Geophysical Survey
Geophysical survey is employed within archaeology as a non-invasive, non-destructive aid to
understanding sub-surface features and conditions. No physical interventions (such as trenches or
pits) are required to conduct a survey, since the geophysical information can be collected remotely
from the ground's surface. Geophysical survey thus plays a vital role in the evaluation of
archaeological areas - allowing the archaeologist to assess a feature or site without risking
damage to the resource, or gauge the placement of a trench to maximise its interpretive potential.
From very specialist, scientific beginnings, geophysical survey has become an integral part of the
archaeological process, and is now employed widely in both commercial and non-commercial
archaeology. The discipline's profile has been raised thanks to this general uptake and its key
involvement in television programmes such as Time Team, but it must be remembered that the
term 'geophysical survey' covers a range of techniques, each suited to slightly different conditions,
situations and budgets. This section will thus briefly outline the field's development and subdisciplines, and discuss the practicalities involved in its employment. Three of the most widelyused techniques of geophysical survey - resistivity, magnetometry and ground penetrating
radar - will also be examined (see menu, left).
Introduction to Geophysics
From early on in the development of organised archaeological study, techniques were being
developed to give archaeological practitioners an understanding of sub-surface features without
the need for invasive excavation. However, the inaccuracy and poor adaptability of these early
techniques meant that decades passed where excavation was the only real way to visualise and
understand archaeological sites. But archaeological excavation is essentially a 'one-chance'
undertaking, for while it provides unparalleled understanding and tangible evidence it is also
ultimately destructive: stratigraphy, relationships and assemblages are destroyed and can never
be rebuilt. With hindsight, many archaeological sites may have yielded valuable information with
the use of modern analytical techniques, and indeed future generations of archaeologists may
lament our under-use of the archaeological resources we destroy through excavation. Thus,
techniques that can 'remotely sense' sub-surface features have been gradually developed,
especially in the second half of the 20th century, and today their role in both commercial and noncommercial archaeology is vital.
Geophysics is widely-employed branch of 'remote sensing' (a term covering many disciplines from
satellite sensing to dowsing!), and spans disciplines such as geology, town planning and
environmental studies in addition to archaeology. Indeed, many early developments were geared
towards these other fields, but specific advances in technology have helped archaeologists tailor
methods and equipment to suit archaeological requirements. Today, a geophysical survey can be
conducted relatively accurately and quickly with the aid of automatic data-loggers and precise
sensors, but this was not always the case. What follows is a brief historical overview detailing the
development of remote sensing, and especially geophysics, within the field of archaeology.
Current legislation, such as PPG16 (Planning Policy Guidance note number 16), promotes noninvasive archaeological investigation where the archaeology itself is not under direct threat from
natural conditions or development.
Historical Overview
Pre-1950
1893 Pitt-Rivers & Sonic 'bosing'
Augustus Pitt-Rivers employs sonic 'bosing' - striking the ground
to judge relative sound responses - at Hadley Down, Dorset, UK.
1920s Phosphate Analysis
Development of phosphate analysis to map areas of habitation
and/or use, Scandinavia, Egypt
1938 Mark Malamphy: USA
Canadian geophysicist Malamphy conducts first systematic
geophysical survey using resistivity soundings at Bruton Parish
Church, Williamsberg, USA.
1946 Resistivity at Dorchester, Dorset, UK
First popular use of resistivity for archaeological purposes (and
widely considered the first archaeo-geophysical survey) by
Richard Atkinson in an attempt to map sites under threat from
gravel extraction. Equipment: Megger Earth Tester used in
engineering geophysics; Method: 'leapfrog' survey developed for
2d mapping of deposits.
The origins of remote sensing lie with techniques such as dowsing, which has been practiced for
many hundreds of years as a way of locating sub-surface resources such as water or minerals.
Debate continues over the scientific basis of dowsing, but the technique has certainly been applied
to archaeological situations and is the subject of many journal articles and studies (eg. Martin
Aitken's 1959 study). It is unclear whether any significant archaeological decisions have been
made as a result of dowsing evidence, but as a fore-runner to more proven techniques it is worthy
of mention.
The beginnings of organised archaeology saw a new role for remote sensing, with pioneers such
as Augustus Pitt-Rivers using bosing at Hadley Down in Dorset, UK. Bosing entails striking the
ground with a heavy wooden mallet or a lead-filled container on a long handle to judge relative
sound responses, and was found to be an effective method of identifying silted pits and ditches
from surrounding natural soils. However, emphasis must be placed on bosing as a relative
technique: it does not provide data or evidence that can be compared accurately to other sites (or
even other areas of the same site), for sonic responses of the ground can change due to many
variables (eg. geology, water content, soil compaction, etc). Bosing is thus useful only within small
areas to delimit areas of 'different' ground.
What archaeology required was a technique of accurately obtaining measured evidence that could
be analysed and compared, and it was the field of chemistry that provided such an advance with
development of phosphate analysis. Used to identify areas of habitation, occupation or use,
phosphate analysis allowed archaeologists to detect the phosphorus residues left by such
activities via chemical analysis. Developed and tested on archaeological sites in Sweden and
