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.
Figure 1.1 The structure of the earth (www.earthonlinemedia.com).
The structure of earth can be defined in two ways: by mechanical properties such as rheology, 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 geologic component layers of the earth are at the depths below the surface as shown in table 1.1 (Jordan, 1979).
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 bar magnet, 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).
1.3.1 Main magnetic field
Main magnetic field is produced in the core of the earth and accounts for the very large regional variations in field intensity and direction. From the measurement of the magnetic field it became clear that the ﬁeld has both internal and external sources, both of which exhibit time dependence.
The earth’s internal magnetic field is a superposition of the field generated by the geodynamo in the liquid outer core (main field) and the field of magnetized rocks in the crust and upper mantle. The internal ﬁeld has two components: the crustal ﬁeld and the core ﬁeld.
The crustal ﬁeld
This portion of the field is associated with magnetism of the crustal rocks and contains magnetism caused by induction from the Earth’s main field and from remnant magnetization. More is known about the crustal ﬁeld than about the core ﬁeld since we know more about the composition and physical parameters such as temperature and pressure and about the types of magnetization.
The core ﬁeld
The temperatures are too high for permanent magnetization. The ﬁeld is caused by rapid (and complex) electric currents in the liquid outer core, which consists mainly of metallic iron. Convection in the core is much more vigorous than in the mantle: about 10 times faster than mantle convection (i.e, of the order of about 10 km/yr) (Merrill and McElhinney, 1983).
The external field is produced by electric current in the earth’s ionosphere consisting of particles ionized by solar radiation and put into motion by the solar tidal force (Robinson and Coruh, 1988). The strength of the ﬁeld due to external sources is much weaker than that of the internal sources. Moreover, the typical time scale for changes of the intensity of the external ﬁeld is much shorter than that of the ﬁeld due to the internal source. Variations in magnetic ﬁeld due to an external origin (atmospheric, solar wind) are often on much shorter time scales so that they can be separated from the contributions of the internal sources.
1.4 Magnetic component
The earth’s magnetic field is a vector quantity; at each point in space it has strength and a direction. To completely describe it we need three quantities. These may be:
- three orthogonal strength components (X, Y, and Z);
- the total field strength and two angles (F, D, I); or
- two strength components and an angle (H, Z, D)
D and I are measured in degrees. All other elements are measured in nanotesla (nT; 1 nT = 10-9 tesla).
The seven elements are related through the following simple expression Declination, D = , Inclination I = , H2 = + = X = H Y = H ,
F2 = + +
1.5 International geomagnetic reference field (IGRF)
The International geomagnetic reference field (IGRF) is a standard mathematical description of the Earth’s main magnetic field and its secular variation. It is the product of a collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world.
The mathematical models of the earth’s main magnetic field and its secular variation comprises a set of spherical harmonic (or Gauss) coefficients, and in a series expansion of the geomagnetic potential (Barton, 1997). The negative gradient of a scalar potential V can be represented by a truncated series expansion (Equation 1.0); (1.1) Okiwelu and George (2009),where, r,θ,λ, are geocentric coordinates (r is the distance from the centre of the earth, θ is the colatitudes and λ is the longitude), R is a reference radius of the earth (6371.2km);
are the Schmidt semi –normalized associated Legendre functions of degree, n and order, m.
An online calculator is available from National Oceanic and Atmospheric Administration (NOAA) which allows easy evaluation of the most recent (11th generation) IGRF model at any location and time between 1900 and 2015. IGRF models are standardized for a particular year, reflecting the most accurate measurements available at that time, and indicating a small-scale, slow time variation of the earth’s overall magnetic field.
1.6 Location of the study area
The study area is in Kogi state, north-central, Nigeria. It is bounded by latitude N and N, longitude E and E. It is accessible through a railway road which is mainly used for the importation of raw material used for steel production. The area is also served by a highway passing through it to Lokoja via Anyigba and Okene. Number of streams arises from the topographic heights and flow into the river Niger. The river Niger formed a confluence with river Benue in Lokoja. The river is navigable overflowing it banks in rainy season and becoming shallow during dry season.