Schlagwort-Archive: magnetogram

How magnetizable are you? – Magnetization in archaeological prospection

Veröffentlicht am von
Antworten

In my latest blog post, I discussed how to read magnetograms and what we have to keep in mind regarding the Earth’s Magnetic Field and the location of the concerning site. Another important factor to approach the comprehensive interpretation of our data is the environment, esp. the geology, geomorphology and formation processes of the region. For magnetometry, it is especially the knowledge about magnetic properties (ferromagnetism) of rocks, minerals and soils.

The magnetogram shows the total magnetization, which is composed by the induced and the remanent magnetization (Fig. 1). The relation of induced and remanent magnetization is described by the Koenigsberger ratio (Q-ratio). It informs us not only about the quality of the rock sample for paleomagnetism, but also if we are dealing with archaeological objects.

Figure 1: Magnetization for rocks with induced and remanent amounts (Lowrie 2007, 321 fig. 5.40 a).

The induced magnetization exists with an applied external field only, e.g. the Earth’s Magnetic Field, and mostly goes along with the direction of the Earth’s Magnetic Field. For interpreting our magnetograms, it is helpful to conduct additional magnetic susceptibility measurements in the field, which tells us more about the induced magnetization. The magnetic susceptibility describes how magnetizable a sample/material is in an applied field. It is affected by the type of contained minerals as well as their grain size. The resulting values are unit less and can be negative (diamagnetic) or positive (ferromagnetic, ferrimagnetic, paramagnetic). Susceptibility can be measured in the field as well as in the laboratory, where more precise measurements are possible and additional parameter can be investigated.

The remanent magnetization is a permanent magnetization, independent of an external field, and important in paleomagnetism and archaeomagnetism. The natural remanent magnetization is the sum of the remanent magnetization and can be composed by several elements. For archaeological prospection, one of the most important remanent magnetizations is the thermoremanent magnetization (TRM). It is formed through heating of material over Curie temperature and cooling in an applied magnetic field, whose direction (Declination) is saved. Kilns, ovens and burnt objects like pottery or bricks etc. are the best examples for TRM. Chemical remanent magnetization (CRM) can be found in sedimentary or metamorphic rocks, whereas detrital remanent magnetization (DRM) develops during sedimentation of small magnetic particles in smooth water. Isothermal remanence (IRM) is the reason why we can detect also lightning strikes (LIRM) in our magnetograms. Although remanent magnetization is usually permanent, several factors could alter it, such as weathering.

But why are some materials/rocks more magnetic or magnetizable than others? It depends on iron oxides. Iron oxides are not only responsible for magnetization but also playing a role which colour a material has. The most important iron oxides regarding archaeological purposes are magnetite, maghemite, greigite, hematite, goethite as well as titanomagnetites, occurring in soils. While magnetite and maghemite are showing up to 1000 times higher susceptibilities than hematite, the latter is responsible for the typical red colour. Pedogenic, anthropogenic, lithogenic, and bacterial processes are responsible for the enhancement of soils, esp. top soils. Additionally, originally nonmagnetic materials can show enhanced magnetization: magnetotactic bacteria in organic materials are generating magnetite so that already gone posts, palisades etc. can be detected by magnetometry.

Magnetic susceptibility measurements in the field can be carried out selective e.g. for scattered objects and rocks on the surface or areal (separate geophysical prospection method). They can be used to define the extension of archaeological sites, activity zones or features in human made environments. Furthermore, they help understanding the morphology, formation processes, erosion and sedimentation as well as stratigraphic sequences for climate research and soil formation processes. Usually, top soils as well as archaeological soils are showing higher magnetic susceptibility values caused by enhancement of magnetic minerals due to the use of fire and fermentation. That is the reason why we can detect areas of human activity, e.g. settlements, and determine their extension.

How can we transfer this knowledge to the MUAFS concession area? The geological map of the Nile valley shows mostly sandstones, siltstones and mudstones accompanied by metavolcanic rocks as well as colluvium, sand sheets and dunes. For the volcanic rocks, we can assume high magnetic susceptibilities, whereas for the sandstones, siltstones and mudstones weak magnetic susceptibility is rather likely, depending on the contained minerals.

During the first geophysical campaign, we conducted spotty magnetic susceptibility measurements on our sites as well as the environment. Therefore, we used the handheld Kappameter SM-30 (Zh-instruments, Fig. 2). While magnetometry is a passive method, magnetic susceptibility meter are active instruments with a small coil included. Sampling the scattered rocks and archaeological objects like mudbricks gives an idea what we can expect in the magnetogram. Furthermore, the method can be applied in excavation trenches e.g. to distinguish stratigraphic layers or walls/floors and susceptibility maps can be produced. Due to magnetic susceptibility it is feasible to differentiate sources of raw material e.g. for mudbricks.

Figure 2: Measuring the magnetic volume susceptibility of a mudbrick laid out in the sun for drying with the Kappameter SM-30, showing a value of 2.22 [10-3 SI] (M. Scheiblecker).

