Geophysical Survey of LAND NORTH OF THETFORD

LAND NORTH OF THETFORDFor Crown Estates

Helen Woodhouse MPhil

Geophysical Survey of

Geophysical Survey of

LAND NORTH OF THETFORDClient: Crown Estates

Local Authority: Breckland District Council

NGR: TL 86600, 85100

Author(s): H. Woodhouse

Illustrations: H. Woodhouse

Doc Ref: LP0460E-GSR-v1.0

Date: November 07

www.lparchaeology.com

A trading name of the L � P : Partnership Ltd.

The Compound | 79 Cowley Road | Cambridge, CB4 0DN | +44 01223 423 969

TABLE OF CONTENTSTable of Figures

Table of Plates

Table of Appendices

Abstract

1. Introduction and Scope of Study

2. Geology and Topography

3. Archaeological and Historical Background

4. Site Conditions

5. Methodology

6. Results & Interpretation

7. Conclusions & Discussion

8. Statement of Indemnity

REF: LP0460E-GSR-v1.0

TABLE OF FIGURESFigure 1 - Site Location General

Figure 2 - Site Location Detailed

Figure 3 - Interim unprocessed greyscale plot of geophysical survey results

Figure 4 - Interim processed greyscale plot of geophysical survey results

Figure 5 - Interim interpretative diagram of geophysical survey anomalies

Figure 6 - Interim interpretative diagram of geophysical survey anomalies superimposed upon greyscale plot of processed geophysical survey results

TABLE OF PLATESPlate 1 - General view across the area being surveyed

TABLE OF APPENDICESAppendix 1 - Sources Consulted

Appendix 2 - OASIS Form

Appendix 3 - Summary of Geophysical Prospection Methods


AbstractThis interim report presents the results of a geophysical survey carried out on land to the

north of Thetford, NorfolK. The work was undertaken on behalf of the Crown Estate as

the first phase of a geophysical survey which forms part of a strategy of non intrusive

investigation at the site. The survey was undertaken with the aim of determining the

potential extent of archaeological features in the vicinity of the Fison Way enclosure

beyond the area subjected to excavation in 1980-1982. This report specifies the

methodology employed together with a brief discussion and interpretation of the results.

The survey was successful in identifying a series of geophysical anomalies to the west and

north of the area excavated over the enclosure.


1. Introduction and Scope of Study

1.1. This geophysical survey report has been prepared by Helen Woodhouse of L - P :

Archaeology, the report was commissioned by the Crown Estate.

1.2. The report considers an area of land to the north of Thetford (FIGURE 1 & 2). The

survey was commissioned as part of an overall strategy of non intrusive survey at the

site designed to elucidate the potential for archaeology and to identity hot spots to

better locate subsequent intrusive investigations.

1.3.This report comprises an interim examination of the geophysical survey results and

seeks to address the following issues:

To assess the level of response of the geophysical survey equipment to the

archaeology

To assess the potential archaeology in the surveyed area

1.4. The discussion of the results presented here represents an initial analysis and

interpretation of the geophysical survey results. A more detailed interpretation and

contextual discussion will form the conclusion of the final report. The purpose of

this document is to present the results of the initial phase of the survey, assess its

potential as a survey method across the remainder of the site, and briefly discuss

some possibilities for interpretation.


2. Geology and Topography

2.1.GEOLOGY

2.1.1. The study site is located within the unique landscape phenomena of the

Breckland region which lies in the south west of Norfolk. Breckland is in

general a geological formation of free-draining aeolian sandy soils overlying

Cretaceous chalk.

2.1.2. Geophysical survey employed in the local area has generally demonstrated a

variable response upon the local geology. Magnetometer survey carried out in

1980 on the same site recorded only a weak and fragmentary response to the

outer ditch of the known enclosure in addition to ditches and pits on the

western side of the field (BARTLETT 1981).

2.2.TOPOGRAPHY

2.2.1. The study area comprises a plot of arable land c. 22 hectares in size, which is

relatively flat to the west and south but which slopes downwards in its north

eastern corner. The site is bounded to the north by the route of the A11 and

by the south by the Fison Way industrial estate.

