Methodical research on magnetometer, - and magnetic susceptibility prospecting: Case histories from archaeological sites
J.W.E. Fassbinder
Bavarian State Department of Monuments and Sites
Introduction
Magnetic prospection - for the first time applied in 1956 [Belshé, 1957; Aitken, 1958] - has become one of the most important archaeological methods for the detection and mapping of large archaeological sites [Ôðàíòîâ, Ïèíêåâè÷, 1966; Aitken, 1974; Scollar et als., 1990; Clark, 1996; Neubauer et als., 1999; Gaffney et als. 2000; Benech, 2005; David et als., 2008, Fassbinder, 2007]. The magnetic methods are extremely sensitive and with respect of the characterization and detection of iron oxides much more than any other chemical analysis. Therefore it should be emphasized here, that many details of the soil layers and archaeological structures in soils can be discovered, visualized and interpreted only by the "magnetic eye" and by the full understanding of their magnetic properties [Fassbinder, 1994; Schleifer et als., 2003; Fröhlich et als., 2003; Schleifer, 2004]. It is self evident that the entire archaeological interpretation needs also all available archaeological background information, as well as surface findings. However many more crucial details can be derived by a well elaborated soil magnetic analysis of the data and many new archaeological questions arise namely by the geophysical prospecting results.
While for a long time it was a firm conviction of archaeologists that geophysical prospecting results on their own would be only of limited use to resolve archaeological problems [Aspinal et als., 2008; Schmidt, 2002]. Today it has become common sense, that the start up and the initiation of a modern archaeological excavation without previous geophysical prospecting is utterly impossible.
The great success of magnetic prospection in general is due to the fact that almost all soils of the world show an enhancement of magnetic minerals such as maghemite or magnetite in the top soil [Le Borgne, 1955; 1960; Fassbinder et als. 1990]. Except of very rare examples mostly on sites with dammed up water and soil wetness, there exist no limiting geological factor for the application of magnetic prospecting. Enrichment of these minerals in the archaeological soil layers and namely in fireplaces, but also in ditches, pits or postholes is caused by the neo-formation of these minerals either by natural or man made fires, as well as by pedogenic processes [Taylor et als., 1987] and by magnetotactic soil bacteria [Fassbinder et als., 1990; Stanjek et als., 1994]. However it should be clear that the use of fire plays the major role in the enhancement of magnetic minerals in soils since this occurs on nearly all sites from the Palaeolithic to modern times.
Instrumentation
There exist a large range of suitable but very different instruments for the measurement of the Earth's magnetic field [Lenz, 1990]. However for the special application of archaeological prospection a robust, but also sensitive and fast measuring field instrument is required [Becker, 1995; 1997; Gaffney et als., 2000]. In general one has to differentiate between vectormagnetometer and scalarmagnetometer. While the vectormagnetometers measure only the intensity of one direction (the x, y or z-component) of the Earth's magnetic field, the scalarmagnetometers measure the total intensity of the Earth's magnetic field more or less independently of the orientation of the probe.
The diurnal variations of the Earth's magnetic field are sometimes in the same range and intensity than those of the archaeological anomalies. Since vectormagnetometer can be operated in a gradiometer mode, these disturbances are removed automatically. Scalarmagnetometer need to be applied as a differential or as a variometer system. Else they need a complex data treatment for the removal of these daily variations.
The decision for the application of a distinct magnetometer system should be carefully balanced between the advantages/disadvantages of each system and depend very much on the situation and the specific aim of the survey. Vectormagnetometers that are designed for the measurement of the z-component are quite independent on disturbances in the x-y-component. Hence these instruments can be more easily operated in the vicinity of a vehicle and nearby technical installations. On the other hand the sensitivity is limited to other factors, namely to the use of a gradiometer system. Although SQUID magnetometers (sensitivity of the tensor component ± Femtotesla [fT]) are physically the most sensitive instruments, in practice (like other vectormagnetometers) they can operate only in a gradiometer mode [Chwala et als., 2001; Schultze et als., 2008]. While fluxgate magnetometers operate with a gradient of 50-100 cm, SQUID's are restricted by other physical reasons to measures in a gradient of 4 cm.
Modern total field instruments like caesium-magnetometers have a sensitivity of ± 0,1 Picotesla (pT), but can be operated in a "variometer" or "duosensor" configuration and therefore reach an overall sensitivity in the same range than the SQUID's [Becker, 1995; 1997]. By this configuration the reference value of the Earth's magnetic field is set to infinity, therefore the magnetic anomalies can be measured with their full intensity (fig. 1). The advantage is that the resulting image gives us more information from the deeper parts of the archaeological structures. On the other hand it is "disturbed" by geological features and very much by narrow technical installations.
