What is geophysics?

Geophysics is useful in locating and identifying archaeological features. It works on the basis of identifying variations or “anomalies” in natural properties of the local environment, such as the local magnetic field or resistance of the earth. The contrast between the geophysical properties of the archaeological features and the natural background allows their detection.

Why is it useful?

Geophysics is a non-intrusive technique. Archaeological features can be detected without having to dig. It is a fast and effective way of assessing large areas where large scale excavation is not desirable.

What types of survey are there?

There are many techniques that can be used to find archaeology. The two main techniques used at J. M. Leigh Surveys are magnetometer and resistance surveys.

How do these techniques work?

· Magnetometer Survey

This technique works by finding variations in the local magnetic field, which are caused by human activity.

What is the Earths magnetic field and what does it do?

The Earths magnetic field can be simplified to a dipole (bar magnet) magnetic structure. It is thought to be produced by chemical and electrical activity within the earths core.

The Earths field is measured in polar co-ordinates, as inclination, declination and strength. Inclination and declination describe the direction at which the field passes through the earth. The strength of magnetic fields is measured in Tesla (T). The Earths magnetic field is approximately 50 mT (0.00005 T). Some archaeological features cause variations in the strength of the natural local magnetic field. These variations are very subtle and can be measured in nano-Tesla (nT), where 1 nT = 0.000000001 Tesla.

What affects the Earths Magnetic Field?

· Solar wind from the sun. (Diurnal or Daily)
· Geology - Rocks have a wide range of magnetic influences from pure magnetite (loadstone) which your steel cap boots will almost stick too, through to sandstone, which is virtually non magnetic.
· Man made ferrous objects (buildings, cars, fences, pipes, drums, etc.)

Isodynamic map showing field strength across the world (x103 T)

Why are archaeological soils magnetic?

Oxidation and reduction of iron rich minerals form ferri-magnetic iron oxides in the topsoil. Burning generally causes this process but natural inorganic and bacterial processes also contribute. In the case of ditches that are cut into the subsoil, they become filled with the more magnetic topsoil. This added with any burnt occupational material, which may have been dumped into the ditch over time, makes them more magnetic than the surrounding soil and detectable as magnetic anomalies.

Burning is a key factor in the detection of archaeological occupational remains. When a material is burnt magnetic grains in iron rich minerals move freely and align with the ambient magnetic field. As the minerals cool down the grains become “fixed” in the same direction as the ambient field, thus becoming magnetic, this is known as the Le Borgne effect.

These minerals are ferri - magnetic. Some archaeological features, such as hearths and kilns, will have sustained high temperatures during use and are relatively strongly magnetized. This magnetism is known as Thermo Remnant Magnetization (TRM).

At J. M. Leigh Surveys we use a Fluxgate gradiometer to measure these subtle variations. (Gradiometers measure the direction and strength of a magnetic field). The instruments used by J. M. Leigh Surveys have two magnetometer sensors each measuring the ambient (local) magnetic field at different heights. The bottom sensor detects the magnetic field close to the ground and the top sensors measure the constant background geomagnetic field. The instrument calculates the gradient (difference) between these two measurements and gives a relative reading of any variations.

What can be detected with a magnetic survey?

Magnetometer surveys are good at locating buried ditches, walls and burnt materials. These types of features tend to be associated with archaeological occupation.

Earth Resistance Survey
This technique relies on the water content of the soil and how archaeological features affect the water content, and as such can be greatly affected by weather conditions. Survey must be planned in accordance to this.

How is resistance measured?

Resistance surveys apply the basic principles of Ohms Law.

A small current is passed though the ground and the resultant potential difference is measured. The resistance is calculated and displayed on the resistance meter. Resistance is measured in Ohms (). An archaeological ditch will tend to hold more water than the surrounding soil and the current will pass easily through the soil. A low resistance anomaly will be detected. In the case of a buried wall, little water will be present and the current will have difficulty passing through. A high resistance anomaly will be detected.

Resistivity, , is measured in ohm-meters and is a material property. Its value will be constant for a given material, where as the measured resistance may vary according to instrument set up and weather conditions. When resistivity is discussed in terms of an archaeological survey it is often referred to as apparent resistivity, as archaeological soils are rarely homogenous. The resistivity reading is an approximation.

What can be detected with a resistance survey?

• Building remains
  
• Ecclesiastical Sites
  
• Buried Paving / Floors
  
• Brick Foundations
  
• Souterrain Features

How is the data collected?

Survey is collected in grids. Each grid is typically 20m x 20m and the sample interval (amount of data to be collected) is decided before the survey commences. For example: a gradiometer survey is set out to collect data every 1m with a traverse interval of 1m. This means the data can be displayed in 1m2 blocks within a survey grid, as bellow.

How is the data interpreted?

Interpretation of the data can be made by examining the responses recorded. Experience indicates that certain archaeological features often produce similar, recognisable responses. Different formats of the data are examined. Usually, as XY-trace plot and dot density plot, along with the greyscale images. Responses are classed into different groups, dictated by their shape and strength. Responses that are clearly archaeological and have other evidence to imply the responses are such are termed Archaeology.
Responses which may be archaeological are termed ?Archaeology. Responses that are modern in origin, or believed to be natural also have their own category. Modern material can often be differentiated as having a large ferrous response. Natural responses often have a sinuous incoherent form. The data is interpreted by the geophysicist using his or her experience.

This interpretation provides the archaeologists with an understandable and useful plot of the geophysics results. Following a geophysical survey a mitigation strategy can be formulated by the archaeologists, depending on the type of archaeology and the reason for the geophysical survey.

What survey methodologies are there and which one is right for my site?

There are several different survey methodologies which can be implemented on a site. This will depend on the survey requirements; whether the survey is commercial or research based, what type of archaeology is to be expected, how large the area to be assessed is, what the geology and local soils constitute etc. All these factors must be considered to ensure the correct methodology is chosen, and consultation with the survey geophysicist is recommended to ensure that the optimum methodology is chosen for a particular site.
·

Typical Methodology
For a magnetic survey there are two main phases of a geophysical survey fieldwork. Phase I is a preliminary investigation, identifying areas of possible interest. Phase II is a more detailed investigation conducted to investigate any anomalies identified during Phase I. Phase I usually consists of gradiometer ‘scanning’. This is a fast method of covering large areas to identify possible areas of archaeological potential. It involves broad traverses of the application area with the gradiometer instrument. Any fluctuations or ‘anomalies’ are noted and can be further investigated with detailed survey, as part of Phase II investigations. Phase II is a detailed investigation where data is recorded to investigate the anomalies detected during Phase I. Phase II may also include resistance survey, depending on the scanning results and if there is any suggestion of building remains. A further Phase II may also be considered which may require fine detail survey. This is often required for survey on research work. Commercial projects often only require Phases I and II field work. It is not advised that Phase I is conducted independently, and Phase II should always be applied to test the results of the gradiometer scanning. The phases of geophysical survey are analogous to phases of other archaeological techniques (table below).

Although a general methodology can be suggested here, it is important to note that the methodology should be site specific, with consideration of additional information, such as any known archaeological remains, local geology, and aims of the project. Methodology should be site specific, and an incorrect methodology may result in an ineffective geophysical survey and disappointing results.