Electrical resistivity tomography (ERT)

Electrical resistivity is the inability of a material to pass an electric current and is specific to a material. In practice within soil it is affected by a myriad of factors including pore size, hydration and clay content. When exploited for electrical resistivity tomography it is used for vertical imaging or profiling and is very effective for measuring the thickness of layers of material. An expansion of the technique is to stack profiles together (2.5D) or conduct a full 3D survey to yield a 3D model of the subsurface.

ERT is frequently used in palaeo-environmental surveys for imaging former river channels in cross section and for detecting the depth of overburden or to bed rock. ArchaeoPhysica uses two systems, the Tigre64/128 and Iris Instruments Syscal Pro, which is a multichannel version ideal for 2.5D and 3D survey. Examples of use have included the detection of the bedrock foundation of islands beneath alluvium and for imaging former water courses beneath historic structures.

Campus Tigre 128 in use profiling overburden in advance of quarrying

Depth to bedrock showing measured and calculated (

Former stream (blue) under a subsiding building

ERT probe array set up for high resolution imaging

ERT can be made to work in a variety of conditions as long as adequate probe contact is possible

The technique is best used across fairly small areas, although long profiles (> 100m) are easily achieved when necessary. The raw apparent resistivity data is used to mathematically generate a model of the true distribution of resistivity which has informative of the nature and distribution of materials within the ground.

Geophysical survey

Shallow geophysics is our specialism and we offer a complete range of techniques singly or in combination to a bespoke specification suited to clients' needs. We pair this with professional implementation by qualified and experienced geophysicists who keep up to date with technological and theoretical developments from across the world. We guarantee high quality data from collection onwards.

For information on individual techniques:

Landscape survey & prospecting

We are able to offer a complete landscape prospection package working at a variety of spatial scales from analysis of remote sensing data to high resolution geophysical and walkover surveys. Our upland survey experience and training means we are well placed to undertake primary reconnaissance in large remote areas across Europe. Several remote areas exceeding 30 square kilometers size have been surveyed to date.

Survey outputs include fully digital photographic recording and data acquisition with customisation for direct entry to national and local databases. Full GIS usage throughout means various types of national and local mapping can be incorporated and analysed, new maps produced, fly-throughs generated and output as video and stills and advanced spatial analyses performed. At the most basic level, monuments can simply be located according to a national co-ordinate system, catalogued and plotted on a map; more detailed or followup work might include detailed topographic and geophysical surveys.

Part of a fly through model of the ridge of Tal Trwynau, Wales

Archaeological sites in upland Wales overlaid on nineteenth century mapping

Monuments overlaid on a fly through model of Pen Cerrig-calch, Wales

ArchaeoPhysica has extensive experience of researching complex landscapes

Open upland areas devoid of landmarks can be rapidly assessed from the air and systematically on foot

Systematic survey has been carried out in well over 100 square km of mountainous terrain

Topographic survey

ArchaeoPhysica has provided high resolution elevation and slope data for many years, recently adding topographic data capture capability to our pioneering multi-sensor platform.

For detailed conventional survey our investment in Leica 1205+ and CS15 technology supplements existing equipment to allow us to quickly and efficiently provide detailed surveys. Whatever the purpose of the survey, data collection can be accomplished reliably, quickly and efficiently. Data is then imaged and made available in a variety of formats via GIS, allowing it to be merged with other data, plotted as stand-alone contour maps or rendered as local relief via hachuring.

3D model of St Catherine's Hill, Abbotsbury, Dorset showing medieval cultivation terraces

5cm contour model of part of a deserted medieval settlement in Herefordshire

Contour model of a former military camp on the south coast of England

Topographic map of St. Just Plan an Gwary, or medieval theatre, Cornwall

High resolution data is analysed and imaged using similar processes used for LiDAR, e.g. slope analysis, local relief, hill shading, etc.

Upland survey

Survey of normally fairly (and occasionally very) remote upland areas requires training and experience beyond normal landscape survey. Additional navigational, leadership and survival skills are paramount to safety and success in areas prone to rapid changes in weather and frequently hostile conditions. In addition, simply locating monuments amongst coarse vegetation, across rocky mountainsides, wetland areas and vast grasslands can itself be a major challenge.

ArchaeoPhysica has experience of working above 400m across many tens of square kilometers and proven ability to operate effectively in hostile conditions. Recording, safety, navigation and survival procedures have been developed that permit working in remote open areas in all upland terrains.

Isolated quarry building

Remains of a transhumance hut serving what were once Summer pastures

Small clearance cairn with possibly a prehistoric core

The low walls of a medieval longhouse ingrown by upland bog

Sheep shelter to help protect remote herds from wind in this isolated spot

A natural looking hillside but actually extensive limekilns and quarries

Much of our work has been undertaken in Wales through the Winter months, including the highest reaches of Snowdonia. Across a number of years we have found and recorded over a thousand new monuments including mines, prehistoric and early medieval settlement, standing stones, tracks, transhumance shelters and animal folds. We are well equipped via our remote sensing analysis capabilities to collate data from dispararate sources and create high resolution maps where necessary, where to support field operations in poorly mapped areas or to aid presentation and understanding of the subsequent data.

