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The location of the Stour buried tunnel-valley using geophysical techniques; discussion and reply

J. D. Cornwell and R. D. Barker
The location of the Stour buried tunnel-valley using geophysical techniques; discussion and reply
The Quarterly Journal of Engineering Geology (1986) 19 (2): 207-208

Abstract

In their use of geophysical methods to supplement sparse borehole data for locating a buried tunnel-valley in the Chalk of East Anglia, the authors include an interesting application of detailed gravity surveys. The thick boulder clay in-fill of the narrow valley gives rise to small Bouguer anomaly highs and a density contrast is derived of +0.26 t/m (super 3) between this material and the Chalk. The sand and gravel in-fill is concluded to have a negligible density contrast. The authors then interpret gravity profiles over parts of the tunnel-valley with both types of in-fill, by adopting an "effective density contrast" weighted according to the proportion of boulder clay present in adjacent boreholes. This approach to the gravity modelling of the overall form of an anomalous multi-component body is, however, only valid if the dimensions of the individual components of the body are significantly smaller, perhaps by an order of magnitude, than that of the body itself. On the evidence of Fig. 2 in the example described, the "effective density contrast" appears not to be valid for the whole tunnel-valley, as there is only one horizon of boulder clay and, at depth in the tunnel-valley, one horizon of sand and gravel in both the Cavendish and Glemsford boreholes. If the sand and gravel has no density contrast with the Chalk then it is surely not possible to derive a model from the gravity data for the bottom part of the valley, below the boulder clay. The adoption of an incorrect density contrast additionally ... This 250-word extract was created in the absence of an abstract.


ISSN: 0481-2085
Coden: QJEGA7
Serial Title: The Quarterly Journal of Engineering Geology
Serial Volume: 19
Serial Issue: 2
Title: The location of the Stour buried tunnel-valley using geophysical techniques; discussion and reply
Affiliation: Br. Geol. Surv., Keyworth, United Kingdom
Pages: 207-208
Published: 1986
Text Language: English
Publisher: Scottish Academic Press; Geological Society of London, Edinburgh, United Kingdom
References: 2
Accession Number: 1989-005501
Categories: Applied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: For reference to original article by Barker, R. D. and Harker, D., see Q. J. Eng. Geol., London, Vol. 17, p. 103-115, 1984
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 1989
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