GEOTHERMAL HEAT FLOW ANALYSIS OVER THE UPPER SOKOTO BASIN, NIGERIA USING AEROMAGNETIC DATA

ABSTRACT

Geothermal Heat Flow Analysis over the Upper Sokoto Basin Nigeria, using Aeromagnetic Data was carried out to determine the geothermal heat flow in the area. The study area extends from latitude 12.50°N to 13.50°N and Longitude 4.00°E to 6.00°E. Polynomial fitting method was applied in the regional – residual separation from the digitised data. The observed magnetic residual anomalies map was Fourier transformed after dividing the area into 24 overlapping sections for spectral analysis and interpretation. The Spectral plot reveals two layers of magnetic sources. The Basement depth (D2) varies from 0.505 km to 1.77 km with an average value of 1.240 km, while the Centroid depth (D3) varies from 3.495 km to 8.110 km with an average of 4.763 km. Thus D3 values were used to obtain an approximate contour map of the basement surface in the basin. The curie – point depth estimated from the spectral analysis varies from 5.720 to 14.840 km with average of 8.285 km. Contour map of the curie- point depth and its corresponding 3-D Model were generated. The calculated geothermal gradient and heat flow for the study area varies between 20.216 to 52.447 oC/km and 36.388 to 94.405 mW/m2 respectively. The general trend shows that the area with the highest heat flow value (94.405 mW/m2) found in the north-eastern region of the study area correspond to the area with the highest geothermal gradient and vice-versa, also with shallow curie-point depth. This area is likely a geothermal source potential zone.

CHAPTER ONE

1.0      INTRODUCTION

1.1       Geophysics

Geophysics is a branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth; such phenomena include the earth’s magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity. The scope of geophysics also broadly includes outer-space phenomena that influence the earth, the effects of the sun on the earth’s magnetic field; and manifestations of cosmic radiation and the solar wind. (Finkl, 2000).

The scientific record of mining and exploration geophysics began with the publication of the famous Treaties De Re Metalica by Georgius Agricola in 1965 (Telford, 1976). The initial step in the application of geophysics to the search for minerals probably was taken in 1843, when Von Wrede pointed out that the magnetic theodolite used by Lamout to measure variations in the earth’s magnetic field might also be engaged to discover magnetic ore bodies (Telford, 1976). The constant increase in the demand for metals of all categories and the massive increase in the use of oil and natural gas during the previous years have led to the expansion of many geophysical techniques with increasing sensitivity for the discovery and mapping of hidden deposits and structures. Improvements have been particularly rapid during the previous decade or so because of the expansion of new electronic devices for field equipment and widespread application of the digital computer in the interpretation of geophysical data (Telford, 1976).

Geophysical exploration, commonly called applied geophysics or geophysical prospecting, is conducted to locate economically significant accumulations of oil, natural gas, groundwater and other minerals. Geophysical investigations are also employed with engineering objectives in mind, such as predicting the behaviour of earth materials in relation to foundations for roads, railways, buildings, tunnels, and nuclear power plants. Surveys are generally identified by the property being measured namely: electrical, gravity, magnetic, seismic, thermal, or radioactive properties. Electrical and electromagnetic surveys map variations in the conductivity or capacitance of rocks measured by special tools lowered into holes drilled for oil and gas, conductivity variations  provide  geophysicists  with  clues  from  which  they  can  judge  the hydrocarbon-bearing potential of rock strata. Direct and alternating electrical currents are measured in ground surveys, but the lower radio frequencies are used both in ground and in airborne electromagnetic surveys. Gravity surveys measure density variations in local rock masses, used mainly in petroleum exploration, these surveys are based on use of a device called a gravimeter, gravity surveys are made on land, at sea, and down boreholes.

Geothermal surveys concentrate on temperature variations and the generation, conduction, and loss of heat within the earth. Geothermometry is also important to volcanologic studies as well as to locating geothermal energy resources (Finkl, 2000).

The choice of techniques applied to locate a certain mineral depends upon the nature of the mineral and of the surrounded rocks. Occasionally a method may give a straight signal of the occurrence of the mineral being sought, for instance, the magnetic method when used to find magnetic ore of iron or nickel; at other time the method may only indicate whether or not the condition are encouraging to the occurrence of the mineral sought for example the magnetic method which is frequently used in petroleum exploration as an investigation tool to discover the depth to the igneous basement rocks and so examine where the sediments are thick enough to permit exploration for petroleum (Finkl, 2000).

1.2       Aeromagnetic Survey:

Aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard 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 (Burger et al, 2006).

