Application of Geo-Electrical Resistivity and Gravity Methods for Geo-Hydrological Investigation in Matuu: Machakos County, Kenya

Abstract

In Kilango-Matuu area, ground water potential zones cover a small area because the region receives low amounts of rainfall of about 800 millimeters (Approximately) annually. Hard rocks dominate the area that result to small region ground water potential zones. In regions with such characteristics, chances of ending up with a dry well are high when drilling for ground water before carrying out correct and accurate preliminary processes. In this research, the main aim was to delineate ground water potential zones within Kilango Matuu–Machakos. In order to achieve this, a proper understanding of the subsurface formation and faults/fractures, which are the main ground water conduits, was inevitable. Therefore, Geo-electrical technique and gravity methods were deployed over a 25 square kilometer area. Subsurface water bodies conductivity measurements were taken as well to help in determining the porosity of the region. Azimuthal square array was deployed and data obtained was utilized in generating a graph of the apparent anisotropy versus the bedrock anisotropy, which displayed a slight change in bedrock anisotropy against a significant change in apparent anisotropy. This array required just 35% of the surface area used for an equivalent survey using other geo-electrical resistivity arrays. These two advantages justified the choice of this array over the most commonly utilized Wenner and Schrumberger arrays. Graphical and analytical analysis displayed presence of a fracture within the porous zone oriented along NE-SW direction. The crossed square array data detected a fracture strike along 0390 azimuth. In summary, Kilango-Matuu region has a possibility of having several fractures with the main one being oriented along 049.566220 orientation. The geometric resistivity values of interest were qualitatively analyzed and a low resistive and a less porous zone was detected in the western part of the study area. Computed porosity had values ranging from 0.1648 to 0.2922. In gravity survey, 156 stations were established and corrections were carried out on the observed gravity values. A bouguer anomaly contour map was generated which indicated presence of a subsurface formation with low density along the western side of the study area. A cross-section AA’ was cut across the zone of interest and imported to Euler software and Euler deconvolution was done, which indicated a discontinuity covering about 600 meters distance. In order to understand the stratigraphy of the study area, the data obtained from the cross-section AA’ was uploaded on Grav2dc and a 2D forward model was attempted. Results from this study indicate that, Matuu-Kilango area has a fractured basement rock, which is capable of holding and transmitting ground water.