Magnetic data analysis of shallow geological structures using selected magnetic filters and comparison of the results: Baharini well field, lake Nakuru basin, Rift valley, Kenya.

Abstract

Lake Nakuru is a closed hydrological basin situated in the floor of Rift Valley which is about 160 km from the Kenya’s capital city; Nairobi. Baharini well field is located north of Lake Nakuru which also harbors a national park of international importance. Since there is no sufficient supply of rainfall in the area, residents rely greatly on groundwater for sustenance. With increasing population, more efforts and resources are being focused on groundwater exploration and development. Due to land use changes that have occurred in the basin over the past four decades resulting into microclimate, there is need for more information on the well field. The aim of the project is to determine the shallow structures in the volcano-sedimentary formation that influence hydrology in Baharini area of Lake Nakuru drainage Basin. This further helped in delineating areas susceptible to pollution and subsidence. Geophysical technique (magnetic method) was used in the study of these structures. A total of 12 magnetic profiles were taken. Magnetic anomalies are useful in the study of basement structures since sediments are weakly magnetic and do not contribute to the observed anomalies. The anomalies are caused by subsurface structures such as buried faults, fissures. Rift floor structures are due to thickening due to pre-rift crustal types and processes. Reduction methods used were first vertical derivative and upward continuation, analytical signal and reduction to the pole. Each pair was compared thereafter. The programs used for the filtering are Lahey ED developer and oasis montaj. Residual magnetic data were corrected for diurnal variation using Lahley ED developer program. Total magnetic intensity map was then plotted. First vertical derivative and downward continuation anomaly maps were constructed analyzed and compared. Analytical Signal and reduction to the pole pair was plotted and their anomaly maps compared. Downward continuation filter calculate the magnetic field with the measurement plane closer to the sources. Anomalies result in less spatial overlap and thus are more easily distinguished from the other due to increased amplitude of shallow sources. The first vertical derivative computes the vertical rate of change in the magnetic thereby suppressing the long wave lengths. Reduction to the pole filter reconstructs the magnetic field of a data set as if it were at the pole. This means that the data can be viewed in map form with a vertical magnetic field inclination and a declination of zero. Analytical Signal filter works on the phase as well as the amplitude the power spectrum of the reduced field and the shape of the amplitude portion of the filter is not very useful but the resulting magnetic field map is. The resulting shape of the analytic signal is independent of the orientation of the magnetization of the source and is centered on the causative body. Stratigraphically, old rocks are characterized by thick layers of loose sands of volcanic origin overlaid by tough volcanic with few bands of loose sediments. The youngest is composed of volcani-clastic sediments and recent soils. The project helps in understanding the rest of the basin. For integrated management of water in the basin and environmental conservation, it is important to understand the fault system to ensure that solid waste, waste water and hazardous pollutants from industries are not disposed near fault zones or permeable zones. The contaminants may end up in groundwater pollution. Contamination impact is very expensive and irreversible. Structural studies should be done regularly using other geophysical techniques that penetrate deeper for effective groundwater protection.