Stratigraphic successions in the Sokoto Basin, after .
In this chapter some preliminaries evaluation were outline briefly based on exploring the use of potential gravity field method in characterizing regional trends of Earth’s sequence system interms of gravitational potentials and fields, viz-a-viz., the background, instrumentation, theoretical model, gravity data reduction, geological framework, Bouguer reduction density, density acceptable models, free-air anomaly and interpretation of Bouguer gravity anomaly. Therefore, application of potential field method (gravity) explore these relationships by focusing on the formation and fill of a continental rift basin in characterizing regional trends of the Earth’s system interms of data processing, interpretation and Earth’s modelling. The study was carried out with the aim to understand and characterized the structural styles and regional trends of the Earth’s sequence beneath the Sokoto Basin and its surroundings. Results from the Bouguer gravity anomaly revealed gravity high of characteristic feature on the Bouguer map with a strong positive oval shape of causative bodies anomaly (> −14.0 mGal) having E-W trend. On contrary a number of gravity minima (−43.31 mGal) and maxima (−39.54 mGal) can be found ENE, N-W parts which are almost defining locations of deep basinal areas. The anomalous features shows negative as well as the lineaments pattern are virtually oriented in the NW-SE, ENE and E-W trends.
- potential field
- gravity method
- regional trends
- Earth’s characterization
- Sokoto Basin
- interpretation model
Earth scientists explore and investigate the structures of the Earth using diverse means, such as tectonic mapping, solid minerals, groundwater and hydrocarbon or for the harvest of geologic structures. Earth scientists may be interested in the determination of, for example, the thickness of sedimentary sequence, depth to basement structures and delineation of fractures (shallow and deep plate sources) for appropriate use in resource evaluation. For example, the identification and mapping of geometry, scale and nature of basement structures are critical in understanding the influence of basement during rift development, basin evolution and subsequent basin inversion. From regional gravity data, information such as tectonic frame work and other aforementioned information can be obtained. The geophysical information invariably combined with geological data are essential for a better understanding of the subsurface and characterizing regional trends of the Earth’s structures. The use of gravity, can powerfully lead to a better detection and geological interpretation of structural features and has the potential of constraining quantitative details and reducing the ambiguity of geological interpretation. Geophysical method involving gravity are commonly used in the structural interpretation of sedimentary basins because of their better spatial resolution . Potential field gravity method has proved very effective for providing useful information known to guide various exploration campaigns, be it regional studies, economic mineral or oil and gas exploration [2, 3].
Meaningful reconnaissance and detailed geological information have been generated by the analyses of gravity data for defining basin’s tectonic framework, gravity survey is the primary method in geophysical exploration as a regional and local structural mapping tool [4, 5, 6, 7, 8, 9, 10, 11, 12]. The effectiveness of gravity survey depends on the existence of a significant density contrast between altered rocks or structures and their host rocks. Moreover, gravity survey not only reflects the shape of major granitoids, but also a correspondence between the tectonic lineaments and regional fault systems . The present chapter guide and explore on the use of the acquired gravity data in characterizing regional trends of the Earth system in some parts of the sedimentary terrain of Africa (i.e. the Sokoto Basin of Nigeria, The Agnes of Egypt as well as Kenya). It’s evidence that the gravity method depends on the different earth materials which have different bulk densities (mass) that bring out variations in the measured gravitational field. The variations can be interpreted through the use of enhancement techniques to determine the density, geometry and depth which causes the gravity variations in gravitational field. The Earth’s gravitational field anomalies results from lateral variations of subsurface materials density and the distance from the measuring instruments, the general problem in geophysical surveying is the ambiguity in data interpretation of the subsurface geology. This arises because many different geologic configurations could reproduce similar observed measurements (Figure 1). The method can infer location of faults, permeable areas for tectonic movement. It is however, more commonly used in determining the location and geometry of Earth’s system characterisation (Figures 1 and 2).
