geophysical study contribution to the rio del rey ... · anomalies of the sedimentary basin of rio...

European Journal of Scientific Research

ISSN 1450-216X / 1450-202X Vol. 151 No 4 February, 2019, pp. 393-409

http://www. europeanjournalofscientificresearch.com

Geophysical Study Contribution to the Rio Del Rey Sedimentary Basin based on Aeromagnetic Data Interpretation

Alain Rodrigue Nzeuga Department of Physics, Faculty of Science

University of Yaounde I, P.O. Box 812, Yaounde, Cameroon

Françoise Enyegue à Nyam Department of Physics, Faculty of Science


Robert Nouayou Department of Physics, Faculty of Science


James Derek Fairhead School of Earth and Environment

Faculty of Environment, University of Leeds, UK

Abstract

The complex tectonics of coastal sedimentary basins results from the great

geodynamic activity of the Earth planet from the Cretaceous to the Neogene. Among these

basins, we have that of Rio Del Rey the tectonics of which highlights the fault system

responsible of the fracturing of its base during the opening process of the South Atlantic.

Using the filtering operations applied on the map of the total magnetic intensity field, a

structural map showing the main faults will be drawn based on the analysis of the local

maximums of the horizontal gradient of the total magnetic intensity field reduced to pole

and analysis of the local maximums of the analytical signal of the total magnetic intensity

field. The use of aeromagnetic data for the characterization of the structures of this basin

gives many informations that the results of previous studies did not take into consideration.

A quantitative study of anomalies along a profile on the map of the total magnetic intensity

field reduced to the pole shows the presence of strongly magnetized bodies. These results

justify the granito-gneissic nature of the Precambrian basement and the perfectly induced

nature of magnetism in this basin. A table of the fracturing directions and the rosace of the

structural directions have been drawn up to correlate the results of the Euler’s

deconvolution map. Finally, a comparison of the results obtained shows that the fracturing

of the basement of the Rio Del Rey basin took place along two major directions. The

complexity of the structural map compared to the one drawn up by Dumort in 1968 shows

that these results could serve as a guide and open up other lines of research in the future.

Keywords: Aeromagnetic, reduction to pole, horizontal gradient, analytical signal,

lineaments, faults, basaltic intrusion.

Geophysical Study Contribution to the Rio Del Rey Sedimentary

Basin based on Aeromagnetic Data Interpretation 394

1. Introduction The subsoil acknowledgement from geophysical methods has become a major challenge for the global

scientific community, with scientific and economical objectives. From the scientific point of view,

since the great tectonic activity that gave rise to the different oceanic and continental plates, the

knowledge of the structural tectonics of the subsoil is of capital interest in the search for solutions

related to the great energy need that the world knows.Thus, a scientist is always on the search for new

solutions to either improve the results of the previous studies, or to completely change one theory in

favour of another considered to be more appropriate. Eonomically, the main objective is to multiply

the sources of energy and optimize them to be more profitable.

This great tectonic activity is known as the break-up of Pangea during the lower Cretaceous due to

convective movements in the mantle which is at the origin of the birth of several sedimentary basins among

which the sedimentary basin of Rio Del Rey that was formed successively in three great tectonic phases

between the lower Cretaceous and the Tertiary (Benkhelil et al., 2002) . In the Early Cretaceous, an initial

rifting phase generates extensive basement faults, followed by a Drift phase, thus establishing short-term

growth faults and compression phase during the Late Cretaceous and Tertiary (Coughlin et al., 1993).

Following previous work by several authors in the field of stratigraphy, structural geology,

petrography and geophysics such as Belmonte (1966), Regnoult (1986), Coughlin et al. (1993),

Schiefelbein et al. (2000), Benkhelil et al. (2002), Brownfield and Charpentier (2006), Mvondo Owono

(2010), Koum et al. (2013), it appears that fractures and faults control the main fluid circulation routes, and

the knowledge of stratigraphy and petrology of a basin is necessary to determine its economic potential.

