Garden Field, Earth Resistance Meter Trial Sutton Hoo, Suffolk

Transcription

1 Garden Field, Earth Resistance Meter Trial Sutton Hoo, Suffolk Client: National Trust Date: November 2016 BML 037 Geophysical Survey Report SACIC Report No. 2016/089 Author: Timothy Schofield HND BSc MCIfA SACIC

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3 Garden Field, Earth Resistance Meter Trial, Sutton Hoo, Suffolk BML 037 Geophysical Survey Report SACIC Report No. 2016/089 Author: Timothy Schofield Illustrator: Timothy Schofield Editor: Richenda Goffin Report Date: November 2016

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5 HER Information Site Code: BML 037 Site Name: Garden Field, Earth Resistance Meter Trial, Sutton Hoo, Suffolk Report Number 2016/089 Date of Fieldwork: October 24th & 25th 2016 Grid Reference: TM Oasis Reference: Curatorial Officer: Project Officer: Client/Funding Body: Client Reference: TBA n/a Timothy Schofield National Trust n/a Digital report submitted to Archaeological Data Service: Prepared By: Timothy Schofield Date: May 2016 Approved By: Rhodri Gardner Position: Director Date: May 2016 Signed:

6 Contents Summary 1. Introduction 1 2. Geology and topography 3 3. Archaeology and historical background 3 4. Methodology 4 5. Results 6 6. Conclusion 7 7. Archive deposition 8 8. Acknowledgements 8 9. Bibliography 9 List of Figures Figure 1. Location plan 2 Figure 2. Earth resistance meter trial, survey grid, station & 'Bromeswell Bucket' location 11 Figure 3. Earth resistance meter trial, raw greyscale plot 12 Figure 4. Earth resistance meter trial, processed greyscale plot 13 Figure 5. Earth resistance meter trial, processed xy trace plot 14 Figure 6. Earth resistance meter trial, interpretation plot of anomalies 15 List of Appendices Appendix 1. Appendix 2. Appendix 3. Metadata sheets Technical data OASIS form

7 Summary In October 2016 Suffolk Archaeology Community Interest Company (SACIC) along with National Trust volunteers and visitors to Sutton Hoo undertook a reconnaissance earth resistance meter survey on land set aside as pasture for rare breed rams at Garden Field, Sutton Hoo, Suffolk. The geophysical survey successfully recorded high and low resistance anomalies indicative of archaeological ditches and pits, natural variations within the superficial geology and a modern service trench that disturbed the location of the Bromeswell Bucket.

8 1. Introduction In October 2016 an earth resistance meter survey covering c hectares of land centred on the location of the metal detected Bromeswell Bucket (Fig. 2) found in 2012 was undertaken at Garden Field, Sutton Hoo, Suffolk (Fig. 1). The project was carried out over two days by Suffolk Archaeology Community Interest Company (SACIC) along with volunteers of the National Trust and visitors to the centre. This followed on from the success of the inaugural geophysical survey trials undertaken in March 2016 which employed the non-intrusive techniques of the magnetic susceptibility meter (area survey, 10 x 10m) and fluxgate gradiometer (detailed survey at 0.25 x 1m). Suffolk Archaeology were commissioned to undertake the work by Angus Wainwright on behalf of the National Trust. 1

9 Figure 1. Location plan Norfolk Woodbridge SUFFOLK Essex 0 25km 0 1km N Brown's Planting Home Wood Garden Field Sutton Hoo Visitors Centre Garden Wood Tranmer House TM Dairy Farm 0 200m Contains Ordnance Survey data Crown copyright and database right

