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

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.

On the footsteps of Vila and the archaeology of monumental surveys in northern Sudan

Every archaeologist knows that what we write about the past is mediated by present-day questions, expectations and challenges, but also state-of-the-art documentation techniques. Archaeologists don’t simply reconstruct what happened back in the day. Instead, archaeological and historical narratives are essentially modern constructions that can either be repaired or demolished as scholarship moves forward. Archaeological research is also mediated by complex site and object biographies that span thousands of years; e.g., in our case, from the Neolithic to monumental surveys carried out in northern Sudan in the mid-20th century.

To understand tombs, burials and mortuary objects in the region from Attab to Ferka we need to understand the impact of André Vila’s work in the region, the epistemological framework from which he was reporting and his methodology. How Vila’s work materialise in the landscape directly affect the questions we ask and the methodologies we apply today. Not surprisingly, archaeologists in Sudan usually deal with traces left by earlier archaeologists at various sites, and retracing their steps becomes a fundamental aspect of accessing the past through remaining material culture (e.g., Howley 2018: 86-87).

Vila’s survey can be seen as part of a long tradition of large-scale surveys going back to the first Archaeological Survey of Nubia (see Adams 2007). From Dal (the southern limit of Lake Nubia) to Nilwatti Island, Vila identified 462 sites of which 219 sites are within the MUAFS concession (figure 1). These sites were attributed to cultural units, e.g., Kerma, Christian etc. Even though Vila noted that various sites belonged to one or more cultural units, today archaeologists approach those ‘units’ more fluidly, especially in situations of cultural exchanges, which is especially true for Kerma and New Kingdom sites. On the one hand, revisiting sites surveyed and reported in the 1960s and ’70s requires us to contextualise archaeology to ‘deconstruct’ theoretical biases and ask different questions. On the other hand, Vila’s methodology determines the extent to which sites can be explored.

Figure 1: distribution of sites in the MUAFS/DiverseNile concession area (map: C. Geiger)

In terms of method, Vila’s survey aimed to keep disturbance to sites to a minimum. Test excavations and sampling followed rigid guidelines and excavations were only carried out when cultural affiliations couldn’t be distinguished otherwise, e.g., based on surface finds. Cemeteries were approached in a slightly different way. Cemeteries were usually cleared to determine their extent and number of graves at each site. A few graves were fully excavated and recorded, as well as ‘peculiar’ collective burials (figure 2).

Figure 2: Bagagin Farki, Ginis East. “Egyptian” New Kingdom pit burial containing an individual deposited in extended position together with sherds from two pots and the remains of other six individuals (Vila 1977, Vol. 5: 47)

In Work Package 2, I am responsible, among other things, for reassessing the material from such graves. For example, comparison of items from graves in our concession area with objects from other sites allows us to shed new light onto different roles performed by the same types of objects in different contexts (Lemos 2020). Scientific analysis of pottery also allows us to explore the (potentially) alternative role of objects in rituals (stay tune to Giulia D’Ercole’s blog!). I am currently collaborating with Kate Fulcher from the British Museum on the topic of mortuary rituals in New Kingdom Nubia based on scientific analysis of artefacts (see Fulcher and Budka 2020 for examples of such approaches).

Working with previously excavated material culture poses several theoretical and methodological challenges, mostly related to the lack of information provided by earlier excavation reports and the problematical categories used to classify things. However, revisiting the burials excavated by Vila holds an immense potential for us to ask different questions within larger-scale perspectives on burial communities, the role of (foreign) objects in the constitution of (local) social relations and identity formation strategies. Comparative approaches to sites and material culture allow us to understand different social realities within Nubia, challenging previous homogenising perspectives on cultural interactions focusing on elite centres. Revisiting sites also holds great potential to unveil things under new theoretical perspectives and using state-of-the-art documentation techniques. This is especially the case because disturbance to sites was limited during Vila’s survey, although looting poses an additional challenge to new fieldwork in our concession area.

Ultimately, researching burials and other sites in our concession area and excavating sites firstly worked by Vila presupposes a deep knowledge of the data sets produced by him, what was ignored/discarded, what was considered worth investigating etc. Previous ways of excavating, identifying and reporting sites also determine the extent to which we can revisit them through excavation and comparative analysis.

References

Adams, W. Y. 2007. A Century of Archaeological Salvage, 1907-2007. Sudan and & Nubia 11: 48-56.

