Search

CROW CLOSE

GEOPHYSICAL SURVEY

Grid Nine Geophysics were commissioned by the Bingham Heritage Trails Association (BHTA) to carry out a detailed magnetic and resistance survey of part of the Deserted Medieval Village in Crow Close. The site works and reporting conform to current national guidelines, as set out in the Institute for Field Archaeologists ‘Standards and guidance for archaeological evaluations’ (IFA, 2001) and the English Heritage document ‘Geophysical Survey in Archaeological Field Evaluation’ (David, 1995).

This section is taken with minor editorial change from the report submitted by Grid Nine to the BHTA.

Methodology

A Level II Evaluation geophysical survey (Gaffney and Gater, 1993) using fluxgate gradiometer and earth resistance techniques was chosen as the most appropriate type of survey for the site. Although there can be no preferred recommendation of which technique to use until the merits of the individual site have been assessed, magnetometer survey should usually be the prime consideration (David, 1995). On this occasion magnetometer and earth resistance techniques were employed in an attempt to provide complementary data.

The combination of a superficial loamy and clayey floodplain pedology and a sandstone solid geology are known to give good to average results from magnetic surveying per se. Clay deposits can give good to average results depending on the depth and nature of the anomaly. The response over sandstone is variable, which is true of most sedimentary parents (Gaffney and Gater, 2003; Clark, 1996).

The response of this type of geology to resistance surveying is more difficult to quantify as there are many variables that can affect the survey (see later), although, generally, sandstones and clays respond reasonably well per se (Clark, A 1990, Gaffney and Gater 2003).

The geology of the site is relatively common in Nottinghamshire and results from geophysical surveys over it are well documented and many cases are held in the English Heritage Geophysical Survey Database (EHGSD). A cursory but specific search of the EHGSD provided many surveys reporting successful results from both magnetic surveys and earth resistance techniques.

Magnetic surveying measures very small changes in the Earth's magnetic field which can be created by man-made or geological changes in the magnetic properties of the soil and/or underlying geology. Magnetic surveying can usually detect magnetically enhanced features such as areas of anthropogenic activity, pits, ditches, hearths and kilns, but also will react to buried 'modern' items such as nails, agricultural equipment fragments, wire fences and generally any ferrous material in the immediate area.

The geology of the site can play an important role in how successful a magnetic survey will be. If the local geology is inherently magnetic then it may not be practicable or possible to undertake a magnetic survey. Similarly, buried services can have an adverse effect on the data.

The magnetic ‘signature’ from certain anomalies, for example from a ditch or kiln, is often very characteristic to that type of known feature. This can assist with providing an informed, but quantitative rather than qualitative interpretation to certain anomalies. It should be noted that geomorphological features can give both positive and negative responses.

The basis of earth resistance surveying is that electric currents are fed into the ground and the resistance to the flow of these currents is measured. Where they ‘meet’ buried wall foundations, high resistance readings are (usually) recorded. If silted-up ditches, which tend to be wetter than their surroundings, are encountered, low resistance readings ensue. By mapping zones of high and low resistance it is possible to identify, for example, the layout of buildings or the size and orientation of a ditched enclosure. The interpretation of resistance data is more difficult than magnetic data as there are more variables that can alter the moisture in the ground, which can in turn alter how features respond to resistance surveying.

The basic concept of how certain high or low-resistance features respond to a resistance survey are reasonably simple. Walls, rubble spreads, trackways and made up surfaces, for example, usually respond as high resistance anomalies. Low resistance anomalies can be caused by features such as silted up ditches, pits, drains and gullies. However, as the responses rely on moisture content in the ground, they can vary with the seasons. Optimum conditions for a resistance survey, therefore, are difficult to predict. It should be noted that geomorphological features can give both high and low responses. The variations in the general background resistance values during a survey will tend to reflect the underlying geology and soils (Scollar et al, 1990), so prior knowledge of the type of geology can be as important as the seasonal timing of the survey.