2.2.2. An area c. 5.3 hectares in size was surveyed for this initial phase of

investigation. The area consisted of a tract of land around the western and

northern edges of the field in the areas closest to the A1066 towards Thetford

and the A11.


3. Archaeological and Historical Background

3.1. A detailed discussion of the archaeological and historical background to the site and

its relevance to the interpretation of the potential archaeological features in the

geophysical survey will form part of the final report. A high level of archaeological

deposits have been found in the surrounding area, details of which can be found in

the archaeological desk-based assessment report produced prior to this survey (MORSE ET AL 2006).


4. Site Conditions

4.1. SITE CONDITIONS

4.1.1. The site was covered with stubble which presented no real problems for the

execution of the survey. The only restriction to the survey area was an area of

thick scrub around the edge of the field that prevented the cart upon which the

geophysical survey equipment was mounted from being wheeled smoothly

and at a regular pace.


Plate 1 - General view across the area being surveyed

5. Methodology

5.1.The decision to use magnetometry on the site was based on its efficiency as a survey

technique suitable for detecting buried remains of materials derived from human

occupation with distinctly different magnetic susceptibility to the geological

background, and in particular those which have been subjected to heating or

burning processes such as the firing of clay for brick or ceramic goods (GAFFNEY ET

AL 1991:6, DAVID 1995:9). The results of this method are, however, severely restricted

in areas of modern disturbance and by the presence of ferrous material (GEOSCAN

RESEARCH 1996; SCOLLAR ET AL 1990:362).

5.2.Magnetometry has not proven to be consistently successful in obtaining good data

from either this site or from comparable sites on similar geological backgrounds

(e.g. BARTLETT: 1981), but given more recent advances in technology and the speed

with which large areas can be surveyed in comparison with alternative techniques it

was deemed suitable to apply in this instance.

5.3.Although a number of alternative geophysical survey techniques could have been

applied to the site (APPENDIX 3) magnetometry represents the best compromise

between speed and quality of data retrieval.

5.4.The geophysical survey grid based on 50m x 50m grids was set out prior to the

initiation of the survey with a Leica Smart Rover GPS system following a north south

alignment.

5.5.Readings were taken at a sample rate of 10 Hz along parallel traverses of 1m spacing.

At normal walking pace this ensured that readings were taken at 0.25cm intervals

along each traverse. Care was taken to ensure that readings were evenly spaced by

keeping the rate of walking even along each traverse. The G823 provides high

sensitivity at fast walking speeds enabling the survey to be completed rapidly.

Walking at a normal pace and sampling at 10Hz the machine has a sensitivity of

about 0.05nT.

5.6.The magnetometry survey was undertaken using a pair of Geomatrix G823 caesium

vapour magnetometers arranged in gradiometer mode. Caesium vapour equipment

has been shown to improve sensitivity to deeply buried weakly magnetised features


(e.g. LINFORD ET AL :2007). The sensors were mounted upon a non-magnetic cart

which both ensured that they were maintained stable and at a constant height above

the ground and separated them from the magnetic equipment utilised to record the

data.

5.7. The data was recorded on a field computer attached to the sensors and downloaded

onto the PC and converted to ASCII format via Trackmaker 823.

5.8. Utilising the sensors in gradiometer mode negated the need to correct for diurnal

variation via use of a second set of concurrent readings from a static base station.

The vertical subtraction from the total field readings recorded was performed on the

raw data in Open Office 2.1 immediately after download and prior to import into

the processing software.

5.9. The magnetometry data was imported into Geoplot 3.01t by Geoscan Research for

processing and analysis.

5.10. The graphical plots were geo-referenced using AutoCAD Civil 3D 2008. From this

set of geophysical data an interpretation plan was drawn (FIGURE 5) which shows the

discernible anomalies present across the survey area. This interpretative plan forms

the basis of the discussion of the results presented below


6. Results & Interpretation

6.1. The overall response of the survey area to magnetometry was reasonable with a

series of potential archaeological features showing against the geological

background. However, consultation of the plan of known archaeological features

from the excavated area demonstrated that many of the smaller more ephemeral

features were not visible in the results. Backfilling after the excavation may have

contributed to the depth of burial of the archaeological features, which in

conjunction with the variable response due to the local geology is probably

responsible for the weak response to the survey technique.

