Magnetometer surveying is used to detect and map small changes in the earth's magnetic field caused by concentrations of ferrous-based minerals within the soil and subsoil, and by materials buried beneath the surface. While most of these changes are too small to affect a compass needle, they can be detected and mapped by sensitive field equipment. During surveys the different magnetic properties of top-soils, sub-soils, rock formations and archaeological features are recorded as variations against a background value. Subsequently magnetic anomalies resulting from potential archaeology can be identified and interpreted.
Bartington grad601-2 gradiometers
A gradiometer is a type of magnetometer and is sensitive to relatively small changes in the earth's magnetic field. Our primary surveying instruments are Bartington Grad601-2 (dual sensor) fluxgate gradiometers with automatic data loggers. They are specifically designed for field use by archaeologists. The Bartington gradiometers provide proven technology in archaeological magnetic surveying and offer fast, accurate set-up and survey rates.
Multiple sensor arrays
A technique relatively new to commercial archaeological surveying but well understood in academic circles involves the use of multiple magnetometer sensors towed behind a quad bike or other off-road vehicle. With multiple sensors and the use of on-board GPS units, it is possible to achieve faster survey rates at competitive commercial rates provided the ground is suitable for the array and the cost-benefit analysis matches our clients needs.
Magnetometer surveying process
Typical magnetic surveysCurrently, the standard instrument used for magnetometer surveys in the UK is a Bartington 601-2 dual sensor gradiometer. This instrument is carried by the operator and is used to detect and systematically map magnetic anomalies up to 1.5m beneath the ground surface with an optimal recording depth of up to 1m. These magnetic anomaly maps are then examined for anomaly groups that may be the result of ground disturbance from past human activities. The end product is a report that includes a georeferenced map of potential archaeological deposits and features with an indication of their possible nature and the level of confidence assigned to this interpretation.
- Using an accurate digital GPS surveying instrument, the site is laid out in a regular, contiguous series of square grids, usually 30m by 30m on each side although smaller grids are occasionally used.
- The data is collected by walking these grids in a systematic direction and speed to record the data at 1m traverse intervals and 0.25m sample intervals along each traverse using an automated data logger. The traverse and sampling intervals can be altered depending on the types of archaeological feature being targeted.
- The recorded data is downloaded to an on-site laptop computer at regular intervals throughout the day.
Factors that influence the speed of survey:
- The speed of walking each grid by the instrument operator. This varies by operator and terrain but typical speeds are between 1.4 and 1.8 metres-per-second. The compromise is that the quality of the data over the day noticeably deteriorates if the pace of survey is set towards the high end of this range.
- The traverse intervals; the narrower the traverse interval the longer each grid will take to complete.
- The number of instruments and operators used. This will always increase the speed of survey although not quite in direct proportion to the number of instruments used. The cost-effectiveness of the number of instruments deployed depends on the size of survey area, the number of personnel used and other site-specific cost parameters.
- The use of a multiple sensor array as discussed elsewhere on this page.
Survey speeds and data quality
Substrata has been trading since late 1999. When undertaking gradiometer surveys, we have found that consistent, high quality surveys with guaranteed survey times is best achieved using a combination of survey speeds and data collection rates that realise a consistent 2ha to 3ha per day per gradiometer survey team depending on local conditions. Faster speeds are possible using a multiple sensor array and area coverage of up to 10ha per day is possible when the terrain is suitable.
With the increasing understanding of the power of good archaeological geophysical surveys by planning authorities throughout the UK and, critically, the cost incurred later in a civil engineering project as a result of missing significant archaeological remains at this relatively early stage make the judicial combination of speed and good quality data a vital element in any archaeological assessment.