Detection of clay filled sink-holes in the chalk by geophysical methods

Summary The significance of sink-holes in chalk to engineering structures is introduced and the available techniques for locating these features are briefly reviewed. Particular consideration is given to the rapid and relatively inexpensive geophysical techniques that are available. The main text is concerned with clay-filled sink-holes, which are common in chalk areas, particularly in Southern England. The site of a large engineering structure, where drilling had indicated the presence of clay infilled sink-holes, has been used to compare the appropriate geophysical methods, and also to develop suitable field procedures. It was found that the magnetic method could locate and delimit the clay filled sink-holes very rapidly. The central electrode configuration was the most suitable for electrical resistivity traversing. The cost and effectiveness of these geophysical methods, compared with conventional drilling, supports a combined approach to the site investigation.