Seeing beneath the farmland, steppe and desert soil: magnetic prospecting and soil magnetism
Graphical abstract
Introduction
Magnetic prospecting is among the most developed archaeological methods for the detection and mapping of archaeological sites. While the very first magnetometer measurements from Aitken (Belshé, 1957, Aitken, 1958) with proton magnetometers, pointed to the detection of thermoremanent magnetic anomalies, it turned out very soon however, that the more subtle magnetic induced anomalies are considerably more common. The great success of this prospecting method is based on the fact that worldwide almost all soils exhibit an enhancement of magnetic susceptibility in the top soils (Le Borgne, 1955, Le Borgne, 1960, Mullins, 1977, Maher and Taylor, 1988, Fassbinder et al., 1990, Fassbinder and Stanjek, 1993, Dalan, 2008, Fassbinder and Bondar, 2013). While early measurements were done on pre-gridded areas with single sensor or single gradiometer systems smarter electronics and further developments in computer technology and satellite navigation now allows the application of multisensory instruments with real-time GPS (Becker, 1995, Gaffney et al., 2000, Trinks et al., 2013). Contemporary development took place with regard to the sensitivity of magnetometers (Linzen et al., 2009). Digital visualization techniques and fusion of data from different sources and prospecting methods combined with extensive knowledge of soil and rock magnetic parameters, allows interpretation of significant archaeological structures with great detail. A multitude of excellent prospecting results seem at first glance easy to interpret and induces many archaeologists to publish and to draw archaeological conclusions. Geophysical and namely magnetic anomalies however are not self-explanatory, but require a wide knowledge of the physical properties of archaeological sediments. In this paper I show with a range of selected case histories the variety of possible magnetic anomalies and their soil magnetic interpretation.
Section snippets
Soil magnetism and magnetic prospecting
Deviations of the intensity and/or direction of the normal Earth's magnetic field are at large and generally speaking caused by the magnetic contrast between the archaeological features and the adjacent soils and sediments. To understand magnetic anomalies it is first necessary to discriminate between induced and remanent based anomalies.
Magnetically induced anomalies
The enhancement of ferrimagnetic minerals in the topsoil is a common property of almost all soils worldwide (Le Borgne, 1955, Mullins, 1977, Fassbinder and Stanjek, 1993, Armstrong et al., 2012). It can be observed even on highly magnetic soils of volcanic origin and background (Tucker, 1952, Fassbinder and Gorka, 2009a).
Enrichment and separation of these heavy ferrimagnetic minerals can occur mechanically simply by wind or by water (Fassbinder et al., 2005), as well as by pedogenic processes
Formation of (ferri-) magnetic minerals in soils
Enhancement of ferrimagnetic minerals in top soils was first recognized and described by Le Borgne, 1955, Le Borgne, 1960 and ascribed to the widespread use of fire, either during forest clearance or the widespread use of fire by people. But very soon it became clear that many archaeological features previously detected by magnetometers as positive anomalies were never exposed to fire (Fassbinder, 1994). Tite and Linington (1975) showed that the climate also has a huge influence on the
The remanent magnetization in archaeological structures
Nearly every rock and sediment displays a measurable remanent magnetization (Dunlop and Özdemir, 1997) but also archaeological soil layers and traces of wooden palisades and post holes can acquire substantial primary natural remanent magnetization (Fassbinder, 1994). A primary natural remanent magnetization (NRM) can be acquired by four basic processes:
- a)
Thermoremanent magnetization (TRM)
If rocks, sediments or soils of archaeological sites or features are exposed to high temperatures they become
Interpretation of the magnetometer data and of the magnetogram image
In the simple form, the magnetogram gives an easily intelligible picture according to the geometry 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 and Eder-Hinterleitner, 1997; Fassbinder and 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, any pit, ditch or wooden posthole refilled by topsoil 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, solid rocks or other material, will cause a
Negative magnetic anomalies on archaeological sites
Negative magnetic anomalies may exist for numerous simple but also complex geochemical reasons:
- a)
The material of the archaeological structure has a lower magnetic susceptibility than the adjacent topsoil. For example, this will be the case if there are foundations of weakly magnetic 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. This can be exemplified
Remanence-based anomalies
There exists a wide range of examples of lightning-induced magnetic anomalies in archaeological sites. In early surveys, such anomalies were often not understood and were erroneously 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 as anomalies that are more or less parallel to the present Earth's magnetic field, lightning-induced remanence
Stratigraphy of magnetic prospection
Magnetic prospecting is a potential field method and thus not very suitable for detecting different archaeological layers and discriminating them from each other. Nevertheless, there are some case histories that demonstrate where and how it was possible to discriminate at least two phases of an archaeological stratigraphy (Fassbinder and Irlinger, 1998b). The Roman camp of Burgsalach gives such an example. The small Roman camp, ca. 40 × 40 m in size, was fortified by a palisade (visible as a
Changing magnetic properties of soils within short distances
Although enrichment of magnetic minerals is omnipresent in almost all soils worldwide and although there existing numerous papers dealing with magnetic properties of soils (Maher, 2011), there is still no starting point for a magnetic classification of soils. First attempts by Cook and Carts (1962) revealed already that soil magnetic properties can change dramatically within soils in short distances. Further attempts to classify soils by their magnetic parameters have been undertaken by Stanjek
Magnetic prospection close to the geomagnetic equator
The intensity and the direction – here we will limit ourselves to the inclination – of the Earth's magnetic field show wide variation from the geomagnetic equator to the geomagnetic poles (ca. 25.000–70.000 nT (10−9 T and 0° to ±90° respectively)) and hence play a great role in the archaeological interpretation of the data.
Although there are numerous case studies from magnetic prospection of archaeological sites in the northern hemisphere, only rare papers report from sites close to the
Summary
The great success of magnetic prospection is beside omnipresent enrichment of ferrimagnetic minerals such as maghemite or magnetite, titanomagnetite and greigite in archaeological soils layers, become due to new research on environmental magnetism and soil magnetism (Evans and Heller, 2004, Maher, 2011) but also due to the possibilities for the precise mapping of large archaeological sites and in landscape archaeology (Benech, 2005, Herbich, 2012). The development of multi-sensor systems in the
Conclusion
The methodology, instrumental techniques (Schultze et al., 2008, Linzen et al., 2009), sensitivity and the image processing for magnetic prospection is continue to improve (Stampolidis and Tsokas, 2012, Schmidt and Tsetskhladze, 2013, Eppelbaum, 2014). Irrespective of the walkability or other awkward conditions, there are only rare times when magnetic prospection is not worth it. Magnetic methods in general provide an extremely sensitive analysis method. Beside the organic content, the soil
Acknowledgement
The author would like to thank the anonymous reviewer, Andrei Asandulesei and Robin Torrence, Editor of Journal Archaeological Science, whose critical comments and valuable suggestions were helpful in the revision of this paper.
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