Caesium magnetometry

Magnetometry is the most widely used technique in commercial archaeological prospection because it tends to produce useful and rapid results in the widest range of environments. It is best suited to the detection of settlement and industrial sites in open ground.

We pioneered the use of caesium (alkali vapour) magnetometers in UK archaeology in 1999 and have used Geometrics caesium technology continuously since, creating one of the largest archives in Europe of total field data from archaeological sites.

Cart mounted systems have been designed by and deployed by ArchaeoPhysica for almost a decade, in more recent times featuring full GNSS (GPS) integration, remote logging and other technological advances. Our carts were the first to be used in UK archaeology and have travelled many thousands of kilometers since.

In 2010 we introduced another technological advance based around an ATV-towed magnetically silent multisensor platform with bespoke acquisition hardware and software from Harewood Geophysical. This has since revolutionised many clients' work, permitting very rapid high quality survey with simultaneous deployment of different measurement technologies. A ten hectare site can normally be surveyed in one day with final quality images available that same day and interpretation the next.

Bronze Age tumulus and later fieldsystem

Magnetic data collected over an Iron Age settlement

Caesium magnetometer configured as 0.5m vertical gradiometer

Extensive Romano-British settlement under medieval cultivation

Romano-British settlement imaged using our multisensor platform

Birdoswald Roman fort western vicus, military way and vallum

Multisensor platform carrying G858 magnetometers

Our routine use of high sensitivity non-gradiometric magnetometers followed by informed interpretation has allowed our clients access to reliable data across a wide range of areas where others have struggled, e.g. deeply alluviated environments, magnetic geology and low contrast sites. A recurring comment is that our data 'looks so clear'.

Electromagnetic survey (magnetic susceptibility)

Magnetic susceptibility, the ability of a substance to become magnetised, is invaluable in archaeological contexts for understanding the pedogenesis of soils. Most soils contain some iron and various chemical processes driven by heating and cooling, or fermentation, can bring about an increase in susceptibility through alteration of iron oxides. It can be used to locate certain sorts of buried monument although a more powerful (and reliable) application is the analysis of soils within sites.

The instruments most commonly used are the Bartington MS2D / MS2F and the Kappameter KT5 / KT9. Both of these have small diameter coils (20cm and 6cm respectively) that irradiate a small volume of soil permitting high measurement resolution. For landscape prospection, the former is often used to collect measurements on approximate 10m centres; however, while it can detect settlement and industrial areas response is strongly variable across soil types. Another use is the high resolution mapping of stratigraphy and excavated surfaces because even tiny (invisible) quantities of heated soil can be detected. It can also be used to indentify buried topsoils and industrial waste materials (e.g. hammerscale), again where there is often no visible trace.

Susceptibility cross section of a visually uniform section ditch fill

Detailed study of susceptibility within an excavation

Collecting high resolution susceptibilty data to map a ditch section

A Geonics EM38 with DGPS used to prospect parkland for buried settlement remains

For magnetic susceptibility to be most effective a thorough knowledge of the local natural soils, geology and often hydrology is necessary, however, even basic intra-site comparions tend to be useful. In an ideal world, susceptibility data would be collected for every context found during excavation and used to map trench sections.

Ground probing radar (GPR)

Radar is one of the true 3D survey methods, capable of producing high resolution reflection data from a wide range of buried structures including pipes, voids, masonry, floors and stratigraphic detail in certain cases. However, it does not work so well in electrically conductive environments, e.g. clay or wet ground. It is the principal method of imaging through the floors and walls of buildings.

For archaeological survey frequencies of between 250 MHz and 1GHz are the most commonly used and in good conditions penetration to around 5m is possible for the lowest frequencies, decreasing to less than 1m for for the highest. However, higher frequencies offer the best vertical resolution and are often used for exploring the structure of masonry whereas lower frequencies are better for the detection of structures buried in soil.

ArchaeoPhysica uses GSSI technology, normally the SIR-3000 paired with a 270MHz or a 400MHz antenna. Data is usually collected at around 0.02m intervals along lines 0.5m apart, with a second orthogonal set where small structures are expected. Most of our work has been to locate and map the sites of buildings and to prospect for voids and other structures buried below floors. Data processing includes profile migration and stacking to form 3D prisms which can then be sliced to reveal detail.

Deepening timeslices from left to right showing various structures at different depths

Strong reflections from tanning pits beneath a concrete floor

A fairly large GPR survey showing buried foundations beneath lawns

GPR survey underway within an abandoned medieval chapel

Former planting beds and drives within the antenna

We hold a licence from OFCOM in accordance with UK regulations and we are a member of EuroGPR.