For the settlement site GiE 001 at Ginis East, the scattered rock material shows mostly susceptibility values in a range of 0,302 to 0,826 [10-3 SI], esp. quartz, schist, and sedimentary rocks, while rocks of volcanic origin result in values around 7,5 [10-3 SI]. The surface values are ranging between 4,35 to 5,6 [10-3 SI] and mudbrick from a partly upright standing hut shows values of around 4 [10-3 SI]. For this site, we could expect therefore the following: walls made of sedimentary rocks would cause negative anomalies in the magnetogram, while the use of volcanic rock would result in positive anomalies. Mudbrick or galus walls (of stamped mud) are more difficult to predict; depending on their mineral composition, they are revealing positive or negative anomalies. Fire installations or the use of fired bricks would be easily recognizable because of their high values.

A profile along a street close to the Nile River shows the different sedimentation layers very nicely: different colours as well as susceptibility values can be seen in Figure 3.

Figure 3: Magnetic susceptibility values (in 10-3 SI) of a profile next to the Nile (M. Scheiblecker).

This example shows the complexity of magnetic susceptibility in combination with colours; darker and brighter layers show similar values, whereas the surface reaches the highest value. The layer with pebbles reveals the lowest value due to the included pebbles of probably sedimentary origin. For understanding the environment of archaeological sites and their formation processes, it is important to consider not only the survey, excavation and magnetometry results itself. Furthermore, knowledge in geology, geomorphology as well as the investigation of their parameters add details for a comprehensive picture of an archaeological landscape.

At the end, if you are asking yourself how magnetizable you are: without any ferromagnetic items from your clothes or e.g. glasses, the magnetic susceptibility would be almost zero or even negative, as the human body consists mostly of water.

References

Aspinall, A.; Gaffney, C.F.; Schmidt, A. (2008): Magnetometry for Archaeologists. Geophysical methods for archaeology 2. Lanham: AltaMira Press.

Butler, R.F. (1998): Paleomagnetism: Magnetic Domains to Geologic Terranes: Electronic Edition. Boston: Blackwell.

Dalan, R. (2017): Susceptiblity. In: Allan S. Gilbert, Paul Goldberg, Vance T. Holliday, Rolfe D. Mandel and Robert Siegmund Sternberg (eds.): Encyclopedia of Geoarchaeology. Dordrecht: Springer Reference (Encyclopedia of Earth Sciences Series), 939–944.

Fassbinder, J.W.E.; Stanjek, H.; Vali, H. (1990): Occurrence of Magnetic Bacteria in Soil. Nature 343 (6254), 161–163.

Fassbinder, Jörg W. E. (2017): Magnetometry for Archaeology. In: Allan S. Gilbert, Paul Goldberg, Vance T. Holliday, Rolfe D. Mandel and Robert Siegmund Sternberg (eds.): Encyclopedia of Geoarchaeology. Dordrecht: Springer Reference (Encyclopedia of Earth Sciences Series), 499–514.

Lowrie, W. (2007): Fundamentals of Geophysics, Cambridge: Cambridge University Press.

More than greyscale: How to read magnetograms in the MUAFS concession area

Veröffentlicht am von
Antworten

The magnetic data collected at our first campaign in the Attab to Ferka region in 2018/2019 was first processed and interpreted directly after the field season. After the first excavation campaign in 2020, focusing on two of the four geophysically investigated sites, a reconsideration of the data took place. It is based on the excavation results, the photogrammetric data and new kite images.

But before looking at the data, you have to know where exactly on earth the data was generated! The Earth’s magnetic field is a complex system, which is protecting us against ultraviolet radiation, as it is deflecting most of the solar wind, which is stripping away the ozone layer. The earth’s magnetic field can be visualized as a three-dimensional vector: Declination (angle in ° to geographic north, X), Inclination (horizontal angle in ° or magnetic dip, Y) and Intensity (measured in T “Tesla” resp. nT “Nanotesla”, Z). In archaeomagnetism, all components are measured to be compared to the single curves of the region. For magnetometry and interpreting these data, the Inclination is the most important value besides the Declination, which helps for example to detect in situ burnt features. The Inclination describes the angle in which the Earth’s Magnetic Field meets the surface of the Earth itself. Therefore, the angle is changing depending on your position e. g. if you are closer to the magnetic poles or to the magnetic equator.

The geomagnetic field changes all the time, every second, every day, and every year! For Munich resp. Fürstenfeldbruck you can follow the alterations simultaneously here. The geomagnetic observatory there is part of the Ludwig-Maximilians-Universität and the Department of Earth and Environmental Studies. As you may know, the magnetic poles are wandering as well. The magnetic north pole did it that fast in the last years that the navigation map had to be changed before the standard interval of five years in 2019. This world magnetic model (WMM) is available online.

