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

Abstract

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.

The geophysical investigation of Quaternary geological environments

“…For example the often quoted illustration that pocket deposits in chalk create detectable magnetic anamolies (McDowell 1975) does not mean that all clay-withflints deposits in chalk are magnetically distinct from the host rock.…”

A Hoxnian interglacial doline infilling at Slade Oak Lane, Denham, Buckinghamshire, England

“…Apparent resistivity is not only effected by lithology but also by other variables such as degree of water saturation and pore water salinity. Despite these limitations McDowell (1975) has successfully used this technique to delimit clay filled sinkholes on Chalk.…”

“…Clay-filled depressions in Chalk are usually recognized by their anomalously low resistivity (McDowell, 1975). However, the resistivity contrast between sandy gravels and chalk is likely to be considerably less.…”

“…Small-scale solution features such as swallow holes and pipes are very common in areas of Chalk outcrop (Jones, 1981, pp. 148-51;Edmonds, 1983;McDowell, 1975;Wilmot & Young, 1985;Chartres & Whalley, 1975). They have been found close to the study area by Hare (1947) around Burnham Beeches (8.75 km to the west), by Higginbottom & Fookes (1971) between High Wycombe and Beaconsfield (9-12 km to the northwest), by McGregor & Green (1983) at Gerrards Cross, and from beneath fluvial gravel and sand at Gerrards Cross (4.5 km to the west) and similar aggradations throughout the area including the Gerrards Cross member by Gibbard (1985) and by Hawkins (1953).…”