The objectives of our study is to bring out the fault system responsible for fracturing in the Rio

Del Rey basin, to estimate their position relative to the surface of the earth, their extension and their

depth. Also, from a geological basement model, we will highlight the pockets of magmatic intrusions

in the earth's bedrock justifying the presence of an opening of the earth's crust on the mantle. To do

this, we shall start from the aeromagnetic data collected in the sedimentary basin of Rio Del Rey

processesd in three successive stages: the first stage is devoted to the filtering of the magnetic

anomalies of the sedimentary basin of Rio Del Rey, followed by the qualitative interpretation

transformed maps. In the second step, we shall use the Euler deconvolution method to carry out a

quantitative study of the magnetic anomalies observed in order to get an idea of the main fracturing

directions and finally the third step will be to use the Blakely and Simpson method (1986) to determine

the maximums of the maps of the horizontal gradient of the total magnetic field reduced to the pole and

of the analytical signal of the total magnetic field which will be superimposed in order to bring out the

structural map of the basin. To verify the validity of our results, we shall draw up the rosace of the

fracturing directions and make a quantitative study of the magnetic anomalies by modeling along a

profile on the map of the anomaly of the total magnetic field reduced to pole of the Rio Del Rey basin.

2. Study Area 2.1. Geographical Setting

The Rio del Rey sedimentary basin (RDR) is a large, open, shallow bay at the bottom of Biafra Bay (Ala and

Selley, 1997). It is situated at a watershed estuary in Cameroon, in its border with Nigeria, in the eastern Niger

River (Olivry, 1986). It is located on the West African margin and belongs to the Tertiary age basins series of

the Gulf of Guinea (Figure 1), specifically in the Southwest Cameroon region, Ndian Department between

latitudes 4° N and 5° N, and longitudes 8.5° E and 9.3° E. It is particularly bordered by the Cameroon volcanic

line of (CVL) in the East and by the Atlantic Ocean at the South West. It is the first coastal producer basin of

Cameroon the western parts of which belong to the domain of the Bakassi peninsula. It covers an area of about

140,000 ha including 7,000 km2 offshore and provides nearly 9/10th of the national production of crude oil from

55 fields (SNH, 2005). Together with the Douala and Kribi-Campo Basins, it was formed during the Aptian-

Albian Times (Agyingi et al., 2006).

395 Alain Rodrigue Nzeuga, Françoise Enyegue à Nyam, Robert Nouayou and James Derek Fairhead

The Rio Del Rey Sedimentary Basin is a mangrove area in the coastal area, consisting of long

benches of solid land a few meters high on which forest vegetation rests. Soils in the study area are ferralitic

(Gavaud and Muller, 1980), yellowish in color, and

subsoils. Agricultural areas have been extensively cultivated for at least the last 200 years (Gartland, 1986).

Figure 1:

2.2. Geological and

The geological history of any sedimentary basin in general and the sedimentary basins of Ca

particular are

the existence of the secondary Pangea era . Figure 2

African and South American continents gave birth to coastal sedi

Figure 2:

Alain Rodrigue Nzeuga, Françoise Enyegue à Nyam, Robert Nouayou and James Derek Fairhead





Geo-referenced map showing the location, drainage and sampling sites in the Rio

Inset: Cameroon map showing study area (shaded re

Geological and Geophysical Setting


particular are rich, beautiful, and integrate



Paleogeography of the South Atlantic (Cretaceous

Project (Scotese, 2001).






referenced map showing the location, drainage and sampling sites in the Rio


Geophysical Setting


rich, beautiful, and integrate




Project (Scotese, 2001).








Geophysical Setting


rich, beautiful, and integrate harmoniously with th







(Gavaud and Muller, 1980), yellowish in color, and varying from clayey, silty, sandy to lateritic clay





harmoniously with th

the existence of the secondary Pangea era . Figure 2 presents in a global way the paleogeography of the






varying from clayey, silty, sandy to lateritic clay



Inset: Cameroon map showing study area (shaded rectangle). (Wotany et al., 2013)


harmoniously with those of Africa and the wolrd based on

presents in a global way the paleogeography of the

African and South American continents gave birth to coastal sedimentary basins in Cameroon.

Paleogeography of the South Atlantic (Cretaceous - Lower Tertiary) according to: PALEOMAP







ctangle). (Wotany et al., 2013)


of Africa and the wolrd based on


mentary basins in Cameroon.

