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SSuubbssuurrffaaccee IImmaaggiinnggCharacterizing sand and gravel deposits in three dimensions

By Jonathan Chambers, British Geological Survey

High-quality site investigationinformation is an essentialprerequisite for accurate reserve

estimation, effective quarry planning andefficient operations. To this end, the BritishGeological Survey (BGS) has been developingstate-of-the-art ground-imaging technologyfor sand and gravel deposit investigationthrough the ‘IMAGER’ (Investigation andMonitoring of Aggregate Deposits UsingGeo-Electrical Resistivity Imaging) project.The project has been funded by the Mineral

Industry Sustainable Technology (MIST)programme, in partnership with Tarmac,Hanson, Lafarge, CEMEX, AggregateIndustries and the Minerals IndustryResearch Organisation (MIRO). Its primaryaims have been to demonstrate electricalresistivity tomography (ERT) as a nationallyapplicable high-resolution imagingmethodology for non-invasivelycharacterizing sand and gravel deposits, andto raise awareness within the mineralindustry of the economic and environmentalbenefits of applying ERT technology.

WWhhyy uussee EERRTT??ERT is an imaging technique that isanalogous to many of those used for medicalapplications, eg magnetic resonance imaging(MRI) and electrical impedance tomography(EIT), and, as such, allows the user to seeinside the object or region of interest,thereby reducing reliance on invasiveinvestigations. In the context of sand andgravel deposit characterization, ERT issensitive to compositional variations in thesubsurface and can, therefore, be used todistinguish between different material types,eg clean gravels and clay bedrock, and tospatially map three-dimensional structures,such as overburden and mineral volumes,and quality variations within deposits.The principal advantage of ERT is that

it provides fully spatial/volumetricinformation; this complements intrusivesampling methods, which typically provideinformation only at discrete locations. Forcomplex sites that display significantgeological variation between boreholes, the

spatially continuous data provided by ERT canbe particularly valuable. Likewise, ERToffers the possibility of providing groundinformation at sites that are undrillable, egdue to a high proportion of cobbles andboulders. Deployment of ERT is also possiblein many areas that can be difficult to accessfor drilling, including woodland, croppedfields, soft or marshy ground and steepslopes.

SSuurrvveeyy eeqquuiippmmeennttThe survey equipment used for ERT surveysis lightweight and portable and can bedeployed from an estate car or 4x4 vehicle.To undertake a survey, lines of stainless steelstake electrodes are positioned along theground surface and connected by multi-core cable to an ERT imaging system, whichis used to automatically run a predefinedmeasurement cycle, typically consisting ofmany thousands of measurements. Surveylines can be anything from a few metres tohundreds of metres with depths ofinvestigation of several tens of metres. For

ERT survey of a sand and gravel deposit on agricultural land, with crop

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site-scale surveys, a typical field crew willconsist of between two and four people, andwill include a geophysicist and fieldassistant(s).Most of the costs associated with ERT field

surveys are related to staff time. PreliminaryERT results for individual survey lines can beproduced in the field within moments of datacollection finishing, using image-reconstruction software running on a fieldlaptop. This provides an initial assessmentof data quality and significant subsurfacestructures. Detailed data processing,topographic corrections and visualization are

undertaken post-survey.Two fundamental survey modes can

be used; these are 2D and 3D ERT. Thesimplest type of ERT survey is 2D and isundertaken using a single line of electrodes,the data from which is used to construct avertical 2D slice through the subsurface. 3Dsurveying is also undertaken using lines ofelectrodes, but requires multiple linelocations in close proximity to one anotherthat are surveyed consecutively; themeasurements from each line are mergedinto a single dataset, which is processed toproduce a 3D image.

RRaappiidd rreeccoonnnnaaiissssaanncceeFor rapid reconnaissance surveys, 2D ERT isan ideal tool. Coverage of hundreds ofmetres to a few kilometres per day isachievable with a crew of two or threepeople. Results can be delivered shortlyafter the survey in the form of a descriptivereport and annotated sections.Surveys can be used as an initial

reconnaissance tool to provide a broadindication of geological variability and toidentify drilling targets. Equally, 2D imagingcan be used post-drilling to provide improvedcharacterization in areas where drillingdata has not provided sufficient lateralresolution of the geological structure.Recent examples of 2D ERT

reconnaissance survey include:characterization of ‘feather-edge’ depositsin Upper Thames terrace deposits whichdisplayed very significant thickness variationand were, therefore, difficult to characterizeusing intrusive methods alone; spatialdefinition of a channel structure incisedinto bedrock; mapping the spatial extent ofclay interburden in a river terrace deposit;overburden thickness determination at a sitewith dipping Folkestone Formation bedrocksands below a clay capping layer; andmapping compositional variability inheterogeneous glacial sand and gravel.

