grimes graves, norfolk. report on geophysical surveys...

GRIMES GRAVES, Norfolk.

Report on geophysical surveys, March 2007.

Summary

A geophysical survey was required at Grimes Graves, Norfolk, to assist with plans to provide new visitor facilities and inform a wider research project at this important English Heritage site. The objective of the survey is to extend the current magnetic and earth resistance survey in an attempt to define and characterise any detectable archaeology remains or geomorphological features.

Keywords Neolithic, Electrical Resistivity, Geophysical Survey, ARP, dry valley, blown sands.

GRIMES GRAVES, Norfolk.

Report on geophysical surveys, March 2007.

Introduction

A geophysical survey was required to cover an area to the north of the visible mine workings to investigate the possible continuation of the site towards the course of a dry valley. Previous geophysical survey at the site has provided mixed results, but may provide useful information to inform the geo-archaeological interpretation of the site, particularly the dry valley. It is hoped that results from the current survey will inform subsequent investigations usingGPR and an auger transect.

Grime’s Graves is the only Neolithic flint mine open to visitors in Britain. First named Grim’s Graves by the Anglo-Saxons, after the pagan god Grim, it was not until some of them were excavated in 1870 that they were found to be mines dug over 5,000 years ago. This valley contains deposits of wind blown sand up to ~ 1.5m deep that may well obscure significant occupation and, possibly, further mining activity.

The detailed geophysical survey was conducted over two parts of the site on the 3rd and 4th March 2007. Weather conditions during the geophysicalfieldwork were wet and sheep grazed the field. The site was surveyed in lessthan two days with the ARP system covering an area of 7.2 ha andcorresponds to a tender set by English Heritage. The ground surface was very tough and made driving conditions with the quad-bike very difficult. Our average speed of acquisition was 6 km/h on both areas (normally the average speed on agricultural plots is 15 km/h). During the survey, we had a visit and preliminary results were shown to Peter Topping (English Heritage) who decided to change the location of the surveyed areas from the original specification.

Method

Preliminary definition of soil electrical resistivity

Electrical resistivity ( ) is a well-known parameter within the geophysicalcommunity (Ward, 1990). Soil scientists are more used to its inverse, the electrical conductivity ( ), in order to define soil salinity, water or matrix mineralization (Rhoades and Ingvalson, 1971; Corwin et al., 2003). Soil electrical resistivity is a measurement of the ability of a current to flow into the soil. Measurement units are Ohm.m ( .m). This ability is closely linked tointrinsic parameters of soil such as: clay content, water retention, texture,

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CaCO3 content, stoniness, soil depth and of course the depth and type of substratum (Abu-Hassanein et al., 1996; Heiniger et al., 2003). Mapping of apparent soil electrical resistivity is consequently an indirect mapping of thespatial variability of these intrinsic parameters (Bourennane et al., 1998;Dabas et al., 2001).

Multi-depth electrical surveying. The ARP device.Multi-electrode devices are used to describe both the vertical and horizontal variations of electrical resistivities. Electrical sounding is used for the vertical variations: the distance between electrodes controls the depth of investigationof these systems (Zhou et al., 2001). By increasing (symmetrically to a fixed point) the distance between electrodes, one increases the depth ofinvestigation. An electrical sounding at a single point is obtained. If the distance between the electrodes is fixed and the electrodes are movedmanually along a transect, a profiling is obtained. By walking along several profiles, a mapping can be obtained.

Geocarta uses the ARP (Automatic Resistivity Profiling) device which is apatented mobile multi-electrodes system: several electrodes are automaticallyinserted in the soil and roll along the surface. A profile and simplified soundings are obtained simultaneously. Data acquisition is very fast: up to 100 ha can be surveyed in a day (distance between transects of 10 m).

Continuous soil electrical mapping at three depths is obtained by the ARP multi-electrode system. Three maps corresponding to three different depths of investigations are derived. Soil units can then be defined by looking at both the vertical and horizontal variation of apparent electrical resistivities.

