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Memorial University of Newfoundland (1995)

Groundwater investigations using geophysical techniques at Marophe, the Okavango Delta, Botswana

Laletsang, Kebabonye

Titre : Groundwater investigations using geophysical techniques at Marophe, the Okavango Delta, Botswana

Auteur : Laletsang, Kebabonye

Université de soutenance : Memorial University of Newfoundland (Canada)

Grade : Master of Science (1995)

Résumé
Results from a seismic reflection survey at Marophe, the Okavango Delta, are presented. The study aims to locate and characterize potential Karoo aquifers near Maun, as a supplement to the municipal water supply. The groundwater resources for Maun comprise the Upper Kalahari beds and alluvial sand aggregates. The Lower Kalahari formations contain saline groundwater. The shallow aquifers in the Upper Kalahari beds do not meet the present water demand, and are prone to nitrate pollution since they are unconfined and close to the town. Alternative groundwater resources are therefore presently being sought in the underlying Karoo strata. A variety of geophysical methods have been used to investigate the Okavango, particularly regarding the existence of Karoo beds. These studies have been inconclusive, in part due to the lack of stratigraphic boreholes required to calibrate potential field data. The present seismic reflection survey is a first attempt to use the seismic reflection method to investigate the Okavango region for hydrogeological purposes. — The geology of Ngamiland comprises Archean granitoid gneiss, the Damara metamorphic belt, and the Ghanzi-Chobe fold belt. Karoo rocks are inferred to overlie the Archean basement throughout the Okavango Delta. The geological structure was interpreted by re-analysis of the 1978 aeromagnetic data. — A seismic reflection survey was conducted along an existing cut-line 14.5 km west of Maun. The survey consisted of a symmetric split-spread with 48 recording channels at 9.0 m intervals and recording at 0.5 ms sampling rate. The signal source was a Betsy seismic gun firing 12 gauge shells at the ground surface. The data set collected during this survey is discussed in this thesis. — Refraction statics analysis of the field records reveal the presence of a 15 m unsaturated zone underlain by an impermeable layer. AVO inversion clearly images the base of the Kalahari beds, which manifests itself as a bright spot on the slacked section. Pseudo-acoustic impedance and velocity inversion of the bright spot show that 377 m of sediment exist south of the Kunyere fault. These comprise 191 m Kalahari and 186 m Karoo beds. To the north, the stacked time section indicates 570 m of sediment comprising 155 m Okavango beds, 230 m Kalahari, and 185 m of Karoo. The Kunyere fault zone comprises a number of faults with a total NW throw of 155 m. The basement at Marophe comprises a mafic igneous body. — The seismic survey has established that potential Karoo aquifers exist below Tertiary cover at Marophe but that no confining Karoo basalt occurs above the sediments. This allows direct recharge from the overlying Kalahari beds, but also salt contamination of the aquifer. The survey has also shown that the seismic reflection technique has direct applicability to groundwater investigations in the Okavango area.

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