Fulcher, K. and J. Budka. 2020. Pigments, incense, and bitumen from the New Kingdom town and cemetery on SaiIsland in Nubia. Journal of Archaeological Science https://doi.org/10.1016/j.jasrep.2020.102550.

Howley, K. 2018. Return to Taharqo’s Temple at Sanam: the inaugural field season of the Sanam Temple Project. Sudan & Nubia 22: 81-88.

Lemos, R. 2020. Material Culture and Colonization in Ancient Nubia: Evidence from the New Kingdom Cemeteries. Encyclopedia of Global Archaeology, ed. C. Smith. https://doi.org/10.1007/978-3-319-51726-1.

Vila, A. 1977. La prospection archéologique de la Vallée du Nil, au Sud de la Cataracte de Dal (Nubie Sudanaise). Fascicule 5: Le district de Ginis, Est et Ouest. Paris: CNRS.

On the tracks of ancient cultures: Archaeological Geophysics

As a team member of the first MUAFS season 2018/2019 responsible for magnetic investigations I would like to introduce the geophysical methods used for archaeological purposes. These methods will also be highly relevant for the DiverseNile project.

In the last decades, geophysics became a substantial part of archaeological projects. Depending on several factors, the most suitable geophysical method is chosen: the environment (desert, steppe, swampland etc.), the archaeological period and the used archaeological materials (stone, mudbrick etc.), but also the questioning (settlement layout and extension, cemetery detection etc.).  Additionally, the decision is influenced by available time, financial means and sometimes the season.

Still the fastest and most effective geophysical method in archaeology is magnetometry. It provides getting an overview of a site as well as its environment, extension and layout. Magnetic prospecting enables us to distinguish between settlement and burial sites, their structure, special buildings, open areas, as well as fortifications. Depending on the chosen sensors, we can learn more about the geology and environment and their changes over time. Accompanying measurements of magnetic susceptibility deliver information about magnetic properties of scattered objects and building materials as well as archaeological sediments and can be used in archaeological excavations as well.

Magnetic gradient investigations in Ginis 2019, site GiE 001 (Photo: Giulia D’Ercole).

But how does it work? Magnetometers are recording the intensity of the earth magnetic field with high-resolution. Nowadays, the earth magnetic field in the Attab to Ferka region has an intensity of around 39.400 Nanotesla (nT). With sensitive total field magnetometers, magnetic anomalies of less than 1 nT can be detected during archaeo-geophysical surveys, displaying even archaeological features like mudbrick walls or palisades.

Magnetic investigations benefit from varying magnetic properties of archaeological soils and sediments as well as materials. Every human activity regarding the surface is detectable because of different magnetic response. For example, digging a ditch, building a wall or using a kiln is changing or disturbing the actual earth magnetic field. What else can be detected? Architecture, streets, canals and riverbeds, ditches, pits and graves can be revealed just as palisades, posts and fire installations. Additionally, more information about geological and environmental conditions can be collected using magnetometry, e. g. paleo channels or former wadis.

For detecting stone architecture and for example voids, resistivity (areal or profile) and Ground Penetrating Radar (GPR) methods are applied, partly in addition to magnetic investigations. While magnetometry gives an overview about buried features beneath the surface in a ‘timeless picture’, GPR and ERT (Electrical Resistivity Tomography) provide more information about the depth and preserved height of the features. Of course, more than one geophysical method can be applied to get a comprehensive dataset for more complete interpretation of the results. Combined with archaeological work – survey and excavation – we can increase our knowledge and understanding of physical properties of archaeological and geological features as well as improve our interpretation.

Geophysical prospecting was originally developed for military purposes to detect submarine boats, aircrafts or gun emplacements. Furthermore, natural and especially mineral resources can be located. Geophysical methods and first of all magnetometry are used in archaeology since the late 1950s, when Martin Aitken detected Roman kilns in the UK. In Sudan, magnetometry is used since the late 1960s when Albert Hesse started investigations at Mirgissa in Lower Nubia. Since then, instruments as well as software programs for data collecting, processing and imaging have been developed and improved and offer detailed mapping of sites. First, geophysical prospecting can be applied fast, nondestructive and comprehensive. For magnetic prospection there is a variety of configurations to use, from handheld one/two-sensor instruments to motorized and multisensory systems but also different types of sensors. Through geographic information systems (GIS) geophysical investigations are benefiting from integrating high-resolution satellite images, drone images and models, survey and excavation data for a comprehensive interpretation of results.

