Fracture mapping in clays, using gas geochemistry

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

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Background, design of a mobile laboratory, and surveys in England and Italy
Nuclear energy and safety

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-THE-INFORMATION-SOCIETY-AND-MEDIA-DI
• Nombre de lectures: 11
• Langue: English
• Poids de l'ouvrage: 4 Mo

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Commission of the European Communities
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Fracture mapping in clays, using gas geochemistry:
Background, design of a mobile laboratory,
and surveys in England and Italy
Report
EUR 13150 EN £
Commission of the European Communities
nuclear science
and technology
Fracture mapping in clays, using gas geochemistry:
Background, design of a mobile laboratory,
and surveys in England and Italy
R. G. Gregory, G. A. Duddridge
Gas Geochemistry Unit, Department of Geology
The University of Exeter
North Park Road
Exeter EX4 4QE
United Kingdom
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Final report
Work performed under cost-sharing contract No FI1W/0088-UK
with the European Atomic Energy Community
in the framework of its third R&D programme on
'Management and storage of radioactive waste' (1985-89),
Part A, Task 4, 'Geological disposal studies'
Directorate-General
Science, Research and Development
EUR 13150 EN
1991 ^yfSo^c 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 92-826-2222-3 Catalogue number: CD-NA-13150-EN-C
© ECSC-EEC-EAEC, Brussels • Luxembourg, 1991
Printed in Belgium Executive Summary
This report covers the design and testing of a mobile gas geochemistry laboratory for the analysis
of soil gases and the results of surveys undertaken in England and Italy.
The migration of terrestial gases from within the Earth to the atmosphere takes place preferentially
along faults and fractures in the upper crust. Anomalous soil gas concentrations occur in the soil
above these faults and fractures. Such anomalies may occur even where the soil is developed on
an intervening layer of unconsolidated material, provided any lateral flow of groundwater through
this layer is not too rapid. The location of soil gas anomalies can therefore be used to determine
the position of faults and fractures which may be masked by a cover of young sediment or soil.
The detailed nature of the soil gas signature can identify relative permeabilities of the faults and
fractures, and reveal channelways of ultra-high permeability within such linear zones.
Although most soil gas anomalies are due to the presence of fracturing in the crust, other features
such as hidden cavities (man-made or natural) and the presence of coal mining and landfill sites
create additional problems in interpretation. These factors have been studied in trial surveys
to evaluate the technique, before applying the methodology devised here to specific sites for the
study of gas migration in fractured clay terrane.
The choice of gases to be analysed was based upon their origin, occurrence, and behaviour in the
soil environment. Gases selected for study were helium (4He), radon (220Rn and 222Rn), carbon
dioxide (CO2), oxygen (O2) and methane (CH4). These gases are suitable indicators of fracturing,
and their behaviour can be used to assess the relative permeability and fluid-bearing nature of
the fracture. In addition, they can provide information on biological activity in the soil and can
detect the presence of man-made or natural cavities. CH4 concentrations can highlight specific
problems in areas of coal-mining activity or at landfill sites.
The vehicle chosen for conversion to a mobile laboratory was a Renault Trafic T1000 petrol-
engined panel van. This represented the most cost-effective vehicle with regards to the role to be
fulfilled, namely that of a self-contained gas geochemistry laboratory.
The choice of soil gases to be analysed determined the layout of the laboratory and its fittings.
The heart of the laboratory is a 4He mass spectrometer. The support equipment and electrical
requirements of this analyser, including portable petrol generators and a fully-sealed transportable
liquid N2 dewar, established the space available for other fittings. The other on-site analysers for
Rn, CO2 and O2 were man-portable. These, and the sampling equipment required suitable storage
space only. Sufficient storage was available for the inclusion of other types of analytical equipment
as and when they were required or developed. Such developments involving gas chromatography