The measurement device consists of an electronic system with real-time acquisition and processing of several parameters such as electrical resistivities, GPS messages, and Doppler radar data. Horizontal and vertical positioning of the ARP system is obtained by a GPS with differential corrections (StarFire, John Deere). The whole system is controlled in real-time by a ruggedized PC which enables acquisition, processing, storage andvisualization of electrical resistivities by the operator while driving. Horizontal precision is normally as defined by the DGPS manufacturer (for StarFire 30 cm minimum). Vertical precision is of the order of 60 cm. The precision of resistivities is 1 %.

The mechanical part of ARP system consists of four dipoles : the first oneinjects a stabilized current into the ground, the following three dipolesmeasure the potential derived from the injection of the current. The distance between the three dipoles increases with the distance from the currentinjection. Distance is respectively 0.5 m, one and two meters (cf. Figure 1).Numerical simulations have shown that the depth of investigation is of the same order as the distance to the injection dipole. The amplitude of thecurrent is 10 mA. Whatever the speed of motion, this device can make a measurement at a fixed distance interval (20 cm).

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For example, driving at a one meter interval (cross-line interval), more than 180 000 measurements can be acquired in less than two hours. The whole system is patented (CNRS, Paris VI University).

Injection(current)

Measurements(potential)

for 3 depths

Injection

1.7 m

1 m

50cm

SolSoil

Figure 1 – Diagram of ARP principle

Figure 2 – ARP-03 device at work

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Results

All the raw data have been processed by a 1D median filter along transect (20 centimeters distance interval, 1 meter between transect) first and then interpolated by a spline bicubic process on a square mesh (0.50x0.50m). The width of the 1D median filter moving window was set to 1 meter.

North part South part GPS 30177 6523Measures for each channel 317483 76508Average speed 6.35 km/h 6.10 km/h Acquisition time 8h22m57 1h48m43Channel 1 mean 1211.6 ohms.m 512.7 ohms.m Channel 2 mean 1423.8 ohms.m 436.7 ohms.m Channel 3 mean 1242.3 ohms.m 367.2 ohms.m

Though we have selected the best printers and paper, we suggest that interpretation should be done on screen where the visual dynamics are higher.

The objective of the survey is to extend the current magnetic and earthresistance survey in an attempt to define and characterise any detectable archaeology remains or geomorphological features. The dynamic of resistivities encountered over the site is very high: from 100 to 5000 ohm.m. The high resistivities clearly correlate with the blown sand deposit which are encountered at the bottom of the slope in the axis of the dry valley [Figure11: r1]. Both maps clearly show well depicted geomorphological features which show up as “stripes” perpendicular to the elevation contours curves (Figures 5 and 9) [Figure11: r2]. These features were already mapped in the previous earth resistance survey (Bartlett-Clark Consultancy, 2006) but are also partially visible from aerial photography (Figure 10). The upper part of the survey, close to the existing shaft is conductive and can be probably explained by loamy cover and/or the presence of wet chalk closer to the surface [Figure11: r3]. From that point of view, this could imply that the white stripes are old channels where sand was washed down. Another possibility is accumulation of fine elements which retain more water. Even at two meter depth, the image is still very clear and the image of the stripes extends evenmore towards the south. The high resistance encountered means that we have probably an even greater depth of investigation. In the north part, NE-SW faint linear features can also be seen and correspond probably to oldagricultural/forestry directions [Figure11: r4]. The geophysical signature of a flint shaft is difficult to predict. As the expectedvoid with coarse stone packed into, it should be resistive. But if loamymaterials are trapped into it, the resistivities should be lower. Looking at“circular” features as potential candidates, we can pinpoint a number ofisolated conductive features to the North border [Figure11: r5]. Looking to the area to the west, two nearly circular resistive structures are visible [Figure11: r6-7]. These features are close to the known shaft to the North and also close to a linear anomaly runnig NNE-SSW.

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Conclusions

The electrical survey conducted at Grime’s Graves using the new towed ARP system proved to be successfull even with two drawbacks : a very bumpy surface made the progress of the quad-bike very difficult (see movie file) and a very wide dynamic of resistivities (100 – 5000 .m).The whole area (7.2 ha) was surveyed in nearly 10 hours of acquisition time.Three maps of apparent electrical resistivities were obtained and a preciseDigital Elevation Model (DEM) was computed. The results show a clear sand blown environment with a dry valley, consitentwith previous electrical measurements (Figure 10) and a conductive chalkdome ridge where shaft are located [Figure11: r3]. Even at two meters depth,the landscape does not seem to change. Depth of these sand deposit make of course possible the existence of burried and well-preserved stratigraphies.Also the shape of this landscape may have played an important role in the choice of the setting place of the neolithic people.

