Electrical Resistivity Tomography (ERT) is a rapidly developing geophysical imaging technique that is now widely

used to visualise subsurface geological structure, groundwater and lithological variations. It is being increasingly used

in environmental and engineering site investigations, but despite its suitability and potential benefits, ERT has yet to

be routinely applied by the minerals industry to sand and gravel deposit assessment and quarry planning. The

principal advantages of ERT for this application are that it is a cost-effective non-invasive method, which can provide

2D or 3D spatial models of the subsurface throughout the full region of interest. This complements intrusive sampling

methods, which typically provide information only at discrete locations. Provided that suitable resistivity contrasts are

present, ERT has the potential to reveal mineral and overburden thickness and quality variations within the body of

the deposit.

Here we present a number of case studies from the UK illustrating the use of 2D and 3D ERT for sand and gravel

deposit investigation in a variety of geological settings. We use these case studies to evaluate the performance of ERT,

and to illustrate good practice in the application of ERT to deposit investigation. We propose an integrated approach

to site investigation and quarry planning incorporating both conventional intrusive methods and ERT

Aumonier, J.

Bailey, E.

Chambers, J.E.

Griffiths, N.

Kuras, O.

Matthews, B.

Meldrum, P.I.

Ogilvy, R.D.

Penn, S.

Wardrop, D.

Weller, A.

Wilkinson, P.B.