Lower Tertiary) according to: PALEOMAP






referenced map showing the location, drainage and sampling sites in the Rio del Rey Basin.

ctangle). (Wotany et al., 2013)

The geological history of any sedimentary basin in general and the sedimentary basins of Cameroon in










del Rey Basin.

meroon in







Three tectonic phases contribute to the setting up of the Rio Del Rey basin (Benkhelil et al.,

2002). In the Early Cretaceous, an initial rifting phase generates extensive basement faults, followed by

a drift phase, as a result, establishing short-term growth faults and compression phase during the Late

Cretaceous and at the Tertiary. According to the types of deformation, three structural provinces are

individualized (Coughlin et al., 1993): the province of growth faults in the North (zone of the Bénoué

ditch), the province of clay wrinkles in the South (zone downstream of the delta, characterized by

normal NS-axis faults set up during the diapses) and the province of the frontal berm or reverse faults

in the central South zone (area where gravity leads to compression of structures).

Their geological history begins in the early Cretaceous by the dislocation of South America and

Africa to the lower Eocene with the accumulation of deposits. It has a variety of geological setting

(Figure 3) comprising of Cretaceous limestones, Tertiary and Quaternary sediments which are

essentially clastics consisting of sands, sandstones, conglomerates, limestones, shales, clays and

alluvium which are terminated by basaltic lava flows from the Rumpi Hills and by Precambrian

basement rocks composed of gneisses, micaschists and quartzites (Dumort, 1968, Obenesaw et al.,

1997, Njoh and Petters, 2008). The lithologic description made by Njoh and Petters (2008) describes

the cretaceous sedimentary rocks in this area as poorly made conglomeritic sandstones, dark micaceous

silt-stones, mudstones and shales. Directly overlying the Cretaceous offshore sediments is the lowest

Tertiary unit known as the Isongo Formation which is equivalent to the Akata Formation in the Delta

Niger (Njoh and Petters, 2008).

Figure 3: Simplified geological map of the Rio Del Rey sedimentary basin (Dumort, 1968)

The stratigraphy of the Rio Del Rey is similar in broad outline to that of the Niger Delta. It is

very variable according to the wells. The Rio Del Rey Basin contains two units dated from the

Paleocene to the Miocene (the pre-deltaic series from the Paleocene to the Miocene and the deltaic

series from the Miocene to the present).

In the pre-deltaic series from Paleocene to Miocene, the Paleocene unit is clayey in nature with

the presence of volcanic layers while the Eocene unit is made of marine clay with the presence of some

traces of volcanic material. The deltaic series from the Miocene to the present consists of three main

diachrone formations (Coughlin et al., 1993): the formation of the Isongo or Akata formation,

composed of the lower Isongo (alternations of sandstone and polygenic sands of turbiditic origin, with

volcanic elements and gray-black clays), medium Isongo (mainly clay) and upper Isongo (the main

source rocks in the Rio Del Rey basin), the deltaic alternations or formation of Agbada, which is an

alternation of sand and silts designated by S and clays designated by M. Let us note that the sands S


constitute the main reservoirs and the clays M the covered rocks, the formation of Benin, consisting of

freshwater sands containing volcanic and detrital elements which constitutes the summit of the

sedimentary series. Its deposit occurs at the beginning of the Pliocene.

The oil system in the Rio Del Rey Basin includes hydrocarbon traps, reservoir rock, bedrock,

source rock, ripening and migration (Regnoult, 1986). The main rock is represented by the clays of Akata

and Agbada. Organic matter is composed essentially of a mixture of kerogen type I and II (Mello and Katz,

1997). The reservoir rocks are exclusively represented by Agbada sands and turbidities, channels and sand

bars in the Akata offshore domain. Currently, oil production is mainly carried out in the Agbada reservoirs.

The rock cover is ensured by the clays of the Akata and the intercalations clay Agbada. The migration

occurred after sediment deposition and after tectonic structuring of the basin. It has several types of

structural and stratigraphic traps. The most common are anticlinal faults, bevels vs. faults and contractures,

normal and inverted faults, clay diaphragm traps and regional lenses.

3. Tools and Methods 3.1. Data Acquisition

The aeromagnetic data used in this work was collected between the longitudes 8.5 ° and 9.5 ° E and the

latitudes 5.3 ° and 6 ° N and came from the database of the British geophysical company GETECH

Group and were collected in 1981 by the company CGG on behalf of the French company ELF,

according to profiles spaced 1000 m apart. The calculation of the theoretical International Geomagnetic

Reference Field (IGRF) at each measurement point on January first, 1981 shows that the values of the

magnetic anomaly provided by the company has already been corrected.

3.2. Data Processing by Qualitative Analysis

To perform qualitative and quantitative analysis of the magnetic anomalies observed in a region and

give a geological model of the subsoil highlighting the intrusions of igneous bodies, filtering

operations are needed. This operation is also valid for the determination of geophysical lineaments.

