fixed-frequency radio-wave imaging of subsurface archaeological features: a minimally invasive...

8/13/2019 FIXED-FREQUENCY RADIO-WAVE IMAGING OF SUBSURFACE ARCHAEOLOGICAL FEATURES: A MINIMALLY INVASIVE TE

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Archaeometry 47, 1 (2005) 159173. Printed in Great Britain

* Received 27 June 2003; accepted 18 July 2004.

University of Oxford, 2005

BlackwellPublishing,Ltd.Oxford,UKARCHArchaeometry0003-813XUniversityofOxford,2005February2005471ORIGINALARTICLERadio-wave imaging of subsurface archaeological featuresL. SOMERS ETAL.

FIXED-FREQUEN CY RADIO-WAVE IMAGING OF

SUBSURFACE ARCHAEOLOGICAL FEATURES:

A MINIMALLY INVASIVE TECHNI QUE FOR STUDYINGARCHAEOLOGICAL SITES*

L. SOMERS,

1

N. LINFORD,

2

W. PENN,

3

A. DAVID,

2

L. URRY

4

and

R. WALKER

5

1

Geoscan ResearchUSA, Sea Ranch, California, USA2

Archaeometry Branch, English Heritage Centre for Archaeology, Fort Cumberland, Eastney,

Portsmouth PO4 9LD, UK3

Department of Electrical Engineering, Syracuse University, Syracuse, New York 132441240, USA4

Department of Radio Astronomy, University of California at Berkeley, Berkeley, California 947203411, USA5

Geoscan Research, Heather Brae, Chrisharben Park, Clayton BD14 6AE, UK

In memoriam Tony Clark

Sadly, this research marked the last field visit to the test site that the authors were able to

share with their colleague Tony Clark. His enthusiasm, experience and encouragement

inspired us to complete this project that, to many, must have appeared as grown men at play

in a sand pit.

A new approach to subsurface exploration has been developed, based on fixed-frequency

radio-wave illumination from a buried transmitter. Data are collected from a buried,

continuous wave source with a phase-coherent surface-scanning receiver and recorded in a

digital archive from which images of the archaeological features are subsequently produced.An important feature of this approach is the opportunity to separate the data collection and

archive functions, which form an uncompromised record of the site, from the more subjective

image formation function. This paper reports the results of a feasibility assessment programme

and discusses the future application of the method to real archaeological sites.

KEYWORDS:

RAYLEIGHSOMMERFELD, PHASE COHERENCE, IMAGING,

ARCHAEOLOGY, GROUND-PENETRATING RADAR

*Received27June 2003;accepted18 July2004.

INTRODUCTION

The full potential of microwave investigation techniques in archaeology has yet to be realized,as the majority of studies to date have utilized non-coherent impulse sources (e.g., Vaughan

1986; Stove and Addyman 1989; Conyers and Goodman 1997). The purpose of the present

study has been to explore a fundamentally new approach that utilizes a fixed-frequency con-

tinuous radio-wave source to illuminate the subsurface from below using a buried transmitter.

The transmitter would be introduced beneath the target archaeological features through a

minimally invasive borehole with the receiver antenna positioned on the site surface. This

approach is well matched to the geometry and propagation characteristics of many archaeolog-

ical sites, and offers the opportunity to image features exhibiting a contrast in conductivity or

dielectric constant.


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Many archaeological sites are characterized by a large area-to-depth geometry, and the

materials and soils from which they are formed are often semi-transparent to radio waves of a

suitable frequency. In addition to being semi-transparent, the electromagnetic properties of the

archaeological materials alter the amplitude, phase and polarization of the incident electro-

magnetic radiation. The variation of these three properties allows the construction of an imagein the same manner as an optical lens, and offers the potential for more detailed material

recognition and in situ

analysis where a diagnostic alteration of amplitude, phase or polarization occurs.

