Reconnaissance geophysics to locate major faults in clays

European Commission Directorate-General For The Information Society And Media Directorate-General For Research And Innovation

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Nuclear energy and safety

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 10
• Langue: English
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Commission of the European Communities
nuclear science
and technology
Reconnaissance geophysics to
locate major faults in clays
Ü 1
MÊ:
f
Report
EUR 13203 EN o
Commission of the European Communities
nuclear science
and technology
Reconnaissance geophysics to
locate major faults in clays
P. D. Jackson,1 J. R. Hallam,2 M. G. Raines2
2
M. P. Rainsbury,1 P. G. Greenwood,2 J. P. Busby
1 Regional Geophysics Research Group
2 Engineering Geologyhp
British Geological Survey
Keyworth
Nottingham NG12 5GG
United Kingdom
Contract No FI1W/0085
Topical report
Work performed under contract in the framework of the
R&D programme on management and storage of radioactive waste
(shared-cost action) of the European Atomic Energy Community
PASRL. EUROP. Biblioth
Directorate-General NC./COM
Science, Research and Development
iipioorw«!-ffe
1991 Published by the
COMMISSION OF THE EUROPEAN COMMUNITIES
Directorate-General
Telecommunications, Information Industries and Innovation
L-2920 Luxembourg
LEGAL NOTICE
Neither the Commission of the European Communities nor any person acting
on behalf of then is responsible for the use which might be made of
the following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1991
ISBN 92-826-2344-0 Catalogue number: CD-NA-13203-EN-C
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1991
Printed in Belgium PREFACE
Clays are being considered as potential hosts for radioactive waste repositories in a number
of countries. Two physical properties of clays have stimulated this interest. Firstly clays
generally have very low hydraulic conductivities and consequently small advective
groundwater flows; secondly clay minerals are considered to sorb radionuclides strongly.
Faults are known to occur in most sedimentary environments, but the extent to which such
faults change the physical properties of the sediments, and the clays in particular, is poorly
understood. Potentially, faults could act as near vertical conduits for direct groundwater flow
from the repository zone to the surface, effectively forming a short circuit in a larger, less
active, regional groundwater flow system. The role of clays as barriers to fluid migration also
has important implications for hydrocarbon exploration because the sealing properties of clay
cap rocks may be reduced or enhanced due to faulting.
The British Geological Survey, in cooperation with ISMES of Italy, is carrying out a
research programme into the properties of faults cutting clay formations. The programme has
two major aims; first, to develop geophysical techniques to locate and measure the
geophysical properties of a fault in clay; second, to measure the hydrogeologicai properties of
the fault and its effect on the groundwater flow pattern through a sequence of clays and
aquifers. The research programme commenced in April 1986 and work started at the Down
Ampney site in February 1987. This is one in a connected series of topical reports on the
research carried out at Down Ampney and further reports will be issued from time to time.
Previous reports in this series are listed below :-
1. Geological sequence at the Down Ampney Fault Research Site. A. Horton, K. Ambrose
and B.M. Cox. Report of the British Geological Survey, No. WE/89/7.
Ill The location of the Down Ampney site and a plan of the boreholes at the research site.
- IV -CONTENTS
Page
PREFACE III
EXECUTIVE SUMMARY VI
LIST OF FIGURES X
1. INTRODUCTION 1
1.1 Objectives
1.2 Background
2. GEOLOGICAL SETTING OF TWO POTENTIAL SITES 2
2.1 Selection criteria 2
2.2 Down Ampney (Wilts) 3
2.3 Wombleton (N. Yorks.)
3. DESIGN OF THE GEOPHYSICAL SURVEYS 4
3.1 Technique selection
3.2 Design of electrical resistivity survey 5
3.3 Theoretical modelling of proposed electrical
resistivity survey 6
3.4 Other first phase reconnaissance methods 9
4. INITIAL GEOPHYSICAL RECONNAISSANCE OF BOTH SITES
4.1 Down Ampney Airfield
4.1.1 General 9
4.1.2 Geophysical fieldwork 10
4.1.3 Geological interpretation2
4.2 Wombleton (N. Yorks.)
4.2.1 . General
4.2.2 Geophysical fieldwork3
4.2.3 Geological interpretation5
4.3 Comparison of the two sites
- V 5. MAIN GEOPHYSICAL RECONNAISSANCE SURVEY AT DOWN AMPNEY 17
5.1 Survey objectives 1
5.2 Progression of field survey7
5.3 Two-dimensional numerical modelling 19
