A geophysical investigation of the structural controls along the southern margin of Lake Ngami, northwestern Botswana using seismic refraction and DC resistivity

A shallow seismic refraction survey and DC Resistivity Soundings were undertaken at southeastern margin of the Palaeo Ngami Lake to determine the structure and shallow stratigraphy of the lake; determine the amount of fault displacement at the border fault of the lake. The results of the seismic refraction survey indicate that the there is a low velocity zone (~500m/s) in the area which is thinner outside the lake (less than 10m) and much thicker toward the centre of the lake (more than 30m).This low velocity zone is underlain by a higher velocity (3125 m/s) layer. Augering results within the lake indicate that the low velocity zone consists of dry clays, diatomaceous earth/silts. The high velocity layer is believed to be made up of sandstone/siltstones. Fault displacement at the margin of the lake is about 21m of downthrow. Another downthrow (about 19m) occurs at a distance of about 1300m from the margin into the lake, thus resulting in a total throw of about 50m. The results of the DC resistivity sounding indicates that the low velocity zone within the lake is characterized by 3 distinct layers; the top layer which is less than 1 m thick, has a resistivity of about 1300 Ωm (dry hard clays), the second layer about 5 m thick and has a resistivity of 34 Ωm (diatomaceous earth) and the third layer has a resistivity of about 11 Ωm (whitish soft clays interlayered with silty units).