After collecting magnetic data in the field, the files are downloaded and processed to get an idea of the first results. With that the field measurement proceeding can be adjusted as well as excavation trenches can be chosen. The detailed processing and analyzing of the collected field data are conducted back home on the desk.

References

Campana, Stefano; Piro, Salvatore (eds.) (2009): Seeing the Unseen. Geophysics and Landscape Archaeology. London: Taylor & Francis.

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, 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.

Herbich, Tomasz (2019): Efficiency of the magnetic method in surveying desert sites in Egypt and Sudan: Case studies. In: Raffaele Persico, Salvatore Piro and Neil Linford (eds.): Innovation in Near-Surface Geophysics. Instrumentation, Application, and Data Processing Methods. First edition. Amsterdam, Oxford, Cambridge: Elsevier, 195–251.

Schmidt, Armin; Linford, Paul; Linford, Neil; David, Andrew; Gaffney, Chris; Sarris, Apostolos; Fassbinder, Jörg (2015): EAC Guidelines for the Use of Geophysics in Archaeology. Questions to Ask and Points to Consider. Namur: Europae Archaeologia Consilium (EAC Guidelines, 2).

The 2020 season in pictures

We should just have landed in Munich these minutes… but since a sandstorm in Khartoum prevented the airplanes to land yesterday, our flight was cancelled and Jessica and me are now stuck in a hotel close to the airport…

Well, we are still waiting for information when we will have the next try and thus I thought I use the time to go through some of the pictures we took this season.

The 4.5 weeks of work at Ginis East and sourroundings were intense and varied, in terms of tasks, weather, nimiti and other things. Here are some impressions with a huge load of thanks to my great 2020 team! Looking much forward to the next season!

First day in the field, setting up the trenches (photo: C. Geiger)
Veronica setting on of the posts for our trench (photo: C. Geiger).

Giulia at work (photo: C. Geiger).
Surface cleaning at GiE 001, Jessica and Huda (photo: J. Budka).
Jessica documenting the church of Mograkka (photo: C. Geiger).
Cajetan at work – image based documentation of the church of Mograkka (photo: J. Distefano).
Giulia, Veronica and Huda at GiE 006 (photo: J. Budka).
The 2020 Team at Ginis East.
The Nile and riverbank at Ginis (photo: J. Budka).
Me busy with surface documentation at GiE 001 (photo: J. Distefano).
Photographing Trench 2 at GIE 001 (photo: J. Distefano).
Jessica taking survey points with the total station (photo: J. Budka).
The landscape at Mograkka East (photo: J. Budka).
Jessica looking for diagnostic pottery sherds at one of the Christian cemeteries in Mograkka (photo: J. Budka).
Jessica taking measurements at the curch of Ferka East (photo: J. Budka).
Me photographing Post-Meroitic tumuli at Ferka (photo: J. Distefano).
Huda, Jessica and me after we finished the survey in Mograkka.

Short update on the 2020 survey

The MUAFS 2020 season will be officially closed today – we arrived safely in Khartoum yesterday and will now finish all the paperwork.

The last days at Ginis were busy, finishing off the survey, packing and registering the finds from this season. A more concise summary of the 2020 season will follow shortly, but here are some observations regarding our survey.

Like in 2019, one particular focus of our survey was on the state of preservation of the sites nowadays – unfortunately, at almost all sites, we observed modern destruction and/or plundering. Especially drastic were destructions because of road building, the electricity posts and modern gold working areas.

One particular striking example is the large tumulus within the Post-Meroitic site 3-P-1 at Kosha East. This monumental tumulus, comparable to the ones at Ferka, but also to the famous tombs at Qustul and Ballana, has completely gone by now. According to information kindly given by local villagers, it was removed in 2008.

One of the sad discoveries of this season: the huge tumulus at Kosha was completly dismantled.

Where Jessica is standing in this photo, there used to be an elite tomb monument dating back to Post-Meroitic times. Large parts of cemetery 3-P-1 are now under modern fields; the line of electricity cuts the southern extension of the site. The nearby Kerma cemetery was affected by the construction work of the road to Wadi Halfa and the Neolithic sites located in the hills above the Kosha plain just 300m to the southeast are strongly influenced by modern gold working.

Altogether, as successful as our 2020 survey was, re-locating 40 sites of the ones documented by Vila and finding a number of previously unrecorded sites, we were also faced with very frustrating news and massive destruction of the archaeological monuments. There clearly is the urgent need to undertake cultural heritage actions in the region, but this is something where MUAFS will need help and support from several authorities.