5.4 Depth interpretation of traverses 20
5.5 Survey assessment 21
6. CONCLUSIONS AND SUMMARY2
7. ACKNOWLEDGEMENTS3
8. REFERENCES
VI EXECUTIVE SUMMARY
Clay formations have been identified by many countries as potentially suitable for the siting
of radioactive waste repositories. Whilst faults are known to occur in most sedimentary strata
environments, the extent to which the hydrogeologicai properties of the sediments and the
clays are changed in the vicinity of such faults in particular is poorly understood. The work
reported here forms part of a research programme to study the hydrogeological implications
of faulting in clay formations.
The detection of faulting in low-lying clay vales, frequently covered with a veneer of recent
superficial deposits, is virtually impossible by conventional geological field mapping. The
use of boreholes alone would be prohibitively expensive.
One important objective of the programme is to establish an efficient geophysical
methodology for the detection and delineation of such faults and the measurement of their
displacements, to an accuracy adequate for their detailed investigation by borehole drilling.
The geophysical reconnaisance, which is reported here, falls into two parts :-
1. The appraisal of two short listed sites to contribute to the final site selection,
together with an initial evaluation of the proposed geophysical techniques.
2. An extensive survey of the chosen site to select the general drilling location, with a
refinement of the geophysical methodology.
The two sites short-listed, at Down Ampney in South Gloucestershire (Oxford Clay) and
Wombleton in the Vale of Pickering, North Yorkshire (Kimmeridge Clay), had been selected
on geological, hydrogeological and logistical criteria, in the expectation of being able to locate
a fault with a minimum clay thickness of 30 metres on the upthrow side, a minimum
displacement of 20 metres and a maximum drilling requirement of 100 metres, to the
underlying aquifer.
Faulting causing the displacement of plane interfaces between two regions of differing
physical properties may be detected by geophysical techniques. The techniques should be
sensitive to the property changes as part of a three-dimentional geophysical target. For
example a very small relative displacement of an interface may not be detectable even though
VII there is an order of magnitude change in properties. The geological setting at both Down
Ampney and Wombleton indicated a relatively large change in clay thickness overlying
limestone, with the expected throw of the fault equal to or greater than the thickness of clay
on the upthrown side. These relatively large throws could be detected by both seismic and
electrical resistivity methods because the contrast in properties is very large in both cases,
being two-fold and ten-fold respectively.
The primary geophysical technique, Schlumberger resistivity traversing, modified as
necessary, was selected on the grounds of high lateral resolution, logistical efficiency and its
capability of almost immediate field interpretation and appreciation as the survey progressed.
Schlumberger resistivity sounding, shallow seismic refraction and electromagnetic profiling,
were available as secondary techniques.
An extensive search was made for existing geological, geophysical and borehole data in the
environs of each site. This data was used to define the survey areas, to optimise the survey
techniques and to aid interpretation of the results.
For each site a profile of anticipated geological formations, thicknesses and resistivities was
compiled. From these, computer models were generated to estimate the composite field
resistivity response ('apparent resistivity') for the rninimum and maximum clay thicknesses
anticipated at each site. This confirmed that at both sites a resistivity spread length ('AB') of
approximately 300m would maximize the resistivity contrast from a probable fault and would
be insensitive to variations in any superficial formations. Graphs were also prepared for the
instant conversion of field readings to approximate clay thicknesses.
For the appraisal survey at Down Ampney, the main traverse line of c.1500 metres was
orientated across the anticipated direction of faulting, at each end terminating near an existing
borehole. The resistivity traverse revealed a very marked anomaly, coincident with an abrupt
change in clay thickness from about 30 to 50 metres, which was confirmed by seismic
refraction. A less marked anomaly was detected by electromagnetic profiling at the same
location. A second parallel line, alongside the perimeter road of the disused airfield was
degrad