Egypt, this geochemical survey method can be used to great effect when employed in a grid
layout, and can complement geophyscial forms of survevy. However, such surveys are slow to
conduct due to the practicalities of obtaining samples and they are thus usually implemented over
relatively small areas.
While this technique was useful for analysing already-identified areas of archaeology, similar
advances were being made in archaeological prospection. In 1938, in Williamsburg, USA,
exploration geophysicist Mark Malamphy of the Canadian firm Hans Lundberg Ltd conducted what
is understood to be the first systematic geophysical survey, using an equipotential map to display
the ground resistivity of a colonial-period churchyard. The survey was designed to pinpoint the
location of a buried stone vault within the churchyard, but the high-resistivity feature thought to be
the vault was instead discovered to be natural soil contrast - opening the possibilities for the
technique to be employed to identify soil-based features.
The first popular archaeological use of resistivity survey was by Richard Atkinson, who employed
the technique in his investigation of a henge monument in Dorchester, UK. Equipped with a
Megger Earth Tester, common to engineering geophysics, he utilised a 'leapfrog' survey
developed for 2d mapping of deposits rather than the usual method at the time which involved
profiles represented by graphs.
1950s
1950s Lerici Foundation: Milan, Italy
Archaeological Prospection Section established within the Foundation an important organization in the development of geophysics in Europe.
1956 Martin-Clark resistivity meter
First instrument designed specifically for archaeological work: used at
Cunetio Roman town, Wiltshire, to trace circuit fortification wall.
1957 Thermoremnant Magnetisation
Introduction of dating technique by John Belshe for ceramic materials,
quickly followed by the application of this principle to kiln sites by
Martin Aitken and Edward Hall, Oxford Research Laboratory.
1958 1nT Proton Magnetometer
Prototype proton magnetometer, sensitive to 1nT, introduced by Aitken
and Hall at Water Newton to search for kiln sites prior to A1 Motorway
construction. Discovery that this method also generated responses
from infilled pits and ditches.
1958 Irwin Scollar & Micro-computers
Irwin Scollar at University of Bonn pioneers use of micro-computers in
data filtering and mapping.
1959 Commercial Availability
Martin-Clarke resistivity meters and two versions of proton
magnetometers (Maxbleep and Minbleep) available commercially.
The use of such non-invasive survey technologies was also developing on continental Europe, and
a major step was the establishment in 1947 of the Institute of Applied Geophysics at Milan
Polytechnic - formed from the Centre of Geomineral Prospecting on the initiative of Ing. Carlo
Maurilio Lerici. From the mid-1950s the Institute undertook inter-disciplinary studies of the
Etruscan necropolises of Tarquinia, Cerveteri and Vulci, and this combination science and
humanistic studies led to the formation of the Ing. Carlo Maurilio Lerici Foundation to focus on
the development, testing and application of geophysical, geological and remote sensing
techniques to archaeological prospection.
Meanwhile, in the UK the first archaeologically-specific geophysics equipment was beginning to
develop in the form of the Martin-Clark resistivity meter. An early employment of the new
technology was at Cunetio Roman town in Wiltshire, where Martin Aitken and Edward Hall of the
Research Laboratory for Archaeology and the History of Art, Oxford, used the equipment to
trace the town's circuit fortification wall. While much slower to assemble, operate and process data
than modern equipment, these early geophysics machines formed the basis from which more
complex models developed.
Alongside the advancements in resistivity-based geophysical survey, another technique was
beginning to emerge based on the understanding that buried objects and features produce small
distortions in the earth's magnetic field. These magnetic survey methods developed from the
principle of thermoremnant magnetisation - the theory that when heated to around 700°C, iron
oxide particles align themselves to whatever orientation the earth's changable magnetic field is in
at the time. The technique, using a proton magnetometer, proved highly useful for identifying
areas of burning, kiln sites and metal objects, and was employed with success by Aitken and Hall
at the Roman city of Durobrivae, near Water Newton, UK, prior to the A1 motorway construction.
See the following 1965 article (Adobe .pdf format):
Marriner, E.H., The Proton Magnetometer: New Tool for Archaeologists and Treasure
Hunters Radio Electronics, September 1965
Geophysics equipment was soon available commercially, with the Martin-Clarke resistivity meter
and two versions of proton magnetometer (Maxbleep and Minbleep) leading the way. The
magnetometers utilised a gradiometer mode, with two sensors mounted vertically to help
eliminate background geomagnetic variations. But advances were not confined to geophysical
survey equipment. Irwin Schollar of the University of Bonn, Germany, helped filter and map the
geophysical data obtained through survey with the use of micro-computers, ushering in a new era
of computer-based analysis that could handle much more data with more accuracy than traditional
techniques.
1960s
1960s Proliferation & Development
A pivotal time for the proliferation of archaeological geophysics, with
key developments in several countries, eg:
USA: Elizabeth Ralph at MASCA (Museum Applied Science Centre
for Archaeology, Pennsylvania)
France: CNRS under Albert Hesse and Alain Tabbagh (Centre de
Recherches Geopysiques, Garchy)
Germany: Rheinisches Landesmuseum, Bonn, (Irwin Scollar)
Italy: Lerici Foundation (Richard Linnington, Carlo Lerici).
Work extensively published in the journals Archaeometry, and
Prospezioni Archeologiche.
1964 Fluxgate Gradiometer
Fluxgate Gradiometer developed by John Alldred (Oxford Research
Laboratory). Designed to increase efficiency and speed of recording,
but also had the benefit of balancing for background variations.
1967 Ancient Monuments Laboratory Geophysics Section
Introduction and experimentation with a number of new instruments,