6.2.The strongest anomaly within the results represents one of the inner ditches of the

Fison Way enclosure [1]. The outermost ditch is also visible in the results but as a

much weaker anomaly [2].

6.3. Relatively few dipolar anomalies (paired positive and negative anomalies) were

recorded in the results. These sometimes result from the presence of fired material

in kilns and hearths but dipolar readings of the nature recorded here are more

usually interpreted as resulting from modern disturbance at the site such as the

presence of ferrous material on the surface or in the topsoil.

6.4. An equally small number of negative geophysical anomalies were recorded. Isolated

negative responses such as these usually correspond to the subsurface presence of

features of low magnetic susceptibility such as sedimentary rocks or voids.

6.5. The most numerous type of geophysical anomaly visible across the area surveyed

are positive maculae. Isolated positive anomalies such as these can be interpreted as

pits. When the readings suggest a particularly intensive response this may

correspond to hearths, ovens or kilns. However it is also possible for these

anomalies to result from naturally occurring features, so they are best interpreted

within the context of supporting information.

6.6.Comparison of the graphical summary of the interpretation and the plan of the

excavated area suggests that in this case it may not be appropriate to interpret the

majority of these anomalies as archaeological pits. Few of the anomalies correspond

with pits but they often correspond to the presence of intersecting features. It is


possible therefore that many of these anomalies result from the variable response

over the archaeology and should be interpreted as possible evidence for

archaeological activity rather than as specific individual features.

6.7.Two particularly large positive anomalies are worth mentioning at this stage [3] and

[4] which are located c. 20m apart towards the north western corner of the field and

have a diameter of c. 5m.

6.8.In addition a series of linear and curvilinear positive anomalies are visible in the

results some of which can be identified within the plan of excavated features. These

anomalies have the highest potential to result from archaeological activity and are

likely to represent features such as ditches, earthworks or gullies associated with

human occupation at the site, although some of them may represent modern field

drains. These will become easier to interpret once the survey area has been

completed but at this stage it is possible to suggest that the larger of these [e.g. 5 &

6] represent part of a coaxial field system which can be associated with the main

enclosure due to their consistent alignment.

6.9. A series of roughly parallel linear anomalies [7] are visible running broadly west-

east in the central section of the survey which corresponds to the downward slope

into the north eastern corner. These probably represent modern plough furrows

which were particularly evident in this area of the site.

6.10.A series of curvilinear anomalies in the north eastern corner of the survey area [8]

are potentially geological or natural, resulting from deposition at the base of the

slope in this area.


7. Conclusions & Discussion

7.1. The results demonstrate that although the response to magnetometry is not

exceptional, a range of features many of which are potentially archaeological are

visible in the survey results.

7.2. The known major archaeological features have so far been visible within the survey

which suggests that any further unknown major features may respond well enough

to the survey to be identified.

7.3. The main concentration of potential archaeological anomalies in the northern and

western region of the survey area corresponds with the results of the magnetic

susceptibility survey carried out in conjunction with the Fison Way excavation

(GREGORY 1991:186-187).


8. Statement of Indemnity

8.1.Whilst every effort has been made to ensure that interpretation of the survey

presents an accurate indication of the nature of sub-surface remains, any conclusions

derived from the results form an entirely subjective assessment of the data and the

importance attributed to certain features is a product of the judgement of the

surveyors. Geophysical survey facilitates the collection of data relating to variations

in the form and nature of the soil. This may only reveal certain archaeological

features, and may not record all the material present.


FIGURES

DOC REF: LP0460E-GSR-v1.0

0 1 km

FIGURE 1 // Site Location - General

CLIENT // The Crown Estate

PROJECT // 0460L- North Thetford

DESCRIPTION // Site Location - General

DOC REF: LP0460E-GSR-v1

PROJECT // 0460E - North Thetford

Reproduced by permission of the controller of HMSO, Licence AL52941A


FIGURE 2 // Site Location - Detailed

0 500 m

DESCRIPTION // Detailed Site Location




FIGURE 3 // Unprocessed Plot of Results

DESCRIPTION // Unprocessed greyscale plot of geophysical survey results

0 100m-3SD Mean 3SD




FIGURE 4 // Processed Plot of Results

DESCRIPTION // Processed greyscale plot of geophysical survey results

0 100m-3SD Mean 3SD

[1]