As the aircraft flies, the magnetometer records tiny variations 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 latter being due both to the regional magnetic field, and the local effect of magnetic minerals in the Earth’s crust. By subtracting the solar and regional effects, the resulting aeromagnetic map shows the spatial distribution and relative abundance of magnetic minerals (most commonly the iron oxide mineral magnetite) in the upper levels of the crust, because different rock types differ in their content of magnetic minerals, the magnetic map allows a visualization of the geological structure of the upper crust in the subsurface, particularly the spatial geometry of bodies of rock and the presence of faults and folds. This is particularly useful where bedrock is obscured by surface sand, soil or water. Aeromagnetic data was once presented as contour plots, but now is more commonly expressed as coloured and shaded computer generated pseudo-topography images. The apparent hills, ridges and valleys are referred to as aeromagnetic anomalies. A geophysicist can use mathematical modelling to infer the shape, depth and properties of the rock bodies responsible for the anomalies (Burger et al, 2006).

Aeromagnetic surveys are widely used to aid in the production of geological maps and are also commonly used during mineral exploration. Some mineral deposits are associated with an increase in the abundance of magnetic minerals, and occasionally the sought after commodity may itself be magnetic (e.g. iron ore deposits), but often the elucidation of the subsurface structure of the upper crust is the most valuable contribution of the aeromagnetic data.

Aeromagnetic surveys are now used to perform reconnaissance mapping of unexploded ordnance (UXO). The aircraft is typically a helicopter, as the sensors must be close to the ground (relative to mineral exploration) to be effective. Electromagnetic methods are also used for this purpose. (Burger et al, 2006)

Aeromagnetic surveys  are extensively used  as  investigation tools and  there has been an increasing acknowledgment of their value for evaluating potential areas by virtue of the unique information they provide. Sharma (1987) outlined the roles of aeromagnetic survey as follows:

1.   Explanation of volcano-sedimentary belts under sand or other recent cover, or in strongly transformed terrains when recent lithologies are otherwise unrecognisable.

2.   Recognition and interpretation of faulting shearing and fracturing not only as potential host for a variety of minerals, but also an indirect guide to epigenetic stress related mineralisation in the surrounding rocks.

3.   Identification and delineation of post tectonic intrusion, typical of such targets are zoned syenite or carbonatite, complexes, kinerlites, tin-bearing granites and mafic intrusions.

4.   Direct detection of deposits of certain iron ores.

5.   In prospecting for oil, aeromagnetic data can give information from which one can determine depths  to  basement  rocks  and  thus  locate  and  define  the  extent  of  sedimentary  basins.

Sedimentary rocks however extent such a small magnetic intensity in measurable at the surface result from topographic or lithologic changes  associated with basement or from igneous intrusion (Dobrin, 1976).

1.3       Earth Crust

The crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically separate from the underlying mantle. The crusts of Earth, Moon, Mercury, Venus, Mars and other terrestrial bodies have been generated largely by igneous rock. Below is the Earth cutaway from core to exosphere.

The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity (Patchett and Samson, 2003). Earth’s crust occupies less than 1% of Earth’s volume.

The oceanic crust of the sheet is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi) (Patchett and Samson, 2003) thick and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick, and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust “float” on the mantle, because the continental crust is thicker, it extends both above and below the oceanic crust, much like a large iceberg floating next to smaller one. (The slightly lighter density of felsic continental rock compared to basaltic ocean rock also contributes to the higher relative elevation of the top of the continental crust.) Because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. The continental crust and the oceanic crust are sometimes called sial and sima respectively. Due to the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima and the dividing line is called The Conrad Discontinuity.   The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C (Patchett and Samson, 2003) (392 °F) to 400 °C (752°F) at the boundary with the underlying mantle. The crust and underlying relatively rigid uppermost mantle make up the lithosphere. Because of convection in the underlying plastic (although non-molten) upper mantle  and  asthenosphere,  the lithosphere is  broken  into  tectonic plates  that  move. The temperature increases by as much as 30°C (about 50°F) for every kilometre locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust

Theory aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago. The primordial crust was very thin, and was probably recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system.

The Earth has probably always had some form of basaltic crust, but the age of the oldest oceanic crust today is only about 200 million years. In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years  and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield. A few zircons with ages as great as 4.3 billion years have been found in the Narryer Gneiss Terrane.

The average age of the current Earth’s continental crust has been estimated to be about 2.0 billion years (Patchett and Samson, 2003). Most crustal rocks formed before 2.5 billion years ago are located in cratons. Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in the earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny or mountain building; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana.  The  crust  forms  in  part  by aggregation  of  island  arcs  including  granite  and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle.

1.3.1    Continental Crust

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima due to its mafic, or basaltic rock. (Based on the change in velocity of seismic waves, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.) Consisting mostly of granitic rock, continental crust has a density of about 2.7 g/cm3 and is less dense than the material of the Earth’s mantle, which consists of mafic rock. Continental crust is also less dense than oceanic crust (density of about 3.3 g/cm3), though it is considerably thicker; mostly 25 to 70 km versus the average oceanic thickness of around 7–10 km. About 40% of the Earth’s surface is now underlain by continental crust. Continental crust makes up about 70% of the volume of Earth’s crust (Armstrong, 1991).

