Mapping Spontaneous Ground Fissures using Seismic Refraction, Reflection, and Ground Penetrating Radar in the Letlhakeng Area, Kweneng District, Botswana

摘要

Infrastructure in the Letlhakeng area is currently being damaged by ground fissures. Geophysical profiles of seismic refraction and reflection, and ground penetrating radar (GPR) were recorded at the areas affected by the ground fissures to map their depth extent and subsurface configuration. The GPR survey used 50 MHz antennas with an inline transmitter and receiver 2 m apart and a sample spacing of 30 cm. A reversed seismic refraction profile with two offset shots was recorded. Twenty four geophones were placed 2 m apart to make a total profile length of 46 m. The seismic reflection data were recorded with an equal source and receiver interval of 1 m and a total spread length of 23 m across a ground fissure. The GPR profile shows that the ground fissures are sub-vertical and extend to a vertical depth of ~5.0 m. The seismic refraction profile shows a two layer velocity model comprising overburden with a seismic velocity of 400-600 m/s lying over a substratum with a seismic velocity of 1200-1400 m/s. The top layer consists of the loose sand which is imaged to a depth of 5 m. The seismic reflection profile shows that the ground consists of subhorizontal layers from 0-15 m depth, which have all been broken by the ground fissure observed at the surface. The geophysical results show that the ground fissures persist from the surface to depths of 15 m.