including different resistivity arrays (square frame and Wenner).
Development of a continuous recording single-operator Fluxgate
Gradiometer - the standard for the next 15 years.
The 1960s saw the general proliferation of geophysical survey within archaeology, testified by
many key developments from numerous research institutes across Europe and North America.
Elizabeth Ralph of the Museum Applied Science Centre for Archaeology, Pennsylvania, USA,
undertook key geophysical investigations at the Palace of Malqata in Thebes, Egypt, while Albert
Hesse and Alain Tabbagh undertook geophysical research at the CNRS in France.
The Lerici Foundation continued its developmental work in the discipline and communicated with
the other major research centres specializing in archaeological prospecting: Pennsylvania
University (USA), Oxford Research Laboratory (England), CNRS Geophysical Research Centre,
Garchy (France) and the Rheinisches Landesmuseum of Bonn (Germany). The archaeological
geophysics community was further cemented with the signing of permanent collaboration and data
exchange agreements between these centres.
The discipline had now reached a sufficient level of use and popularity to warrant its own forums,
which duely arrived in the form of the journals Archaeometry, and Prospezioni Archeologiche.
New advances and projects that incorporated geophysical survey were published extensively via
these media, in addition to other, more general archaeological journals.
Among the most important advancements of this era was the Fluxgate Gradiometer, developed
by John Alldred of the Oxford Research Laboratory. Not only did this equipment significantly
increase efficiency and speed of recording, but it also allowed for adjustments, or balancing, for
background magnetic variation.
Further developments, including different resistivity probe arrays such as Wenner and squareframe, were made at the Ancient Monuments Laboratory Geophysics Section in the UK. Also to
come out of this institution was the continuous recording, single-operator Fluxgate Gradiometer,
which was to become the standard piece of equipment for this type of survey for the next 15 years.
1970s - Present
Universities
Focus change in Britain and Europe from research and development of new
instruments and methodologies to the teaching of techniques and geophysics'
place in archaeology.
Postgraduate degree in Scientific Methods in Archaeology at Bradford in 1971
under Arnold Aspinall.
Bradphys twin Probe resistivity meter developed, establishing the new
generation of equipment still in use today.
Survey equipment coupled with portable field computers (eg. Epson PX20),
transforming methodology of data logging and field procedures.
By 2000 nearly all University archaeology depts. teach geophysical survey new generation of archaeological practitioners who are familiar with
geophysical equipment, methods and results.
Commercial & Popular Use
British Gas appoint full-time archaeologist, and in 1979 a full time
Geophysicist.
Commercial possibilities presented by new generation of instruments
(designed by Roger Walker, Geoscan, 1984) combining large scale data
collection with ease of use.
Developer-funded, tender-based commercial archaeology fuels a
archaeogeophysics boom, leading to over 450 commercial surveys in 200
alone.
Television exposure, such as Time Team, boosts coverage further,
popularising and promoting the discipline to new levels.
Archaeological geophysics in Britain and Europe could be said to have 'come of age' during the
1970s and 80s, with less emphasis placed on technological and methodological development and
more focus upon teaching and theoretical research. Universities played a large role in this focus
shift, and in 1971 the postgraduate degree Scientific Methods in Archaeology was established
at Bradford University for students who wished to follow this specialist path. The degree is still run
today, and Bradford University has gained a reputation for cutting-edge scientific research and
degree courses in archaeology. Out of this scientific community at Bradford developed companies
such as Bradphys, whose twin-prode resistivity meter is still used today.
But in the 1980s it was clear that archaeological theory and practice was changing - moving away
from the traditional site-wide focus towards more encompassing landscape studies. Geophysics
was quick to move with this trend, and as Heron and Gaffney wrote in their 1987 paper
'Archaeogeophysics and the site: ohm sweet ohm?':
'There is more to archaeophysics than locating habitation structures for excavation. Archaeologists
do not follow walls these days, why should archaeogeophysicists?'
Meanwhile, significant developments were also occuring in commercial archaeology as heritageaware restrictions were placed upon planning and development. British Gas was one of the first
companies to employ a specialist, full-time archaeologist, and they soon complemented this
appointment with a specialist geophysicist. Those sectors that could not warrent employing
specialist archaeological staff, such as buildings developers, were serviced by a quickly growing
number of archaeological contractors, many of which offered (if not specialised in)
archaeogeophysics. Commercial archaeogeophysics surveys, once too time-consuming to
conduct, process and synthesis, were now a commercially-viable venture thanks to a new
generation of quick and efficient equipment pioneered in the mid-1980s by Roger Walker at
Geoscan.
In reponse to the 'development boom' of the 1980s and 90s, government planning policy had
shifted to favour the non-intervention and preservation of archaeology over excavation.
Archaeogeophysics was thus in greater demand than ever, being key to the 'evaluation' stage of
the planning process - ie. after the initial desk-based assessment established a possible threat to
archaeology, an evaluation comprising geophysics and test trenches (if required) was undertaken
to gauge extents and the need for a full excavation. This unique set of developer-funded,
commercial tender-based conditions present in Britain meant that by 2000 over 450
archaeogeophysical surveys had been commissioned by developers for that year alone.
Fundamental Principles
Raw Data Capture
At a Glance

