FLUXGATE GRADIOMETRY
for archaeological prospection
Fluxgate gradiometers have been developed specially for the detection of buried archaeological remains. The instrument measures the local vertical gradient field by using two fluxgate sensors 0.5m or 1m apart. They have been in use since the mid 1970s and are now used routinely in archaeological evaluation. The instrument is extremely sensitive being able to detect changes as small as 0.5nT.
The gradiometer can be used for both "scanning" and detailed recorded survey. The former method involves traversing the site along regularly spaced linear transects (usually about 10m apart) and observing the fluctuations in magnetic signal. It enables a rapid initial assessment of large sites and identification of areas of potential interest which can then be investigated in more detail. Collection of detailed data is speedy and with the introduction of dual sensor instruments, up to 3ha can be investigated in one day with readings being taken at 1m by 0.25m intervals. The results of the survey are known while still in the field.
The interpretation of fluxgate gradiometer data can be used to position excavations in the "best" place, or even to re-design proposed developments, thereby reducing excavation costs.
non-archaeological shallow prospection
The advantages of speed and field display make this technique useful in disciplines other than archaeology. The range (sensitivity) of the instrument can be adjusted to make it more suitable for the detection of large ferrous objects and it can also be used to locate certain natural features.
Archaeological Targets Detected by Fluxgate Gradiometry
- Kilns, furnaces and hearths
- Buried ditches and pits
- Areas of occupation and ritual sites
- Industrial sites
- Palaeochannels
- Ferrous pipes and cables
- Buried ferrous material; tanks, foundations etc
- Clay land drains
- Palaeochannels
- Mine shafts
Any rural or semi-urban site free of excessive surface ferrous material
ELECTRICAL RESISTIVITY SURVEY
for archaeological prospection
Area surveys undertaken using electrical methods of investigation have been used since the 1940s. The "Twin-Probe" method has been developed for archaeological purposes and is routinely used in GSB surveys. The basis for this method involves the injection of a small electrical current through the earth and the measurement of the subtle sub-surface variations in resistance. The technique is particularly suited to the detection of buried buildings and structural remains, although it can be used to locate a variety of other features, including football pitches! As with gradiometer surveys, the interpretation of resistivity or resistance data can be used by archaeologists and developers to formulate their project designs.
Targets Detected by Electrical Resistivity Survey
- Masonry and brick foundations
- Floors and cavities
- Buried megaliths
- Palaeochannels and geological changes
Any rural or semi-urban site free of excessive surface contamination and where a good electrical contact can be made with the ground.
GROUND PENETRATING RADAR
for shallow prospection
Although airborne radar has been in use for many decades, Ground Penetrating Radar (GPR), is a relatively new technique. Over recent years vast improvements have been made to both the hardware and software making it a versatile technique suitable for use on a wide variety of sites.
The method utilises the absorption and reflection of electromagnetic waves at contrasting interfaces. GPR records detailed vertical time sections which can provide a wealth of stratigraphic information and clearly define any discontinuities. The primary advantage of GPR is that it provides a good estimation of the depth of a target. The complex nature of archaeological deposits can present a complicated picture when viewed as individual radargrams. However, data collected along a series of closely spaced parallel traverses can be combined to form a series of time slice maps - horizontal slices through the ground at different time/depth intervals - enabling a 3D image of the survey area. This type of data processing allows more subtle features and relationships between features to be analysed more easily.
Targets Detected by GPR
- Mines and Shafts
- Voids
- Services; pipes, cables, culverts etc
- Foundations
- Ditches
- Bedrock
- Water table
- Relatively level and even surface
- Most rural areas, with the exception of saturated peat or clay
- Most urban sites, with the exception of heavily reinforced concrete
- Minimal electromagnetic noise i.e. radio transmitters, certain machinery
SEISMIC REFRACTION SURVEY
for shallow prospection
Seismic refraction has been used extensively in the exploration industry for many years, primarily for determining the depth of the overburden to formulate corrections for reflection data. However, in recent years the sensitivity of shallow refraction instruments has been improved. This, combined with improved software packages, has made it a useful method for determining the nature of shallow stratigraphy.
The primary advantage of seismic refraction is that it produces a vertical section which provides an accurate estimation of the depth of a target. In addition, seismic refraction works well on sites with a clay soil, which may preclude the use of Ground Penetrating Radar.
Targets Detected by Seismic Refraction
- Depth to bedrock
- Buried channels
- Soil and/or grass cover to enable insertion of electrodes
- Sufficient distance either side of target; dependant on depth
- Minimal vibration noise, i.e. passing traffic
ELECTRICAL IMAGING SURVEY
for shallow prospection
Electrical imaging techniques have wide applications and were developed primarily for the exploration industry. The techniques have been scaled down to provide valuable information about the shallow subsurface. The method is commonly used to determine the nature and depth of potential archaeological features and has great potential for environmental and engineering applications.
The primary advantage of electrical imaging is that it produces a vertical section which provides a good estimation of the depth of a target. It is slightly more versatile than seismic refraction surveys and ground penetrating radar as it is less dependant on the soil conditions.
Targets Detected by Electrical Imaging
- Water table
- Depth to bedrock
- Buried channels and banks
Sites suitable for Electrical Imaging
- Soil and/or grass cover to enable insertion of electrodes
- Sufficient distance either side of target, dependant on depth