were planned for the analysis of CH4, which is currently carried out at Exeter.
The mobile laboratory has been fitted out with steel box-section frames and plywood and formica
panels to give a durable finish to storage cupboards and work surfaces. The spectrometer was sim­
ilarly supported, with isolating rubber shock-mountings for transit. All equipment not contained
within the lockable cupboards was securely fastened to the walls and floor. Storage of petrol to
power the portable generators was provided in a steel cabinet mounted on the full-length roof
rack.
The man-portable equipment was selected both for accuracy and for ease of routine use. Analysis
of Rn, as total Rn, and as the isotopes 220Rn and 222Rn, was achieved by a-particle scintillometry.
Analysis of CO2 utilised a portable infra-red spectrometer, while O2 was determined paramagnet-
ically. The equipment used for CII4 analysis was a laboratory-based gas Chromatograph equipped
with a flame ionisation detector.
Evaluation of the equipment selected for soil gas analysis was carried out at sites near Exeter
and were chosen to provide a diverse range of soil gas signatures. These related to fractured
III and mineralised terranes, rock cavities in limestone, and areas of intense coal mining activity.
The behaviour of the soil gases studied over these terranes supplements prior knowledge of gas
migration in fractures, and aids in the modelling of soil gas transport mechanisms.
At Exmouth (Devon), high total Rn activities clearly defined a set of fractures which indicated
potential back-scarps of future cliff-failures. At Gooseford (Devon),s were also observed.
These were both mineralised (Cu and As sulphides) and non-mineralised in nature. The presence
of shallow man-made exploratory workings can also be detected using soil gases, and these areas
can be differentiated from natural fractures. Results over the fractures were comparable for simple
fractures and mineralised ones, indicating that the processes responsible for soil gas anomalies
are not necessarily dependent upon mineral reactions in the fractures or soil, but do reflect the
reactivity of gases migrating along such fractures. Measurements at Buckfastleigh and Chudleigh
(Devon) extend the examination of cavities to natural features in limestone terrane. The pres­
ence of cavities in the rock underlying the soil cover introduces air with a composition closer to
atmospheric than that observed over solid rock. This appears to be due to infiltration, and can
be observed in the soil gas signature over the cavity, regardless of whether the cavity is artificial
or natural. The presence of coal seams at the surface has been detected in the overlying soil gas
using CH4 measurements at Rhôs-wen (Glamorgan), although as with all these procedures, it is
the relative variation which is important. Higher CH4 concentrations have been observed at sites
devoid of carbonaceous deposits.
Combination of soil gas measurements into an integrated sampling scheme shows that interpre­
tations made on the basis of a single gas measurement can be invalidated by comparison with
the variation of other gas species. Absolute soil gas variation is site-specific in nature, and is
dependent upon the soil and drainage conditions. Since these are in part allied to the presence or
absence of freely-draining fractures, it can be seen that the benefits of integrated soil gas surveying
are clear. Soil gas CO2 and O2 measurements can be used to highlight fracture positions with
ease. 4He and isotopie Rn activity variation describes the relative permeability of the channelway,
and its moisture-content. CH4 concentrations complement these results, and have specific uses in
areas of mining activity and landfill.
At Down Ampney airfield, Gloucestershire, England, a faulted sequence of Jurassic sediments
comprising Cornbrash (limestone), Kellaways Beds (sands and clays) and Oxford Clay (clay) is
covered by about 5m of Pleistocene gravel capped by lm of soil. In 1988 soil gas samples from
a depth of 0.5m were taken at intervals of 25m along one traverse and at 12.5m from a further
eight traverses, all but one running north-south. The samples were analysed for 4He, Rn, CO2
and 02- Anomalous concentrations of these gases were found in a number of linear zones trending
approximately east-west which define the position of fractures.
It was not possible with soil gas measurements alone to positively identify which of these fractures
exhibit vertical displacements, but the most significant feature occurs about 50m north of the
east-west runway. This is bounded by less well-develop e d features, so that the