ABSTRACT
Aeromagnetic data over Abakaliki area of the lower Benue trough of Nigeria was interpreted qualitatively and quantitatively using Oasis montaj software (version: 6.4.2 H.J).The interpretation unveiled basic intrusive bodies like dyke, lacolyte and batholyte in the study area. It also showed fault zone which trends North East to south western (NE-SW) part of the study area.
Quantitative interpretation of the area was carried out by source parameter imaging and forward and inverse modeling. Source parameter imaging unveiled predominance of deep seated bodies in the south western part of the area, while shallow bodies are predominant in the south eastern part of the study area. Depth obtained by source parameter imaging ranged from 99.50m to 5930.78m. Forward and inverse modeling was carried out using potent Q software which is an extension of the Oasis montaj software used in the work. The magnetic anomalies over the area were modeled by bodies in the form of sphere and ellipsoid by varying the total magnetic intensity parameters such as susceptibility, inclination, declination, depth of burial and by varying the length, width and height of the bodies used in the model. The radius of the spherical bodies are 1457m, 17704m and 4883m for models A, B and D representing Obubra, Abakaliki and Ameka respectively. Lengths of the ellipsoids are 6099m and 5341m.Width of the ellipsoids are 411m and 2203m while Heights of the ellipsoids are 6017m and 275m (for models C and E representing Enyigba and Ameri respectively). Depth obtained by forward and inverse modeling ranged from 477m to 6366m. Depth obtained for model A (Obubra) is 546m with susceptibility of 0.0180SI (signifying limestone). Height obtained for model B (Abakaliki) is 50m (likely the out crop near college of Agricultural sciences-CAS EBSU) with susceptibility of -0.0017SI (signifying calcite). Depth obtained for model C (Enyigba) is 956m with susceptibility of -0.0134SI (signifying Rock salt). Depth obtained for model D (Ameka) is 6366m with susceptibility of -0.009SI (signifying Quartz). Depth obtained for model E (Ameri) is 477m with susceptibility of -0.006SI (signifying Calcite). These values correlate with some of the works done in the study area. The presence of out crop, intrusive and minerals like calcite, pyrite, rock salt and limestone were delineated in the area.
CHAPTER ONE
INTRODUCTION
The study of geophysics has helped man to access hidden treasures in the sub-surface of the earth. These treasures usually appear as anomalies which could be accessed by different geophysical survey methods such as magnetic method, gravitational method, seismic method, electromagnetic method etc.
In geophysical exploration during the last decade, potential field methods have a renewed interest in the search for solid minerals and hydrocarbons. In the gravity and magnetic method data processing, the first and the most crucial step is the removal of the effect of deep-seated structures from the observed Bouguer gravity or from the observed total magnetic fields, in order to enhance the signatures of shallow bodies (Ndougsa et al., 2007). These shallow bodies are associated in solid mining exploration firstly to precious metals (gold, diamond) which have a different density with the surroundings (gravity exploration) and secondly to substances such as magnetite, hematite, which have contain an iron ore deposit (magnetic exploration) (Ndougsa et al., 2012).
Magnetic survey investigates the subsurface based on variation in the earth’s magnetic field that results from the magnetic properties of the underlying rocks. Magnetic survey can be carried out on land, sea and in air. However, magnetic survey is in principle similar to gravity survey. Aeromagnetic survey aids in indicating major basement surface structures which reveal encouraging exploration areas that could be studied in broader detail using seismic geophysical survey method. The earth’s magnetic field acts on the magnetic minerals in the crust, inducing a secondary field which reflects the distribution of the minerals. The main magnetic field induces a field which varies slowly from one place to another while the crustal field which is the portion of the magnetic field associated with the magnetism induced by the earth’s main magnetic field varies more rapidly (Reford, 1962).
The traditional role of aeromagnetic studies over continental areas is to establish geologic and tectonic frame works and to explore for minerals. The magnetic method is a relatively inexpensive method of learning about geologic hazards such as seismically active faults, shallow magma chambers and volcanic centers. Induced magnetic method may be the only way to study hazardous structures in places where they are concealed beneath young deposits, water and vegetation. Aeromagnetic data have long been used by the petroleum industry to map structures and to enhance depth to magnetic basement (Steenland, 1965).
The use of computers in the 1960s for processing and interpreting geophysical data has application in estimation of depth to basement and mapping of basement structures. It has been demonstrated in literature and other forms that high resolution aeromagnetic data provide valuable data that solves problems in petroleum exploration.
Aeromagnetic survey has application also in mapping of potential ground water resources in the arid and semi-arid areas of the world. The fact that most sedimentary rocks and surface cover formations like water are effectively nonmagnetic implies that the observed anomalies are attributable to the underlying igneous and metamorphic rocks.
