Earth resistance surveying is an excellent tool for detecting buried archaeology. Its relatively slow rate of survey compared to magnetometer surveys means that it usually employed in commercial surveys when a detailed understanding of buried building remains is required. This technique measures changes in the electrical resistance of the ground being surveyed. In practice, the recording of differences in the electrical resistance of near-surface deposits and structures allows the detection and interpretation of masonry and brick foundations, paving and floors, drains and other cavities, large pits, building platforms, robber trenches, ditches, graves and similar buried features. It is also a useful tool for distinguishing archaeological deposits from geological and other natural features.
Resistance to electrical current flow in the ground depends on the moisture content and structure of the soil and other materials buried beneath the surface. For example, the higher the moisture content of a soil, the less resistant it is to electrical current flow. A ditch completely buried beneath the present ground surface is likely to have an infill soil different to that surrounding the ditch in terms of compactness and composition. As a result, the soil filling the buried ditch will retain moisture in a different way to the surrounding soil which means it will have an electrical resistance at variance with the surrounding environment. By passing a small current through the ground it is possible to detect, record, plot and interpret such changes in electrical resistance.
Substrata uses the Geoscan Research RM15 series multi-probe resistance meter with build in automatic data-logger. The Geoscan MPX15 multiplexer is added to the instrument configuration on all surveys to create multi-probe arrays which speed up survey area coverage rates and, if required, facilitate simultaneous multiple-depth data collection . We've been around a while and the RM15 has been replaced by later derivatives as you will see if you follow the link above. Our RM15 remains a reliable instrument and a credit to Geoscan Research. The rate of progress and the quality of data in a typical commercial survey between the RM15 and its later variants is unchanged.
Typical resistance surveys
Currently, the standard instrument used for earth resistance surveys in the UK is the Geoscan Research Ltd RM series of resistance meters used by Substrata. This instrument is moved over the survey area by the operator in a systematic pattern and is used to detect and map electrical resistance anomalies typically up to 1.5m beneath the ground surface with an optimal recording depth of up to 1m. The resistance anomaly maps generated 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.
The typical survey process:
- Using an accurate digital GPS surveying instrument, the site is laid out in a regular, contiguous series of square grids, typically 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 typically at 1m sample intervals along each traverse by physically placing the probes slightly into the ground. The readings are recorded 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 is limited by the recording rate of the adjustable automated data logger. The compromise is that the quality of the data is reduced when the speed of recording is set at the high end of the range.
- The traverse and sampling intervals; the narrower the traverse interval and the smaller the sampling 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 mobile sensor platform supplied by Geoscan Research Ltd. This system has yet to make a significant impact on the commercial archaeological geophysical survey market and Substrata has not evaluated the system.
Survey speeds and data quality
Since 2000 Substrata has been delivering earth resistance surveys in non-urban environments. We have found that consistent, high quality surveys with guaranteed survey times is best achieved using a medium automated data collection rate which yields a maximum daily survey rate of 0.6ha per RM15 survey team depending on the terrain when using 1m spaced traverses and 1m sampling intervals.
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.