Surveyed by: M Dabas Date of survey: 03-04/03/2007 G Hulin

M Gruel (student volunteer)

Reported by: A Favard Date of report: 30/03/2007M Dabas Date of revision: 25/05/2007

Geophysics Team, Geocarta.

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List of figures.

Figure 1 Location plan of geophysical surveys areas superimposed over the base OS map (1:3500).

Figure 2 Detailed map of channel 1, ARP survey, North Part (1:2000).



Figure 5 Detailed map of Digital Elevation Model with superimposed contour lines (1:2000) and 3D vizualisation of channel 2, North Part.

Figure 6 Detailed map of channel 1, ARP survey, South Part (1:1500).



Figure 9 Detailed map of Digital Elevation Model with superimposed contour lines (1:1500) and 3D vizualisation of channel 2, South Part.

Figure 10 Aerial photo with superimposed ARP Survey Channel 2, Bartlett-Clark survey and contour lines (1:2250).

Figure 11 Aerial photo with superimposed ARP Survey Channel 2, Bartlett-Clark survey and annotations (1:2250).

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References

Abu-Hassanein Z.S., Benson C.H., and Blotz L.R., 1996. Electrical resistivity of compacted clays, Journal of Geotechnical Engineering, pp. 397-406.

Bourennane H., King D., Le Parco R., Isambert M. and Tabbagh A., 1998. Three-dimensional analysis of soils and surface materials by electrical resistivity survey. European Journal of Environmental and Engineering Geophysics, 3, pp. 5-23.

Corwin D.L., Lesch S.M., Shouse P.J., Soppe R. and Ayars J.E., 2003. Identifying Soil Properties that Influence Cotton Yield Using Soil Sampling Directed by Apparent Soil Electrical Conductivity, Agronomy Journal, 95, pp.352-364.

Dabas M., Tabbagh J. and Boisgontier D., 2001. Multi-depth continuous electrical profiling (MuCEP) for characterization of in-field variability, 3rd

European Conference on Precision Agriculture, Montpellier-France, June 18th-21st, 2001, pp. 361-366.

Heiniger R.W., McBride R.G. and Clay D.E., 2003. Using soil electrical conductivity to improve nutrient management, Agronomy Journal, 95, pp. 508–519.

Rhoades J.D. and Ingvalson R.D., 1971. Determing salinity in field soils with soil resistance measurements, Soil Sci. Soc. Amer. Proc., 35, pp. 54-60.

Ward S.H., 1990. Resistivity and induced polarization methods. In geotechnical and environmental geophysics, vol. 1, Ward, S.H., Editor,Society of Exploration Geophysiscist, pp. 147-190.

Zhou Q.Y., Shimada J. and Sato A., 2001. Three-dimensional spatial and temporal monitoring of soil water content using electrical resistivity tomography. Water Resour. Res., v. 37, n°2, pp. 273-285.

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This map is based upon Ordnance Survey material with thepermission of Ordnance Survey on behalf of the Controllerof Her Majesty's Stationery Office © Crown copyright.Unauthorised reproduction infringes Crown copyright andmay lead to prosecutionor civil proceedings.English Heritage 100019088 2007.

FIGURE 1GRIME'S GRAVES: Location of ARP Surveys, March 2007

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FIGURE 2GRIME'S GRAVES: NORTH PART, ARP Survey, Channel 1, March 2007


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FIGURE 5GRIME'S GRAVES: NORTH PART, Digital Elevation Model, March 2007


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FIGURE 6GRIME'S GRAVES: SOUTH PART, ARP Survey, Channel 1, March 2007


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FIGURE 9GRIME'S GRAVES: SOUTH PART, Digital Elevation Model, March 2007


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FIGURE 10GRIME'S GRAVES: Aerial photography with superimposed ARP Survey Channel 2, Bartlett-Clark Survey and contour lines


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FIGURE 11GRIME'S GRAVES: Aerial photography with superimposed ARP Survey Channel 2 and annotations


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