Geophysics takes readings that can be reconstructed on graphs or maps.

Readings can be taken individually or continuously depending on the type of equipment used.

Geophysical surveys conducted on grid layouts across measured transects.

Geophysical survey equipment has no spatial awareness, and can be either active or passive in type.

Software is used to interpret geophysical data and assemble it correctly for presentation.

Geophysics is not usually used in isolation, but rather as part of broader study - definitive results should
not be expected.
Most geophysical equipment used in archaeology today revolves around the basic principle of
taking readings (either individually or as a continuous set) from the earth's surface that can be
spatially reconstructed on either a graph or a map. To properly space and order the data, the
readings from a geophysics survey are taken at set intervals along transects of a measured grid.
If the readings are taken individually, as with a resistivity survey, the intervals are measured by
distances (eg. 50cm), while if the readings are automatic (continuous) then the intervals are
measured in periods of time (eg. one reading every second).
A common factor of most geophysics equipment, both past and present, is that they are purely
data-capture machines and do not have any spatial or locational awareness. Where a GPS
understands its position via satellite readings, or an EDM-equipped theodolite can measure
distances, a geophysics resistivity meter or magnetometer has no concept of its present location or
spatial position. Usually, all it knows is the number of readings it is to take per transect and the
number of transects that make up a grid square. Its only purpose is to take readings and thus an
un-processed results file from a geophysical survey, if viewed in a standard text processor, is
simply a long string of numbers. This principle is important to understand, since errors in the maps
or graphs generated from the processed results are often not due to faulty equipment or poor
conditions, but rather the way in which the readings have been reassembled.
Geophysics equipment can be divided into two broad categories:


Active: those which induce an action in the earth and produce a response to that action,
such as resistivity or magnetic susceptibility.
Passive: those which record physical properties which already exist without any external
influences, eg. magnetometer.
These categories do not influence the design of a survey, for decisions are usually based on the
strengths and weaknesses of the type of equipment to be used, eg. resistivity meter or
magnetometer. However, generally speaking it is usual that active forms of equipment take longer
to operate, with each action (and thus reading) being induced individually. Readings for passive
forms are often taken automatically.
[Image of data processing of results in field]
Whereas in the past geophysics results have been processed manually, today computer software
exists that can interpret the results and place them in the correct order to create a map or graph.
By setting the number of readings per transect and the number of transects per grid square, the
software can reassemble the data into the correct order - but mistakes when specifying this
information will lead to incorrectly reassembled, and thus misleading, output! Most software
programs also allow for further processing of the raw data, helping to isolate certain features and
remove background noise. The correct use of these software post-processing techniques is often
as vital as the data collection itself, since only this method can extract the full range of features
and patterns within the data (see Processing Data below).
Geophysical survey data is generally collected over relatively large areas, out of which a
meaningful result may or may not be generated. This approach is essentially objective, allowing for
the return of inconclusive or patchy results as well as positive interpretations. Those who conduct
geophysical surveys must therefore remember not to expect conclusive results. Geophysical
survey should be used as part of a coordinated investigation and will rarely present faultless
results in isolation.
Processing Data
[Image of dipolar anomaly/poor contrasting features]
In order to produce meaningful results we must be capable of detecting contrast between those
readings that we believe to be archaeological and those readings that occur naturally (termed
'background' or 'noise'). In some situations this easily done, for example in magnetometry where a
kiln or fragments of highly fired material will give distinctly different signals. However, more typical
archaeological anomalies are generally characterised by small wavelength and amplitude signal unfortunately similar to the background noise levels. A process of signal enhancement is therefore
often undertaken to augment the distinction between archaeology and background noise.

Identification of background noise (random/ systematic/ short range/ long range)
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Modelling and characterisation of data and anomalies
Filtering applications to separate signal from noise
Production of thematic interpretation of probable signal
Resistivity
Amongst the first modern geophysical techniques to be devised and tested (see Geophysics
Historical Overview), resistivity is possibly the most widely-used method of geophysical survey
within archaeology. This popularity stems largely from the broad range of situations/conditions to
which the technique can be applied, while the equipment itself is relatively inexpensive compared
to other techniques.
The basic principles involved are straight-forward: an electric current is passed between two
probes which are placed into the ground. The resistance that the current encounters in the ground
while passing between the probes is then measured (in ohms) to give a reading. The moisture
content of the particular area of ground will affect the electrical resistance - eg. a relatively loose,
un-compacted area of soil (such as the fill of a ditch) will hold more moisture and thus conduct an
electric current well; a compacted or solid area of ground (such as a wall) will hold less moisture
and thus conduct the current less well (see Principles and Overview for more). This section will
outline the principles behind resistance geophysics, the situations and conditions to which it can be
sucessfully applied and outline the steps involved when planning and conducting such a survey.
Principles and Overview
First employed in 1938 in what is thought to be the first systematic geophysical survey (see
Geophysics Historical Overview), resistance survey has developed much over the decades
leading to increases in accuracy, efficiency and speed. Yet the primary underlying principle of the
technique remains the same: a material's resistance to an electrical current can be measured and
compared. The pioneering 1938 survey applied this principle in a churchyard to find a buried stone
vault and thus understood the possibilities of the technique to distinguish soil from a solid
structure. However, the survey greatly expanded these possibilities when it was found that
differences within the soils could be also be identified. This phenomenon was subsequently
discovered to be largely a result of the differences in moisture content betweeen soils: those that
had a high moisture content were able to conduct the electrical current more easily than those that
had little moisture - ie. those deposits with less moisture has greater electrical resistance.
At a Glance

Geophysical resistance survey able to differentiate soils and features based on their moisture content by
passing an electric current through them.

Resistance is a combined measure of the potential difference and the current passing through an object or
substance.

Resitivity equipment measures both the potential difference and the current to obtain a resistance reading using two sets of probes: 'static' or remote probes and mobile probes.