Geophysical lineament means any uniform geophysical coverage, independent of the topography,

which makes it possible to distinguish the superficial and deep structures of the crust. It is a structural

alignment of varying size that corresponds to a crustal earthquake (usually a fault) the influence of

which can be depicted over a long period of time (O'Leary et al., 1976, Richards, 2000). ). These filters

are applied to the anomaly map of the total magnetic field and can be the upward continuation, the

vertical gradient, the horizontal gradient, the analytical signal, the Euler's deconvolution or others.

3.2.1. Anomaly Map of the Total Magnetic Field The anomaly map of the total magnetic field was drawn using the software Oasis montaj 6.3 with a

constant step of 0.01 degree, ie about 1.1 km. Represented in Figure 4, it highlights the sum of the

effects of all the magnetized bodies, whatever their orientation, their nature and intensity of

magnetization. On this map, we can see areas of strong magnetic anomalies up to 580 nT. A superficial

analysis of this map shows that the magnetic relief of the Rio Del Rey Basin is disturbed by numerous

anomalies of different wavelengths, most of which are superimposed on outcrops of the Precambrian

granitogneissic basement. In the center of the basin, we observe a large positive magnetic anomaly

with peaks of magnitude reaching 580 nT, direction NE-SW. This is the largest visible positive

anomaly that would represent aphyric basalt outcrops in the Rio Del Rey sedimentary basin as

indicated by the geological map. A similar anomaly is visible in the northeast of the basin and

represents the paleocene unit of clay nature with the presence of volcanic layers. North of the basin, we

observe a strong negative anomaly with local minimums reaching -270 nT, N70 ° E general direction

that would represent outcrops of gneiss in the Precambrian basalt sandstone formed at the origin of the

separation of American and African plates and at the opening of the South Atlantic at Cretaceous.



Figure 4: Map of the anomaly of the total magnetic field of the Rio Del Rey sedimentary basin

3.2.2. Anomaly Map of the of the Total Magnetic Field Reduced to the Pole The reduction to the pole operator allows to determine the magnetic field that would be observed if the

magnetization was vertical (inclination I = 90°). It therefore acts by placing the anomaly directely

above the source, thus making the anomaly monopolar as in gravimetry. From the values of the

inclination and the declination obtained from the IGRF at the date quoted above (I = -16,272 °, D = -

5,402 °), the reduction to the pole operator was applied using the Oasis Montaj software on the map of

the anomaly of the total magnetic field and the result is shown in figure 5. This map shows in a more

precise way the distribution of the sediments compared to the Precambrian basement and corroborates

the information collected on the geological map. In fact, the sediments that would be the alluvium are

better distinguished from traces of volcanic material in the basin, and are distributed both on the

continental plate and on the oceanic plate.

Figure 5: Map of the anomaly of the total magnetic field reduced to the pole of the Rio Del Rey sedimentary

basin on which it is represented the profile P.

P


3.2.3. Upward Continuation Maps The transformation by upward continuation is equivalent to a smoothing operation of the total magnetic

field map reduced to the pole. It consists to eliminating the high frequencies of the field associated with the

effects of superficial magnetic structures, to show only the effects of deep structures.

In order to highlight the deep or superficial nature of the causative sources of anomalies, the

upward continuation has been applied to the map of the total magnetic field reduced to the pole and the

results are shown in Figures 6 and 7. These show that the positive and negative anomalies observed do

not fade quickly, thus stating that the sources of these anomalies would be deeps. Also, the positive

anomaly observed at the east of Mundemba is a recent outcrop of basalt formed on an opening part of

the Earth's crust on the mantle, while the negative anomaly surrounding the Mundemba Zone is

believed to be Precambrian gneiss formed during the creation of the basin. All these informations

justifies among others those observed on the geological map of the region.

Figure 6: Map of the total magnetic field anomalies reduced to the pole, upward to 2 km of the Rio Del Rey

sedimentary basin

Figure 7: Map of the total magnetic field anomalies reduced to the pole, upward to 3 km of the Rio Del Rey

sedimentary basin



3.2.4. Map of the Vertical Gradient of the Magnetic Field Reduced to the Pole The vertical gradient operator makes it possible to better distinguish the different anomalies, because

when several structures are close enough and located at comparable depths, the measured signal shows

the existence of a single anomaly.