[2]

[3]

[4]

[7]

[8]

[5]

[6]




FIGURE 5// Interpretative diagram of anomalies

DESCRIPTION // Interpretative diagram of geophysical survey anomalies

0 100m

Positive Linear

Positive Curvilinear

Natural or Geological

Modern Ploughscars

Positive Maculae

Negative Maculae

Dipolar Maculae

[1]

[2]

[3]

[4]

[7]

[8]

[5]

[6]




FIGURE 6// Interpretative diagram of anomalies

DESCRIPTION // Interpretative diagram of survey anomalies with plot

0 100m

Positive Linear

Positive Curvilinear

Natural or Geological

Modern Ploughscars

Positive Maculae

Negative Maculae

Dipolar Maculae

-3SD Mean 3SD

SOURCES CONSULTEDAPPENDIX 1


BIBLIOGRAPHICBARTLETT ADH, 1981. Gallows Hill Thetford. Unpublished Ancient Monuments Laboratory Report 3405.

CLARK AJ, 1990. Seeing Beneath the Soil. Batsford. London

DAVID A, 1995. Geophysical Survey in Archaeological Field Evaluation. English Heritage

GAFFNEY C, GATER J, & OVENDEN S. 1991. The Use of Geophysical Survey Techniques in Archaeological Evaluations. Institute of Field Archaeologists Technical Paper No. 9

GEOSCAN RESEARCH. 1996. Fluxgate Gradiometer: Instruction Manual Version 1.2

GREGORY T, 1991. Excavations in Thetford 1980-1982, Fison Way. Volume 1. East Anglian Archaeology Report 53: Norfolk Field Archaeology Division.

LINFORD N. LINFORD P. MARTIN L. & PAYNE A. 2007. Recent Results from the English Heritage Caesium Magnetometer System in Comparison with Recent Fluxgate Gradiometers. Archaeological Prospection 14; 151-166

MORSE C, YOUNG J AND EVE S, 2006. Archaeological Deskbased Assessment of Land at Kingsfleet, Thetford. Unpublished Archive Report L – P : Archaeology.

SCOLLAR I. TABBAGH A. HESSE H. & HARZOG I. 1990. Archaeological Prospecting and Remote Sensing. Cambridge University Press: Cambridge

SOIL SURVEY OF ENGLAND & WALES 1983. Soils of England and Wales. Sheet 4, Soils of Eastern England. Ordnance Survey


OASIS FORMAPPENDIX 2


SUMMARY OF GEOPHYSICAL PROSPECTION METHODSAPPENDIX 3


The following presents a summary of two geophysical prospection methods which are

available to the archaeologist in determining the extent and nature of sub-surface structures

and remains. Details of survey methodology are dealt with elsewhere (Geoscan Research

1996) and so will not be discussed here.

Resistivity Survey

Resistivity survey is based on the ability of sub-surface materials to conduct an electrical

current passed through them. Differences in the structural and chemical make-up of soils

affect the degree of resistance to an electrical current (Clark 1990, 27).

The technique involves the passing of an electrical current through a pair of probes into the

earth in order to measure variations in resistance over the survey area. Resistance is measured

in ohms ( ), whereas resistivity, the resistance in a given volume of earth, is measured inΩ

ohm-metres ( m). Ω

Four probes are generally utilised for electrical profiling (Gaffney et al. 1991, 2), two mobile

and two remote probes. Resistivity survey can be undertaken using a number of different

probe arrays; twin probe, Wenner, Double-Dipole, Schlumberger and Square arrays.

Twin Electrode Configuration

This array represents the most popular configuration used in British archaeology (Clark 1990;

Gaffney et al. 1991, 2), usually undertaken with a 0.5m separation between mobile probes.

The twin probe array configuration utilises two probes on a mobile frame, with two remote

probes located at least 30 times the distance between the mobile probes away from the

mobile frame.

