Amagnetic study of the shallow structure in the olobanita wellfield lower baringo basin, kenya rift: implication on the hydrogeology of the basin

Abstract

This study focuses on the application of the magnetic method as a mapping tool for determining the depth of volcano-sedimentary formations and detection of subsurface structural features in Olobanita wellfield basin, Central Kenya rift. The frame work of the study looks at the prominent fault systems within the Olobanita wellfield basin and alongside the soft loose volcano-sedimentary, therefore this study is meant to establish subsurface shallow structures that are controlling and influencing the groundwater movement. The objectives of this study are to determine the extent to which the structures and depth to volcano-sedimentary formation influence the hydrogeology of the basin and map the shallow structural features beneath the basin. In total twenty one magnetic profile lines were established. Magnetic anomalies are particularly useful in the study of basement structures of sedimentary basins because sediments are usually weakly magnetic and do not contribute to the observed anomaly. The structure of the rift floor influences the axial geometry and the surface process. Simiyu and Keller (2001) interpret the rift floor structures due to thickening related to the pre-rift crustal type and processes. The methods used in this study are Euler deconvolution method and analytic signal. The Euler deconvolution was implemented for analysis of magnetic data to establish depth to magnetic sources, the effect of the complexity of the magnetic anomaly shapes has direct influence on the depth results. Analytic signal used to grid the magnetic data and revealed the amplitude of magnetization. Pronounced magnetic anomaly map was produced, the signal shape confirmed that the magnetic anomalies are caused by buried fault systems and the analytic signal map depicts some well oriented shaped anomalies related to the subsurface structures (faults). Interpretation of the results quantitatively and qualitatively revealed four normal faults, minor faults and fractures zone that dissect the study area in N-S direction. The depth to magnetic sources was established at depths ranges from 5m to 1000m bgl. The results obtained indicate possible deeper aquifers can be reach at depth of approximately 400m bgl. The geology of the Olobanita wellfield is not much different from rift valley areas and is comprises of Tertiary to Quaternary volcanic suites and sediments units intercalated within the succession of the Lakes bed and therefore the volcano- sedimentary formations are the main source of groundwater.