The Lacoste and Romberg model gravity meter was used in data acquisition for this study. It has an advantage of repeatability of 3 mGal (980,000,000 mGal is the Earth’s gravitational field) reading and is one of the preferred instruments for conducting gravity surveys in industry. It has a reading precision of 0.01 mGal and a drift rate less than 1 mGal per month (model G569 manual). Measurements were also made along designated areas to further check the behaviour of the instrument. All necessary routine checks on level adjustments and sensitivity of gravimeter were carried out as described in gravimeter manual (Figure 3).
3. Gravity data reduction
It’s understood, the Earth’s is slightly irregular oblate ellipsoid which means that the gravity field at its surface is stronger at the poles than the equator. The density distribution is irregular, particularly in an inelastic crust, which causes gravity to vary from expected value as the measurement position changes. Therefore, the variations are expressed as gravity anomalies. Mapping the gravity anomalies gives an understanding the structure of the Earth’s [1, 5]. It’s therefore essential to identify the reasons gravity varies and that it can be corrected while using gravity method in exploring and characterizing regional trends of the subsurface . In this present survey, correction for the tide was not made because loops were closed at interval of about 2 hours or less. Also, since the area is relatively flat, there was no need considering excess mass or mass deficiency, hence terrain correction was not carried out. The results of gravimeter measurements are gravity differences between an arbitrary reference point and a series of field stations. The measured values at each station have some influences which completely mask the desired effect if they were not removed. Therefore before gravity measurements may be useful in possible indications of subsurface conditions (The observed gravity differences must be corrected for those various large influences). The objective of data is to remove the known effects caused by predictable features that are not of the target. The remaining anomaly is then interpreted in terms of subsurface variations in density. Each known effect is removed from observed data. The various corrections are described below:
3.1 The latitude correction
Both the rotation of the earth and its slight equatorial bulge produce an increase in gravity with increase in latitude (Figure 4). Therefore it becomes necessary to apply latitude correction for stations at different latitudes. The value of gravity increases with the geographical latitude . With advance of Earth’s rotation, the Earth’s is not spherical but is flattened at poles thus the distance factor causes the ‘
Where is the latitude of the station concerned in degrees.
3.2 The free air correction
Free air anomaly is obtained from the difference between the measured or absolute gravity of a station,
where is the vertical gradient and it is the station elevation above mean sea level in meters and its value is 0.3086 Gal m−1 [1, 5]. In practice the value of 0.3086 mGal/m is the only value used after deriving from Eq. (2) thus, assumed that the Earth’s is spherical and non-rotating. Finally, the correction considers only elevation differences relative to a datum and does not take into account that the mass between the observation point and datum as the station were suspended in free-air, not sitting on land (Figure 5). These serve as the reason that the correction termed as free-air correction (Figure 5). In general the datum used for gravity surveys is sea level and gravity decreases 0.3086 mGal for every meter above sea level [1, 5].
3.3 Bouguer correction
This is the difference between the observed gravity and the theoretical gravity at any point on the earth corrected for the mass of materials between the point and the datum plane (Figure 6), its value 0.04188
Where BC is the Bouguer correction.