10 2. Geology and topography Garden Field is situated in the grounds of Tranmer House, adjacent to the Suffolk town of Woodbridge in the parish of Bromeswell (Fig. 1). The survey area (centred on NGR TM ) comprises the south-eastern corner of a single field currently set aside to pasture and grazed by rare breed rams. Topographically the ground slopes up from the western boundary from c. 20 up to c. 30m AOD where it plateaus within the central and eastern half of the field. The Bedrock geology is described as Thames Group clay and silt, formed 34 to 56 million years ago in the Palaeogene Period as shallow water sediments redeposited as graded beds. Superficial geology is described as River Terrace sand and gravel formed up to 3 million years ago in the Quaternary Period deposited as sand and gravel detrital material in channels to form river terrace deposits with fine silt and clay forming floodplain alluvium and some bogs depositing peat (BGS, 2016). 3. Archaeology and historical background Garden Field is located close to the famous Sutton Hoo ship burial, immediately to the northwest of the Visitor Centre and the Treasury. Archaeological fieldwork undertaken over the last thirty years on Garden Field has most significantly recovered remains of a rare imported artefact of eastern Mediterranean origin known as the Bromeswell Bucket in the southeastern corner of the field, dating from the 6th century. Survey work undertaken in the preceding years includes fieldwalking in 1984 and 1986 by Suffolk County Council Archaeological Service (SCCAS), confirming the presence of early Anglo Saxon pottery alongside later Ipswich ware in two areas of Garden Field (Gardner and Sommers 2013). In 1997 Field Archaeology Specialists Ltd (Copp, 1997) undertook a topographic survey in Garden Field as well as geophysical surveys in fields to the east and south of the site. In 2001 English Heritage (Linford, 2002) carried out a geophysical survey over a large part of Garden Field. Perpendicular running positive linear trends interpreted as former field boundaries, two short and narrow irregular linear anomalies that could represent 3

11 structural remains or linked rubbish pits and a plethora of positive discrete pit-type anomalies were prospected. One curving broad linear anomaly indicative of a geological response was also recorded. A strong dipolar linear trend orientated northwest to southeast is likely to delineate the route of a buried ferrous service run. Further fragments of the Bromeswell Bucket were recovered during the 2012 metal detecting survey (Gardner and Sommers, 2013); the bucket has been associated with a potential burial that may have been unintentionally damaged by the construction of the buried service run recorded in the preceding geophysical survey. Forty other objects were recovered, eleven of which could be dated. Thirteen were described as droplets, which may indicate the presence of a metalworking site nearby. Two objects were identified as modern replicas of Anglo Saxon objects, a consequence of re-enactment activities undertaken on the field over recent years. In March 2016 SACIC staff along with National Trust volunteers and visitors to the centre undertook the inaugural geophysical survey trial. The primary aim of this was to find out whether visitors to the centre could get involved in undertaking non-intrusive geophysical surveys to enhance the Sutton Hoo experience. The trial was a resounding success with very positive feedback from the participants and volunteers. The secondary aim of the trial day was to prospect two areas located in the northwest and southwest corners of Garden Field that were not surveyed by English Heritage CfA in 2001, employing a magnetometer. The results revealed the continuation of the modern ferrous service pipe that disturbed the Bromeswell Bucket, continuation of a broad linear anomaly recorded as a conduit of the River Deben, pits, thermoremanent responses indicative of a kiln and positive linear anomalies interpreted as enclosure and trackway ditches on a variety of alignments, all of which suggests that multi-phase activity was present below the topsoil. 4. Methodology Geophysical survey trial One geophysical surveyor was in charge of the instrument s operation and helping the visitor with data collection. The second surveyor was in charge of the dissemination of information; this included explaining how the instrument and survey technique worked, describing the results of the previous trials and fielding questions posed by the visitors. 4

12 National Trust volunteers kept the visitor s experience positive by welcoming them on to site, signing them on to the activity and guiding them to the survey area. The volunteers also handed out feedback forms that were completed. These will be collated to evaluate the day, which will allow improvements to be implemented before any future geophysical survey trials are undertaken. Instrument type A Geoscan Research RM85 Advanced earth resistance meter was employed to undertake the reconnaissance geophysical survey, covering an area of 0.64 hectares centred on the location of the Bromeswell Bucket. Instrument calibration and settings The weather was sunny following a period of precipitation, the soil moisture content was found to be relatively low, predominantly due to the free draining nature of the sand and gravel geology underlying a relatively shallow topsoil. To keep the instrument weight down and the degree of manoeuvrability up, a single (twin pole-pole) array was employed. The sampling interval was set to 1m along 1m traverses. Gain was decreased to x1 in order to keep the readings within instrument range, at a frequency of 122.5Hz, with an output voltage of 45v, the auto-log delay was set to 300ms and the high pass filter was programed to 13Hz. Station readings were equalised when moving the remote probes to allow consistent data matching between the survey grids. Survey grid layout A 20m grid was employed (Fig. 2, magenta grid), orientated east to west and geolocated employing a Leica Viva GS08+ Smart Rover RTK GLONASS/GPS, allowing an accuracy of +/- 0.01m. Data were converted to the new horizontal National Grid Transformation OSTN15. Data capture Earth resistance meter survey points were recorded on an internal data logger that were downloaded and checked for quality at midday and in the evening, allowing grids to be re-surveyed if necessary. A pro-forma survey sheet was completed to allow data 5