English

NERC Open Research Archive

CHARACTERISING SAND AND GRAVEL DEPOSITS USING ELECTRICAL RESISTIVITYTOMOGRAPHY (ERT): CASE HISTORIES FROM ENGLAND AND WALESJ.E. CHAMBERS1, P.B. WILKINSON1, A. WELLER1, P.I. MELDRUM1, O. KURAS1, R.D. OGILVY1, J. AUMONIER2,E. BAILEY3, N. GRIFFITHS4, B. MATTHEWS5, S. PENN6, AND D. WARDROP71British Geological Survey, Keyworth, Nottingham, NG12 5GG.2MIRO, Concorde House, Trinity Park, Solihull, Birmingham, B37 7UG.3Aggregate Industries, Bardon Hall, Markfield, Leicestershire, LE67 9PJ.4Hanson, Ashby Road East, Shepshed, Nr Loughborough, Leicestershire, LE12 9BU.5Tarmac Limited, Stancombe Lane, Flax Bourton, Bristol, BS48 3QD.6Cemex UK Operations, Wolverhampton Road, Oldbury, Warley, West Midlands, B69 4RJ.7Lafarge Aggregates Ltd, Panshanger Park, Hertford, Herts., SG14 2NA.INTRODUCTIONElectrical resistivity tomography (ERT) is a groundimaging tool, which can complement conventional pointsampling approaches by providing fully volumetricinformation on ground conditions. Todate, ERT has notbeen routinely applied by the minerals industry to sandand gravel deposit investigation due, in part, tounfamiliarity with the technique and a lack ofdemonstration studies. Here we describe a programme ofresearch, which was established to begin the process ofdeveloping ERT specifically for sand and gravel reserveassessment. Our principal aims and objectives have been to:ABSTRACTElectrical Resistivity Tomography (ERT) is a rapidly developing geophysical imaging technique that is now widelyused to visualise subsurface geological structure, groundwater and lithological variations. It is being increasingly usedin environmental and engineering site investigations, but despite its suitability and potential benefits, ERT has yet tobe routinely applied by the minerals industry to sand and gravel deposit assessment and quarry planning. Theprincipal advantages of ERT for this application are that it is a cost-effective non-invasive method, which can provide2D or 3D spatial models of the subsurface throughout the full region of interest. This complements intrusive samplingmethods, which typically provide information only at discrete locations. Provided that suitable resistivity contrasts arepresent, ERT has the potential to reveal mineral and overburden thickness and quality variations within the body ofthe deposit.Here we present a number of case studies from the UK illustrating the use of 2D and 3D ERT for sand and graveldeposit investigation in a variety of geological settings. We use these case studies to evaluate the performance of ERT,and to illustrate good practice in the application of ERT to deposit investigation. We propose an integrated approachto site investigation and quarry planning incorporating both conventional intrusive methods and ERT.Chambers, J.E., Wilkinson, P.B., Weller, A., Meldrum, P.I., Kuras, O., Ogilvy, R.D., Aumonier, J., Bailey, E.,Griffiths, N., Matthews, B., Penn, S., Wardrop, D., 2011. Characterising sand and gravel deposits using electricalresistivity tomography (ERT): case histories from England and Wales. Pp. 166-172 in Hunger, E. and Walton, G. (Eds.)Proceedings of the 16th Extractive Industry Geology Conference, EIG Conferences Ltd, yyypp.e-mail: jecha@bgs.ac.uk• design and carry out controlled field-tests at well-characterised sites to prove the suitability of ERT as arapid, cost effective tool for non-invasive imaging ofsand and gravel deposits,• determine how ERT methods can be integrated withconventional site investigation methodologies,• estimate the likely cost benefits of using ERT,• develop good practice guidance for the use of ERT,• facilitate knowledge transfer and raise awarenesswithin the minerals industry of the benefits of applyERT technology.Characterising sand and gravel deposits using electrical resistivity tomography (ERT)166J.E. Chambers, P.B.Wilkinson, A.Weller, P.I.Meldrum,O. Kuras, R.D.Ogilvy, J. Aumonier, E. Bailey, N.Griffiths, B.Matthews, S. Penn,D.Wardrop167Figure 1. Schematic illustration showing the basic components of an ERT imaging survey, and targets associated with sand and graveldeposits. A 3D survey comprises a network of survey lines (i.e. a grid of electrode positions), whilst a 2D survey is carried out using a singleline of electrodes. ERT has the potential to distinguish between mineral, bedrock and overburden, identify quality variations in the mineral(e.g. high concentrations of fines), and monitor the level and quality of groundwater.OVERVIEW OF ELECTRICAL RESITIVITY TOMOGRAPHY(ERT)ERT is used to generate models of subsurface electricalproperty distributions, from which subsurface geologicalstructure and hydrogeological variations can be identified(Chambers et al., 2002; Kuras et al., 2008). This techniqueis analogous to medical imaging techniques, such as MRIand CT, which are used to image the internal structure ofthe body. The principal benefits of ERT are that it is arapid non-invasive method, which employs highlyportable lightweight field equipment (Figure 1), that canprovide fully 2D & 3D spatial models of the subsurface atthe site scale. This is in contrast to intrusive samplingmethods, which typically provide information only atdiscrete locations. ERT is sensitive to compositionalvariations in the subsurface, and can therefore be used todistinguish between different lithologies, e.g. cleangravels (high resistivity) and clay (low resistivity) bedrockor overburden, and can also be used to spatially map 3Dquality variations within the mineral deposit (e.g.resistivity typically reduces with increasing clay content).When used in time-lapse mode (i.e. repeatedmeasurements) ERT is very sensitive to changes inmoisture content (i.e. resistivity decreases with increasingmoisture content) and the electrical conductivity ofgroundwater (which is usually a good measure of waterquality). Due to the high temporal and spatial resolutionof the technique it can be used to monitor not only slowseasonal changes, but also rapid changes, includingdrawn down surfaces that could be associated withquarry dewatering.One of the earliest references to the use of 2D ERT forsand and gravel resource studies is by Barker (1997), inwhich he describes a survey from the Trent Valley, UK.Other published examples are rare, but include Beresnevet al. (2002), Hill (2004) and Lucius et al. (2006). Thereare currently no published examples of the use of 3DERT for this application.CASE STUDIESA set of 18 field sites for controlled testing werechosen to include a range of geological settings andgeological complexity (Figure 2). Most of the sites wereassociated with existing sand and gravel quarries and allwere well characterised with ground truth data (e.g.boreholes, trial pits) used to calibrate and assess the ERTmodels.2D ERT2D ERT surveys were undertaken at thirteen sites,including Washington (West Sussex), Clifton(Worcestershire), Blashford (Hampshire) and Borras(Flintshire). These surveys were intended to demonstrate2D ERT as a rapid sand and gravel reconnaissance tool,and to extend our knowledge of the electrical propertiesof the deposits and bedrock in these areas. In all cases2D ERT was successful in imaging the sand and graveldue to the clear resistivity contrasts between the sand andgravel, overburden and bedrock materials (Figure 3). Thebedrock materials in each case were more clay rich thanCharacterising sand and gravel deposits using electrical resistivity tomography (ERT)168Figure 2. BGS ERT sand and gravel survey sites (green circles).J.E. Chambers, P.B.Wilkinson, A.Weller, P.I.Meldrum,O. Kuras, R.D.Ogilvy, J. Aumonier, E. Bailey, N.Griffiths, B.Matthews, S. Penn,D.Wardrop169Figure 3. A selection of 2D ERT images from 4 study sites in England and Wales. The dashed white lines show the inferred interfacesbetween mineral and bedrock, and mineral and overburden. Elevation is given as either depth below ground level or metres aboveOrdnance Datum (AOD).170Characterising sand and gravel deposits using electrical resistivity tomography (ERT)of clay till. However, it is essentially unproven due to thehigh proportion of cobbles and boulders in the deposit,which prevent the successful deployment ofconventional drilling methods. The 3D ERT survey(Figure 4) was effective in identifying the distribution ofoverburden across the area and revealing the thickness ofthe gravel deposit. A surface defining the base of thegravel was calculated from the ERT model; as with othersurveys we have undertaken, this surface was in a formthat could be directly incorporated into terrain modellingpackages for reserve calculation.Ingham is a complex site difficult to characterise usingboreholes, because of the spatial variability of thedeposit. 3D ERT was used to image the extent andthickness of till overburden and the sand and graveldeposit (Figure 5). Quality variations were also observedin the mineral, with high resistivities (yellow to red) likelyto be associated with coarser materials, and lowerresistivities (green) indicative of higher fines content.SUMMARYERT was an effective ground investigation techniquefor all but one of the 18 sites considered during theprojects due to the good resistivity contrasts observedbetween mineral and bedrock. Economic sand and graveldeposits are by definition relatively clean (i.e. low clayFigure 4. Masham 3D ERT model, including calculated bedrock surface. The interface between the mineral and bedrock is shown by awhite dashed line. Elevation is given as metres above Ordnance Datum (AOD).the sand and gravel, resulting in a lower resistivity.The Washington survey included a thickening wedgeof clay overburden, which was successfully identified inthe resistivity image as a low resistivity zone (<50Ohm.m). At Clifton, river terrace sands and gravels,including a clay interburden, were imaged above MerciaMudstone bedrock. The Blashford survey revealed asignificant channel structure in the bedrock, whichresulted in significant thickness variations in the deposit.The glaciofluvial deposits at Borras were shown to behighly variable, with clean coarse (i.e. lower finescontent) areas of the deposit characterised by particularlyhigh resistivities (>1000 Ωm).3D ERTProof-of-concept surveys using 3D ERT wereundertaken at Masham (North Yorkshire), Ingham(Suffolk), Norton Disney (Lincolnshire), Chelmsford(Essex), Nottingham (Notts), and Willington(Bedfordshire) (Chambers et al., 2012). Two surveys aredescribed here.The Masham site represents a relatively simplegeology, which consists of Namurian sandstone ormudstone bedrock, overlain by fluvio-glacial sand andgravel, which in turn is overlain by a thin variable cover171J.E. Chambers, P.B.Wilkinson, A.Weller, P.I.Meldrum,O. Kuras, R.D.Ogilvy, J. Aumonier, E. Bailey, N.Griffiths, B.Matthews, S. Penn,D.Wardropparticularly for complex deposits or sites that are difficultto drill (Chambers et al., 2011). Whilst ERT should notreplace drilling, it has the potential to reduce the numberof intrusive sample points required, and will enablebetter targeting of boreholes. Moreover, by revealing thestructure of the deposit between intrusive sample pointsit can provide additional information that cannot beidentified from borehole data alone.The modes of ERT field deployment (i.e. 2D and 3Dimaging) provide for significant flexibility in terms ofsurvey design and survey objectives. 