archaeology techniques

7/27/2019 Archaeology Techniques

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Archaeology Techniques

Resistivity

The electrical resistance of the Ground is almost entirely dependant upon theamount and distribution of moisture within it. Buried remains affect this

distribution and can be detected with instruments. Stone, for example, is more

moisture resistant than a clay subsoil or the filling of a ditch. These resistivity

differences can be detected and when overlaid on a map will often give a plan of

buried remains.

Resistivity survey methods have been used to detect both natural and

archaeological features since the techniques discovery in 1946.

When one encounters a soil resistivity survey in progress, several thoughts springto mind. How can objects deep under the ground be detected by electrodes

inserted only a few centimetres? And why are four (sometimes five) electrode

probes needed? Do differing types of soil affect the precision of the readings?

This guide aims to provide sufficient background information to allow the lay

person to understand the basic principles of resistivity survey for archaeology.

First Principles

Those who remember physics lessons in school may remember that when an

electrical voltage is applied between the ends of an electrical conductor such as

wire, a current flows through it; the size of the current depending on the

resistance of the conductor. The symbol for resistance is R, measured in ohms

(often represented by the Greek letter omega).

Water, in its natural state, is an insulator. However, with a little salt added, it

soon allows current to flow. Chemicals which have this effect on water are

known as electrolytes. The resistance of soil is almost entirely dependant on its

water content and the electrolyte "mix" it contains. Most of its other components,

such as stone, are largely insulators. Buried wood generally tends to attract water and so reduces the resistance in that area.

Resistivity is a uniform measure which allows the resistance of different

substances to be compared. It is defined as the resistance of a cubic meter of

material when a 1 volt charge is applied between the two opposite faces of the

cube. The unit of resistivity is the ohm metre, its symbol is the Greek letter rho.



The resistivity of soil can vary from 1-10 ohm-metres; porous rocks 100-1,000

ohm-metres and non porous rocks anything from 10k to 10m ohm-metres.

Contrary to how it may first appear, current does not flow through soil as a direct

path. Current flowing between two electrodes in soil will spread out into a

myriad of paths rather like the force bands surrounding a magnet. The totalresistance is a sum of the resistance offered down each path. It can be seen

therefore that a ditch cut through a rock base will show lower resistance in

surface measurements than the natural soil and rock layer around it. In fact, the

lower the resistance the deeper the current will travel - due to the like charged

particles repelling each other, thus causing a wider spread of current.

Technically, resistivity measurements could be made using a household

resistance meter. A calculation would be required to work out the resistivity and

indeed some of the first resistivity devices used by archaeologists were based on

an electrician's "Megger" - a device normally used to certify domestic electricitycircuits. However, due to design limitations these devices are generally

unsuitable for soil resistivity measurement.

Measuring Soil Resistance

Measuring the resistance of soil presents us with problems. The electrodes

applying the current have a small contact area compared to the volume of ground

to be measured. At its surface soil tends to be dry, thus providing a poor contact

medium. These effects create a much higher resistance in the immediate area of

the electrodes, which would tend to cancel out any reading from the ground in between.

The solution to this problem was found by creating a probe with four electrodes.

Known as the Wenner system, these are placed at equal distance in a line - the

outer two apply the current, the inner two measure the voltage of the ground.

These two measurements - voltage and current - are used to calculate the ground

resistance (R=V/I).

In addition to this a high impedance measuring circuit helps take into account

variations in surface contact conditions. An AC circuit is used as DC currentwould effectively turn the soil into a battery and mess up the readings. A further

refinement uses a phase-sensitive rectifier to cancel out other interference.

Types of instruments

Manual Balance Instruments



These early instruments used an on-board dial to allow the resistance to be

matched and noted; often the probes were pushed in individually. In some cases a

rotary switch and a five probe design allowed a measurement to be taken each

time a single probe was moved. Often two skilled operators were required.

Automatic Instruments

In general, manual balance systems have been consigned to the past, thanks to

electronic devices such as data loggers which automatically sample and store the

measurements, and the creation of a probe 'cradle' which allows an individual to

survey a field at near walking speed.

Data Loggers

With a data logger, every time the probe cradle is inserted into the soil, a button

is pressed to take a sample. The data logger - an electronic device attached to thecradle - takes the resistance readings and stores them in sequence.

Later, either on site or in the office, these samples can be plotted against a map to

provide a clear picture of the resistivity changes of the subsoil, often giving

feature markings so clear that little confirmation excavation is needed.

Laptop Computers and Beyond

The advent of the laptop and sub-laptop computer, together with the ability to

provide data logging and sampling onboard, will ultimately create a cradlecapable of giving a "live" display of the underground resistance topology. The

technology for this advance exists currently, but will need a little development

before it can be realised.

Resistivity for Archaeology

A feature of high resistance buried in the ground will cause the resistance of the

overall local area to increase, this is known as a "positive anomaly". Conversely,

a feature such as a rock cut ditch will lower the overall resistance and is known

as a "negative anomaly".

In the early days it was assumed that based on the above, features such as stone

foundations and walls would always give high resistivity readings and therefore

be positive anomalies. However experience has shown that whilst this can be the

case, often other factors such as the features geometry, associated deposits, soil

moisture content and electrode configuration can cause complications to this rule.



