ABSTRACT:

In Belgium, some part of the country is covered by chalky cretaceous formations surrounded by 6 to 10 meters of quaternary silts. At the top of the chalk, a thin (less than 2 meter thick) phosphate layer has been exploited as fertilizer for centuries. As a new High Speed Train line has to cross an hazardous area, a microgravity survey has completed the set of historical (old maps, mines administrations archives) and geotechnical investigations. The results consist of a 9 km long profile showing the variations of the gravity. A signal treatment method has been applied to distinguish between the various kinds of anomalies. Some of the local anomalies fit very well with the archives data, some other not. Based on this information, some additional surveys have been carried out confirming the conclusions. During the excavation works, a nice underground quarry has been brought to light, allowing its topographical survey. A mathematical model has been built to verify the good accordance between the measured anomalies and the theoretical anomalies caused by the observed underground quarry.

INTRODUCTION

The HST railway in Belgium crosses an area where the underground is composed of quaternary silts, with thickness ranging from about 6 to 10 m, overlaying flint conglomerate and Cretaceous chalk. Between the conglomerate and the top of the chalk is sometimes included a phosphate bearing layer which has been locally extracted in the past. The whole area is therefore subject to collapse phenomena due not only to the voids related to underground phosphate quarries, but also to the natural dissolution of the chalk. A microgravity survey has been performed in 1993 on the most hazardous 9 km in order to detect the lacks in density related to the above-mentioned facts.

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