Electromagnetic survey (magnetic susceptibility)
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.
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.
For information on individual techniques:
- Magnetometry (caesium)
- Magnetic susceptibility
- Electrical resistance (2D planar)
- Electrical resistivity tomography (2D, 2.5D & 3D)
- Electromagnetic (quadrature electrical conductivity)
- Ground probing radar
Landscape survey & prospecting
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.
See also Upland Survey
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.
High resolution data is analysed and imaged using similar processes used for LiDAR, e.g. slope analysis, local relief, hill shading, etc.
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.
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.
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.
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'.
Ground probing radar (GPR)
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.
We hold a licence from OFCOM in accordance with UK regulations and we are a member of EuroGPR.
Electrical resistance (twin probe)
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.
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.
See also Resistivity Tomography
Electrical resistivity tomography (ERT)
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.
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.
See also Electrical Resistance
Electromagnetic survey (quadrature electrical conductivity)
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.
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.
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.
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
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.
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.