2D ERT is a bestapplied as a rapid reconnaissance tool for characterisinggeological structures over large distances, whereas 3DERT is ideally suited to detailed high-resolutioninvestigations for reserve assessment. Both 2D and 3DERT data can be translated directly into operationallyrelevant information (e.g. bedrock surfaces) for use byminerals geologists and planners.A framework for good practice guidance for 3D ERTmineral reserve assessment survey has now beingdeveloped, based on previous studies, and from thefindings and experience gained during this and otherresearch (e.g. Chambers et al., 2011).Figure 5. Ingham 3D ERT model. The dashed white lines show the interfaces between the mineral and overburden, and mineral andbedrock. Elevation (x) is given in metres below ground level. Mineral quality variations are indicated by the resistivity variations, i.e. highresistivities (yellow to red) are likely to be associated with coarser materials, whilst lower resistivities (green) indicate a higher fines content.content), and are therefore typically more resistive thanweathered mudstone (e.g. Mercia Mudstone Formation),chalk, and clay (e.g. Oxford & London Clay) bedrock thatunderlies many important UK deposits. The success ofERT in the diverse range geological settings consideredso far greatly increases our confidence that it will bemore generally applicable to UK sand and gravelresources.It should be noted that the ERT has a number oflimitations (also see Chambers et al., 2008). In particular,ERT provides indirect (non-invasive) information in theform of images, which can contain distortions caused byresistivity variations in regions adjacent to the survey lineor area; consequently, some intrusive investigations arealways required for verification and sample analysis. Itprovides smoothed images in which precise interfacelocations can sometimes be difficult to determine, andalso displays a decreasing resolution with depth. ERTsurveyed should therefore by undertaken by suitablytrained operators, with the results interpreted alongsideother appropriate forms of subsurface information.Initial comparisons have shown that ERT survey costsare likely to be broadly similar to those of drilling(assuming a ≤100 m spaced grid of boreholes),Characterising sand and gravel deposits using electrical resistivity tomography (ERT)172ACKNOWLEDGEMENTSThe research was funded by DTI TechnologyProgramme Grant (DTI Project No: TP FastTrackJ1556K)and Mineral Industry Sustainable Technology ProgrammeGrants MA/6/1/008 & MA/7/G/1/007, and in-kindcontributions from project partners. This paper ispublished with the permission of the Executive Directorof the British Geological Survey (NERC).REFERENCESBarker, R.D. 1997. Electrical imaging and its application in engineeringinvestigations. Pp. 37-44, in McCann, D.M., Eddleston, M., Fenning,P.J., and Reeves, G.M., (Editors), Modern Geophysics in EngineeringGeologyThe Geological Society, LondonBeresnev, I.A., Hruby, C.E., and Davis, C.A., 2002. The use of multi-electrode resistivity imaging in gravel prospecting. Journal ofApplied Geophysics, 49, 245-254.Chambers, J.E., Ogilvy, R.D., Kuras, O., Cripps, J.C., and Meldrum, P.I.,2002. 3D electrical imaging of known targets at a controlledenvironmental test site. Environmental Geology, 41, 690-704.Chambers, J.E., Wilkinson, P.B., Meldrum, P.I., Wardrop, D., Matthews, B.,Penn, S., Joel, P., Aumonier, J., Weller, A.L., Benham, A.J., Bricker, S.H.,Kuras, O., Cave, M.R., and Ogilvy R.D., 2011. Investigation andMonitoring of Aggregate Deposits Using Geo-Electrical ResistivityImaging (IMAGER). British Geological Survey Commissioned Report,CR/11/001. 94pp.Chambers, J.E., Wilkinson, P.B., Wardrop, D., Hameed, A., Hill, I.,Jeffrey, C., Loke, M.H., Meldrum, P.I., Kuras, O., Cave, M., Gunn, D.A.,2012. Bedrock detection beneath river terrace deposits using three-dimensional electrical resistivity tomography, Geomorphology, inpress.Chambers, J.E., Wilkinson, P.B., Weller, A.L., Aumonier, J., Ogilvy, R.D.,Williams, J.D.O., Meldrum, P.I., and Kuras, O. 2008. DeterminingReserves of Aggregates by Non-invasive Electrical Tomography(DRAGNET): MIST Project MA/6/1/008. British Geological SurveyCommissioned Report, CR/08/040. 64pp.Hill, I.A. 2004. Geophysical Rapid Archaeological and Mineral Surveys(GRAMS): Final Report of MIST Project MA/3/01/001. University ofLeicester.Kuras, O., Pritchard, J., Meldrum, P. I., Chambers, J.E., Wilkinson, P. B.,Ogilvy, R. D., and Wealthall G.P. 2008. Monitoring hydraulicprocesses with Automated time-Lapse Electrical ResistivityTomography (ALERT). Comptes Rendus Geosciences - Special Issueon Hydrogeophysics, 341, 868-885.Lucius, J.E., Langer, W.H., and Ellefsen, K.J., 2006. An Introduction toUsing Surface Geophysics to Characterize Sand and Gravel Deposits.U.S. Geological Survey, Open-File Report 2006-1257.

Characterising sand and gravel deposits using electrical resistivity tomography (ERT) : case histories from England and Wales

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Characterising sand and gravel deposits using electrical resistivity tomography (ERT) : case histories from England and Wales

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