But why do we have to know especially the Inclination of the area we are working in and doing magnetometry? The shape and intensity of every single anomaly is depending especially on the Inclination! The shallower the Inclination the wider the anomaly is visible in the magnetogram. Additionally, the dipole (positive/black – negative/white) components are changing. The closer we are measuring to the geomagnetic equator (not the geographic equator), the larger gets the negative part of the anomaly and the lower are the amplitudes of the magnetic signal. Figure 1 illustrates the differences in Inclination for a single anomaly.

Figure 1: Anomaly strength of the total field intensity as north-south traverse through the anomaly’s centre for different Inclinations (Ostner et al. 2019, 181 Fig. 2).

While the Inclination in Munich is around 64°, the Inclination in the MUAFS concession area is 27-28° and shallower. The components of the Earth’s Magnetic Field at the MUAFS concession area are illustrated in Figure 2, showing a Declination of almost 4° and a total field intensity of around 39.000 nT (Munich: 48.585 nT). The measured archaeological and geological features, visible in the magnetogram, are showing contrasts of sometimes less than 1 nT. Due to different Inclinations, the same archaeological feature would result in a different anomaly in Sudan compared to Munich. While the anomaly in Sudan would be wider (see the red curve, Fig. 1) than in Munich (ca. the blue curve, Fig. 1), it would cause lower intensities as well as showing more negative parts than the Munich one. This means while in Bavaria the negative part of an anomaly is regarded more as a small “white shadow”, in Sudan it would be almost equal to the positive part of the anomaly. Furthermore, depending on the depth of the buried feature, the shift in locating the feature could be larger with shallower inclination.

Figure 2: The Earth’s Magnetic Field in Sudan after World Magnetic Model (WMM) 2019, with the MUAFS concession area in red (M. Scheiblecker).

Regarding the used magnetometer – a gradiometer, the intensities are additionally lower than for example with a total field magnetometer, which makes it more difficult to interpret the data and why sometimes low value-features like pisé walls are not detectable with gradiometers. Furthermore, with wider anomalies closer to the geomagnetic equator like in Sudan, it is more possible that anomalies are overlapping so that it is not easy to distinguish features lying next to each other or from different periods.

Usually, magnetograms are shown in greyscale to avoid confusion and “pseudo-limitations” of different values and colors. For interpreting the data, one can play around with the minimum and maximum values as well as inverting of the greyscale version. On magnetograms of measurements with the total field magnetometer usually a high-pass filter is applied, which can be overlayed with the total field data as well.

In rare cases it is helpful to use color scales for the magnetograms additionally to show special features better or to highlight some very high or low values. If the magnetogram is disturbed by high magnetic anomalies like metal fences, iron rubbish on the site etc., color scales are not useful anymore, because they are showing especially the disturbances due to their high amplitudes and less of the archaeological features itself. Nevertheless, it is possible to adjust the color scale as needed for every site separately.

Illustrating the mentioned methods, I would like to show the magnetogram of GiE 002, where a cemetery is located.

Figure 3: Magnetogram of GiE 002 in greyscale (M. Scheiblecker).

The usual greyscale (Fig. 3) shows clearly the traces of the recent and former wadi/khor, tumuli-like features in the very south as well as lots of features of different shape in the northern part of the magnetogram, interpreted as graves. They are resulting in positive anomalies, accompanied by negative anomalies of different amplitudes.

Figure 4: Magnetogram of GiE 002 in blue to red color scale (M. Scheiblecker).

To understand more of the single burials it is helpful to change to a blue-red color scale (Fig. 4). In this way, it is easier to differentiate the single anomalies consisting of the positive (red) and negative (blue) part.

Figure 5: Magnetogram of GiE 002 in highlighted grey scale, showing maximum values in red as well as minimum values in yellow (M. Scheiblecker).

Highlighting the minimum and maximum values – in yellow resp. red – helps e. g. focusing on the probably best-preserved archaeological features located in the center of the measured area, visible in Figure 5.

The magnetograms of GiE 002 show clearly that it is worth playing around with different color scales and that there is more than one magnetogram important for interpreting the data for archaeological and geological purposes.

References

Fassbinder, J. W. E. (2017): Magnetometry for Archaeology. In: Allan S. Gilbert, Paul Goldberg, Vance T. Holliday, Rolfe D. Mandel and Robert Siegmund Sternberg (eds.): Encyclopedia of Geoarchaeology. Dordrecht: Springer Reference (Encyclopedia of Earth Sciences Series), 499-514.

Livermore, P.W.; Finlay, C.C.; Bayliff, M. (2020): Recent north magnetic pole acceleration towards Siberia caused by flux lobe elongation. Nature Geoscience 13, 387–391.

Ostner, S.; Fassbinder, J. W. E.; Parsi, M.; Gerlach, I.; Japp, S. (2019): Magnetic prospection close to the magnetic equator: Case studies in the Tigray plateau of Aksum and Yeha, Ethiopia. In: James Bonsall (ed.): New Global Perspectives on Archaeological Prospection. 13th International Conference on Archaeological Prospection. 28 August – 1 September 2019. Sligo – Ireland. Oxford: Archaeopress, 180-183.