13 composites to be created. Data were filed in unique project folders and backed-up onto an external storage device and then a remote server in the evening. Data software, processing and presentation High quality raw survey data was collected despite the uneven terrain, ploughed fieldwalking area and brambles, allowing minimal data processing to be required. High contact resistance readings were collected where large flint stones were present just below the surface, low contact resistance readings were recorded where voids created by recent ploughing activity for a fieldwalking exercise were extant. Datasets were composited and processed using DW Consulting s Terrasurveyor v ; the raw grid files will be stored and archived in this format. Minimal processing algorithms were undertaken on the raw (Fig. 3) and processed (Fig. 4) datasets and schedules are presented in Appendix 1. Data composites were exported as raster images into AutoCAD. An interpretation plan based on the combined interpretations of the raw, processed and xy trace plots (Figs. 3, 4 & 5) has been produced in Figure 6. Survey grid restoration Three virtual survey grid stations are recorded on survey grid nodes along the baselines of the survey (Fig. 2); these will allow the position of the grid and geophysical anomalies to be accurately relocated. 5. Results An earth resistance meter trial reconnaissance survey was undertaken over two days employing a Geoscan Research RM85 Advanced, with participants consisting of Sutton Hoo visitors, aided by National Trust volunteers and overseen by the SACIC geophysical survey team. Nineteen grids were surveyed over two days by 105 visitors, 10 National Trust Volunteers, 2 SACIC staff members and 1 work experience student. This trial followed on from the successful inaugural geophysical survey trial undertaken in March

14 Low resistance anomalies were recorded throughout the survey and can be broadly classed as either linear or discrete in nature. A very straight linear anomaly bisects the survey on a northwest to southeast alignment delineating the route of a modern service trench that disturbed the Bromeswell Bucket, this anomaly was also recorded during the magnetometer surveys undertaken in 2001 and A further four linear anomalies, measuring a maximum of 20m and a minimum of 7m in length, are potentially of an archaeological origin. Three were orientated northwest to southeast and perpendicular, with the shortest aligned approximately east to west. Nine discrete low resistance anomalies present in the southern half of the dataset have been interpreted as archaeological pits, or sandy patches of geological derivation. One low resistance curvilinear anomaly surrounds an associated high resistance curvilinear anomaly in the southwestern corner that may comprise a large pit. Two narrow high resistance parallel linear anomalies are recorded in the dataset, orientated north to south, that are likely to be of agricultural derivation, potentially compacted furrows. To the east of the parallel linear anomalies is a curving broad high resistance linear response, possibly associated with a pathway that is located on Ordnance Survey mapping. Two broad high resistance linear trends are recorded in the southwestern corner that are potentially gravel deposits on the edge of the former river valley located to the south of Garden Field, or alternatively are indicative of wide banks of potential archaeological origin. One further high resistance curvilinear response, located to the north of the two broad linear trends, may prove to be the bank of a ditch, or a seam of naturally occurring gravel. Eight discrete high resistance anomalies recorded in the dataset are most likely to be caused by naturally occurring patches of gravel present within the geological horizon; however an archaeological origin cannot be discounted. 6. Conclusion The earth resistance meter was found to be particularly suited for undertaking visitor participation geophysical surveys and is relatively simple to operate despite its weight and cable attachments. Under close SACIC survey team supervision good quality data was recorded that was suitable for analytical interpretation. 7