The anomalies observed on the map of the total magnetic field reduced to the pole are formed

in blocks and the application of the first order vertical gradient operator on this one would better

distribute them in space. Figure 8 represents the map of the first order vertical gradient of the total

magnetic field reduced to the pole, on which we can see that the large positive anomaly which

appeared in a block in the Northeast on the map of the total magnetic field reduced to the pole is

individualized on the first order vertical gradient map. These anomalies are in most cases oriented N70

° E which is a direction parallel to the shear line of Cameroon. This direction justifies the creation of

the Rio Del Rey sedimentary basin at the Cretaceous during the opening of the South Atlantic.

Figure 8: Map of the first order vertical gradient of magnetic field reduced to the pole of the Rio Del Rey

sedimentary basin

3.2.5. Map of the Horizontal Gradient of the Magnetic Field Reduced to the Pole The horizontal gradient (HG) of the intensity T of the total magnetic field is given by the relation:

HG = �� + ��

� (1)

In low latitudes, for the existence of a horizontal gradient to correspond to the presence of a

magnetic susceptibility contrast, we must apply the horizontal gradient operator to the map of the

magnetic field reduced to the pole rather than the total magnetic field map itself.

The map of the horizontal gradient of the magnetic field reduced to the pole represented in

Figure 9 shows high gradient areas which appear in various forms with peaks of magnitude up to 0.13

nT. On this map, we observe areas of strongly positive anomalies in the northeast and east of the study

area. These anomalies are associated with deep and shallow structures whose boundaries correspond to

tectonic accidents in the study area. These anomalies are in most cases parallel to the N70 ° E

direction, which would be the major structural direction observed in the Rio Del Rey sedimentary

basin. The rest of the basin is formed of areas of negative anomalies that could correspond to marine

clays and sandstones. Thus, we can conclude that these results corroborate those of the

lithostratigraphic studies carried out in the basin, stipulating that the sedimentary basin of Rio Del Rey

is of clayey nature with the presence of the volcanic rocks.


Figure 9: Map of the horizontal gradient of the total magnetic field reduced to the pole of the sedimentary

basin of Rio Del Rey

3.2.6. Map of the Amplitude of the Analytic Signal The amplitude of the analytic signal (or amplitude of the total gradient 3-D) denoted AS of the

anomaly T of the magnetic field is defined in Cartesian coordinates by the relation (Roest et al., 1992):

AS = �� + ��

� + �� (2)

In contrast with the horizontal gradient, the amplitude of the analytic signal depends very little

on the direction of magnetization (Roest et al., 1992, Blakely, 1995), and is completely independent

when the sources are vertical (Nabighian, 1972).

Figure 10 shows the map of the analytic signal of the total magnetic field of the sedimentary

basin of Rio Del Rey. It makes it possible to identify the strongly magnetized geological structures

present in the basin. The positive anomalies are in most cases oriented N70 ° E and the major maxima

underlined on the map of the horizontal gradient are well represented on the map of the analytic signal,

reaching peaks of amplitude of 0.19 nT. In general, the results obtained corroborate well those obtained

on the map of the horizontal gradient.

Figure 10: Map of the amplitude of the analytical signal of the total magnetic field of the Rio Del Rey

sedimentary basin



3.2.7. Data Processing by Quantitative Analysis

The quantitative analysis of the magnetic anomalies observed in the sedimentary basin of Rio Del Rey

will aim to justify first the results of the qualitative study, then to highlight the lineaments essential to

the realization of its structural map. This quantitative analysis includes the Euler’s deconvolution and

the determination of the maximums of the anomaly field.

3.2.8. Euler’s Deconvolution Map The purpose of the Euler’s deconvolution map is to highlight contact or fault structures in order to

determine the main fracturing directions of a studied area. That of the sedimentary basin of RDR

shown in Figure 11 was drawn up with the following parameters: structural index N = 1, tolerance Z =

15%, dimension of the window W = 10 x 10. On this one, we can observe that the directions of

fracturations obtained are in most cases oriented NO-SE and NE-SO, which would be the major

fracturing directions of the Rio Del Rey basin and corroborate the results of the qualitative analysis.

Figure 11: Euler’s deconvolution map of the magnetic anomalies of the Rio Del Rey sedimentary basin

3.2.9. Map of Maximums The maximums of the anomaly field are determined by the method of Blakely and Simpson (1968).

Figures 12 and 13 represent respectively the map of the maximums of the horizontal gradient of the

total magnetic field reduced to the pole upward to 2 km and that of the maximums of the analytical

signal of the total magnetic field upward to 2 km.

