A microgravity survey over deep limestone bedrock

Abstract

A microgravity survey had been carried out within a residential area. A total of 141 gravity values was obtained over an area of about 500 by 300 m 2 • These data were obtained along the roads within the study site. The boreholes within the area indicate that the area is underlain mainly by fine grained material namely silty clay, silty sand, clayey silt and sandy clay. Near the bedrock coarser material such as pebble and gravel may be present. The density of this clayey material is 1.4 to 1.8 gc/m3. The bedrock is limestone and has density of2.64 to 2.75 gc/m 3. From the borehole data the limestone within the study area is at about 27 to 33 m deep. A major problem within the study area is the presence of subsurface mass movement. This is indicated by the formation of large scale undulation on the ground both on the road and within the house areas and the cracks on the building structure including drains, walls and pillars. Both lateral and vertical movement occur on these structures indicating lateral and vertical mass movement. It has been suggested that collapsing of cavity roof within the limestone can be the cause. The microgravity survey was aimed at trying to determine the presence of the cavity. With the borehole data, the gravity modelling is very well constrained. The modelling indicates that the subsurface topography of the limestone is made up of a series of ridges and valleys. Possible occurrence of the cavity within the ridges has also been modelled.