Abstract:
A 2D controlled source shallow seismic refraction survey was carried out in Magadi basin in the southern part of Kenya to determine the characteristics of the low velocity layer. This involved mapping the thickness and corresponding velocities in the weathered zone which has considerable influence in processing and interpretation of deep seismic reflection data. The thicknesses and velocities are suitable for determining static corrections for deep seismic reflection surveys. The data was collected in eight profiles using 1C-24 channel geophones of 10 Hz at an interval of 3 km between stations whose spread length was 108 m at each station. Sledge Hammer acted as a source of seismic waves. Stack of 10-15 shots were made at each station for reverse and forward shooting to minimize background noise effect. Data was created on Vscoope database and exported to SEG-Y file which were transcribed to 2D Vista Seismic Processing software for first break picking. Picked first break time were loaded into spreadsheet where layers were picked and velocities calculated using time intercept method. A two layer model of the low velocity zone was obtained; the weathered zone and the semi-weathered zone. The third layer is a consolidated/ bedrock zone only observed from the velocities. Line 3 was observed to have the least thickness of approximately 16.0 m and while line 5 was the thickest averaging 34.4 m. The weathered zone of layer 1 of line 6 had the highest average velocity of 381 m/s while layer 1 of line 1 had the lowest velocity of 262.6 m/s. Layer 2 of line 2 had the highest average velocity of 1328.8 m/s while that of line 8 had the least average velocity of 878.2 m/s which all lie within the weathered zone of less than 1500 m/s. A qualitative interpretation of isopachs and isovel results show higher values of velocity and thickness in the north, north eastern and eastern region of study. Low velocity values are observed in the southern region of study. These are areas around Shompole and Nasurrana swamp as well regions towards Lake Natron. The thicknesses of low velocity layer are generally lower in the southern part but increase towards the north, around Lake Magadi and towards the east. It is observed that applied refraction statics due to low velocity layer is necessary for clear resolution of deep reflection seismic sections as processed seismic sections free of statics do not show clear structural and stratigraphic reflections seismic sections, while seismic reflected sections whose statics are applied show clear structural, stratigraphic and lithological sections. These results show that the weathering zone could result in adverse effect on deeper reflection seismic data since it exists in Magadi basin. Therefore, to map deep features for hydrocarbon location, it is recommended that static correction be applied when processing deep seismic reflection data in this region.