Alterations can be made to suit different conditions. A number of factors may affect the

interpretation of twin probe survey results, including the nature and depth of structures, soil

type, terrain and localised climatic conditions. The response to non-archaeological features

may lead to a misinterpretation of the results, or the masking of archaeological anomalies. A

twin probe array of 0.5m will rarely recognise features below a depth of 0.75m (Gaffney et al

1991). More substantial features may register up to a depth of 1m.


With twin probe arrays of between 0.25m and 2m, procedures are similar to those for the

0.5m twin probe array. Although changes in the moisture content of the soil, as well as

variations in temperature, can affect the form of anomalies present in resistivity survey results,

in general, higher resistance features are interpreted as structures which have a limited

moisture content, for example walls, mounds, voids, rubble filled pits, and paved or cobbled

areas. Lower resistance anomalies usually represent buried ditches, foundation trenches, pits

and gullies.

Multiplex Resistivity

Modification of a standard resistivity array through the use of multiplexing hardware enables

a series of readings to be taken at each point in each traverse of the survey. The basic

principles of the survey are identical to standard two probe resistivity survey however the

multiplexing hardware enables the resitivity kit to be used for a number of additional

applications. Firstly although perhaps least commonly it is possible to carry out basic

resistance tomography with a series of probes placed along a fixed traverse. Although this

method of prospection is often used in geological surveys it’s applicability to archaeological

geophysics is limited. More common within archaeological geophysics is to use the

multiplexer to enable a series of different readings to be taken at varying probe separations at

each point in a survey grid. This effectively allows a series of resistance plots to be derived at

different depths depending on the probe separations employed. Whilst this technique does

not provide a true sectional picture of the archaeological deposits surveyed it does have the

advantage of being able to see under anomalies in the upper layers potentially allowing a

sequence to be established, or for overlaying anomalies to be defined as individual entities.

Magnetic Survey

Magnetic prospection of soils is based on the measurement of differences in the magnitude of

the earth’s magnetic field at points over a specific area. The iron content of a soil provides the

principal basis for its magnetic properties. Presence of magnetite, maghaematite and haematite

iron oxides all affect the magnetic properties of soils.

Although variations in the earth’s magnetic field which are associated with archaeological


features are weak, especially considering the overall strength of the magnetic field of around

48,000 nano-tesla (nT), they can be detected using specific instruments (Gaffney et al. 1991).

Of the various types of magnetometer available to the archaeologist; Fluxgate gradiometers are

the most widely utilised instruments in British archaeological geophysics.

Fluxgate Gradiometer

Fluxgate instruments are based around a highly permeable nickel iron alloy core (Scollar et al.

1990, 456), which is magnetised by the earth’s magnetic field, together with an alternating

field applied via a primary winding. Due to the fluxgate’s directional method of functioning,

a single fluxgate cannot be utilised on its own, as it cannot be held at a constant angle to the

earth’s magnetic field. Gradiometers therefore have two fluxgates positioned vertically to one

another on a rigid staff. This reduces the effects of instrument orientation on readings.

Fluxgate gradiometers are usually sensitive to 0.1nT depending on the instrument. The depth

of the features which can be located is dependent upon the sensor spacing of the individual

instrument and usually varies between 0.5m and 1.5m below the surface of the ground.

Archaeological features such as brick walls, hearths, kilns and disturbed building material will

be represented in the results, as well as more ephemeral changes in soil, allowing location of

foundation trenches, pits and ditches. The results are however extremely dependent upon the

geology of the particular area, and whether the archaeological remains are derived from the

same materials.

Caaesium Vapour or Optically Pumped Magnetometer

An electron in orbit has an electrical charge and therefore a small magnetic moment. This

moment means that the energy of the electron will vary dependent upon the direction of its

spin axis relative to an ambient magnetic field. When monochromatic light passes through

the magnetic field of a material the electromagnetic properties of the light are altered. By this

principle measurement of the variations induced in the light when passed through the

electrons of a material can be utilised to measure changes in the magnetic field. In the highly

sensitive Caesium Vapour magnetometer the light is passed through the single electron in the

outer most electron shell of the vaporised caesium atom.


Geophysical Survey of LAND NORTH OF THETFORD

Documents

Transcript of Geophysical Survey of LAND NORTH OF THETFORD