Initially, it is useful to consider how the proposed imaging system differs from a conven-

tional bistatic impulse radar (GPR). As the name suggests, an impulse GPR emits a brief pulse

of electromagnetic radiation from a dipole antenna (transmitter) to penetrate below the ground

surface. This pulse is comparatively broadband, containing a range of frequencies to either

side of a centre frequency determined by the physical design of the antenna (e.g., Conyers and

Goodman 1997, fig. 8). If the transmitted pulse is incident upon a buried object with differing

material properties (with respect to the transmission of electromagnetic radiation) to the

surrounding soil, a reflection of the pulse will be created. These secondary reflections arereceived by a separate dipole antenna (the receiver, generally mounted together with the trans-

mitter antenna) back at the ground surface that records both the amplitude of the reflected

signal and the time delay from the initial incident pulse, known as the two-way travel time.

To create a three-dimensional image from such timeamplitude data, a series of measure-

ments must be made, injecting a pulse of energy over a regular survey grid established on the

ground surface. The vertical dimension or depth is obtained from the two-way travel time

between the initial pulse and secondary reflections, calibrated to a physical depth through an

estimate of the average velocity of the electromagnetic pulse through the ground surface at the

site. Such data is often displayed through a series of time slice images constructed from the

horizontal variation of the reflection amplitude recorded through a given window of two-waytravel time across a number of closely spaced parallel traverses (e.g., Meats 1996; Conyers and

Goodman 1997; David and Linford 2000; Neubauer et al

. 2002).

The currently proposed system differs from an impulse GPR by utilizing a continuous,

rather than a pulse, source, at a fixed frequency produced by a transmitter introduced by means

of an invasive borehole beneath the site under investigation. Measurements are then made

with a passive dipole receiver antenna over a survey grid established on the site surface. This

receiver records the amplitude and phase of the signal from the buried transmitter at every

sample station over the survey grid. As the energy emitted by the transmitter is continuous,

there is no pulse and thus no travel time associated with secondary reflections from this

pulse received at the ground surface. Data from such a phaseamplitude system are moreclosely analogous to an optical imaging device, where a lens is used to provide a variable

plane of sharply defined focus at a certain distance.

Conceptually, the proposed system is analogous to an optical light transmission microscope,

and Figure 1 (a) illustrates this in conventional optical terms (Gaskill 1978; Goodman 1996;

Born and Wolf 1997). In the case of an optical microscope, the semi-transparent target object

under investigation is introduced on a cover slide between the microscope lens and an under

stage source of illumination. The image formed in the microscope eyepiece is created through

the modulation, in terms of phase and amplitude, that occurs as light from the under stage

source of illumination passes through the semi-transparent target object. The aperture of the

microscope lens collates incident light from across the target object and produces a focusedimage within the eyepiece. Note that altering the focus of the lens, to better image an aspect

of the target object, does not change the energy incident upon the lens aperture but reprocesses


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Figure 1 A conventional optical microscope imaging system (a), illustrating the illumination of the target object by a light sou

from this source is modulated, in terms of amplitude and phase, by the target object and falls on the pupil plane of the microsinto a coherent image. Note that the ability of the lens to focus on differing elements of the target object is due to the optical

lected from the pupil plane that, for a fixed sample stage and light source, remains invariant. The proposed radio-frequency (RF)

system, is shown in (b). Here, the RF source is introduced through a vertical borehole beneath the target archaeology and th

where a scanning receiver antenna can record the modulated phase and amplitude of the illuminating source once it has pass

semi-circular lines on this figure represent the divergent spherical wavefront propagating from the buried RF transmitter. C

produced through numerical processing of the amplitude and phase data set (in this case, using the RayleighSommerfeld inte

mation within a conventional optical lens.


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this information to create the new image in the eyepiece. In this respect, the lens of a micro-

scope (or any other optical system) functions as an optical computer that may be simulated by

a mathematical model to demonstrate the concept of refocusing the phaseamplitude data

collected over the aperture of the lens into a focused image.

The proposed radio-frequency implementation of this system investigated here is illustratedin Figure 1 (b). In this case, the subject of the microscope is intended to be archaeological

remains illuminated from below by means of a fixed-frequency radio source introduced

through a borehole from the surface. Radio waves from this source propagate up to the site

surface, and their amplitude, phase and polarization are modulated by the electromagnetic

properties of the subsurface, including any archaeological remains, through which they may

pass. The resultant modulated wave function is recorded at the ground surface by means of a

receiver antenna that records the amplitude and relative phase of the signal across a two-

dimensional survey area. This information is analogous to the light falling on the pupil plane

of an optical microscope prior to the formation of a focused image through the viewing lens.

