The application of the seismic refraction technique to the study of the fracturing of the Middle Chalk at Mundford, Norfolk

Abstract

Variations in seismic velocity with depth have been measured in the Middle Chalk at the proposed 300 GeV proton accelerator site at Mundford, Norfolk, where they could be compared with the results obtained from a geological and geotechnical survey. It is shown that the seismic velocity increases with depth in clearly defined steps, and that these steps broadly correspond with some of the grades in the engineering classification adopted by the geotechnical survey. Correlation has also been obtained between the seismic results and the thickness of the overlying sandy drift, and with tectonic structures in the chalk. On a mesuré les variations de la vitesse de propagation des ondes sismiques en fonction de la profondeur dans le Calcaire Moyen du site qui a été proposé à Mundford (Norfolk), pour l'accélérateur de protons de 300 GeV où on pouvait les comparer aux résultats obtenus à partir des études géotechniques et géologiques. On montre que la vitesse de propagation augmente en fonction de la profondeur suivant une série de valeurs bien définies, et que ces valeurs correspondent approximativement à certaines catégories résultant de la classification visuelle de l'étude géotechnique. On a également obtenu une corrélation entre les résultats des vitesses de propagation et l'épaisseur de la couche détritique sableuse de surface, ainsi qu'avec les structures tectoniques du calcaire.

Setting the standard for geophysical surveys in site investigation

“…The seismic refraction method is widely used in site investigation. A typical example of its use in the study of the continuity of geological structure was given by Grainger et al (1973), who described a seismic refraction survey carried out in the Middle Chalk at Mundford, Norfolk, UK. They showed that the compressional wave velocity increased in well defined steps, that broadly correlated with the rock mass classification system adopted by Ward et al (1968).…”

“…The correlation was established in an area of lithologically uniform chalk on seismic lines that had good borehole control, both to classify the rock mass in engineering terms from the seismic classification, and to study the continuity of the seismic section. Grainger et al (1973) emphasized that the seismic classification was only applicable in the specific lithological section for which it was derived and should not be extrapolated outside the immediate calibrated area. Cratchley et al (1976) carried out a detailed seismic refraction survey along the line of a tunnel constructed as part of the Foyers pumped storage hydro-electric scheme near Inverness, Scotland.…”

Geometry of Quaternary sediments along the north Norfolk coast, UK: a shallow seismic study

“…The lowest velocity claimed for the unit is 1800 m s -1 at Holkham, where part saturation is another possible explanation for the velocity. Chalk at Mundford in Suffolk (Grainger, McCann & Gallois, 1973) shows velocities from 700 to 2300 m s -1 depending on these factors and on fracturing, although below the water table the velocities were in excess of 1700 m s -1 .…”

“…It has been estimated to be up to about '50 feet' thick elsewhere in north Norfolk (Woodward, 1884). A study at Mundford in Suffolk (Grainger, McCann & Gallois, 1973) showed that the chalk can have a wide range of velocities depending on saturation, weathering and jointing. Seismic basement is assumed to be at the top, or close to the top, of the Chalk, but the large chalk rafts seen in the cliffs of northeast Norfolk (Chatwin, 1961;Hart & Boulton, 1991) include (apparently) fresh rock, and it would not be possible to distinguish the top of a raft from true basement in a refraction survey.…”

Field testing methods for engineering geological investigations

“…The compressional wave velocities may be raised appreciably by the presence of water (Grainger et al 1973). D' Andrea et al (1965) compared the compressional wave velocities measured for a range of American rocks with their uniaxial compressive strengths.…”