Several configurations of mobile probes are in use today, with differing levels of
resolution and depth perception.
The term 'electrical resistance' can be understood as:
" the measure of the degree to which an object or substance
opposes the passage of an electric current"
However, resistance is comprised of two factors: the potential difference (essentially the 'power'
required to move the current) across the object or substance (measured in volts); divided by the
current passing through the object or substance (measured in amperes). It can be understood by
the following equation:
To obtain resistance readings, which are measured in ohms per metre (ohms/m), we therefore
require measurement of the two factors of the equation. A typical resistivity meter will therefore
have two sets of probes:


the primary set of probes, often mounted on a manoeuvrable frame and thus known as
the mobile probes, are used to measure the electric current passing through different
areas of ground;
a secondary set of probes is used as a potentiometer to measure the relative voltages
required to pass the current through the object or substance (ie. the potential difference).
These are placed in the ground slightly away from the survey area and do not move - and
are thus known as the 'static' or remote probes.
Most geophysical resistance equipment employed within archaeology adheres to this basic setup,
but several different alignments, or configurations, for the mobile probes can be used. Variations in
the number, spacing and placement of mobile probes used can aid resolution or depth perception
in different situations, and some practioners favour certain setups over others. However, the most
common is the standard two-probe setup, which is described as part of the RM15 Resistivity
Meter Survey section.
Gradiometry/Magnetometry
Magnetometry is the most common form of geophysical survey in the UK for assessment and
evaluation, mainly thanks to the speed and adaptability of equipment such as the fluxgate
gradiometer. Developed largely in the late 1950s, the technique has proved extremely useful in
detecting sub-surface objects and features across a range of conditions.
Magnetic surveys operate by examining the local magnetic fields associated with soils or objects,
and comparing them to the earth's magnetic field in the surrounding area. Soils and buried objects
can be subject to two types of magnetisation 1) magnetic susceptibility or the ease at which
soils/objects are magnetised when subjected to a magnetic field and 2) remnant magnetisation
which is a function of the magnetic strength of certain mineral in soils which have enhanced
magnetic signals in comparison to surrounding soils as a direct result of physical processes such
as heating or fermentation. Anomalies that stand out from their surroundings can be identified on
the strength and alignment of the magnetisation they have acquired.
The most common type of magnetometer surveys for archaeological purposes employ fluxgate
gradiometers - a passive technique which senses the remnant magnetism of soils and objects
without subjecting these targets to an induced magnetic field. Conversely, magnetic
susceptibility surveys use instruments that subject soils or objects to a magnetic field in order to
measure the potential for magnetisation of that material (also used infield archaeology and in lab
based soils analysis).
Like other forms of geophysical survey, magnetic survey can provide excellent results - but it
should ideally be used as part of a survey strategy in conjunction other techniques, such as
resistivity. Each form of survey has its own strengths and weaknesses, and only a combination of
these can provide a full picture.
Principles and Overview
The first experiments with magnetometry as a survey technique were undertaken by John Belsche
in the mid-late 1950s, and were further enhanced by Martin Aitken and Edward Hall (two of the
foremost pioneers of archaeological geophysical survey) to aid in the location of kiln sites. Kiln
sites were areas where high-temperature burning (ie. the firing of pottery) had occured, and were
excellent examples of the magnetic phenomenon known as thermo-remnant magnetisation.
The principle of thermo-remnant
magnetisation is reasonably
straightforward. When heated to
around 600-700°C (known as
the Curie point), materials begin
to
lose
their
inherent
magnetisation: they become demagnetised.
Once
demagnetised, the iron oxide
particles present in the material
can
align
themselves
to
whatever orientation the earth's
changable magnetic field is in at
the time, and as the material
cools the new magnetisation is
retained. However, an important
concept is that the earth's
magnetic field changes over
time. Indeed, the actual 'magnetic north pole' (as opposed to the north pole at the end of the
earth's spinning axis) wanders relatively quickly - causing compasses in equatorial countries to
change their 'north' reading by upto a degree a decade. Thus, those materials that have
experienced high temperatures will retain the earth's magnetic alignment at that particular time which is likely to be different to alignments present in surrounding material and the present
magnetic alignment.
Magnetic survey is useful for identifying many other features in addition to high-temperature objets
and areas. Metal objects with a high ferrous (iron) content can act as strong localised magnetic
fields, and usually provide high contrast with the surrounding soils. Weaker magentic fields can
also be generated by general occupation, since discarded pottery, ash, organic matter and soils
containing iron-rich minerals can be distinguished from surrounding soils. The fills of pits and
ditches around occupation and settlement centres are thus also identified by magnetic survey.
What to Expect when commissioning or
conducting surveys
Geophysical survey is often expected to meet the requirements of a range of very different
specialisms. Field archaeologists, heritage managers, planners and buildings specialists all look to
geophysical data to provide them with the knowledge and understanding of a site or area that they
need to do their job.
1.
Resolution: generation of general data
 Much like field collection except relating a different story and will depend on
resolution
 survey design and objectives
 reconnaissance
 evaluation
 specific research question
 Most geophysical data is machine/ method specific
 No one will ever give you a full picture
 Integrated data is strong data: geophysical surveys are enhanced by data from
other survey/lab/excavation data
General information