Digital image processing
Digital image processing of geophysical data was a milestone in the field of archaeological prospecting and a crucial step to make these results intelligible to both camps the geophysicist and the archaeologists. The potential of digital image processing of geophysical data compared to the isoline display was recognized and applied already very early in the history of archaeological prospecting [Scollar, Krückeberg, 1966]. Meanwhile there is a range of sophisticated programs available that enable us to treat the data by a multitude of correcting and processing tools with respect to the requirements of the different instruments in archaeological prospection [David et als., 2008; Schmidt, 2002].
Step by step the survey area has to be measured at least by a sampling interval of 25 x 50 cm or better 25 x 25 cm. The magnetometer data (the relative intensity versus an x-y-plane) is then converted to a grey-shade image and displayed on the computer screen, then exported and stored as a tiff-file. The reason for the display as a grey-shade image is twofold:
1) The intensity of the magnetic field is a mono parametric data set. The only physically serious display of such data must be performed by a mono parametric scale.
2) The human eye is capable to discriminate up to 60 greyscales. Hence the display of the magnetometer data as a grey shade plot of 256 greyscales (from white to black) is a crucial tool for the success of this prospection method.
The popular denotation of the resulting image of the magnetometer data was introduced in the literature as a magnetogram, although this term is also used in geomagnetism for the variation of the Earth's magnetic field with time.
Soil magnetism and archaeological soils
Deviations of the intensity and/or direction of the normal Earth's magnetic field are simply provoked by the magnetic contrast between the archaeological features and the adjacent soils and sediments and hence enable us to detect archaeological sites beneath the ground. The enhancement of ferrimagnetic minerals in the top soils is a common property of almost all soils worldwide [Le Borgne, 1955; Mullins, 1977; Fassbinder, Stanjek, 1993]. It was observed even on very high magnetic soils of volcanic origin and background [Tucker, 1952; Fassbinder et als., 2009].
Enrichment and separation of these heavy ferrimagnetic minerals can occur mechanically simply by wind or by water [Fassbinder et als., 2005], as well as by a "pedogenic" formation in soils, but first and foremost by the heating of soils during natural fires, wood fires and more intensively by the use of fire by man. Once produced and originated in the topsoil these minerals end up in ditches, pits, palisades or postholes and will originate a magnetic anomaly above the ground.
The formation of magnetic minerals in soils
Enrichment of ferrimagnetic minerals in top soils was recognized and described by Le Borgne [1955; 1960] and ascribed to the widespread use of fire either during forest clearance and the widespread use of fire by man. But very soon it was also clear, that many archaeological features previously detected by magnetometers as a positive anomaly were obviously never exposed to fire [Fassbinder, 1994]. Tite and Linington [1975] showed that also the climate has a huge influence on the susceptibility and hence on the formation of magnetic minerals in soils.
Here it must be emphasized that the distinction between magnetite and maghemite is of great importance. The presence of one of these minerals may give also valuable information about the fate and history of an archaeological site.
Le Borgne ascribed the formation of maghemite either to burning events (high-temperature process) or to a low temperature reduction-oxidation cycle [1955; 1960].
Formation of maghemite in soils after Le Borgne:
1) By fermentation: Reduction by the decomposition of organic materials in anaerobic soils, followed by reoxidation to maghemite during dry weather periods under aerobic conditions
2) by natural and man made fire:
α - Fe2O3 ==> Fe3O4 ==> γ - Fe2O3
hematite ==> magnetite ==> maghemite
reduction ==> oxidation
Both processes start with hematite, have magnetite as intermediate phase and should finally yield maghemite. Synthesis experiments and observations in nature, however, indicate that the processes forming maghemite are different and more complex. Four precursors are known for maghemite:
a) Magnetite inherited from parent rock or sediment oxidizes (partially) to maghemite. These maghemites have usually grain sizes in the range of millimetres. This process has been observed e.g. for titanomagnetites [Fitzpatrick, Le Roux, 1976].
b) Depending on the particle size, lepidocrocite (γ - FeOOH) dehydrates between 260°C and 300°C to maghemite [Scheffer et als., 1959; Schwertmann, Taylor, 1979].
c) In the presence of organic matter, goethite is transformed to maghemite during bush fires [Schwertmann, Fechter, 1984; Anand, Gilkes, 1987; Stanjek, 1987].
d) Siderite (FeCO3) oxidizes readily to maghemite, when it is gently heated [Van der Marel, 1951; Schwertmann, Heinemann, 1959].
