Magnetic Surveying in Archaeology (book)
Encyclopedia
Magnetic Surveying in Archaeology (Wormianum, 2008, ISBN 978-87-89531-29-8) is a book written by Russia
n archaeologist T. N. Smekalova together with O. Voss and S. L. Smekalov. In the book researches collected information about magnetic prospecting of archaeological site
s. 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.
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.
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 rock
s and mineral
s 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.
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.
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 (building
s, fence
s, electric power line
s, small modern metal objects on a surface of a site, pipeline
s and natural features (magnetic (granite
etc.) bedrock
, solar storm
s, 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).
- 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 dipole
s, 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 proton
s 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 sensor
s 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)
. 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 kiln
s. 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, posthole
s), 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.
, Syria
, Denmark, Ukraine
, Russia, Greece
, Norway
.
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
n archaeologist T. N. Smekalova together with O. Voss and S. L. Smekalov. In the book researches collected information about magnetic prospecting of archaeological site
Archaeological site
An archaeological site is a place in which evidence of past activity is preserved , and which has been, or may be, investigated using the discipline of archaeology and represents a part of the archaeological record.Beyond this, the definition and geographical extent of a 'site' can vary widely,...
s. They have based most of their examples on the use of the Overhauser magnetometer
Magnetometer
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature...
-gradiometer
Gradiometer
A gradiometer measures the gradient of a physical quantity, such as a magnetic field or gravity.-Types of gradiometer:There are at least two types of gradiometer to measure magnetic fields:...
GSM-19WG from GEM Systems. In their book the researches concentrated mostly on practical aspects of magnetic survey
Magnetic survey (archaeology)
Magnetic survey is one of a number of methods used in archaeological geophysics. Magnetic surveys record spatial variation in the Earth's magnetic field. In archaeology, magnetic surveys are used to detect and map archaeological artifacts and features...
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.
The Earth's magnetic field
Magnetic fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
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
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
areas (Breiner, 1973).
The method of magnetic survey is a passive geophysical
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
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.
Magnetic anomalies
If the earth were composed of uniform material, the magnetic lines of forceLine of force
A line of force in Faraday's extended sense is synonymous with Maxwell's line of induction. According to J.J. Thomson, Faraday usually discusses lines of force as chains of polarized particles in a dielectric, yet sometimes Faraday discusses them as having an existence all their own as in...
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
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...
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
Magnetic anomaly
In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material...
(Breiner, 1973).
The anomalies from archaeological objects or naturally occurring rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s and mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s are due chiefly to the presence of the most common magnetic mineral, magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, 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
Data collection
Data collection is a term used to describe a process of preparing and collecting data, for example, as part of a process improvement or similar project. The purpose of data collection is to obtain information to keep on record, to make decisions about important issues, to pass information on to...
. 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.
Limitations
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 (building
Building
In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
s, fence
Fence
A fence is a freestanding structure designed to restrict or prevent movement across a boundary. It is generally distinguished from a wall by the lightness of its construction: a wall is usually restricted to such barriers made from solid brick or concrete, blocking vision as well as passage .Fences...
s, electric power line
Power Line
Power Line is an American political blog, providing news and commentary from a conservative point-of-view. It was originally written by three lawyers who attended Dartmouth College together: John H. Hinderaker, Scott W. Johnson, and Paul Mirengoff...
s, small modern metal objects on a surface of a site, pipeline
Pipeline transport
Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
s and natural features (magnetic (granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
etc.) bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
, solar storm
Solar storm
Solar storm can refer to:*Solar flare, a large explosion in the Sun's atmosphere*Coronal mass ejection , a massive burst of solar wind associated with solar flares*Geomagnetic storm, the interaction of the Sun's outburst with Earth's magnetic field...
s, lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
). 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).
Magnetometers
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
Proton magnetometer
The proton magnetometer, also known as the proton precession magnetometer , uses the principle of Earth's field nuclear magnetic resonance to measure very small variations in the Earth's magnetic field, allowing ferrous objects on land and at sea to be detected.It is used in land-based archaeology...
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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
atom in a sample of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
fluid (water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...
, alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, etc.) to measure the total intensity of the field. The spinning protons, which behave as small magnetic dipole
Magnetic dipole
A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not complete. In particular, a magnetic...
s, 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 proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s 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
Polarization
Polarization is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acoustic waves in a gas or liquid do not have polarization because the direction of vibration and...
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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
. 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 sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s 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
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
to provide a correction for the temporal change in the earth’s field. It could be connected to the console
Console
- Computing and video games :* System console, a physical device to operate a computer** Virtual console, a user interface for multiple computer consoles on one device** Command-line interface, the typical use of the computer 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
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
. 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
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
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 DenmarkDenmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
. Magnetic methods were used in Danish archaeology in two different ways: first, for archaeomagnetic dating
Archaeomagnetic dating
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials. These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool below the Curie point, freezing the magnetic moment...
, 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
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
site Drengsted in Southern Jutland
Southern Jutland
Southern Jutland is the name for the region south of the Kongeå in Jutland, Denmark. The region north of the Kongeå is called Nørrejylland . Both territories had their own ting assemblies in the Middle Ages . South Jutland is mentioned for the first time in the Knýtlinga saga.In the 13th century...
, 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 kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
s. 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
Jutland
Jutland , historically also called Cimbria, is the name of the peninsula that juts out in Northern Europe toward the rest of Scandinavia, forming the mainland part of Denmark. It has the North Sea to its west, Kattegat and Skagerrak to its north, the Baltic Sea to its east, and the Danish–German...
, 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, posthole
Posthole
In archaeology a posthole is a cut feature used to hold a surface timber or stone. They are usually much deeper than they are wide although truncation may not make this apparent....
s), 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
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
) is large enough (see Table below), and, second, that the noise level is rather low.
Part 2. Results
At the second part of the book many examples from field work are represented. Among others you can find magnetic surveys from EgyptEgypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
, Syria
Syria
Syria , officially the Syrian Arab Republic , is a country in Western Asia, bordering Lebanon and the Mediterranean Sea to the West, Turkey to the north, Iraq to the east, Jordan to the south, and Israel to the southwest....
, Denmark, Ukraine
Ukraine
Ukraine is a country in Eastern Europe. It has an area of 603,628 km², making it the second largest contiguous country on the European continent, after Russia...
, Russia, Greece
Greece
Greece , officially the Hellenic Republic , and historically Hellas or the Republic of Greece in English, is a country in southeastern Europe....
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
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