15 The results reveal that there is good archaeological potential in the southeastern corner of Garden Field. Surveying the whole of the field with the RM85 however will greatly aid in the interpretation of the anomalies recorded thus far. Areas that are considered to have a special archaeological importance could also be targeted with tighter centres (0.5m sample interval x 0.5m traverse interval); this would allow an increase in resolution that could further aid in the interpretation of the geophysical anomalies. 7. Archive deposition The paper and digital archive will be kept at the SACIC office in Needham Market, before deposition in the Suffolk County Council Stores. 8. Acknowledgements The fieldwork was carried out by A level student Oliver Rose, SACIC geophysical field team members Ed Palka and Tim Schofield and the survey was directed by Tim Schofield. Project management was undertaken by Rhodri Gardner. The report and illustrations were created by Tim Schofield and was edited by Richenda Goffin. 8

16 9. Bibliography Ayala, G., et al., 2004, Geoarchaeology; Using Earth Sciences to Understand the Archaeological Record. English Heritage. Abbot, C., and Breen, A., 1997, Sutton Hoo Visitors Centre Archaeological Evaluation (BML 018). SCCAS Unpublished Report 97/11. Brown, N., and Glazebrook, J. (Eds), 2000, Research and Archaeology: a Framework for the Eastern Counties, 2. Research Agenda and Strategy. East Anglian Archaeology Occasional Paper No. 8. Chartered Institute for Archaeologists, 2014, Standard and Guidance for Archaeological Geophysical Survey. Clark, A, J., 1996, Seeing Beneath the Soil, Prospecting Methods in Archaeology. BT Batsford Ltd. London. Copp, A., et al., 1997, Geophysical and Topographic Survey, Sutton Hoo House area, Suffolk. Field Archaeology Specialists Unpublished Report, University of York. David, A., et al., 2014, Geophysical Survey in Archaeological Field Evaluation. Historic England. Dearing, J., 1999, Environmental Magnetic Susceptibility; Using the Bartington MS2 System. Fern, C, J, R., 2015, Before Sutton Hoo: The Prehistoric Remains and Early Anglo- Saxon Cemetery at Tranmer House, Bromeswell, Suffolk. East Anglian Archaeology 155. Gaffney, C., Gater, J., and Ovenden, S., 2002, The Use of Geophysical Techniques in Archaeological Evaluations. IFA Technical Paper No.6. Gaffney. C., and Gater. J., 2003, Revealing the Buried Past, Geophysics for Archaeologists. Tempus Publishing Ltd. Gardner, R., Sommers, M., 2013, Garden Field Metal Detecting Survey, Sutton Hoo, Bromeswell. SCCAS Unpublished Report 2012/197. Historic England, 2015, Management of Research in the Historic Environment (MoRPHE). Gurney, D., 2003, Standards for Field Archaeology in the East of England. East Anglian Archaeology Occasional Paper No 14. Linford, P., 2002, Sutton Hoo, Suffolk. Report on Geophysical Survey. English Heritage, Centre for Archaeology. Medlycott, M., (Ed)., 2011, Research and Archaeology Revisited: A revised framework for the East of England. EAA Occasional Paper 24. Ordnance Survey, 1983, Soils of England and Wales': Soil survey of England and Wales, sheet 4 Eastern England 1: 250,000. Harpenden. Schmidt, A., 2001, Geophysical Data in Archaeology: A Guide to good Practice. Archaeology Data Service. Oxbow books. SCCAS, 2010, Deposition of Archaeological Archives in Suffolk. SCCAS, 2011, Requirements for a Geophysical Survey. 9

17 Topham-Smith, C., 2000, A second Anglo-Saxon cemetery at Sutton Hoo, Saxon 33, 1 3. Schofield, T, P., 2016, Garden Field, Sutton Hoo, Suffolk. (BML037). SACIC Unpublished Report 2016/035. Simpson. T., 1999, Geophysical Survey, Sutton Hoo House, Suffolk. Field Archaeology Specialists Unpublished Report, University of York. Witten, A, J., 2006, Handbook of Geophysics and Archaeology. Equinox Publishing Ltd. London. Websites British Geological Survey, 2016, 10

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19 N STATION EASTING NORTHING Bromeswell Bucket Fragments STN STN STN TM Crown Copyright. All rights reserved. Licence no Figure 2. Earth resistance meter trial, survey grid, station and 'Bromeswell Bucket' locations m

