- Magnetic Surveying in Archaeology (book)
Magnetic Surveying in Archaeology (Wormianum, 2008, ISBN 978-87-89531-29-8) is a book written by Russian archaeologist T. N. Smekalova together with O. Voss and S. L. Smekalov. In the book researches collected information about magnetic prospecting of archaeological sites. They have based most of their examples on the use of the Overhauser magnetometer-gradiometer GSM-19WG from GEM Systems. In their book the researches concentrated mostly on practical aspects of magnetic survey for the investigation of archaeological sites situated in different geographical and geological conditions. Therefore, they submitted as many examples from their field work as possible.
Part 1. Principles of magnetic surveying
The method of magnetic surveying
The Earth's magnetic field
Magnetic field exists around us. We could not see and feel them, but we can measure them with sensitive instruments, called magnetometers. The Earth’s magnetic field is approximately the same as would be produced by a large bar magnet near earth’s centre oriented with the positive end towards the North Pole and inclined at an angle of 10˚ to the axes of rotation. The field or flux lines of the earth exhibit the usual pattern common to a small magnet. They are vertical on the pole areas and horizontal at the equator areas (Breiner, 1973).
The method of magnetic survey is a passive geophysical technique based on detection of contrasts in the magnetic properties of different materials. In the event that such contrasts do not exist, magnetic prospecting will not be useful. To do magnetic prospecting, one simply measures the Earth's magnetic field with the small measurement spacing and very close to the surface.
If the earth were composed of uniform material, the magnetic lines of force would be evenly distributed between the poles and at the small area would be parallel. However, since various materials in the earth have different magnetic susceptibilities due to their composition, the earth’s magnetic lines of force are distorted. The local disturbances of the global magnetic field are called magnetic anomalies (Breiner, 1973).
The anomalies from archaeological objects or naturally occurring rocks and minerals are due chiefly to the presence of the most common magnetic mineral, magnetite, FeO·Fe2O3, or its related minerals. All rocks contain some magnetite from very small fractions of percent to several percent.
Field procedure of magnetic survey
In the initial stage of an investigation, so called ”free search” is carried out to determine the boundaries of the site and some single magnetized objects. At this stage, the operator measures the magnetic field with the help of a Proton or Overhauser magnetometer without using a regular grid. Instead, the operator meanders while measuring at spacings of about 1-1.5m, and marks with small flags the anomalies which seem to be magnetized objects. The method of“free search”is characterized by a high speed (covering typically 3-4 hectares per day).
The method of detail magnetic surveying of archaeological sites is to measure Earth’s magnetic field point by point with a small step (not more than half a meter), close to the surface, and present the measurements on the magnetic map. A coordinate system is set on the site for data collecting. Usually, if there are no obstacles, there are plots 40 m (or 20 m) wide and as long as is necessary to cover the area of the site. Small wooden sticks are put at every meter along two opposite sides of the plot and 40 m-strings (or 20-meters) with meter marks are used between the sticks.
Since magnetic method, as other geophysical methods, is indirect by nature, the geophysicist can interpret data in the form of anomalies.
Causes of an anomaly can be suggested or speculated upon, however only excavations can clear verify a character of anomaly.
All geophysical techniques are subjected to noise. Noise is nothing more than false signals in the geophysical measurements. These false signals can be caused by cultural features (buildings, fences, electric power lines, small modern metal objects on a surface of a site, pipelines and natural features (magnetic (granite etc.) bedrock, solar storms, lightning). Sources of noise should be identified prior to any magnetic field work, as geophysical surveys can be planned to eliminate or diminish noise (Breiner, 1973).
A special pattern of anomalies of the Earth’s magnetic field is created on the archaeological site, detectable with sensitive instruments - magnetometers. For the archaeological prospection we used:
- Two Overhauser gradiometers GSM-19WG of GEM systems Inc. (Canada, Ontario) as main instruments;
- Cesium magnetometers MM-60, M-33 and PKM-1 (Russia, Saint Petersburg, “Geologorazvedka”);
- Proton magnetometer MMP-203 (Russia, Saint Petersburg, “Geologorazvedka”).
Proton magnetometer of free precession is one of the most common types of portable magnetometers used today for archaeological purposes. It is so named because it utilizes the precession of the spinning protons or nuclei of the hydrogen atom in a sample of hydrocarbon fluid (water, kerosene, alcohol, etc.) to measure the total intensity of the field. The spinning protons, which behave as small magnetic dipoles, are temporarily polarized by application of a uniform strong magnetic field generated by a current in a coil of wire. When the current is removed, the spin of protons causes them to precess around the direction of the ambient or earth’s magnetic field. The precession protons then generate a small signal in the same coil used to polarize them, and the frequency of this signal is precisely proportional to the total intensity of the magnetic field, which can be measured with a precision of 1nT.
The principle upon which it is based is so elegant and simple that it retains its importance despite many decades of the development of other methods (Scollar, Tabagh, Hesse, Herzog, 1990, p. 450-456). The proton magnetometers have two serious disadvantages. First, erroneous observations may occur where gradients of 300-1000 nT per m are encountered. Also, due to a finite measurement period of time, approximately three seconds, it is quite slow.
Overhauser magnetometer is a variation of the proton-precession magnetometer.