Equipment used

The magnetic survey was carried out using a Bartington Grad601-2 Dual Fluxgate Gradiometer with an onboard automatic DL601 data logger. This instrument is a highly stable magnetometer, which utilises two vertically aligned fluxgates, one positioned 1m above the other. This arrangement is then duplicated and separated by a 1m cross bar. The 1m vertical spacing of the fluxgates provides for deeper anomaly detection capabilities than 0.5m spaced fluxgates. The dual arrangement allows for rapid assessment of the archaeological potential of the site. Data storage from the two fluxgate pairs is automatically combined into one file and stored using the onboard data logger.

The earth resistance survey was carried out using a TR Systems Earth Resistance Meter using the standard twin probe array and an on-board automatic data logger. Although the TR Systems instrument has been designed to be used primarily with the standard 0.5m twin probe array it has been modified by the manufacturer to use different probe arrangements including the Wenner array.

Summary of survey parameters

  • Fluxgate magnetometer
    • Instrument: Bartington Grad601-2 Dual Fluxgate Gradiometer
    • Sample interval: 0.25m
    • Traverse interval: 1.00m
    • Traverse separation: 1.00m
    • Traverse method: Zigzag
    • Resolution: 0.1 nT
    • Processing software: ArchaeoSurveyor 2.3.0.X
    • Surface conditions: Grazing for cattle.
    • Area surveyed: 2.2 hectares
    • Surveyors David Charles Hibbitt and Angela Hazel Hibbitt
    • Data interpretation: David Charles Hibbitt
    • Date of survey: 19th April 2008
  • Earth resistance
    • Instrument: TR Systems Earth Resistance Meter
    • Sample interval: 1.00m
    • Traverse interval: 1.00m
    • Traverse separation: 1.00m
    • Traverse method: Zigzag
    • Electrode spacing: 0.5m standard twin-probe array
    • Processing software: ArchaeoSurveyor 2.3.0.X
    • Surface conditions: Grazing for cattle.
    • Area surveyed: 0.5 hectare
    • Surveyors David Charles Hibbitt and Angela Hazel Hibbitt
    • Data interpretation: David Charles Hibbitt
    • Date of survey: 26th April 2008

Data collection and processing

Magnetic survey

For the magnetic survey, the site was marked out with a series of 20m x 20m grids aligned broadly north-south and loosely based on a plan supplied by the BHTA showing the area of interest to be surveyed. The BHTA area of interest (Figure 6) was adjusted slightly to accommodate the 20m x 20m grids (Figure 7) and also to avoid magnetic interference from the fencing around the playground in the NW corner of the site. A north-south grid alignment is preferable for a magnetic survey as enhancements to the magnetic field caused by buried features are intensified the closer the traverse approaches magnetic north – south direction (Scollar et al., 1990). Data were collected by making successive parallel traverses across each grid in a zigzag pattern, as close to a magnetic north – south alignment as practicable. Three of the survey grid corners were marked with wooden semi-permanent ground markers (SPGM). The locations of these were recorded using a hand-held Garmin GPS60 unit in ‘averaging mode’ with an external Garmin MCX25 external aerial set up to receive WAAS signals. The positions of the SPGMs were also measured in to various fixed points around the site boundary.

Earth resistance

For the more limited earth resistance survey, the site was marked out in a similar fashion to the magnetic survey. The survey grids, however, were positioned across anomalies noted in the magnetic data, rather than within a pre-determined area as with the magnetic survey. The two areas surveyed (Figure 8) were also on the flatter areas of the site which are relatively unaffected by the extant earthworks, although some of the earthworks were traversed by the resistance survey.

Data processing and analysis

The data collected from the survey have been analysed using the current version of ArchaeoSurveyor 2 (2.3.0.X). The resulting data set plots are presented with positive nT and high resistance mapped as black and negative nT and low resistance mapped as white.

The data have been subjected to processing using the following filters:

  • De-spike
  • De-stripe (also known as Zero Mean Traverse or ZMT) (Figure 9)
  • Edge match (earth resistance only)

The de-spike process is used to remove spurious or extreme high intensity anomalies or datapoint values in magnetic data. These are often caused by small ferrous objects (such as modern surface or sub-surface ‘rubbish’, ferrous fence posts, buried services etc), which may affect subsequent filter use, data enhancement and interpretation. The de-spike filter is used in a similar way with resistance data to remove spurious or poor readings due to poor contact with the ground, but is more usually reserved for magnetic data.