The data recorded by the scanning receiver forms a unique digital archive that may then bevisualized computationally into a focused image.

One may consider such an imaging system to be merely a two-dimensional, trans-illumination

methoda shadow plot, if you likemapping the combined attenuation of objects buried

in the subsurface. This would, however, fail to recognize the full potential of the recorded

phaseamplitude data set that may, through the use of an appropriate mathematical lens, be

reprocessed into a series of images sharply focused at an increasing distance, or depth, from

the ground surface. Again, by way of optical analogy, this process represents the ability of a

photographer to focus his or her camera lens either on the bars of the enclosure at the zoo, or

beyond these, to bring the tiger sharply into view. As with impulse ground radar, the image

formed at a particular point of focus will be contaminated by the response due to featureslying above and below the current depth of interest. However, the degree of image degradation

will vary depending on the archaeological features present, with more intrusive materials, such

as stone or brick foundations, having a much greater influence than pits, ditches and decayed

timber post-holes.

Background

Anthropogenic activity and processes alter the electromagnetic properties of the subsurface.

Some of these alterationssuch as the introduction of wall stones that differ in conductivity

or dielectric constant from the surrounding soilare intrusive. Others occur at a microscopicscale through chemical or thermal alterations of the soil particle size, compaction, magnetic

permeability, moisture or soluble ion concentration. It is this record, written on the ancient

landscape, that the archaeologist wishes to read. To enable this process through subsurface

imaging with radio waves, the target archaeological features must introduce sufficient ampli-

tude, phase or polarization modulation to be distinguishable from the surrounding soil and

geological matrix, and the imaging system must provide a direct means of rendering these

modulations as image contrast. Furthermore, the imaging system must deliver archaeologi-

cally useful lateral resolution, in terms of the physical size of the target archaeological features

that may be resolved, and the effective depth of investigation.

Contemporary methods of detecting and mapping the archaeological record, (e.g., Clark1990; Scollar et al

. 1990) suffer loss of lateral resolution with increasing depth of investiga-

tion. Resistivity, conductivity (EM) and impulse ground-penetrating radar (GPR) methods


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Radio-wave imaging of subsurface archaeological features

163

suffer due to the angular divergence of the measuring fields that they introduce into the soil,

and for GPR the attenuation due to the conductivity of the sediment matrix. Magnetic survey

methods suffer because of the approximate 1/

r

2

decay of the field amplitude from magnetic

objects (Blakely 1995), and are often further compromised by the presence of highly magnetic

structures found on the surface of urban sites that completely mask the weaker anomaliesexpected from archaeological features.

In theory, the proposed system provides a comprehensive amplitude and phase data set for

a given observation frequency, polarization and illumination geometry. The lateral resolution

of the system will, if adequately sampled, be constrained only by the observation frequency

used and the attenuation of the signal between the buried transmitter and the receiver on the

ground surface. In addition, the amplitude and phase data have not been subject to any form

of processing and represent a comprehensive archive of the physical properties of the site.

These data may, therefore, be reprocessed at a later date with newly developed algorithms as

these become available. To return to an optical analogy, the proposed system represents the

ability to record and archive the light falling upon the aperture of camera lens (the pupil plane)before this data is processed by the lens to form an image on the film (or digital sensor) at the

image plane. Thus, should a new, higher-quality camera lens become available, then the

original archived pupil plane information may be reprocessed to produce a new negative, with

improved resolution over the original image.

METHOD

To demonstrate this concept, two studies have been performed, one based on a numerical

(computer) model and the other on a radio-frequency field prototype. The numerical model used

a mathematical algorithm to determine the expected amplitude and phase data that would berecorded by a scanning antenna across the receiver plane. This computer-generated model allowed

a range of parameters within the proposed imaging system to be simulated, such as the frequency

of the radio-frequency source, prior to the development of a practical field instrument. Informa-

tion from the numerical model informed the construction of a suitable test site, in terms of the

physical size of target object that might be resolved, for use with the field system as this was par-

tially restricted by the availability of an affordable radio-frequency transmitter. The numerical

model study also provided an idealized data set, uncompromised by the influence of either noise

or attenuation, for comparison with the results of the subsequent field prototype study.