However, a significant amount of research effort has created a range of designs

which offer a robust and reliable surveying tool.

Electrode Configurations

The first experiments with soil resistance were carried out by Frank Wenner in1916. His original four electrodes in a line configuration with two current

electrodes to apply power (C1,C2) and two potential electrodes to measure

resistance (P1,P2) has been adopted and modified by archaeologists based on the

results of extensive testing.

During testing, it became clear that for some features the Wenner and related

probe configurations were not effective at detecting some types of underground

features. Narrow features were found to show double or even treble readings.

As a result of this a wide range of electrode configurations have been evaluatedgiving archaeologists the option of a range of electrode configurations to suit the

ground and type of feature.

The Wenner configuration is still a commonly used configuration, as it offers

good all round functionality for most types of submerged features. The wenner

configuration can sometimes exaggerate the width of the anomaly and is

susceptible to misinterpret some high resistance features.

C1 P1 P2 C2

Wenner electrode configuration

For very shallow features, the Double Dipole configuration has been shown to

give particularly good results, this configuration, also known as the Wenner beta

configuration is created by taking the Wenner configuration and swapping on

current electrode for a potential electrode.

C1 C2 P2 P1

Double dipole electrode configuration

A more recent development, the Twin Electrode configuration sees the Wenner



design cut in half and provides for a half size cradle as well as almost eliminating

some of its inaccuracies. With the twin electrode configuration, two probes are

fixed at a static point to one side of the test area. The other two probes are

attached via a long lead and are moved around the survey site. This design helps

to eliminate the exaggeration of high resistance features.

C1 C1 - - - - - - - - - - - - - - - - - - - -C2 P2

Fixed Mobile probe

Twin electrode configuration

One other electrode configuration worthy of mention is the Square Array, this

was developed as a solution to the poor response given by the Wenner

configuration to small buried objects. With this configuration the probe looks likea small table and tends to be used in more specialist circumstances.

C1 C2

P1 P2

Square Array configuration

The Impact of Soil

Now that we have a better understanding of the relationship between the buried

feature and the types of probes used for resistivity, it is important to understand

the effect different soils and moisture levels with readings.

The structure of soil

Generally, when we are talking about soil, we are considering several different

factors, each of which are inter-related. Firstly, the top layer of soil is usually a

loam type material with varying amounts of other materials either due to natural

deposition or related to the natural bedrock. The actual resistivity of this soil is acombination of this soil composition and the retained moinsture at the time of the

survey. The level of moisture retained by soil is a result of that particular soils

natural drainage, and the drainage provided by the underlying bedrock.

In some times of the year, the soil effectively becomes waterlogged and this will

result in many features being hidden by the overall low resistance of the soil



The following section illustrates the impact of bedrock on a features resistivity

and discusses the impact of rainfall in order to identify the likely results for a

particular feature as well as proposing the best time of year to survey for each

bedrock type.

Bedrock

type Feature Dimensions

Anomaly

type "Season" Best

Sandstone DitchesW1-4m

D3mLow Jun-Sept July

ClayRubbleWall

W10mD1m

High Jun-Nov Sept

LimestoneStn

CoffinW.5mD1.5m

High Jul-Oct Oct

Chalk DitchW18m

D6mLow Dec-Jun

Mar-

Apr

Chalk DitchW2.5m

D1mHigh Jul-Nov Sept

Chalk Ditch W6m D2m Low/High Dec-JuneMar-

Apr

The Best "Season"

The above table gives example responses for types of anomaly and time of year.

The first point to note is that features provide differing levels of response

throughout the year. The main reason for this is the amount of rain held in the

soil. As soil becomes soaked its resistance lowers until the readings from many

features become "swamped" or hidden by this low resistance (low resistancefeatures will disappear once the surrounding soil reaches the same resistance.

Also hi resistance features may go by unnoticed when surrounded by very low

resistance soil).



The amount of moisture retained by the soil is largely dependent on two things -

amount of rainfall and drainage. Rainfall is typically seasonal, thus over the

winter months many sites became waterlogged and unreadable.

The drainage for a particular site is dependent on many localised factors such as

slope of the land. The underlying bedrock however commonly plays a significantrole in determining the soils water content. An impervious rock such as

sandstone will typically retain moisture for longer and therefore the resistivity

"season" is shorter. By the same reasoning each local area will have a best time

to survey depending on the local drainage and recent rainfall.

The size and nature of the buried feature also has an impact on the definition

shown by a resistivity survey. Stone tends to have a higher resistance than the

low resistance soil and even with relatively high resistance bedrock such as clay.

Ditches generally always show a low resistance, the deeper the ditch the lower

the resistance since deep ditches cut through the bedrock, lowering the depth of the low resistance fill.

Chalk can give conflicting results due to its structure. When it is dry, small ditches willtend to show a low resistance reading. However, chalk can act like a sponge when

waterlogged, changing its resistance "form" from damp chalk to that of chalky water. In

these conditions the bedrock becomes very low resistance and the ditch reads as a high

resistance anomaly. Furthermore it has been found that in some cases due to the local water table the anomaly can fluctuate between high - neutral - low readings throughout the year

and it is with this in mind that in chalk areas two resistivity surveys approximately. six

months apart are recommended (spring and autumn).

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