentary rocks in general are nonmagnetic unless locally enriched with magnetite, ilmenite, and pyrrhotite- magnetite-bearing sulfide deposits.
The common causes of magnetic anomalies are intru- sion of mafic and ultramafic dykes, sills, lava flows, and magnetic orebodies. Amplitude varies between as low as 20 nT in limestone and 800 nT in mafic igneous rocks to more than 6000 nT over sulfide orebodies. Magnetic sus- ceptibility caused by variation of rocks or orebodies is superimposed on the geomagnetic field at that location.
Magnetic anomaly is the response signaled by the causative body over regional trends of country rocks.
6.4.5 Survey Instruments
A magnetic survey instrument used during the early 1900s to measure geomagnetic elements was themagnetic vari- ometer. It was essentially based on principles of a sus- pended bar magnet in Earth’sfield. Since then, instruments
are updated to be user friendly and compatible with computer-based processing for easy interpretation with a precision of0.1 nT.
The fluxgate magnetometer was developed during the 1940s and employs two identical ferromagnetic cores of high permeability that provide instantaneous measurements.
The instrument is developed following either “nuclear precision”or“proton Precision”(Fig. 6.13), and consists of a container filled with liquid rich in hydrogen atoms sur- rounded by a coil. The next-generation instruments with higher precision are the optically pumped potassium or al- kali vapor magnetometer and the magnetic gradiometer suitable for airborne, ground, and marine surveys.
6.4.6 Data Reduction
The reduction of magnetic data is essential to remove noises caused by other elements not related to subsurface magnetism. The effect of diurnal variation on ground surveying can be removed by periodic calibration of in- struments at afixed base station. Similarly, an aeromagnetic survey can alternatively be assessed by arranging numerous crossover points in the survey path (refer to Fig. 6.20).
Geometric correction is computed by using the Interna- tional Geomagnetic Reference Field, which defines the theoretical undisturbed magnetic field at any point on Earth’s surface. Terrain correction is rarely applied in magnetic survey due to the negligible effect of vertical gradient of the geomagneticfield.
6.4.7 Applications
Magnetic survey is extensively used for metallic mineral investigation, particularly for iron ore with a higher ratio of magnetite to hematite. It is capable of locating massive sulfide deposits, especially in conjunction with the elec- tromagnetic method (Section 6.6). Aeromagnetic surveying should preferably be programmed at a low-levelflight path
FIGURE 6.13 User-friendly proton magnetometer device compatible to high-end processing with precision.
FIGURE 6.12 Schematic diagram of geomagnetic elements showing the declination (D) and inclination (I) of the totalfield vector F.
(100e200 m above ground) avoiding excessive monsoon, peak summer, and foggy weather. The depth of penetration of an airborne survey will depend on the capacity of the instruments. In a ground survey, traverses are designed across a strike of the formations at a line interval less than the width of the expected causative body. Magnetic anomalies caused by shallow objects are more easily detectable than deep-seated targets. Airborne magnetic and geomagnetic surveys with advanced configuration systems, both in frequency and time domain, with high penetration capacity can identify deep-seated metallic bodies. Appli- cation of the system requires a considerable increase in bandwidth of both helicopter-borne frequency-domain electromagnetic and fixed-wing time-domain electromag- netic (TDEM) systems.
Depth estimates from aeromagnetic data can determine values for broad areas, such as the thickness of the sedi- mentary section in an oil and gas reservoir basin or at a limited number of points within the basin.
Interpretation of 2D and 3D isocontour maps of cor- rected magnetic data provides a qualitative existence of orebodies. The approximate location and horizontal extent of causative bodies can be determined by studying the relative spreads of the maxima and minima of anomalies. A comparison of gravity and magnetic interpretation of rich sulfide orebodies is given inFig. 6.14. Similar studies will be applicable for the Ni-PGE-Cu belt, Sudbury Camp, Canada.