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21 N Ω 234Ω TM Figure 3. Earth resistance meter trial, raw greyscale plot Crown Copyright. All rights reserved. Licence no m

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23 N Ω 112 Ω TM Figure 4. Earth resistance meter trial, processed greyscale plot Crown Copyright. All rights reserved. Licence no m

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25 Scale Interval 125Ω/cm N TM Figure 5. Earth resistance meter trial, processed xy trace plot Crown Copyright. All rights reserved. Licence no m

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27 High resistance anomaly N Low resistance anomaly TM Figure 6. Earth resistance meter trial, interpretation plot of anomalies 15 Crown Copyright. All rights reserved. Licence no m

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29 Appendix 1. Metadata Sheets Raw Data Filename Sutton Hoo 3 raw.xcp Description Instrument Type Resist. (RM85S) Units ohm Direction of 1st Traverse 90 deg Collection Method Parallel Sensors 1 Pair Dummy Value Dimensions Composite Size (readings) 80 x 120 Survey Size (meters) 80 m x 120 m Grid Size 20 m x 20 m X Interval 1 m Y Interval 1 m Stats Max Min Std Dev Mean Median Composite Area 0.96 ha Surveyed Area ha Program Name TerraSurveyor Version Raw Data Schedule Processes: 2 1 Base Layer 2 Clip at 1.00 SD

30 Processed Data Filename Sutton Hoo 3 pro.xcp Description Instrument Type Resist. (RM85S) Units Ohm Direction of 1st Traverse 90 deg Collection Method Parallel Sensors 1 Pair Dummy Value Dimensions Composite Size (readings) 160 x 240 Survey Size (meters) 80 m x 120 m Grid Size 20 m x 20 m X Interval 1m Y Interval 1m Stats Max Min Std Dev Mean Median Composite Area 0.96 ha Surveyed Area ha Program Name TerraSurveyor Version Processed Data Schedule Processes: 4 1 Base Layer 2 Clip at 1.00 SD 3 Despike Threshold: 0.5 Window size: 3x3 4 Interpolate: X & Y Doubled. Display Clip +/- 3SD Grad. Shade

31 Appendix 2. Technical Data Soil resistance The earth s soil has an electrical property known as conductivity or low resistance, that can be exploited by geophysical surveyors when prospecting for archaeological features. Naturally occurring minerals within the soil can be broken down by rain water forming electrolytes, that further break down into positive and negative ions. When a current is inserted into the ground these ions will either attract or repel the current driving it through the matrix along the path of least resistance. Two sets of probes are employed to measure the relative resistance of the soil matrix; the first are the current probes which inject an electrical signal into the soil that is measured by a second set of potential probes recording the current s density. Archaeological features contain varying amounts of soil moisture, for example a loose moisture rich pit or ditch will allow an injected electrical current to pass through it with relative ease, increasing the current density whilst decreasing the potential gradient and recording a low resistance anomaly within the dataset. Conversely a wall or road that is structurally dense, will repel the current driving it above and below the feature on its journey through the matrix, decreasing the current density and increasing the potential gradient recording a high resistance anomaly. Earth Resistance Meters A single twin (pole-pole) probe array was employed to undertake this survey, using one set of mobile probes that along with the instrument box are mounted to the frame, recording individual data points within the survey grid, and remote probes that are located at least 15m beyond the edge of the grid to avoid feedback. The remote probes act as a static control station that the mobile probe readings are measured against. A 50m cable connects the remote probes to the instrument box; to progress the survey the static station will need to be moved. A control reading is taken before and after the remote probes are moved, to enable grid matching from one section to another. The mobile probes are mounted 0.5m apart on the frame, with the remote probes pushed into the ground approximately 3 4m apart. Once the mobile probes are pushed onto the ground surface an electrical circuit is formed between the current electrodes of the remote and mobile probes; the potential gradient between the remote and mobile probes is then automatically recorded by the instrument. Removing the mobile probes