Hence the Bouguer anomaly is determined using the expression:
Where is the assumed crustal density value which is 2.67 × 1011 kgm3 or Bouguer density and
3.4 Bouguer gravity anomaly
A Bouguer gravity anomaly is the difference between the observed acceleration of an object in free fall (gravity) on surface of the Earth’s, and the corresponding value predicted from a model of the gravitational field. If the attraction due to the effect of material between the plane of observation and the m.s.l. (known as the Bouguer correction (Bc)) is subtracted from the free-air anomaly, the corrected gravity field is called the Bouguer gravity anomaly and is given by:
3.5 Drift correction
In the reduction of gravity data, the removal of drift which occurs as a result of elastic creep in the spring of the instrument is very necessary. The instrumental drift of the gravimeter used in this survey was removed using a Geosoft computer Algorithm routine of . It is assumed that there is a linear relationship of the drift with time as given by the drift rate which is expressed as:
If the sums station is reoccupied, then
With the assumption that drift of the instrument is a linear function of time over a short time interval, it was ensured that all observations in a day were tied to the same time origin during a day’s work and the repeat observations at the same station after drift correction was equal to the former. Drift correction was done separately for each altimeter height value using free same cascade drift model. The absolute elevation for each of the stations were determined for each altimeter using the height of the Bench Mark No BM15 to which they were tied. Due to the characteristic behaviour of instrument , the field values recorded front tie altimeters for each station were varying. The observed gravity value at the detailed station is given by:
4. Geological framework
The Sokoto Basin is the Nigerian sector of the larger Iullemmeden Basin which spans parts of Algeria, Benin Republic, Niger Republic, Mali and Libya . The study area falls within the Sokoto basin and lies between Latitudes 3:30 E–5:30 E and longitudes 11: 00 N–13:00 N. It is geographically located in the semi-arid with a zone of savannah-type vegetation as part of the sub-Saharan Sudan belt of West Africa with an elevation ranging from 250 to 400 m above sea level (Figure 7a and b). The area enjoys a tropical continental type of climate. Rainfall is concentrated in a short-wet season, which extends from April to October . Mean annual rainfall is about 800–1000 mm while the mean annual temperature ranges from 26.5 to 40°C. Night temperatures are generally lower. The highest temperature occurs between April and July, the lowest in August (during the rainy season). An average nature of 40% low humidity during the wet season reaches a maximum of 80%, explain the dry nature of the environment in the area of study (Figure 8), which is in agreement of a sharp contrast to a humid environment when compare in the southern parts of Nigeria. The Sokoto Basin is predominantly a gentle undulating plain with an average elevation varying from 250 to 400 m above sea level. The plain is occasionally interrupted by low mesas and other escarpment features [18, 19, 20]. The sediments of the Iullemmeden Basin were thought to accumulate during four main phases of deposition as follows (Figure 7b and Table 1):
The Illo and Gundumi Formations (made of grits and clays) unconformably overlie the pre-Cambrian Basement Complex. This is the so-called pre-Maastrichtian ‘continental interclaire’ of West Africa .
Next on the succession is the Maastrichtian (66–72 Ma) Rima Group (consisting of mudstones and friable sandstones) (the Taloka and Wurno formations) separated by the fossiliferous, calcareous and shaley Dukamaje Formation overlies the Illo and Gundumi Formations unconformably [17, 18, 19].
The Dange and Gamba Formations (mainly shales separated by the calcareous Kalambaina Formation constituting the Paleocene) (56–66 Ma) Sokoto Group overlie the Rima Group .
The sequence cover is the Gwandu Formation of the Eocene (33–56 Ma) age forming the continental terminal .
|Pre-Cambrian||Basement complex rocks|
The sediments dip gently and thicken gradually towards the northwest with maximum thicknesses attainable towards the border with Niger Republic.
5. Accuracy of the Bouguer gravity anomaly in Earth’s system characterization
The computed Bouguer anomalies could have several errors introduced to it. Errors could be as a result of incompleteness of the formulae used and the correctness of the numerical values of the constants occurring in them [13, 15]. The calibration factor of the modern Lacoste and Romberg gravimeter depends only on the quality of the measuring screws and the lever system. Errors which could arise from the calibration factor is thought to be negligible because, the calibration factor does not change perceptibly with time, which eliminates the need for frequent checks of calibration. At each station, errors could arise from several sources. These include: errors in elevation determination (eh), errors in terrain effect (et), errors in base value (eb) errors is assumed which recommended that the most likely to in situ densities of subsurface rock lies between the dry and the saturated densities. The summary of the results for the various rock types identified in the area are shown in Table 2.