Electrical resistance (twin probe)

One of the oldest geophysical techniques, this remains one of the most effective means of mapping masonry and similar relatively impervious structures buried close beneath the surface. It is also effective for mapping buried pit fills and ditches and for locating anomalously damp areas of ground associated with springs and former pools. It's only disadvantage is that it requires insertion of probes for each measurement and is therefore one of the slowest techniques. Recent innovations with greater speed will probably offer consideral benefit.

By passing an electric current through the ground and measuring the electric potentials formed at the ground surface, variations in resistance close to the probes can be mapped. The most common archaeological array used in Europe is the twin probe which produces data sensitised to lateral change but is insensitive to vertical ones. In contrast, the Wenner (or 'standard') array is sensitive to vertical variation and is useful for measuring the depths of deposits.

0.5m multiplexed twin probe survey in use to locate a former settlement

Medieval cemetery south of Henllan chapel, Builth Wells, Wales

Former rampart crossing a Cheshire hillfort

Drains and former terrace structures below a lawn

Roman roads and insulae divisions within a Roman town in Warwickshire

At the risk of being pedantic, this technique is often incorrectly called 'resistivity' but the instrumentation used in archaeology measures apparent resistance and does not take into account the probe configuration necessary to calculate apparent resistivity.

Electromagnetic survey (quadrature electrical conductivity)

Continuous wave electromagnetic (CWEM) survey is widely used for electrical conductivity mapping at various scales. The techique is invaluable for medium resolution rapid coverage without ground contact and the instrumentation is often carried on our multi-sensor platform alongside magnetometers. This is especially useful within alluvial and wetland environments where magnetic response can be contrasted wihth conductivity variations caused by buried channels and islands. A further use of CWEM survey is for rapid detection of buried metals.

Within the archaeological and environmental sectors (excluding hydrological) penetration to about 6m is common using a variety of twin coil Slingram type instruments. The Geonics EM31 (4m long, max. 6m penetration) and the Geonics EM38 (1m long, max. 1.4m penetration) are in common use. The quadrature response corresponds to electrical conductivity while the in-phase is a measure of magnetic susceptibility.

Geonics EM31 in use to detect deeply-buried masonry

Electrical conductivity variation over silts next to wetland

Coastal conductivity variation over estuarine clay and silt under ridge and furrow

Geonics EM31 on multisensor platform with Geometrics G858 magnetometers

We have used electromagnetic survey to effectively map buried palaleochannels and other landscape scale structures including shallow geological formations and former mine workings. The size of the instruments (related to coil separation) limits the lateral resolution of survey but the technique has the advantage of being able to operate in surface conditions too dry or hard for probe-based conductivity measurement.

GIS Services

ArchaeoPhysica has used GIS for all our projects for many years, whether for collating data from geophysical and topographic survey or for hosting spatial databases supporting landscape and upland survey. Not only does it therefore underpin our work but the spatial analysis functionality is exploited to explore relationships between landscapes and features.

Past projects have included plotting 3D site distributions in mountainous areas, simulating flood events, producing detailed maps and various other applications. We are proficient coders of SQL queries and BASIC scripts with the GIS environment which unlocks massive analytical potential, e.g. mapping 3D landscapes in terms of distance from water, exposure to sunlight and similar natural variables. It is also used for thematic coding of data, e.g. distribution maps of monument by type or date.

Interpreted geophysical data from a Roman town visualised within a GIS for analysis

Modelled territories associated with upland sheepfolds with evidence for zoning by altitude

Analytical use of GIS to examine settlement distance from water supplies

Air crash sites coded by size of search area for debris

Geophysical data interpreted within a GIS

Our capabilities include import and processing of most forms of 2D data, whether vector, raster or surface models and a full cartographic service is available.

Remote sensing data analysis

We undertake large (and small) area prospection based upon remotely sensed data, incorporating LiDAR terrain models, high resolution aerial photographs and satelite imagery (visual, radar and spectral) into a GIS environment for generation and update of mapping.

LiDAR data is examined and imaged in a number of ways depending upon conditions and expected targets, including using slope analysis, local relief (terrain filtering) and hill-shading. Traditional contour models are also generated. SAR data is imaged in similar ways although there are certain contraints imposed in urban or forested environments. LiDAR data is preferred for high resolution work, e.g. feature detection within woodland or within urban areas.

Chromatic processing and analysis of aerial photography to reveal variations in crop growth

Pseudo-aerial view of an Iron Age hillfort denuded of trees, constructed from LiDAR data

Conventional colour plot of terrain showing plan form of valleys

LiDAR slope data combined with aerial photography to reveal 3D form

Low earthworks defining an ancient monument highlighted using LiDAR imagery

Colour and spectral imagery is processed according to its source with colour imagery normally converted to pseudo-spectral data and chromatically enhanced where necessary.

These data sets are best for large areas, e.g. tracing low earthworks or palaeo-channels across farmland or virtually modelling mountain sides to extract likely transport corridors, view points and for inter-visibility analyses.