Numerical model study

The numerical study examined the imaging system shown in Figure 1 (b) through the means

of a mathematical model consisting of the following:

(1) A source transmitter beneath the object plane, producing a diverging spherical wavefront

to illuminate the target objects.

(2) Target objects in the object plane, modulating the amplitude and phase of the incident,

illuminating, wavefront.

(3) Diffraction between the object plane and the pupil plane (the site surface).

(4) Sampling of the modelled amplitude and phase in the pupil /receiver plane (the site surface)

at approximately half the wavelength of the RF source.(5) The formation of a focused image from the modelled amplitude/phase data by means of

the RayleighSommerfeld integral acting as a mathematical lens.


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The modelling algorithm was used to study focus and spatial resolution in the proposed

system. This allowed the influence of many parametersfor example, the sample interval inthe receiver plane, and the frequency and depth of the illuminating sourceto be studied prior

to the implementation of a field prototype. From this numerical study, it was a determined that

a practical field prototype of the imaging system could be constructed. However, note that the

algorithm used by the numerical model did not account for signal attenuation in the ground

surface between the source transmitter and the receiver plane.

Images produced by the numerical model are presented for comparison with the results

from the field prototype study in Figures 5 and 6, which are discussed below.

Field prototype study

A prototype imaging instrument was implemented at 137 MHz by means of a phase-coherent

radio illumination receiver system constructed with components from aircraft communication

Figure 2 A block diagram of the field prototype study electronic components.


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radios (Narco 457-A). Figure 2 is a schematic of this system and Figure 3 shows the elec-

tronic equipment deployed in the field. The field prototype study was conducted at the Hall

Aggregates Sand Quarry, Lenham, Kent, UK, a site chosen for the uniformity and depth (

100

m) of the coarse yellowish-green sands of the Folkestone beds sand deposits (British Geolog-

ical Survey 1958) and the ability to construct a resolution test object made up from sheet metal

buried at 3 m below the surface.The test site was constructed by using a 360-degree mechanical excavator to dig a 20 m

7.2 m trench to a depth of 3 m in a suitable area of previously undisturbed sand within the

Figure 3 A photograph of the test site (a), showing the location of the scanning receiver antenna, the wellhead o

the central vertical borehole through which the transmitter source was introduced beneath the target objects and elec-tronic equipment. Detail of the receiver antenna (b) and transmitter antenna (c) being lowered down the borehole are

also shown.


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quarry. The first target object was then constructed by laying a series of 3 m

1 m sheets of

corrugated iron at the base of the trench, following the geometry illustrated in Figure 4. Two

additional sheet-metal bar-type targets at an identical depth of 3 m were also constructed in a

similar manner, with all of the construction trenches finally back-filled with sand, which was

compressed by the mechanical excavator. The bucket of the excavator was then used to smooth

the surface of the site above the target objects that would serve as the receiver plane during the

field experiment. Finally, a PVC-lined borehole was drilled through the centre of the site to

a depth of 10 m and capped with a steel wellhead, to conform with site health and safety

requirements.Amplitude and phase data were collected by the scanning receiver (Fig. 3 (b)) and was

digitized during acquisition for subsequent use with the RayleighSommerfeld integral to

form images of the sheet-metal test objects. The scanning receiver was fixed to a small wooden

sledge and manually manoeuvred over a 0.1 m

0.2 m sample grid across the site surface

(receiver plane). Note that the buried transmitter produced a continuous signal rather than a

short impulse. This allowed the scanning receiver to determine the variation of both amplitude

and phase across the site by reference to a second fixed receiver antenna on the surface. Whilst

the dimensions selected for the sheet-metal test targets were somewhat greater than those of

real archaeological objects, due to the limited resolution that can be achieved when operating

at 137 MHz, it was hoped that a field prototype would prove the practical application of theimaging system and gather essential information to assist with the development of subsequent

instrumentation.

