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

Kebabonye Laletsang Lucky Moffat

doi:10.4172/2329-6755.1000175

Abstract

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.

Highlights

There have been reports by residents of Letlhakeng, Ditshegwane, Metsebotlhoko, and Sesung Villages, in the Kweneng District, that infrastructure in the area is being damaged by periodic ground movement which gives rise to large ground fissures
The ground fissures form during the dry seasons and prolonged droughts, and are closed by debris inflow during the wet seasons to become completely unrecognizable, only to develop again during the dry season
This paper presents the results of an investigation into the cause of the ground fissures in a regional geological context, using geophysical data acquired at different stages during localized seismic and ground penetrating radar surveys

Summary

Introduction

There have been reports by residents of Letlhakeng, Ditshegwane, Metsebotlhoko, and Sesung Villages, in the Kweneng District, that infrastructure in the area is being damaged by periodic ground movement which gives rise to large ground fissures. The infrastructure damage is incremented in the dry season until the structure fails completely, leading to closure and relocation of institutional buildings as it happened at the Mphu, the Junior Secondary School and the customary court building in Letlhakeng village [1] The cause of this destructive ground movement is largely unknown but suspected to be related to either active seismicity or expanding clay underlying the buildings. A number of geotechnical, geological, and geophysical investigations, Geotechnics International Botswana, Laletsang et al [1,2] have been commissioned by the local authorities to establish the cause of the ground fissures damaging institutional property such as the schools, roads, and tribal administration buildings (Figure 2) and to suggest possible remediation measures that can be adopted. The findings of this investigation are intended to assist in the design and subsequent monitoring of civil structures erected on the soils underlying the Letlhakeng village, which is a development planning area

Background

Discussion of Results

Conclusions