The de-stripe process is used to equalise underlying differences between grids or traverses in a magnetic survey. Differences are most often caused by directional effects inherent to magnetic surveying instruments, instrument drift, instrument orientation (such as off-axis surveying or heading errors) and delays between surveying adjacent grids. The destripe process is used with care as it can sometimes have an adverse effect on linear features that run parallel to the orientation of the process. On this particular occasion this adverse effect was an advantage as it was used to remove the prominent topographical effect in the data caused by the well-preserved extant ridge and furrow ploughing.

The edge match process is typically used with resistance data to correct for changes in the fixed (reference) probes. These changes may be due to errors in transferring the probes to new positions or, when a resistance survey is spread over a period of time resulting in different moisture content in the ground, different geological conditions.

Plots of the data are presented in raw linear greyscale (Figure 9, magnetics), processed linear greyscale (Figure 10, magnetics) and (Figure 11, resistance) and trace plot form (Figure 12, magnetics), with any corrections to the measured values or filtering processes noted.

Results

Magnetometer survey

It is apparent from the plot of the magnetic data overlain on the topographic map (Figure 13) that the magnetometer has partially mapped the extant earthworks (shown in grey on the interpretations in Figure 14). Although the magnitude of the earthwork responses is fairly low, peaking at about 4nT, their presence can be seen clearly in the data. It is highly likely that this is due to a topographical effect arising from the passage of the instrument over the earthworks and/or other physical reasons why the earthworks have responded to the magnetometer survey. Unfortunately it is not uncommon for the topographical effect to mask out subtle archaeological responses, and there are many instances from surveys on DMV and SMV sites where this has occurred (Gaffney & Gater, 2003).

The magnetometer survey has mapped a plethora of dipolar responses due to the presence of buried and on-surface ferrous or highly fired detritus. These dipolar responses seem at their most intense in the hollows of the earthworks and along the southern boundary of the survey. Some of the more obvious detritus was removed from the surface prior to starting the survey (such as several lengths of barbed wire), but it proved impossible to remove it all. Groups or clusters of these intense dipolar responses have the potential to mask archaeological responses in a similar way to the topographical effect as they can distort the results by giving magnitudes well outside the operating sensitivity of the instrument (in excess of -3000nT/m to 3000nT/m). The two areas of magnetic disturbance correlate with a metal fence surrounding the playground.

Local residents remember military activity on the site and some of this activity can be seen in the data in the NW corner of the survey area. The magnetometer has mapped the extant shallow circular earthwork [M1] (c.20 metres diameter) as a very faint discontinuous negative anomaly (Figure 14), but has also revealed an inner circular positive anomaly [M2] (c. 4 metres diameter) with a magnitude of 8nT/m. This earthwork is likely to be either a gun or a searchlight emplacement dating to the Second World War. There are several faint, hardly discernible positive and negative linear anomalies in the same general area, which may tentatively suggest ancillary structures associated with the gun/searchlight emplacement. There are two strong positive linear anomalies, one of which [M3] has a peak magnitude of 44nT/m. The latter anomaly is clearly associated with the emplacement and is probably a ferrous pipe due to the characteristic response. The other anomaly [M4] just to the south, has a magnitude of 10nT/m. appears to lead to a smaller extant circular earthwork and may indicate a service to a second gun/searchlight emplacement. The existence of a second emplacement/structure is also strongly suggested in the resistance data over the same area.

The extant ridge and furrow ploughing to the south-east corner of the survey is visible in the data as broad positive and negative striations [M5] running roughly east-west through the survey. The extant ridge and furrow has created a faint topographical effect in the data due to the unevenness of the surface.

Although there is no obvious or conclusive evidence in the magnetic data for structural remains or features such as hearths or kilns, several linear positive and negative anomalies [M6 to M10] have been mapped which correlate with the possible earthwork platforms. It is possible that these linear anomalies represent structural remains or ditches of uncertain use. Localised magnetic responses may also represent anthropogenic activity in and around such features, which may include pits and small areas of burning. The more intense of the pit-type anomalies are [M11 to M13] which have magnitudes of 12nT/m, 10nT/m and 9nT/m respectively. These relatively high magnitudes suggest fills of higher magnetic susceptibility than the surrounding soil, possibly arising from anthropogenic activity.