Figure 4 The test site geometry (a), showing the central vertical borehole for the transmitter source and the target

objects, constructed from corrugated iron sheets, at a depth of 3 m below the site surface. A plan view detailing the

dimensions of the knife-edge and two-bar object is shown in (b). The central 10 m 10 m receiver plane (site sur-

ace), from where the grid of amplitude and phase data was collected, is illustrated in both parts of this figure.


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THEORY

The resulting digital archive from the field prototype study represents the amplitude and phase

data at the receiver plane, and must be processed by a suitable mathematical algorithm to

act as a lens and provide a focused contrast image of the subsurface. In this case, Rayleigh

Sommerfeld diffraction theory (Goodman 1996; Born and Wolf 1997) is used to characterizeand model wave propagation between the object plane (target objects) and the pupil plane (site

surface), as well as between the pupil plane and the final image. As shown in Figure 1 (b), a

Figure 5 Images of the one-dimensional knife-edge scan calculated from the numerical model to simulate the

expected results are shown in (a) and (b) for comparison with images created from actual field data collected from

the pupil (receiver) plane of the knife-edge scan (c, d). The numerical model and actual field data have been pro-

cessed for two incident wavelengths, = 0.5 m and = 0.8 m. The y-axis of each graph represents the relative image

brightness versus distance across the scan traverse (the x-axis). For the field data, this is measured in units of milli-

volts from the amplitude of the received signal at the site surface, arbitrary units being used for the numerical study.


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diverging spherical wavefront (represented by the concentric semi-circular lines in the figure)

is launched from the origin and illuminates the object plane from below. The amplitude and

phase of this wave (

U

0

) is modulated by the archaeological targets as it passes through the

object plane to the site surface. Obviously, in the current study the use of opaque sheet-metaltargets will reduce the amplitude of the incident wave to zero, effectively casting a shadow

on to the receiver plane at the site surface. However, real archaeological targets would exhibit

Figure 6 Grey-tone images of the two-dimensional survey calculated from the numerical model to simulate the

expected results are shown in (a)(d) for comparison with images created from the actual field data (ad), focused

at depths of 1 m, 3 m, 5 m and 10 m. The quality of the images created from the field data has suffered due to a com-

bination of instrument noise, signal attenuation and RF leakage through the transmitter power cable.


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a greater degree of transparency, as evidenced by the results of impulse GPR surveys.

The RayleighSommerfeld integral is used to describe the wave function (

U

1

) at the receiver

plane (site surface):

(1)

In equation (1), U

1

(

x

1

, y

1

) represents the complex wave function at the site surface, U

0

(

x

0

, y

0

)

represents the complex wave function in the object plane and cos(

5

, @

r

01

) is the obliquity term.

Note that a negative exponent corresponds to a diverging wave function. The scalar r

01

is the

distance from any point being evaluated in the image plane (

x

1

,y

1

) to the running point of inte-

gration of the pupil-bounded wave function in the receiver plane (

x

0

,y

0

). This distance is given by:

(2)

where L

B

is the focal distance. Equations (1) and (2) form the basis of the numerical modelused to estimate the variation of amplitude and phase in the receiver plane.

The RayleighSommerfeld integral may also be used as the lens to focus the amplitude and

phase data from the receiver plane for final image formation. In this case, the pupil plane wave

function is launched with the same spatial values with which it was received (

U

1

), but with a

positive exponential (converging wave function). The net effect of this model is to create a planar

lens with 1:1 imaging of the complex object wave function into a complex image wave function.

RESULTS

Numerical model study

Two different amplitude target objects were modelled: the first being an opaque knife-edge,

which forms a one-dimensional object; and the second being an extension of the knife-edge to

include two additional opaque bars, forming a two-dimensional object. Both were chosen for

their simplicity of implementation in the field as well as in the numerical model. The relatively

large object dimensions were necessitated by the chosen operating frequency and the corre-

spondingly long wavelength.