|Rock type||No. of samples||Range of densities × 103 × kgm−3||Mean densities × 103 × kgm−3||Standard deviation|
|Total number of rock samples||228|
5.1 Bouguer reduction density
The objective of gravity survey is to detect subsurface density variations. Observed/measured gravity value at the station includes all kinds of attraction. Remove the effect of attraction except that of subsurface density anomaly (Figure 9a). There are three methods of selection of Bouguer reduction density; one is a “traditional” or standard density with which most regional maps have traditionally been reduced using a value of 2.67 × 103 kgm−3 (Figure 9b). The second is by determining a Bouguer reduction density which minimizes the correlation between the computed Bouguer anomaly and topography. This method is widely used in areas of rugged topography  and which was originally suggested by  and . This second method was not used in this chapter because the area is relatively flat. The third method is to measure the density of representative rock samples just as described and characterized them interms of earth system evolution. The fact is that it is usually difficult to obtain a suite of rock samples that is truly representative [2, 23]. Therefore in order to ensure consistency and compatibility with after regional gravity map in adjacent areas, the standard density value of 2.67 × 103 kgm−3 was used for reduction in this survey purpose.
5.2 Densities acceptable for models
The proper density values used for gravity interpretation depend upon the depth of formation in relation to the water table, which will in turn depend on whether the climate is arid or moist. The age and depths of sediments depend on how long they are buried. If the period is long enough, the sediments usually consolidate and lithify, resulting in a reduction in porosity and increase in density. Limestones and sandstones which are found in the study area increase in density by infiltration of cements without volumetric change [17, 18]. Clays and shales which are compacted clays are the most highly compressible of all sedimentary rocks and they therefore show the greatest amount of compaction. If sandstone and limestones on the other hand are subjected under the same environment, they experience smaller density change [18, 19, 20].
From the Table 2, it can be seen that the range for the rock density in some part of the Sokoto basin (i.e. Argungu, Shanga Kalambaina, Dange), area is from about 1.5 × 103 kgm−3 to 4.5 × 103 kgm−3 and their respective mean densities approximately agree with the published values for similar rock types from other places [1, 8]. Considering the results from Telford et al. for example, the mean densities for limestone, clay, shales, and laterites are (3.45, 2.43, 2.50 and 2.66) 103 kgm−3, respectively and from the table, the same set of rocks have their density values ranging from 1.76 to 5.30 × 103 kgm−3. Since limestone and laterites are the dominant rocks in the study area, density values within this range were used
5.3 Free-air anomaly map and topography
Free-air correction essentially takes care of the vertical decrease of the gravity with increase of Elevation and no account of the materials between the station and the datum plane taken. The variation amounts to −0.3086 mGal/m. The relationship between the free-air anomaly and heights was investigated and explained in the previous Section 5.1 above. The result of the free air anomaly are shown in Figure 10 below. The free-air anomaly map indicates values ranging from a maximum of 11.8 mGal to a minimum of −41.2 mGal and a contour interval of 2 mGals was used for the map. A careful study of the map reveals that major linear pattern is generally in NE-SW direction with exception of few anomalies located at the S-W trend of the area.
5.4 Interpretation of Bouguer anomaly map
The Bouguer gravity map (Figure 11) comprises various low and high anomalies extend in the NW-SE, ENE and E-W trends as consequence with fold patterns in the southeastern part of Iullemmeden basin (Sokoto Basin in particular). These alternated anomalies are primarily due to the density contrast between the sedimentary blanket and some portion of the crystalline basement in Taloka formation. Sokoto Basin gravity high is a very characteristic feature on the Bouguer map with a strong positive oval shape anomaly (> −14.0 mGal) having S-E trend. The structural trend pattern from the map trending in the S-E direction is associated with deep basinal area of the causative (anomalous) body determined from the gravity survey are found to range from (−43.31 mGal) in the west corner of (Figure 11) fold patterns, and (−39.54 mGal) in the N-W part. The Bouguer gravity anomalous shows negative values and the structural lineaments patterns trending E-W (major trends) and NW-SE, ENW (minor trends) (Figure 12). The deep basinal of the causative (anomalous) body are fall within Gwandu, Kalambaina, Dange, Gamba, Wurno and Taloka formation represents two complementary different events; an older event probably of Continental intercalaire and pre-Cretaceous ages which caused major folding and faulting of NE-SW and ENE trends (Figure 12), respectively [17, 18].