There are several faint curvilinear and linear positive anomalies [M14 to M16] and a small group of similar linear anomalies [M17] which all have magnitudes of less than 3nT/m. It is possible that these may represent track ways or ditches, although a geomorphological cause should not be ruled out.

Resistance surveys

Like the magnetometer survey, the resistance surveys have also partially mapped the extant earthworks, as shown on the plot of the resistance data on topographical map (Figure 15). This is due to the variations in moisture content and general make up of the earthwork banks and their ability to retain or lose moisture.

The survey to the north-west (Area 1on the simplified interpretation of the resistance data, Figure 16) has mapped the likely military structures more clearly than the magnetometer survey, revealing a complex of positive linear anomalies [R1] (Figure 16), which are likely to be associated with structural remains. These anomalies appear to radiate to the east and south east from an area of amorphous very high resistance [R2] which is likely to be caused by rubble spreads or possibly by compacted floors.

Several other amorphous areas of high resistance have been mapped, notably anomalies [R3] and [R4]. The anomalies [R5 to R9] display very high resistance values and may be associated with rubble spreads or compacted surfaces. Patterning within these amorphous areas may indicate structural remains, such as walls.

The circular ditch [R10] around the likely gun or searchlight emplacement to the north-west and the second emplacement postulated just to the south responded well to the resistance survey. On closer inspection of the data, there is a suggestion that both structures may have been hexagonal or octagonal rather than circular. This is particularly evident with [R11]. Hexagonal and octagonal emplacements are not unusual and there are several examples around the outskirts of London (Faulkner and Durrani, 2008). They include Monkhams Hall, Essex (hexagonal) and One Tree Hill, Southwark, London (octagonal).

Curiously, the low resistance linear anomaly [R11] is suggestive of a narrow ditch rather than the base of a wall or foundation as would be expected from structural remains. It would be unusual for a wall or stone/masonry foundations to give a low resistance response, but this can happen if the construction of the wall is such that it is retaining more moisture than the surrounding ground. It is also possible that the low resistance response is due to a robbed out wall leaving a narrow trench, or possibly a metal pipe or a drainage gully respecting the shape of a former structure.

The survey to the east (Area 2) has mapped several high resistance linear anomalies, all of which may represent structural remains, plus several amorphous areas of high and very high resistance. Although these amorphous areas could signify rubble spreads or something similar, a geomorphological cause should not be ruled out.

High resistance anomalies [R12 to R16] suggest structural remains, possibly the foundations of buildings, but these might also correlate with paths or tracks.

Several amorphous areas of high resistance have been mapped which could be caused by rubble spreads [R17 to R21]. Like Area 1, there are suggestions of ‘patterning’ within these amorphous areas, especially within [R17] which may represent structural remains. The very high resistance anomalies [R21 to R25] may also represent rubble spreads.

Conclusions

• The non-intrusive methodology employed was appropriate to the scale and nature of the project. The geophysical survey revealed many anomalies of potential archaeological interest within the earthworks complex, including possible World War II installations, but further archaeological investigations would be required to establish their character.


• It is slightly disappointing that the magnetometer survey appears not to have detected any of the thermoremnent features usually associated with habitation, such as hearths or kilns (these have characteristic ‘signatures’ in the magnetic data). However, it has detected several pit-type and ditch-type anomalies, which could be associated with anthropogenic activity. Unfortunately the precise dating of these anomalies is not within the scope of geophysical surveying.

• The resistance survey has been more successful than the magnetometer survey on this occasion for revealing potential archaeological remains. The likely military remains have responded well, and there are linear anomalies throughout the data, which may represent remains of buildings or tracks/paths, although a precise date for the anomalies is not within the scope of geophysical surveying.

• The complementarity of the magnetic and resistance data is well demonstrated (Figure 17). In places, the two methods identified different structures, some of which reinforced the identification made by the other method. This is best shown with regard to the World War II installations where the two methods identified different parts of a single structure.

• The site may benefit from further earth resistance surveying in the future both within the area subjected to the magnetic survey and also outside it.


Home Page | About Us | Links | Contact Us | Newsletter

Site developed by Ambrow Limited | Published by the Bingham Heritage Trails Association | All content is © BHTA
for site credits click here

Back to
top of page