1.4          Geothermal

Geothermal Energy, energy contained in intense heat that continually flows outward from deep within Earth, this heat originates primarily in the core. Some heat is generated in the crust, the planet’s outer layer, by the decay of radioactive elements that are in all rocks. The crust, which is about 5 to 75 km (about 3 to 47 mi) thick, insulates the surface from the hot interior, which at the core may reach temperatures from 4000° to 7000° C (7200° to 12,600° F). Where the heat is concentrated near the surface, it can be used as a source of energy.     (Nemzer, et al., 2009)

1.4.1     Geothermal Geology

The distance from Earth’s surface to its centre is about 6,500 km (about 4,000 mi). From Earth’s surface down through the crust, the normal temperature gradient (the increase of temperature with increase of depth) is 10° to 30° C per km (29° to 8  7°F per mi). Underlying the crust is the mantle, which is made of partially molten rock. Temperatures in the mantle may reach 3700° C (6700° F).  The convective (circulating) motion of this mantle rock drives plate tectonics— the ‘drift’ of Earth’s crustal plates that occurs at a rate of 1 to 5 cm (0.4 to 2 in) per year. Where plates spread apart, molten rock (magma) rises up into the rift (opening), solidifying to form new crust. Where plates collide, one plate is generally forced (sub ducted) beneath the other. As the sub ducted plate slides slowly downward into the mantle’s ever-increasing heat, it melts forming new magma. Plumes of this magma can rise and intrude into the crust, bringing vast quantities of heat relatively close to the surface. If the magma reaches the surface it forms volcanoes, but most of the molten rock stays underground, creating huge subterranean regions of hot rock.

1.4.2    Geothermal Reservoirs

In certain areas, water seeping down through cracks and fissures in the crust comes in contact with this hot rock and is heated to high temperatures. Some of this heated water circulates back to the surface and appears as hot springs and geysers. However, the rising hot water may remain underground in  areas of permeable hot  rock,  forming geothermal  reservoirs. Geothermal reservoirs, which may reach temperatures of more than 350 °C (700 °F), can provide a powerful source of energy.

Geysers are caused when underground chambers of water are heated to the boiling point by volcanic rock. When heat causes the water to boil, pressure forces a superheated column of steam and water to the surface. Because most geothermal reservoirs are capped by overlying rock, the heated water cannot escape, remaining underground instead. If a geothermal reservoir is sufficiently close to the surface, the heated water can be piped to the surface and used to produce energy

1.5       Source of Data for the Present Study

The study area covers the upper Sokoto Basin in the north-western part of Nigeria. The Basin consists predominantly of a gentle undulating plain, underlain by metamorphic rocks. The study area is covered by eight (8) digitised aeromagnetic maps. The digitised aeromagnetic maps were numbered 8, 9, 10, 11, 27, 28, 29 and 30 and the names of the places each map covers are also written on them for easy reference. The maps were produced by Nigerian Geological Survey Agency (NGSA) between 1974 and 1980.

1.6     Aim and Objectives of the Study Area

The main aim of this study area is to use the aeromagnetic maps of the area for depth computation, and applying spectral method for the production of geothermal facts of the study area. The depths are used to map the surface plot of the areas according to depths computed for the area. This map of crustal thickness and thermal data will furthermore provide insight on the crustal resolution as it relates to volcanoes, tectonic and fault system in the study area. Thus, these pieces of information on crustal thickness and temperature can contribute to the improvement of global crustal thickness and heat flow map respectively. This research method corresponded with the Spectral Analysis of the Magnetic Residual Anomalies over Sokoto basin carried out by Shehu et al., (2004).

1.7       The Methodology of the Study

The Procedures involved in this research project are outlined below:

i.          Digitised aeromagnetic maps covering the upper part of the Sokoto basin and the surrounding rocks were obtained.

ii.         All the digitised aeromagnetic maps were combined into a single supper map. This supper map or composite aeromagnetic map of the study area formed the basis for further analysis and interpretation.

iii.       The regional magnetic field map for the area was determined by fitting order polynomial field (because of simple geology and limited spatial extent of the study area) (Spector and Grant 1970) to the total field data using the least square method. The residual magnetic field will then be extracted by subtracting the regional field from the total field.

iv.        Estimations of the depth to layers of magnetisation in the upper Sokoto basin were carried out using spectral analysis of the residual magnetic field. Contour and surface maps of the second layers depths will produced and qualitatively discussed.

v.         The result generated from spectral techniques data set and maps provide a guide to regional crustal thickness reconnaissance studies. It has been understood that average depth of an ensemble magnetic source can be calculated using spectral analysis (Spector and Grant, 1970; Shuey et al., 1977). Therefore estimate of curie-point isotherm depth (Crustal depth) use this depth and thickness information to predict if the area of study is viable for geothermal energy source. The Curie-point at which dormant rocks loosed their ferromagnetic properties provides a link between crustal depth models and models based on the analysis of magnetic sources.

The Curie isotherm may be analysed to produce geothermal gradient and heat flow data, which are major observable parameters for geothermal and mineral exploration.

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