However, it has not been observed so far, that hematite may act as precursor for maghemite or magnetite. Apart from the fact that hematite is not always present in soil, where maghemite was found, it has not been conclusively shown yet that hematite can be reduced to magnetite under such conditions. Mineral assemblages [Anand, Gilkes, 1987; Stanjek, 1987] as well as calculations [Scotter, 1979] limit the maximum temperatures reached in top soils during burning to about 300-400°C, where a reduction of hematite by reducing agents such as organic carbon is unlikely.
The formation of magnetite in soils and sediments is still discussed controversially in the literature [cf. Oldfield, 1992; Dearing et als., 1997]. Two pathways for the pedogenic formation are proposed:
a) Inorganic: In synthesis-experiments the controlled oxidation of ferrous iron yields magnetite [David, Welch, 1956]. This inorganic formation may also take place in soils [Maher, Taylor, 1988].
b) Biologically: The biologically controlled formation of magnetite by soil bacteria has been observed [Fassbinder et als., 1990]. The intracellular formed magnetite crystals may be arranged in chains and have similar size and shape to magnetite that was extracted from soils. Furthermore dissimilatory iron-reducing bacteria such as GS-15 [Lovley et als., 1987] may form magnetite extracellularly in soil.
Formation of greigite (Fe3S4) in soil can occur by two path ways:
a) Inorganic: It was shown that syntheses-experiments under controlled conditions yields greigite [Uda, 1965].
b) Biologically controlled: Evidence of magnetotactic greigite bacteria was found by Mann et als. [1990]. Evidence of biologically unidentified soil bacteria was reported by Stanjek et als. [1994] and Fassbinder & Stanjek [1994].
The formation and transformation process of iron oxides in soils is a rather complex interrelation between geochemistry, temperature, temporary weather conditions and climate [Schwertmann, 1988]. A simplified sketch illustrates the different pathways of the formation and transformation process for magnetite and maghemite that may occur in natural soils and sediments (fig. 2).
Induced magnetization
If a ferro(i)magnetic sample is exposed to a magnetic field, a magnetization Ji will be induced in the sample. Magnetometer measurements in applied field (in the range of the Earth's magnetic field) provide information on the nature and quantity of the magnetic minerals and can therefore monitor changes that occur in a sample through the heating.
Examples of archaeological sites whose resulting magnetic anomalies can be explained by the induced magnetization of the features are shown from archaeological sites in the Nile delta [Becker, Fassbinder, 1999] and from a kiln site near Regensburg, Bavaria [Fassbinder et als., 2011] (fig. 6 and 8).
Table 1.
Occurrence and origin of magnetic iron oxides and sulphides in soils and archaeological layers and objects
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Phase Magnetite Maghemite Greigit Hematite Goethite Ti-Mgt, Ti-Mgh
SP - Super paramagnetic; PSD - Pseudo-single Domain; SD - Single Domain; MD - Multi Domain; Gt - goethite; Lp - lepidocrocite; Mgt - magnetite
Table 2.
List of the specific magnetic susceptibility (nondimensional SI - Units) of natural magnetic minerals and ferrous ironSpecific susceptibilities of various minerals:
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Remanence carrying minerals specific susceptibility (SI units) Ferrous iron 2 x 10E7 Magnetite (Fe3O4) 5 x 10E4 Maghemite (γ - Fe2O3) 4 x 10E4 Pyrrhotite (Fe2S8) 5 x 10E3 Ilmenite (FeTiO3) 200 Lepidocrocite (FeOOH) 70 Goethite (α - FeOOH) 70 Hematite (Fe2O3) 60
Beside the formation and enrichment of fine grained magnetic particles also the remanent magnetization of archaeological features and objects is playing a significant role in the magnetic anomalies.
The exact connection between an anomaly and the archaeological structure (body) is not always clear - usually because the sample is not available without an archaeological excavation. The interpretation of an anomaly depends on the fact, whether the magnetization is only induced by the present Earth's magnetic field (and is therefore in the present field direction) or is mainly remanent (permanent) and parallel to some ancient field - which may be only a small angle to the present field direction if the structure was magnetized in situ by a burning or heating event. The Koenigsberger ratio Qn of remanent to induced magnetization is therefore an important parameter that may help with the archaeological interpretation of the results. While the remanent versus induced magnetization for archaeological soils is in the range of Qn ~ 1, in burned bricks and heated soils it can reach values of > 10 (depending very much on the grain size of the magnetic minerals) and in areas of a lightning strike it can reach values of up to 500-700 (see fig. 7).