DDeettaaiilleedd iinnvveessttiiggaattiioonnFully volumetric images of the subsurfacegenerated using 3D ERT are a powerful toolfor detailed site investigation and reserve �

Prospecting & Surveying

Schematic diagram showing the components of an ERT field survey

2D ERT section showing river terrace deposits overlying mudstone

2D ERT section showing feather-edge deposits of Upper Thames terrace gravels (red-yellow) overlying bedrock (blue)

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estimation. Coverage rates are dependent onsurvey design and ground conditions, buttypically are of the order of 1 to 3ha per day.These surveys are best suited to undrillableor complex deposits for which point samplinginformation does not provide sufficientresolution. 3D imaging does not replaceintrusive methods (ie boreholes/trial pits)because physical sample recovery andanalysis is necessary, and intrusive data isrequired to calibrate and interpret thegeophysical images. However, through

improved characterization of the deposit,ERT has considerable potential to improve thetargeting of intrusive sampling and reduce thenumber of boreholes required. 3D ERT has now been demonstrated on a

number of sand and gravel deposits in the UK,including highly complex river terrace depositsthat had proved impossible to adequatelycharacterize using conventional means, andglacial fan deposits that were impossible todrill due to the high cobble and bouldercontent. The images have been analysed

using interactive visualization software thatallows the geophysicist or geologist toappreciate fully the 3D nature of the deposit.It addition, the software can be used tocalculate mineral or overburden volumes,thereby contributing directly to the process ofreserve estimation and quarry planning.Integration of the technology with currentpractice has been achieved through extractinglayers (eg base of mineral) from the 3Dimages in a format compatible with industrystandard software, such as LSS.

3D ERT survey of a complex river terrace deposit: complete image (top); mineral volume (middle); and bedrock surface (bottom)

3D ERT image of river terrace deposits overlying clay bedrock (left) and calculated bedrock surface (right)

NNaattiioonnaall aapppplliiccaabbiilliittyyTo date, ERT surveys have been carried out on a broad range of important economicdeposits in England and Wales and were shown to be an effective groundinvestigation technique for all of the sitesconsidered, due to the good resistivitycontrasts observed between mineral andbedrock. Economic sand and gravel depositsare, by definition, relatively clean (ie low claycontent), and are, therefore, typically moreresistive than the weathered mudstone (egMercia Mudstone Formation), chalk and clay(eg Oxford and London Clay) bedrock thatunderlies many important UK deposits. Thesuccess of ERT in the diverse range ofgeological settings considered so far greatlyincreases confidence that it will be more generally applicable to UK sand and gravelresources.

GGoooodd pprraaccttiicceeGeophysical surveys should always bedesigned, executed and interpreted by skilledoperators who are able to assess the suitabilityof the technique for the given problem, and areable to communicate the likely limitations ofthe technique to the client. In the case of ERT,it is important to indicate the likely resolvingcapabilities for a particular survey design (theimage resolution decreases with depth), and to highlight potential ambiguities ininterpretation due to, for example, insufficientresistivity contrasts between material types.An assessment of data quality and modellingerrors should always be made and reported;these are essential indicators of theconfidence that can be placed in the surveyresults.ERT should not be used as a stand-alone

tool for site investigation; it is good practiceto use intrusive data and any other availablegeological or engineering information tointerpret and calibrate the geophysicalimages. Instead, ERT should form a component of a structured site investigationprogramme. If used as an initialreconnaissance tool as a means of identifyingdrilling targets, the ERT images should bereassessed after ground-truth informationbecomes available. In this way, additionalinformation may be gained from the originalERT through improving and refining the initialinterpretation of survey results.

CCoonncclluussiioonnElectrical resistivity tomography is a ground-imaging tool which can complementconventional point-sampling approaches byproviding fully volumetric information onsubsurface conditions. Despite the potentialbenefits of ERT, it has not yet been routinelyapplied by the minerals industry to sandand gravel deposit investigation. This is due,in part, to unfamiliarity with the technique anda lack of demonstration studies. Recent workby BGS and industry partners has nowdemonstrated ERT as a cost-effective siteinvestigation tool across a range of economic

deposits, both for rapid site reconnaissanceand detailed reserve estimation. This approachis particularly beneficial at sites that aredifficult to characterize using conventionalapproaches. It is hoped that the necessaryevidence base now exists to facilitate the take

up of ERT, so that the industry can benefitfrom the latest advances in ground-imagingtechnology.For further information contact Jonathan

Chambers at BGS on tel: (0115) 936 3428; oremail: jonathan.chambers@bgs.ac.uk

Prospecting & Surveying

ERT survey at an active quarry site

Location of ERT demonstration surveys (grey circles)

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