32 from contact with the ground resets the instrument ready for the next point, as soon as the probes contact the ground a circuit is once again formed, this point is then autologged by the instrument. Resistance Anomalies Discrete anomalies Discrete anomalies can be recorded with both high and low resistance, those with low resistance are likely to be moisture rich and those with high resistance are likely to have low moisture content compared with the surrounding matrix. Examples of low resistance anomalies include naturally occurring pockets of differing material within the geology, tree hollows or throws, glacial infilling of natural hollows, ponds, culturally excavated and backfilled storage or rubbish waste pits. High resistance anomalies are recorded where naturally occurring stone deposits, structural post pads, kilns, oven and hearth, furnace linings, rubble dumps are encountered. Linear trends Linear anomalies can also be either high or low resistance. Once again those with low resistance are likely to be moisture rich and conversely those with high resistance are likely have a low moisture content. Examples of low resistance linear trends include periglacial troughs, agricultural or settlement ditches, service run trenches. Examples of high resistance linear anomalies include geological rock formation, buried foundations, walls, metalled tracks or road surfaces, ditch banks.

33 Appendix 3. OASIS Form

34 OASIS DATA COLLECTION FORM: England List of Projects Manage Projects Search Projects New project Change your details HER coverage Change country Log out Printable version OASIS ID: suffolka Project details Project name Short description of the project Garden Field, Earth Resistance Meter Trial, Sutton Hoo, Suffolk In October 2016 Suffolk Archaeology Community Interest Company (SACIC) along with National Trust volunteers and visitors to Sutton Hoo undertook a reconnaissance earth resistance meter survey on land set aside as pasture for rare breed rams at Garden Field, Sutton Hoo, Suffolk. The geophysical survey successfully recorded high and low resistance anomalies indicative of archaeological ditches and pits, natural variations within the superficial geology and a modern service trench that disturbed the location of the 'Bromeswell Bucket'. Project dates Start: End: Previous/future work Any associated project reference codes Type of project Site status Current Land use Monument type Monument type Significant Finds Methods & techniques Yes / Yes Development type None Prompt Position in the planning process Solid geology (other) Drift geology Techniques BML037 Sitecode Field evaluation Scheduled Monument (SM) Grassland Heathland 4 Regularly improved DITCHES Uncertain PITS Uncertain NONE None ''Geophysical Survey'' Research Not known / Not recorded Thames Group, clay and silt RIVER TERRACE DEPOSITS Resistivity area Project location Country Site location Study area Site coordinates England SUFFOLK SUFFOLK COASTAL BROMESWELL Garden Field, Bromeswell, Suffolk 0.64 Hectares TM N E Point

35 Height OD / Depth Min: 32m Max: 32m Project creators Name of Organisation Project brief originator Project design originator Project director/manager Project supervisor Type of sponsor/funding body Name of sponsor/funding body Suffolk Archaeology CIC National Trust Tim Schofield Tim Schofield Timothy Schofield National Trust national Trust Project archives Physical Archive Exists? Digital Archive recipient Digital Contents Digital Media available Paper Archive recipient Paper Contents Paper Media available No National Trust ''Survey'' ''GIS'',''Geophysics'',''Images raster / digital photography'',''images vector'',''spreadsheets'',''survey'',''text'' National Trust ''Survey'' ''Plan'',''Report'',''Survey '',''Unpublished Text'' Project bibliography 1 Publication type Title Author(s)/Editor(s) Schofield, T, P. Other bibliographic details Date 2016 Grey literature (unpublished document/manuscript) Garden Field, Earth Resistance Meter Trial, Sutton Hoo, Suffolk Report 2016/089 Issuer or publisher Suffolk Archaeology CIC Place of issue or publication Description URL Needham Market A4 bound report, with A3 fold out figures Entered by Entered on 14 November 2016 Tim Schofield (tim.schofield@suffolkarchaeology.co.uk)

36 OASIS: Please e mail Historic England for OASIS help and advice ADS Created by Jo Gilham and Jen Mitcham, Last modified Wednesday 9 May 2012 Cite only: for this page

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38 Suffolk Archaeology CIC Unit 5 Plot 11 Maitland Road Lion Barn Industrial Estate Needham Market Suffolk IP6 8NZ Rhodri.Gardner@suffolkarchaeology.co.uk