The remanent magnetization in archaeological structures
Nearly every rock and sediment, but also fossil and archaeological soils have a remanent magnetization [Dunlop, Özdemir, 1997; Fassbinder, 1994]. The origin of such a magnetic remanence can be manifold. A primary natural remanent magnetization NRM can be acquired by three basic processes:1) Thermoremanent magnetization (TRM)
If rocks, sediments or soils of archaeological sites or features are exposed to high temperatures they become initially magnetized by a thermoremanent magnetization (TRM). While the constituent magnetic minerals cool through their Curie or blocking temperature in the ambient geomagnetic field, the direction of the magnetization will be aligned along the Earth's magnetic field direction. Additional ferrimagnetic minerals such as maghemite will be created by the transformation of antiferromagnetic minerals such as goethite and lepidocrocite (see fig. 2). The high magnetic intensity namely that of pottery kilns, fireplaces and metal production sites originates from the TRM and can be easily identified in the magnetogram (see also fig. 8).
2) Detrital remanent magnetization (DRM)
As an archaeological soil containing permanently magnetized oxide grains is deposited in water (e.g. in a pit, ditch or in ground depressions) the grains tend to orient themselves in a position of minimum energy i.e., with their magnetic axis aligned along or parallel to the ambient magnetic field direction. Although there could be still some disorientation, a statistical alignment along to the field direction remains, this is called DRM. If there are some grains that are sufficiently small relative to interstitial volumes, alignment will also take place in water trapped in the interstices of the soils and sediments after the deposition. This is referred to as a post-depositional remanent magnetization (PDRM). Although such remanences are rather weak, they can result in a detectable magnetic anomaly, if they are partially destroyed by an excavation and thus by a mechanical demagnetization [Fassbinder, Becker, 2003; Fassbinder, Gorka, 2009b; Fassbinder, 2010a].
3) Chemical remanent magnetization (CRM)
At normal Earth-surface temperatures and sufficiently small magnetic particles thermal fluctuations dominate and the alignment of particles is randomized. But as the particle volume grows through a critical value, magneto-static forces overcome the thermal fluctuations and produce a chemical or crystallization remanent magnetization.
4) Lightning induced remanence (LRM)
A lightning strike can produce a magnetization in the adjacent and immediately surrounding area of rocks, but also of sediments and soils. Such a lightning strike magnetization is easy to recognize by an anomalous high intensity of the magnetic anomalie (> 200 Nanotesla) but also by their typical star shaped structures (fig. 7) [Maki, 2005; Fassbinder, Gorka, 2009a].Interpretation of the magnetometer data and magnetogram images
In the simple form the magnetogram (deviation of the magnetic intensity) gives an easily intelligible picture of the structures beneath the soil. The knowledge of soil magnetic properties combined with the descriptive/comparative method of archaeological understanding is the key to the optimal results of this approach [Neubauer, Eder-Hinterleitner, 1997; Fassbinder, Irlinger, 1999].Positive magnetic anomalies on archaeological sites
The most common situation on nearly all soils of the world is an enhanced magnetization and magnetically enriched topsoil. Hence whenever any pit, ditch or wooden posthole that was refilled by topsoil this will generate a positive magnetic anomaly. If such a structure is refilled by homogeneous topsoil, the intensity and the shape of the anomaly is proportional to the size and volume of the archaeological feature. Any concentration of pottery, ash or burned material, but also solid rocks or other material, will cause a deviation and thus will determine the intensity and the shape of the magnetic anomaly. The Neolithic ring ditch from Steinabrunn (lower Austria) gives us such an ideal example for this case (fig. 3).Features with a negative magnetic anomaly
Negative magnetic anomalies may have numerous reasons:
a) The material of the archaeological structure has a lower magnetic susceptibility than the adjacent top soil. For example: this will be the case, if there are fundaments of limestone or sandstone in the ambient magnetic soil, but also when mud bricks are made from more sandy material than the surrounding mud or the debris of ceramic, pottery and burned materials.