5.5 Regional-residual gravity separation
In the present study, a purely analytical method was used with (Geosoft Oasis montaj V.8.4.3) in which matching of the regional by a polynomial surface of low order exposes the residual features as random errors. A first order polynomial surface was considered adequate for estimating the regional effect. Regional-residual separation process was applied to gravity data-set in order to estimate the amplitude of the regional background. Upward continuation was used to separate a regional gravity anomaly resulting from deep sources from the observed gravity (Bouguer anomaly) (Figure 11). The regional field (Figure 9b) is a plane dipping gently in a NW-SE direction with a gradient of about 1 mGal/km. The regional effect correspond to low frequencies therefore the anomalies are usually of long wavelength showing a gradual change in value while the residual anomalies which are due to local effects may show larger variations [6, 7]. There are several methods of removing the unwanted regional, some approach is entirely graphical while others are analytical. In some cases the graphical methods are incorporated in the analytical methods. The regional gravity values shows the negative entirely and are found to range from a maxima of −21.9 mGal to a minima of −59.3 mGal (Figure 9b).
The residual anomaly at any point is then calculated as the difference between the observed Bouguer anomaly
The residual anomalies at all the points were gridded and contoured through the application of (Geosoft Oasis montaj V.8.4.3). The resulting map (Figure 13) shows the gravitational effect of the near surface and local structures in the study area and the values was found to range from −19.5 mGal (minima) and 16.3 mGal (maxima). The larger features generally show up as trends which continue smoothly over very considerable areas, and they are caused by the deeper heterogeneity of the earth’s crust superimposed on these trends, but frequently camouflaged by them, lie the smaller, local disturbances, which are secondary in size but primary in importance. These are the residual anomalies, which may provide the direct evidence for reservoir—type structure or mineral bodies.
6. Gravity data modelling/advanced processing
This process is aimed in modelling the source of the gravity signal measured at the surface. This can be done through processing of
2D density models
3D density models
As the Bouguer gravity value represents the effect of crustal and upper mantle density variations, the gravity anomalies were used to study the entire lithosphere. The 2D modelling (Figure 14). In quantitative interpretation of gravity data, the objective is to estimate a subsurface structure whose calculated gravity effect satisfactorily approximate the observed gravity field measured on the surface. The magnitude of gravity anomaly caused by any structure depends directly on its volume times its density contrast. Secondly, the amplitude of the anomaly decreases as the depth of the structure causing it increases. If the shape of the structure is irregular or diffused, the observed gravity will be predictable to reduce in sharpness and in magnitude. Quantitative interpretation, generally barely unique or specific as it is always based on geologic implications. Thus, sufficient and adequate information about the geology of the study area becomes necessary for a meaningful interpretation. The study area falls into the sedimentary basin in the northwestern part of Nigeria, and the particular sediments found from the surface (Gwandu formation) which is of the Eocene age have average density of about 2.74 × 103 kgm3 considering the lithologic sequence downward to depth of about 45 m. Underlying it with a slight unconformity are the Sokoto groups (Gamba, Kalambaina and Dange formation) which are of Paleocene age which have average density of about 2.43 × 103 kgm3. These deposits extend to the depth of about 80 m (Figure 14) . Below this occurs continental deposits (fluvial) which were of lower cretaceous or pre-Maastrichtian age. The estimated density value has a first density contrast of 2.4 × 103 kgm3, and the second of 2.92 × 103 kgm3, with respect to the average density of the basement (2.48 × 103 kgm3) used. Therefore almost all the gravity lows in the study area were accounted for by the thickening of the sediments. In the interpretational procedures, the gravitational effect of any assumed initial model is calculated and compared with the observed effect. Changes are made as necessary on the presumed model in order to get a better fit. The common changes usually involve volume, shape and density contrasts. This