Aeromagnetic survey is the oldest potential field method used for hydrocarbon exploration. The aim of the aeromagnetic survey is to detect minerals or rocks that have unusual magnetic properties by causing anomalies in the intensity of the earth’s magnetic field. Aeromagnetic survey is the exploration method of the earth’s magnetic field intensity with magnetometers installed in airplanes or helicopters. The process is to operate the magnetometer continuously along equally spaced parallel flight lines covering the survey area. The principle of the geophysical survey is similar to a magnetic survey carried out with a hand held magnetometer but it allows much larger areas of the earth’s surface to be quickly covered. The aircraft flies in a grid –like pattern with height and line spacing determining the resolution of the data. As the aircraft flies, the magnetometer records tiny variation in the intensity of the ambient magnetic field due to the temporal effects of the constantly varying solar wind and spatial variations in the earth’s magnetic field. The spatial variation of the earth’s field is due to the regional magnetic field and the local effect of magnetic minerals in the earth’s crust. Subtraction of the solar and regional effects reveals the spatial distribution and relative abundance of magnetic minerals.
Aeromagnetic survey covers much larger areas of the earth’s surface quickly for regional reconnaissance than hand-held magnetometer.
The increasing demand for metals of all kinds and the use of petroleum products have led to the development of many geophysical techniques of ever increasing sensitivity for detection and mapping of unseen deposits and structures (Telford et al, 1990).
1.2 Background to the study
Magnetic method is the primary tool in the search for minerals. It is used in many areas such as locating intra-sedimentary faults, defining subtle lithological contacts, mapping salt domes in weakly magnetic sediments. These applications have increased the methods utility in all areas of exploration especially in the search for minerals, oil and gas, geothermal resources, natural hazards assessment, mapping impact structures, underground water survey, engineering and environmental studies. Aeromagnetic methods traditionally are used to Map crystalline basement, igneous rocks at depth and for mapping faults that offset Basin fills and for delineating buried igneous bodies in the near surface (Grauch, 2001).
Aeromagnetic data represents variations in the strength of the Earth’s magnetic field that are produced by changes in magnetization of the crust. Magnetization of rocks is determined by the quantity of magnetic minerals (commonly titanomagnetites) and by the strength and direction of remanent magnetization carried by those magnetic minerals. The quantity of magnetic minerals is measured as magnetic susceptibility and produces an induced magnetization. The remanent magnetization is based on the permanent alignment of magnetic domains within the rock and is measured using paleomagnetic methods (Butler, 1992).
Aeromagnetic surveys respond to the total magnetization of rocks, which is the vector sum of the induced and remanent magnetizations. Igneous and crystalline metamorphic rocks commonly have high total magnetizations compared with other rock types, whereas sedimentary rocks and poorly consolidated sediments have much lower magnetizations (Reynolds et.al, 1990; Hudson et.al, 1999).
The aeromagnetic geophysical method plays a distinguished role in terms of rapid rate coverage of geophysical survey area. The main purpose of aeromagnetic geophysical survey is to detect minerals or rocks that have unusual magnetic properties which reveal themselves by causing anomalies in the intensity of the earth’s magnetic field (USGS, 1997).
Aeromagnetic data was once presented as contour plots but it is more commonly expressed as colour and shaded computer generated Pseudo-topographic images where in the apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. The Aeromagnetic survey is applied in mapping these anomalies in the earth’s magnetic field and it is subsequently correlated with the underground geological structures.
1.3 Geology of the study area
Abakaliki lies in the lower Benue Trough. It is located between latitudes 6o35‘ N to 6o45‘ N and longitudes 8o42‘ E to 8o47‘ E, with average elevation of 117m. The Lower Benue trough is believed to have been formed during the formation of the Benue Trough. A series of tectonic activities characterize the formation of block faulting. This occurred during the separation of the South American Continent (Olade, 1975, Nwachukwu, 1972 and White man, 1982).
The separation of the continents led to an aborted rift (Aulacogen) which was later filled with transgressive and regressive sedimentary deposits. Aside Abakaliki anticlinorium towards the Anambra basin, the Afikpocyncline is also part of the Lower Benue Trough. The sequence of events that led to the formation of the Benue Trough and its component units are well documented (Burke et al, 1971, Benkhelil, 1982, Nwachukwu, 1972, Olade, 1975, Ofoegbu, 1985, Ojoh, 1992).
The Lower Benue Trough underlain by thick sedimentary sequences deposited in the cretaceous and the Precambrian basement complex is essentially made up of granitic and magnetic rocks which are predominant in the eastern part of the study area (Ofoegbu and Onuoha, 1990).
The development of Abakaliki anticlinorium was adversely affected by the folding episode which occurred during the santonian, hence, the Predominantly compressional nature of the fold that occurred during the period was revealed by their asymmetry and reversed faults.
The sediments that occur in the Abakaliki anticlinorium belong to Four (4) geologic formations (Fig 1.1). These include: Asu-river group (Albian), Awgu shale (caniacian),Nkporo shale and Ezeaku shale(Turonian).
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