b) A negative magnetic anomaly may be also occur whenever there was excavated a pit that was immediately refilled by the same material. Such an action can be understood by we comparing it to a mechanical demagnetization of a rock sample. The resulting magnetic field intensity, which is normally always a composition of the induced plus the remanent magnetization, is diminished by the remanent part of the soil. Hence the resulting magnetogram shows a negative anomaly compared to the adjacent intensity above such an area. Similar case histories of magnetic anomalies, which could be explained most probably by such a reason, are reported very rarely [Fassbinder, Irlinger, 1998b; Fassbinder, 2010a]. A comprehensive example for such a case was also found in the steppe of North-Eastern Kazakhstan (fig. 4). Adjacent to a Scythian kurgan, there was an Islamic cemetery of the 17th century. The site consists of an oval ditch that is still 40 cm deep and visible above the ground. Inside the oval, there is a small memorial square place also enclosed by a small ditch. In the remaining area however - there is nothing else visible in the topography - roughly 40 negative (white) anomalies from burials, ca. 1 x 2 m in size are detectable that can very probably be ascribed to the pits of a burial yard. These pits with a depth of ca. 1,8-2,0 m were excavated for the funeral and immediately refilled by the human body and the same loess sediment that was excavated before.
c) A geochemical process such as the partial dissolution of ferrimagnetic particles and the precipitation of iron oxides as goethite, ferrihydrite and lepidocrocite is another basic cause for the occurrence of a negative magnetic anomaly. In soils, where we have stagnant moisture in combination with a temporary changing ground water table, one can observe very often the situation that an originally positive anomaly converts to a negative one. So a former ditch of an earthwork that was originally filled with topsoil and organic material of high magnetic susceptibility, shows up as a negative lane. A good example for this is the Neolithic earthwork of Riekofen (Bavaria). The central settlement is on a slight hill only 1,5 m above a little river. The inner ditch shows us a normal positive magnetization, whereas the outer one was partly flooded by water and shows a "negative" trace (fig. 5) [Fassbinder, Irlinger, 1998a; Schleifer et als., 2003]. Temporary soil wetness is also one of the main reasons why the large ditches of Celtic square enclosures become somehow invisible although they were clearly detected by air photos and aerial prospecting [Fassbinder, 2005].
The ground plan of a palace of Ramses II shows up as a negative magnetic anomaly in huge parts of the wall, but at some areas also as a positive one depending on the magnetic susceptibility of the used mud bricks (fig. 6). The foundations of the columns however were always visible as a negative sign because of their limestone and/or sand fundaments, which were used for this purpose. In situ measurements of the magnetic susceptibility of mud bricks confirmed this hypothesis [Becker, Fassbinder, 1999].
Remanence based features
There exists a wide range of examples of lightning induced magnetic anomalies in archaeological sites. In former times such anomalies remained quite often not understood and were mistakably interpreted. Lightning induced anomalies are typically star shaped and are characterized by their varying direction of the remanence. While thermoremanent magnetization of archaeological features shows up with anomalies that are more or less parallel to the present Earth's magnetic field, lightning induced remanence has almost erratic directions, which follow and show the trace of the electric current in the soil. The highest density of lightning induced anomalies was found on the famous geoglyphes of Nasca, Peru (fig. 7). Although today's most driest place on Earth (average precipitation of less than 5 mm per year), these sites are situated on the borderline between the desert and the fertile land and they were exposed extensively to lightning strikes in the past [Eitel pers. comm.; Eitel et als., 2005]. On a measured area of ca. 100 ha (five trapezoidal Geoglyphs around Palpa, Peru) we counted in total 50 lightning strikes [Fassbinder, Gorka, 2009a].Thermoremanent magnetic anomaly
The example of a kiln, which was discovered by magnetic prospecting near the Roman fort of Großprüfening (city of Regensburg), is shown in fig. 8. The normalized magnetic anomaly of the total field measurement shows us a big positive (black) round shaped anomaly (ca ± 20 nT) with an oval shaped adjacent area, that is an enrichment of fine grained magnetic minerals in the topsoil (± 8 nT). Applying a high-pass filter to the data set enables us to visualize the round shape and the ground map of the furnace in all details, including the opening of the kiln. The traces of the ash deposits, that is located in opposite direction of the opening disappeared by the high-pass filter [Fassbinder et als., 2011].Stratigraphy of magnetic prospection
