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Magnetometry

Magnetometry is the most widely used technique in archaeological geophysics 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, however, it does not work equally well everywhere.

The use of caesium (alkali vapour) magnetometers in the UK for archaeology was pioneered in 1999 by ArchaeoPhysica and the company has since acquired 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.

In 2010 we introduced an ATV-towed multisensor platform with bespoke acquisition hardware and software from Harewood Geophysical. This has revolutionised many clients' work, permitting rapid quality survey, fast turnarounds and simultaneous deployment of different measurement technologies. Rates of 10 hectare / day are normal, with interim images available sometimes the same day.

Our routine use of high sensitivity non-gradiometric caesium 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'.

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.

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

Electromagnetic survey (magnetic susceptibility)

Magnetic susceptibility, the ability of a substance to become magnetised, assists archaeologists in understanding the origin of soils. Processes involving heating and cooling, fermentation, or other drivers of redox reactions, can increase susceptibility through alteration of iron oxides and hydroxides. This enhancement can sometimes help locate sites in a landscape but a more useful application is the analysis of soils within sites.

For soil analyses, mapping of stratigraphy or hammerscale detection use is made of instruments with small diameter coincident coils allowing tightly constrained investigation of small volumes of soil. For the detection and mapping of larger areas use of non-contact co-planar coil instruments is normally better as not only can they traverse faster but the measurement interval is much smaller.

For the effectve use of magnetic susceptibility 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.

Electromagnetic survey (quadrature electrical conductivity)

Continuous wave electromagnetic (CWEM) survey is widely used for electrical conductivity mapping at various scales, providing medium resolution rapid coverage without ground contact. The instrumentation is sometimes carried on our multi-sensor platform alongside magnetometers, a useful multi-method approach to mapping alluvial and wetland environments where magnetic response can be contrasted with conductivity variations caused by buried channels and islands.

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. Depths of investigation of about 6m are possible.

See also Magnetic Susceptibility, Electrical Resistance

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. High frequencies offer the best vertical resolution and are often used for exploring the structure of floors and masonry whereas lower frequencies are better for the detection of structures buried in soil.

Data is usually collected at around 0.02m intervals along lines 0.5m apart, with a second orthogonal set where small structures are expected. Individual 2D profiles are analysed and stacked into 3D prisms.

ArchaeoPhysica is a member of EuroGPR.

Electrical resistivity tomography (ERT)

Electrical resistivity is the inability of a material to pass an electric current and is specific to a material. In 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 profiling and is effective for measuring the thickness of layers of material or depth to bedrock. Adjacent profiles can be stacked together to yield a 3D model of the subsurface.

ArchaeoPhysica has used ERT to detect former islands beneath coastal alluvium, image stratigraphy and to map former water courses beneath historic structures in 3D.

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

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