Ladybridge Farm: Soil Resistance Survey (Intervention 5)

full report (pdf file, 9.74 MB) | method statement

A targeted magnetometer pilot study was undertaken in November 2003 in order to assess the effectiveness of magnetometry on the site. Although the results were not wholly conclusive, a magnetometer area survey of 12 hectares was carried out in Zones A, B, C, and D, during December 2003. Given the relatively disappointing results of the magnetometer area survey, a targeted soil resistance pilot study was undertaken in January 2004. A soil resistance area survey covering 2 hectares was then employed to investigate Zone D during February 2004.

Survey Procedure

The external edges of the survey areas were set out using a total station theodolite, with intermediate points being positioned using tapes. This procedure ensured an internal grid point accuracy of ±0.05m for the survey.

This technique involves passing an electrical current through an area of ground and measuring the ground’s resistance to the current. Although soil particles and stone are insulators and do not conduct electricity, the presence of water which is a good conductor allows these materials to conduct an electrical current. In effect, a soil resistance survey maps moisture content. Archaeological deposits vary in character and composition and have different moisture retention qualities which can often be mapped by a soil resistance meter.

This survey was carried out using a soil resistance meter with digital storage and data transfer facilities (RM15 Advanced - manufactured by Geoscan Research). The RM15 was used with a MPX15 multiplexer connected to a PA5 probe array fitted with three probes. The use of a multiplexer and multiple probe array allows a series of different readings to be taken at the same point. In this case, two readings at 0.5m probe separation and one at 1.0m probe separation were logged at each point. This method produces two data-sets: firstly, a higher resolution data set with readings at 0.5m x 1.0m intervals (0.5m probe spacing), and secondly, a lower resolution data set with readings at 1.0m x 1.0m intervals (1.0m probe spacing). The first data-set produces a higher definition image of soil resistance anomalies, while the second, with wider probe spacing, provides a coarser image of soil resistance at greater depth.

The raw data was processed using Geoplot version 2.02. This involved the adjustment of any differences in the average background reading between individual survey grids, as well as inconsistencies caused by changing climatic conditions, which were removed to facilitate clear presentation of the data-sets. The processed data was transferred to Surfer version 6.2, in which it was prepared for presentation, and the resulting greyscale images were imported into AutoCAD and output on a laser printer. Raw data plots are provided as Appendix C.

Survey Results

Soil Resistance Pilot Study

Due to the relatively inconclusive results of the magnetometer area survey, a pilot soil resistance survey was undertaken. Two areas were targeted, a 60.0m x 90.0m area in Zone A (A1) and a 60.0m x 90.0m area in Zone B (B1). Both of these areas were targeted due to the presence of cropmark features; within Zone A these appeared as linear features and within Zone B the curvilinear features noted prior to the magnetometry were targeted again.

A total of seven anomalies were identified within A1, six low resistance and one high resistance. In addition, an area of high resistance and an area of low resistance were identified and interpreted as variation in the underlying subsoil.

F32 This is a circular low resistance anomaly measuring approximately 4.30m in diameter. The anomaly may represent a pit or sink hole.
F33 This is a circular low resistance anomaly measuring approximately 3.20m in diameter situated 9.80m to the south of F32. The anomaly may represent a pit or sink hole.
F34 This is a linear low resistance anomaly orientated NNW-SSE with a western spur orientated approximately east-west. The anomalies are visible for a length of 94.50m and 12.20m respectively. The weaker response on the 1.0m probe spacing results suggests that this anomaly may be fairly shallow. These anomalies may represent field enclosure or field drains.
F35 This is a circular low resistance anomaly measuring 3.50m in diameter. The anomaly may represent a pit or sink hole.
F36 This is a circular low resistance anomaly measuring 2.35m in diameter situated 4.80m to the southwest of F35. The anomaly may represent a pit or sink hole.
F37 This is a high resistance curvilinear possibly sub-oval anomaly situated in the southern part of A1. The anomaly is orientated approximately east-west and measures c.33.0m across x 18.50m in width. The anomaly may represent a small enclosure or geological variation.
F38 This is a high resistance curvilinear anomaly consisting of a north-south length extending for a distance of 34.40m before turning approximately 90 degrees to the west and continuing for a length of 22.0m. The second element consists of an east-west length measuring 30.70m before it disappears beyond the western limit of A1.

A total of six anomalies have been identified within B1 and allocated F39 to F44. In addition, a high resistance anomaly located in the southeastern corner has been interpreted as geological variation. Two areas of low soil resistance and one of high resistance were identified and interpreted as variation in the natural subsoil.

F39 This is an amorphous low resistance anomaly measuring approximately 5.20m x 4.0m. The anomaly may represent a pit or sink hole.
F40 This is a possible rectilinear low resistance anomaly orientated NNE-SSW and NNW-SSE measuring 34.80m and 16.50m respectively. The anomaly may represent a field enclosure or field drain.
F41 This is a sub-oval high resistance anomaly measuring approximately 34.80m x 16.5m in diameter. The anomaly may represent local variation in the underlying natural subsoil.
F42 This is an amorphous low resistance anomaly measuring approximately 12.50m x 5.60m. The anomaly may represent a pit, sink hole, or variation in the local geology.
F43 This is a group of three small circular low resistance anomalies orientated NW-SE measuring 1.50m, 2.00m, and 1.75m in diameter. The anomalies may represent pits or sink holes.
F44 This is a group of two large circular low resistance anomalies measuring 4.00m and 5.50m in diameter situated 4.30m to the southwest of F43. The anomalies may represent pits or sink holes.

The soil resistance pilot study detected possible field enclosures, isolated anomalies and other possible features. The results of the pilot study indicated that soil resistance may prove to be a more reliable method of detecting a broader range of sub-surface features. As such, a soil resistance area survey was employed to investigate Zone D, which had become available for study, having not been accessible during the magnetometer area survey.

Soil Resistance Area Survey

A cruciform-shaped sample area was designed to target possible linear features in the zone. The survey area consisted of two linear areas which crossed one another. These measured 30.0m x 330.0m north-south and 30.0m x 300.0m east-west.

Only two potentially archaeological anomalies were identified within area D1 and were allocated F45 and F46. Several high resistance anomalies have been identified, as well as broader areas of high and low resistance throughout D1; all of these areas are considered to relate to variation in the local geology.

F45 This is a circular low resistance anomaly measuring approximately 7.50m in diameter and disappears beyond the northern limit of D1. The anomaly may represent a pit or sink hole.
F46 This is a possibly linear ‘Y’-shaped low resistance anomaly visible for the width of area D1 at this point (30.0m), orientated approximately east-west and measures no more than 4.20m wide. The linear anomaly splays towards the east to form two spurs. The feature may represent a field boundary or field drains, or possibly geological variation. The improved definition of this feature on the 1.0m probe spacing data set suggests that this anomaly is not superficial.

The soil resistance area survey did not detect any anomalies which could be interpreted as boundary features. The results of the survey were dominated by large low and high soil resistance anomalies considered to represent variation in the underlying sand and gravel subsoil. As was found with the magnetometer area survey, the soil resistance survey identified few potential features of archaeological origin. The relatively poor results of the geophysical surveys either indicated that few archaeological features were present on the site, or that the two techniques applied do not provide a reliable means of remotely mapping and characterising a full range of archaeological feature types on the site.

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