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
Extent of deposits
Condition of deposits
Spatial Details/ morphology
Depth
Physical properties of underlying geology
Material composition of anomalies
Use and function (occasional)
General dating basted on historical typologies
Considerations





Stratigraphy
Depth
Presence or absence of signal
Importance of ground truthing
(related) integration with other data
Planning and conducting surveys

Planning surveys is extremely important and it is very easy to have unrealistically high
expectation of the likely outcome of surveys unless serious thought is given to the
planning process.

This is as true for those planning the survey as it is for those conducting it, and dialogue
should always remain open.

Expectations should always remain flexible on behalf of those commissioning surveys and
those conducting them should always be realistic about the probably outcome and
articulate this well to.

Be careful of falling into the trap of using geophysics as fallback or a universal answer.

The following are factors that should be considered in any planning.
Equipment and Method

Modern equipment provides a large range of possibilities.

Appropriate choice of equipment is very important for:
 Type of archaeology
 Nature of geology
 Speed and access of surveying
 What type of data can be generated
 How that can be used

Requires quite specific knowledge of capabilities and limitations of equipment as well as
different ways they are used.

Most commonly a combination
Environmental Conditions, Soils and Geology

First: Not all sites suitable
 Trees, crops, building sites, etc.
 Slopes
 Rough ground

Geology





Different responses to different methods: guidelines usual refer magnetometer
surveys (chart).
Most will behave fine, but in conjunction with other factors such as moisture
trends, etc, can be problematic.
For details see G&G and EH guidelines
Igneous geologies
Clays (GPR)

Vegetation image
 Moisture content
 Physical obstacles

Time of year and climate
 Dry/ wet climate resistivity example

Urban vs. rural
 Pipelines/ overhead cables
 Vehicles 20/40 m away
 houses
 Metal and magnetism
 Paving
Nature of Archaeology / Extent of Investigation Area (extremely important)
Survey Framework and Design


Assessment and evaluation level: large area surveys under pressure. Most commercial
surveys done in this mode
 Sampling strategies borrowed from other branches of archaeology: principally,
field walking.
 What strategy
Research based surveys
 Larger area sample intervals
 Severl methods
 Specific questions
Data processing

All data requires some level of optimisation

Raw data and processed data stored together

Graphical representation: appropriate and meaningful

Level of interpretation: caution of bold

Metadata

Interpretation plans
Geophysics Reading & Resources
Books and Articles

Clarke, A (1990) Seeing Benieth the Soil: Prospecting Methods in Archaeology: Routlege

Gater, J and Gaffney, C (2003) Revealing the buried past: geophysics for archaeologists
Stroud: Tempus

Gaffney, C et al. (2002) The Use of Geophysical Techniques in Archaeological
Evaluations (Institute of Field Archaeologists, paper no. 6)

Sarris, A. and Jones, R. E. (2000) ' Geophysical and Related Techniques Applied to
Archaeological Survey in the Mediterranean: A Review' Journal of Mediterranean
Archaeology Vol 13. No. 1
Web Resources

ADS Geophysical Data in Archaeology: A Guide to Good Practice
http://ads.ahds.ac.uk/project/goodguides/geophys/

Archaeological Prospection Resources: Department of Archaeological Sciences, Bradford
http://www.brad.ac.uk/acad/archsci/subject/archpros.htm

Archaeological Prospection (journal)
http://www3.interscience.wiley.com/cgi-bin/jhome/15126

West Yorkshire Archaeology Service Geophysics Archive
http://ads.ahds.ac.uk/catalogue/projArch/wyas/