process is repeated within geologically realistic limits until a new structure whose calculated effect best fits the observed effect was obtained. This approach is referred to as forward modelling (Figure 14). Profile 2 was chosen atleast to cross one major causative (anomalous) bodies identified earlier for interpretation (Figure 13). The computer program used for quantitative gravity interpretation of this profile 2. This interpretation reveals the prominent Gwandu formation and the Sokoto groups (i.e. Gamba, Kalambaina and Dange formation) of sedimentary in-fills have a common origin. The profile runs in the E-W direction and cuts across the causative (anomalous bodies) while modelling, an intrusion lie with density contrast of 2.43 × 103 kgm3 introduced in part of Gwandu formation (Figure 14) at about 20–45 km along the profile before a fit of the computed with the observed was obtained in uppermost part of (Figure 14). While the low gravity at the western side of the profile was accounted for by thickened sediments which has high density contrast of 2.92 × 103 kgm3. The maximum and minimum depths to the top of the sediments in-fills along this profile are 40 and 80 km, respectively. The body it has inward dipping walls and the dips are 45° and 55° on its western and eastern flanks, respectively has been calculated from the GYS-System.
6.1 2D density interpretation
In indirect interpretation, the Earth’s sequence model whose theoretical anomaly can be computed simulates the causative body of a gravity anomaly characterization. The shape of the body can be altered until the computer anomaly closely matches the observed anomaly (Figure 15a and b).
6.2 Gravity contribution to conceptual model
Gravity methods are good in structural mapping in potential exploration for: Earth’s system characterization interms of (imaging the lithospheric structures, dense material in shallow crust), fractures/faults (gravity gradients/slopes), help to identify potential drilling sites, help to identify potential recharge areas, etc.
The chapter were able to explore the potential application of gravity method for Earth’s system exploration interms of regional trend characterization in African tectonic evolution settings. The strength of the potential gravity field method lies in the adequate density mass distribution of gravitization effect within the crustal materials of the Earth in the light of measurable gravity field over them. The Earth’s gravitational field, that is the Earth’s shape and global force, is itself complex. Advanced data processing, analysis, interpretation and modelling provides the means of characterizing the Earth’s regional trends and with such a representation; it is possible to predict the Bouguer anomalies and other densities acceptable for models. The knowledge of the free-air anomaly of the Earth enables the gravity anomaly to be determined over a survey area from measurements of the gravitational field strength. The method were applied to real field measurements of Bouguer gravity data over the Sokoto Basin, Nigeria. The working data were corrected for Bouguer reduction density variation using regional-residual separation model. In particular, the major anomalies of the regional and the observed Bouguer gravity field exhibits majorly trending in the E-W, and NW-SE directions adjacent to the main structural fold patterns of (Figures 9b and 11) in the northwestern parts. The anomaly field which is the summary of the regional field was further processed to obtain the residual gravity anomaly (Figure 13). The regional models show that the crustal structure in the study area consists of normal continental crust, which is divided into lower and upper by the Conrad of a nearly constant depth. The density effect for the sedimentary formations is necessary valuable and extremely critical to interpret the deeper effect of Sokoto Rima groups. Also, it may occur on the edges, in other gravity filtering enhancements which significantly influence the structural fittings. In order to overcome the edge effect, the modelled length is slightly enlarged outside the limits.
I acknowledge the words of encouragement support of my chairman supervisory team for my ongoing Ph.D. Program (Prof. O.K. Likkason) of Physics Program, ATBU, Bauchi, Nigeria as well as members’ supervisory team (Prof. A.S. Maigari & Dr. S. Ali) of ATBU Bauchi, Nigeria. Indeed I’m grateful to the support intervention for Petroleum Technology Development Fund, PTDF, Abuja, Nigeria. Finally, I also wish to acknowledge the reference text citation made within context of this work.
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