The lineaments located here highlight linear contacts that may correspond to faults with major

directions NNE-SSO and NO-SE, thus justifying the results of the qualitative analysis.


Figure 12: Maximums of the horizontal gradient of the total magnetic field reduced to the pole upward to 2 km

of the sedimentary basin of Rio Del Rey

Figure 13: Maximums analytic signal map of the total magnetic field extended to 2 km of the Rio Del Rey

sedimentary basin

4. Results The main results of this study are the determination of the structural map responsible for the fracturing

in the RDR basin and the modeling of the volcanic intrusions along the profile P represented on the

map of the anomaly of the total magnetic field reduced to the pole.

Maximums of the horizontal

gradient of the the total

magnetic field reduced to the

pole upward to 2 km

CAMEROON

Guinea Gulf

Maximums of tha analytical

signal ot the total magnetic field

upward to 2 km

CAMEROON

Guinea Gulf



4.2. Determination of the Structural Map

The objective here will be to determine all existing contacts like fault in the study area at 2 km depth.

The principle consists in superimposing the map of the maximums of the horizontal gradient of the

total magnetic field reduced to the pole upward to 2 km and that of the maximums of the analytic

signal of the total magnetic field upward to 2 km. The methodology used states that when the

maximums of the horizontal gradient and those of the analytic signal are superimposed, this gives the

position and extension of a fault. Also, when the maximums of the horizontal gradient are parallel and

slightly offset, in this case the maximums of the analytic signal represent the position of the fault and

those of the horizontal gradient the limit of the extension of the said fault. A lineament can be drawn

from topographic, hydrographic or geophysical information, or by photo-interpretation of aerial or

satellite images. The interpretation of topographic lineaments dates back to the early twentieth century

with the work of Hobbs (1904 and 1912) in New England. Empirical, spatial, and linear relationships

with several mines in different mining districts in the United States and Australia have also been made

(Marshall, 1978, O'Driscoll, 1990, Richards, 2000).

Figure 14 represents the map of superposition of the maps of Figures 12 and 13 and brings out

only linear contacts that can be linked to faults while Figure 15 represents the structural map of the Rio

Del Rey basin. This highlights all faults existing in the basin at an average depth of 2 kilometers and

these are in most case oriented along the major directions of fracturing observed during the qualitative

study.

Figure 14: Superimposition map of the maximum of the analytic signal of the total magnetic field upward to 2

km and the map of maximums of horizontal gradient of the total magnetic field anomaly reduced to

the pole upward to 2 km of the Rio Del Rey sedimentary basin

Maximums of the horizontal

gradient of the the total

magnetic field reduced to the

pole upward to 2 km

Maximums of tha analytical

signal ot the total magnetic field

upward to 2 km

CAMEROON

Guinea gulf


Figure 15: Interpretative structural map of the Rio Del Rey sedimentary basin

To justify the results obtained on the Euler deconvolution map, we have drawn up the rosace of

the fracturing directions (Figure 16) and calculated the contact directions of the said basin with respect

to the North azimuth which are represented in Table 1.

Figure 16: Rosace of the fracturing directions of the Rio Del Rey basin

Table 1: Principal contact directions interpreted as faults in the Rio Del Rey Basin

Failles Directions

L1 N62°E

L2 N140°E

L3 N110°E

L4 N75°E

L5 N78°E

L6 N70°E

Fault

Territorial limit

CAMEROON

Guinea Gulf



Failles Directions

L7 N71°E

L8 N66°E

L9 N111°E

L10 N103E

L11 N141°E

L12 N65°E

L13 N78°E

L14 N18°E

From these results, we can conclude that the influence of the opening of the Atlantic North on

the tectonics of our study area is very remarkable. In fact, it caused a fracturing of the Precambrian

basement along two major directions namely NO-SO and NE-SO. A comparison of the structural map

obtained with the geological map drawn up by Dumort in 1968 shows the existence of several faults

that were not visible from the methods of investigation used previously. In addition, we can observe

that these results corroborate those of the qualitative analysis and the Euler’s deconvolution.

