CHAPTER ONE
INTRODUCTION
1.1 Introduction
Geomagnetism is the branch of geophysics that studies earth’s magnetic field. The science of geomagnetism developed slowly. The earliest writings about compass navigation are credited to the Chinese and dated to 250 years B.C. When Gilbert published the first textbook on geomagnetism in 1600, he concluded that the earth itself behaved as a great magnet. In the early nineteenth century, Gauss (1848) introduced improved magnetic field observation techniques and the spherical harmonic method for geomagnetic field analysis. Not until 1940 did the comprehensive textbook of Chapman and Bartels bring us into the modern age of geomagnetism (Campbell, 2003).
Magnetic surveying investigates the subsurface based on variations in the earth’s magnetic field that result from the magnetic properties of the underlying rocks. Mostly, the earth’s magnetic field is generated in the fluid outer core of the earth by self exciting dynamo process. Electrical current flowing is the slowly moving molten iron generates the magnetic field which is generally referred to as the main field as observed on the earth’s surface.
Airborne geophysical surveying is a process of measuring the variation of several key physical or geochemical parameters of the earth. The most important parameters measured are conductivity, magnetic susceptibility, rock density, radioactive element concentration, and reflectance spectra. Any change in the earth’s near surface that causes a measurable change in these parameters presents a potential application for airborne geophysics. The systems used to measure these parameters are electromagnetic, gamma-ray spectrometry, magnetic, and gravity. Airborne geophysics has always been at the forefront of technological developments and innovation. Modern exploration systems can measure minute changes in the geophysical properties of the earth with high sensitivity instruments and survey platforms. Exploration projects utilise GPS navigation and timing, laser and radar altimeters, satellite communications and innovative data processing techniques.
An aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer on board or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger areas of the earth’s surface to be covered quickly for regional reconnaissance. The aircraft typically flies in a grid like pattern with height and line spacing determining the resolution of the data (and cost of the survey per unit area) (Olasehinde, 2009).
Airborne geophysical surveys are applicable in oil and mineral exploration, engineering projects, geothermal mapping, land management; they are excellent tools for mapping exposed bedrock, geological structures (such as basements, faults, dikes, sills, kimberlites), sub-surface conductors, paleochannels, mineral deposits and salinity.
1.2 Solid earth structure
The interior structure of the earth is layered in spherical shells, like an onion as shown in fig1.1. These layers can be defined by either their chemical or their rheological properties. The earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of earth’s internal structure is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through earth, measurements of the gravity field of earth, and experiments with crystalline solids at pressures and temperatures characteristic of earth’s deep interior.The structure of earth can be defined in two ways: by mechanical properties such as rheSology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core.
The interior of earth is divided into five important layers. Chemically, the earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.
The layering of earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The outer core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers. The changes in seismic velocity between different layers causes refraction according to Snell’s law, like light bending as it passes through a prism. Likewise, reflections are caused by change in acoustic impedance and are similar to light reflecting from a mirror (Lowie, 1997).
1.3 The geomagnetic field
Earth’s magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the earth’s inner core to where it meets the solar wind, a stream of energetic particles emanating from the sun. It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the centre of the Earth. However, unlike the field of a bar magnet, earth’s field changes over time because it is generated by the motion of molten iron alloys in the earth’s outer core (the geodynamo).
Near the surface of the earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the centre of the earth and tilted at an angle of about 10° with respect to the rotational axis of the earth. The dipole is roughly equivalent to a powerful barmagnet, with its south pole pointing towards the geomagnetic north pole . This may seem surprising, but the north pole of a magnet is so defined because, if allowed to rotate freely, it points roughly northward (in the geographic sense). Since the north pole of a magnet attracts the south poles of other magnets and repels the north poles, it must be attracted to the south pole of earth’s magnet. The dipolar field accounts for 80–90% of the field in most locations (Merrill et al., 1996).
The earth’s field is not constant at any point on its surface but undergoes variations of different periods. From the stand point of applied geophysics, the most important are the diurnal variations and magnetic storm. Their disturbing effect must be suitably eliminated from magnetic survey observations (Parasnis, 1986).
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