Research On The Interpretation Methods Of Remote Sensing,geoelectric,and Aeromagnetic Data For Delineating Hard Rock Aquifers In The Adamawa Region Of Cameroon

In the past,exploitation of water resources in hard rock aquifers was almost impossible due to their inability to store massive quantities of groundwater.This is partly due to a lack of understanding of the hydrogeological setting and geology of the target area,as well as a lack of appropriate techniques for locating large volumes of groundwater.However,this water resource is associated with geological discontinuities(fractures,faults,joints,dykes,and geological contacts)resulting from tectonic activities and weathering processes.Determining the proportion of groundwater in storage and the conditions under which it can be exploited requires knowledge of the geometry and distribution of discontinuities in a given area and the frequency of their occurrence.Recent advances in science and technology have led to the development of a variety of techniques for detecting discontinuous zones in hard rocks,including the combination of two or more techniques.This dissertation systematically examines the detection of these geological discontinuities using an integrated approach combining remote sensing,aeromagnetic,and geoelectric methods.Below are the main contributions and innovations of this work:(1)Using the lineament extraction technique,the remote sensing data reveal the topographic lineaments along which the rivers flow in the area.The distribution of these lineaments according to the lineament density map indicates the degree of permeability or porosity of each region and thus its capability in terms of groundwater potential.The directional analysis of the extracted lineaments shows that the major faults are in the directions of N135 E,N20-30 E,N50-70 E,and N45 E,while the minor faults are in the directions of N-S,N155 E,and N105E(E-W).According to the statistical analysis,their minimum and maximum lengths range from 1.01 to 4.77 km,with an average length of 2.48 km.It was found that the orientation of these lineaments is related to the tectonic deformations in the region as well as to the groundwater flow in the study area.(2)The filtering of the aeromagnetic data allows the construction of a structural map showing different geophysical lineaments interpreted as the fault systems of the study area.Forward magnetic modelling suggests that these faults are the main channels through which magnetized granitoid-type rocks have intruded into the basement and groundwater flows into the subsurface.The orientations of these geological structures correspond to the major axes of the CCSZ and CVL in the AYD.The Euler deconvolution filter indicates that the depth of these lineaments ranges from 200 m to 900 m.The magnetized rocks consist of quartz monzonites,nepheline syenites,and monzodiorites and show a strong magnetic response.(3)The local application of the geoelectric method shows that the study area is highly fractured/faulted and consists of numerous lithological units,including two aquifer layers(clayey sands and fractured schists)with thicknesses of 5 to 15 m and34 to 82 m,respectively.The upper aquifer(clayey sands)is located within the saprolitic horizon,while the lower aquifer(fractured schists)is located within the fissured zone.Two-dimensional geoelectrical cross-sections and three-dimensional modelling show that the subsurface structures consist of horsts,followed by progressive subsidence represented by the NE-trending normal faults towards the southern depression,and grabens bordered by NW-and NE-trending normal faults.The horst structures are most likely the result of compressive stress effects from E-W to NW-SE,which led to the formation of the NE fault.This fault would thus be responsible for the northward uplift of one of the compartments of the zone that led to the identified horst structures and for the probable formation of a basin or graben by the southward subsidence of the other compartment.In the context of the hard rocks in the Adamawa region,these prominent structures(faults/fractures,horsts,and grabens)are considered typical aquifer models.This dissertation includes 51 figures,9 tables,and 347 references.