In the proton magnetometer, the polarization is raised by briefly applying a strong field. The Overhauser magnetometer uses free radicals dissolved in a liquid to raise its apparent susceptibility by pumping with a radio frequency. There is a dipole coupling between the proton spins of a liquid and the electron spins of a free radical dissolved in it. Because of the very great increase of polarization (by a factor of up to 4000 or 5000), very small amounts of fluid can be used, which makes the sensors quite small and therefore also highly resistant to gradients. Sensitivities of the order of 0.01 nT are readily obtained in practice (Scollar, Tabagh, Hesse, Herzog, 1990, p. 450-456).
The main instrument, which we normally use for archaeological prospecting is an Overhauser of GEM systems Inc. (Canada, Ontario). It permits measuring magnetic fields at rates as high as 5 readings per second with a storage capacity of about 32 Mbytes. The sensitivity is from 0.02 nT to 0.015 nT/√Hz with 10,000 nT/m gradient tolerance. The spacing between two sensors in such a gradiometer can be changed and the sensor height can be set at any value. One sensor may be used as a base station to provide a correction for the temporal change in the earth’s field. It could be connected to the console by a long cable (we had a 50-meters long cable).
Cesium magnetometers are highly sensitive type of instrument, their high resolution is about 0.01 nT.
The principle is more complex than that of the proton magnetometer. It operates at the atomic rather than nuclear level. A lamp is used for polarization. When monochromatic light passes through a magnetic field in an appropriate material, there is interaction between the spins of the substance and electromagnetic properties of the light. In contemporary instruments caesium 133 is used. The sensor is glass cell containing metallic caesium. It is heated slightly to vaporize the material. Th e circular polarized pumping light excites electrons in the caesium atoms to a more energetic state. The electrons quickly fall back to their original energy level, but they are continuously re-excited. The magnetic vectors of the atoms precess around the external field, and their moment locks onto one of the rotating components of the field from the coil around a glass cell. This “depumps” the spins and increases the transparency of the cell with a maximum at resonance which occurs at the frequency, proportional to the total magnetic field intensity (Scollar, Tabagh, Hesse, Herzog, 1990, p. 466-469). The sensitivity of the caesium magnetometers derives from its high precession frequencies, which is important for recording small signals. Another advantage of caesium magnetometer - high gradient tolerance makes it useful in measuring of strongly magnetized archaeological objects in a very shallow depth.
Fluxgate gradiometer. The sensor of it consists of two similar parallel strips of an alloy of high magnetic permeability called Mumetal.
They are driven in and out of magnetic saturation by the solenoid effect of an alternating “drive current” in the coils wound round them. Every time they come out of saturation, an external field can enter them, causing an electrical pulse in the detector coil proportional to the field strength. The drive coils of the two strips are switched in opposite directions – so that the drive current has no net magnetic effect (Scollar, Tabagh, Hesse, Herzog, 1990, p. 456-466).
The Geoscan fluxgate instruments have a noise level of about 0.1 nT, that makes surveys in areas of weak magnetic contrasts readily achievable. There are additional advantages of compactness and relative cheapness. Therefore, fluxgate gradiometer with its closely spaced direction-responsive detectors has become “the workhorse - and the racehorse” - of the British archaeological prospecting (Clark, 1996, p. 69)
The use of magnetic prospecting in Denmark
A considerable part of our work is connected to the investigations of archaeological sites in Denmark. Magnetic methods were used in Danish archaeology in two different ways: first, for archaeomagnetic dating, and second for magnetic surveying. The first magnetic survey in Denmark was carried out in 1965 by Olfert Voss and Niels Abrahamsen on the Roman Age iron-smelting site Drengsted in Southern Jutland, which immediately showed the effectiveness of this method for searching for slagblocks (Abrahamsen, 1965).
Other archaeological feature which creates strong anomalies and therefore are prospective objects for magnetic survey, are pottery kilns. Several of them were investigated with magnetometers in the filed and were also archaeomagnetically dated (Abrahamsen et al., 1982; Abrahamsen et al., 1991). Good results were obtained by geomagnetic field measurements over a reconstructed Bidstrup brick kiln (Hansen et al., 1980). Many important investigations, which could clear up the nature of magnetism in diff erent kinds of archaeological material along with age determinations, have been carried out at the Geophysical laboratory of Aarhus University by Niels Abrahamsen, Niels Breiner and their colleagues and students. (Abrahamsen & Breiner, 1990, 1993; Abrahamsen, et al., 1998). Since 1992, systematic magnetic surveys have been carried out in south-west Jutland by the authors, mostly on Roman Age iron-smelting production centers. Several promising magnetic surveys have also been done on other archaeological sites.
For the conditions in Denmark, especially in south-west Jutland, where almost all the land is cultivated, the only parts of archaeological sites still preserved are those which were underground in ancient times: all kinds of pits (garbage pits, pit-houses, postholes), wells, ditches, and also slag-blocks, etc. The usefulness of magnetic surveys on archaeological sites in Denmark is mostly due to the combination of two conditions. First, that the contrast of the magnetic properties of the archaeological material and the surrounding matter (almost nonmagnetic sand) is large enough (see Table below), and, second, that the noise level is rather low.
Part 2. Results
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