4.3. Magnetic Anomaly Modeling

Magnetic modeling aims to determine the geophysical characteristics of the intrusive bodies

responsible for the magnetic anomalies observed in the study area. Three parameters are essential for

the realization of a geological model, namely: the magnetic susceptibility contrast, the depth at which

the anomaly is and its shape. Magnetic susceptibility is obtained from information on geology and

tables of average magnetic susceptibility values of rocks proposed in the scientific literature (Table 2).

The depth of the sources will be given by the spectral analysis and the information on the form of the

anomaly will be obtained after realization of the geological model.

Table 2: Mean magnetic susceptibilities of some rocks (Soulaimani et al., 2006, Telford et al., 1976)

Rocs Ranges of magnetic susceptibility

(x 10-6 CGS unit) Average magnetic susceptibility

(x 10-6 CGS unit)

Sandstone 0 – 1660 30

Gneiss 10 – 2000 130

Shale 25 – 110 120

Granite 0 – 4000 200

Basalt 20 – 14500 6000

Diorite 50 – 10500 7000

Pyroxene 10500

Peridotite 7600 – 15600 13000

For the rest, we will consider that the magnetization is perfectly induced and choose the profile

on the map of the anomaly of the total magnetic field reduced to the pole, because the anomaly is

supposed to be directly above the source. The positive magnetic anomaly observed on the map of the

anomaly of the total magnetic field reduced to the pole of the sedimentary basin of Rio Del Rey on

which is represented the P profile would be a basaltic intrusion in the Precambrian basement. We

attributed to this anomaly a magnetic susceptibility of 0.00985 cgs.

The spectral analysis of the sources of the anomaly around the P profile (Figure 17) on the map

of the anomalies of the total magnetic field reduced to the pole, shows that the roof of the magnetic

anomalies in this zone is about 5 km away.


Figure 17: Spectral analysis of the anomalies around the P profile represented on the map of the total magnetic

field reduced to the pole of the sedimentary basin of Rio Del Rey

The 2-D geologic model of the Rio Del Rey sedimentary basin along the P profile depicted on

the total magnetic field map reduced to the pole was obtained using Oasis Montaj's GM-SYS software.

This represented in Figure 18 has been calculated with the following physical parameters:

• Intensity of the geomagnetic field: 32700 nT (given by the IGRF);

• Inclination: -16,217 °;

• Declining: -5,402 °;

• Magnetic susceptibility: 0.00985 cgs.

Figure 18: Geological model of basaltic intrusion in the basement of the Rio Del Rey sedimentary basin. (a) -

Magnetic anomalies along the profile, (b) - geological model of terrain

These results show that the tectonics evolution of Rio Del Rey sedimentary basin since the

break-up of Pangea would have favored the establishment of a fault in the zone traversed by the profile

and this would have favored a rapid migration of the magma towards the Earth's surface, thus giving

rise to this basaltic intrusion observed on the map of the total magnetic field reduced to the pole.

base basaltic Intrusion base base base

M

ag

ne

tic

An

o

ma

lie

s

De

pt

h

(k

m)

Observed Calcukated Mean square error: 4.665

Ln Ln

DD

Wavenumber (km-1

)



5. Conclusion At the end of this work, the hypothesis of the perfectly induced magnetic anomalies made at the

beginning is justified with the result obtained on the geological model of ground made on the P profile

in the sedimentary basin of Rio Del Rey. The analysis of the map of the maximums of the horizontal

gradient of the field reduced to the pole and that of the maximums of the analytic signal of the total

magnetic field contributed to the realization of the structural map of the said basin and this shows a

predominance of the fracturing directions NO-SE and NE-SO. The spectral analysis of the anomaly

around the P profile depicted on the map of the total magnetic field reduced to the pole contributed to

the determination of a basaltic intrusion in the basement and shows that the tectonic activity was dense

in this region. Finally, the results of the quantitative analysis of field anomalies reduced to the pole are

similar to those of the qualitative analysis and justify the presence of basaltic pockets inside the

Precambrian basement.

The Rio Del Rey sedimentary basin acknowledgement study by the aeromagnetic prospecting

method led to the determination of its structural map between the Cretaceous and the date of data

collection and the realization of a geological model around a profile justified the presence of faults in

the basin. This map illustrates a predominance fracturing directions N60 ° E, N70 ° E, N110 ° E and

N140 ° E from which comes the capital interest in the search for hydrocarbons. This information is

necessary for the determination of its economic potential and optimization, but not sufficient. As

prospective, further studies are reqired to characterize it, but the aeromagnetic method remains the root

of any in-depth exploration study.

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