The divergent wave function for all numerical studies was calculated over a 30 m

30 m

object plane and then integrated to a 10 m

10 m pupil plane (scanning receiver). The dimen-

sions of the objects were identical in both the numerical model and the field prototype study,as were the data sample intervals (0.1 m

0.2 m) in the receiver (pupil) plane.

One-dimensional study

The results of the numerical model study of the one-dimensional

knife-edge calculations using the RayleighSommerfeld integrals are shown in Figures 5 (a)

and 5 (b) for wavelengths of 0.5 m and 0.8 m, respectively. Both numerical models produce

convincing images, with a clear distinction between the shadow caused by the attenuation of

the knife-edge target and the bright, unmodulated portion of the object plane. The monochro-

matic nature of the image is evident in the ripples that are visible in the bright portion. The

wavelengths of 0.5 m and 0.8 m were chosen as best estimates, and reflect the uncertainty in

the measured wavelength encountered during unsuccessful attempts to determine this directlyin the field. A polychromatic image made over a narrow range of frequencies would, of course,

reduce these ripples.

U x y U x y P x y r r x y

jkr

1 1 1 0 0 0 0 0

01

01 0 0

1 01

( , ) ( , ) ( , ) cos( , )=

j

ed d

5 @

r x x y y LB01 1 02

1 02 2 ( ) ( )= + +


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L. Somers et al.

Two-dimensional study

The results of the two-dimensional test object calculations using the

RayleighSommerfeld integral are shown as grey-tone images in Figures 6 (a)(d) for a wave-

length of 0.8 m. This wavelength was chosen as the most likely estimate based on the known

frequency of the transmitter source (137 MHz) and a realistic average velocity of the wave-

front in dry sand (

0.11 m ns

1

; e.g., Reynolds 1997, table 12.3). The pupil plane wave func-tion was calculated and subsequently imaged at a number of focal distances by altering the

value of L

B

in equation (2) to 1 m, 3 m (the object plane), 5 m and 10 m (the illuminating

transmitter plane). The images behave as expected from the optical analogy, with the objects

most clearly focused at 3 m and the illuminating source sharply defined at 10 m. It is of inter-

est to note that although the model fails to fully account for attenuation within the subsurface

medium, the image amplitude decays rapidly at the edges of the plot. Indeed, the long bar

object in the image focused at 3 m (top of Figure 6 (b)) is only partially visible as a linear

feature of much reduced amplitude.

When focused above and below the object plane (Figs 6 (a) and 6 (c)), the images become

more blurred and the ability to resolve the target object is lost. In a similar manner, whenfocused at 10 m the buried source is clearly defined as a central bright feature (Fig. 6 (d)). This

clearly demonstrates the ability to computationally form focused images of target objects at

different depths.

Field prototype study

Figure 4 illustrates the geometry of the test site used for the one- and two-dimensional imag-

ing surveys. In both cases, the amplitude objects were constructed from individual sheets of

3 m

1 m corrugated iron, buried at the bottom of an excavated 3 m deep trench in the sand.

Considerable practical problems were encountered in the construction of the test site, whichled to positioning errors and a series of small gaps throughout both the knife-edge and the bar

objects. These were mainly due to the construction of the target objects from multiple corru-

gated iron sheets that were too easily buckled or misplaced by the bucket of the mechanical

excavator. Due to the required 3 m depth for the target object trenches and the unconsolidated

nature of the sand deposits, it was not possible to allow positioning of the metal sheets by

hand. In addition, the back-filled sand could not be recompressed uniformly, or to its initial

density and moisture content. This undoubtedly altered the conductivity and the dielectric con-

stant above and around the sheet-metal target objects.

The one-dimensional knife-edge object was constructed first and a central 10 m deep bore-

hole (lined with 0.1 m diameter PVC tubing) was drilled to facilitate the introduction of thetransmitter (Fig. 3 (c)). Amplitude and phase data were collected at 0.1 m intervals along a

single traverse across the site surface (receiver plane) perpendicular to the knife-edge sheet-

metal target. Subsequently, the two additional bar objects north of the original knife-edge were

added (Fig. 4) and a detailed two-dimensional area survey conducted at the same sample inter-

val (0.1 m) along parallel traverses separated by 0.2 m. Phase and amplitude measurements

were made over a 10 m

10 m area of flattened sand (receiver plane), with a dipole antenna

in close physical contact with the ground surface (Figs 3 (a) and 3 (b)).