Magnetic prospecting is a potential method and thus not very suitable to detect different archaeological layers and to discriminate them from each other. Nevertheless there are some case histories, that show us some examples of specific features, where it was possible to discriminate at least two phases of an archaeological stratigraphy [Fassbinder, Irlinger, 1998b]. The Roman camp of Burgsalach gives such an example. The small Roman camp, ca. 40 x 40 m in size, was fortified by a palisade (visible as a small narrow ditch), then in a second phase it was overbuild and enlarged to a greater camp of ca. 100 x 120 m. The older trace of the palisade was "overbuilt" by the new one. That becomes visible at the junction of both constructions. This geophysical interpretation is supported by the archaeological interpretations based on the ground map of the site. We can see that the symmetry and the entrances of the later camp are referring to the remains of the smaller and older camp (fig. 9) [Fassbinder, Gorka, 2009b].Celtic square enclosures are an abundant type of earthworks and occur in southern Bavaria, Baden-Württemberg, as well as in France. Magnetic prospecting of nearly 40 sites in Bavaria reveals the main characteristics of these monuments [Fassbinder, 2005]. Almost all consist of an earth wall that was enclosed by a ditch, which was partly filled by water. Some of these ditches contain stagnant water and can hardly be recognized in the resulting magnetogram. Compared to the size (from 1-5 hectares) only a few buildings can be found inside. Many of these buildings were actively destroyed by a fire and the resulting magnetogram of these buildings shows a high intensity. By the help of "archaeological knowledge" and the excavation report of a similar type of building with two phases [Möslein, 2002], it was possible to interpret the magnetogram (fig. 10).
Magnetic prospection near the geomagnetic Equator
Meanwhile there are numerous case studies from magnetic prospection of archaeological sites in the northern hemisphere. However only rare papers report from sites near to the geomagnetic Equator [Tite, 1966; Fassbinder, Becker, 1999; Magnavita, Schleifer, 2005; Schmidt et als., 2009; Fassbinder, Gorka, 2011].
The reason is probably twofold: firstly geophysics is well established as a prospecting method for archaeological fieldwork in Europe, Russia and Northern America, as well as in China and Japan, but poorly in the countries at the equatorial latitudes. Secondly, the results of magnetometer surveys near the geomagnetic Equator are rather complex and require a profound knowledge for a proper archaeological interpretation. The shape of the magnetic anomaly of the same feature varies dramatically with the change of the latitude as well as the type of the magnetometer used (see fig. 11).
A selected case study undertaken at Bolivia may help to demonstrate and explain the difficulties. The archaeological background is: the pre-Hispanic archaeological sites and ring ditches near Bella Vista in the lowlands of the Llanos de Moxos (northern Bolivia) were discovered after large forest clearing in 1999. First excavations by archaeologists of the German Archaeological Institute revealed the occurrence of single burials inside the ditches, but no further structures or traces of settlements. The magnetogram (fig. 12) reveals in an ideal way all the problems with the interpretation of magnetogram near to the geomagnetic Equator. Although there is a full ring, this ditch is only clearly visible in the northern and southern part. This result compares quite perfect with the theoretical prediction and the shape of magnetic anomalies at the Equator (see fig. 11).
Summary
Without magnetic prospecting many details of an archaeological site will probably remain overlooked and unseen in the excavation. However magnetic prospection can only be successful on sites where there has developed a magnetic contrast between the archaeological structures and the adjacent undisturbed soil layers. And so far there is no serious forecast whether or not magnetic prospecting will be successful or not.
Magnetic prospecting provides us with detailed maps of ancient settlements or even whole ancient cities as well as with monumental buildings of different periods. In particular the prospecting of a multiplicity of sites from the same specific category and/or archaeological period, delivers further valuable information both for the archaeological and geophysical research as well as for the monument protection [Fassbinder, 2005; 2010b]. If we take into account mineral and rock magnetic science, it is possible to explain further details about some particulars of the archaeological features. From the part of geophysics, the magnetic anomaly is determined by the intensity and the direction of the anomaly. Further analysis can allow discriminating between induced and remanent magnetization and may yield also information on different archaeological phases. From the archaeological side, it could be possible to introduce a rough dating of the features by comparing the layout and the structures with other findings that were already known from older excavations.
Neither from the methodology nor from the sensitivity and the image processing or from the measuring technique is the development of magnetic prospection yet accomplished. But already nowadays we are able get results that are comparative to the maps of an archaeological excavation. Specific structures and the shape of the features allow a rough dating of a site, without any destruction of it. Anyhow, these results will not substitute or replace an archaeological excavation, but they should be preceding every archaeological field work, in order to maximize the efficiency and to minimize the physical destruction of the archaeological heritage.
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