One-dimensional study

Images based on the one-dimensional field data, a single scan

perpendicular to the knife-edge object, are shown in Figures 5 (c) and 5 (d). Wavelengths of0.5 and 0.8 m have been assumed in the image formation process, both to permit direct com-

parison with the corresponding numerical model studies (Figs 5 (a) and 5 (b)) and to reflect


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the uncertainty in the known wavelength of the source illumination at the site. Images formed

using the (convergent) RayleighSommerfeld integral from both assumed wavelengths produce

convincing step-type responses over the object, with the 0.8 m wavelength image resulting in

the sharpest/steepest response at the blackwhite transition.

Both field data images exhibit a considerable degree of attenuation between the centre and thenorth, or bright, edge of the image, where the field data signal falls, by station 70, to the same level

as the dark shadow over the target object. This apparent loss may be due in part to complex

interference effects arising from energy passing through gaps between the individual corrugated

iron sheets of the knife-edge target object, caused by the construction defects mentioned above.

Two-dimensional study

Images based on the two-dimensional 10 m

10 m field data array

are shown as grey-tone plots in Figures 6 (a

)(d

), again formed by the (convergent) Rayleigh

Sommerfeld algorithm at focus distances, set by LBin equation (2), of 1 m, 3 m, 5 m and 10

m at a wavelength of 0.8 m.

These images have suffered from amplitude and phase distortion introduced by unwanteddirect-path energy from the transmitter power cable. Neither radio-frequency decoupling fil-

ters built into the transmitter and power supply electronics nor a plug of conductive wire mesh

installed inside the PVC tube, to act as a shield to absorb the RF radiation, could fully isolate

this radio-frequency leakage. Furthermore, attempts to computationally reduce the effects of

this corruptionfor example, through interpolation of unaffected data across the corrupted

areahave been only partially successful. Data that could not be cleaned up were manually

removed from the pupil plane and replaced by null values, thus reducing the density of the

data set (U1) used for subsequent image formation. By way of an optical analogy, it is as if we

have formed images with a lens containing an irregular central obstruction.

Ignoring the central portion of the image focused at 3 m (Fig. 6 (b )), a bright rectangularfeature is discernible, which suggests that both the knife-edge and the long bar to the north

have been partially resolved (cf., Fig. 6 (b)). In addition, note the contrast between the two

separate bright features to the west and the single bright feature in the east. This may well

represent an image of the central short bar object.

Despite this degradation and the signal attenuation noted above, images formed using the

two-dimensional data from the field prototype study illustrate the applicability of the proposed

system. In particular, the ability to focus the field data is clearly demonstrated by the similarity

of the two images focused at 10 m on the transmitter plane (Figs 6 (d) and 6 (d )) and the re-

solution of the two bright anomalies to either side of the central bar target in the object plane

(Figs 6 (b) and 6 (b)). Note that an exact correlation between the numerical model and thefield prototype study would be highly unexpected, due to the not inconsiderable limitations

imposed by the available equipment.

DISCUSSION AND CONCLUSIONS

Taken together, the numerical and field prototype studies have demonstrated the feasibility

of fixed-frequency radio-wave imaging and data archive. Unavoidable design constraints

namely, low-frequency operation, a cable-powered transmitter and the field site design

have compromised data quality in the prototype field study. The resulting poor image

quality, in comparison with the numerical model study, further highlights the absoluterequirement for high-quality vector field data. This could be obtained by constructing a com-

pact, high-frequency transmitter with an integral power supply, communicating to the surface


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172 L. Somers et al.

by means of a fibre optic cable. Such a system would be free from RF leakage through a

trailing power cable and could be designed to operate over a range of frequencies to further

enhance the quality of the final image. Advances in the design of modern cellular telephones

demonstrate that the technology is readily available in a highly compact form that may

reduce the diameter of the borehole needed to introduce the transmitter beneath the targetarchaeology.

The choice of corrugated metal sheets for the construction of the test target was necessitated

due to the low cost of the materials and ease of construction in the field. However, the use of

a conducting material will have led to the diffraction of the incident EM signal and the likeli-

hood that the target itself will act as a resonator. This would lead to a standing wave pattern

of highly unpredictable form, due to reflections between the surface/air interface and the

sheet-metal target. Additional reflections may also have been generated between the back-

filled sand in the trench excavated to construct the target and the more consolidated, undis-

turbed sand surrounding the site. It is possible that this resonance introduced an interfering

reflection that may account for the sharp attenuation of response recorded in the knife-edgefield data (Figs 5 (c) and 5 (d)).

A potential solution to these problems could possibly have been to use an RF-absorbent

material, such as is used to line anechoic chambers for antenna testing. However, the cost and

likely thickness (> 1 m) of the RF material required precluded its use at this preliminary stage

of research. More generally, the construction of a test site containing archaeological targets

(e.g., ditches, pits, post-holes etc.) is complicated by the series of interrelated processes (ero-

sion, deposition, physical, biological and geochemical changes) that occur over an extended

period of time, all of which will alter the refractive index of both the archaeological features

and the surrounding soils.

Future work will address data quality by a combination of equipment design and fieldmethod optimization. For example, a thin layer of smoothed sand overlaying the site surface

can be used to minimize the micro-topographic induced phase defects, as well as to provide

an antenna impedance-matching benefit. Both transmitter and receiver antenna optimization

will consider the power pattern and polarization issues. For example, a multi-channel receiver

may be deployed to simultaneously record the polarization of the signal from an array of

antennae, each orientated at a different angle with respect to the sampling transect.

The use of a continuous wave as opposed to an impulse source has also been questioned and

may be justified through the aim of investigating the application of classic optical image-

forming algorithms, such as the RayleighSommerfeld integral used here, for the reconstruction

of ground radar data. In this case, a continuous wave system allowed both the phase and theamplitude to be determined with ease, and an increased signal-to-noise ratio to be obtained

through long-duration exposure at each measurement point. Further development of the system

would certainly aim to increase the range of frequencies used to illuminate the target features.

Such systems will certainly be considered during any future development of this methodology.

In the longer term, image formation and analysis algorithms research will dominate the

development of this method. Propagation path inhomogeneities must be assessed and incor-

porated into the image formation process. One possible approach would use polyfrequency oper-

ation from multiple transmitter locations, combined with iterative propagation path modelling.

In situ identification of archaeological materials by means of phase (dielectric permittivity)

and amplitude (electrical conductivity) contrast imaging and analysis will also be examined.Assuming that the technical limitations can be overcome, the archaeological potential of the

proposed imaging system is worthy of further considerations. This system will always require


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Radio-wave imaging of subsurface archaeological features 173

a minimal degree of intervention to introduce the transmitter source beneath the target archae-

ology, but this should not necessarily limit the application of the technique, particularly if is

applied to a site in advance of partial excavation. In practice, this will require a number of ver-

tical boreholes to be drilled through the site at a spacing determined by physical factors, such

as the depth of the archaeological targets to be imaged and the attenuation of the illuminatingRF source by the subsurface (largely determined through the conductivity of the site). Conven-

tional geophysical techniques, such as earth resistance surveys and impulse ground-penetrating

radar, may also be employed to determine the most suitable site for the boreholes; for example, to

avoid masonry wall footings.

A grid of boreholes at a 2 m spacing may well prove necessary for use on sites with clay-

rich, high-conductivity soils. However, a small-diameter transmitter with an integral power

supply could be constructed for this purpose and introduced beneath by means of a standard

hand auger for soil sampling. Obviously, considerable information could also be obtained

from the boreholes during their construction, including the recovery of soil samples for

detailed physical (e.g., magnetic susceptibility and conductivity) and environmental investiga-tions (e.g., archaeobotanical analysis). Combined with an automated scanning receiver

antenna on the ground surface, the system should prove sufficiently practical to allow data

acquisition from reasonable sized areas, particularly within an urban context, where the

application of conventional geophysical techniques is often highly impaired.

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