1The documentation of the research project “The Oppidum of Manching”, carried out by the Romano-Germanic Commission (RGK) of the German Archaeological Institute (DAI) from 1956 to 2018 and handed over to the archive of the Bavarian State Department for Monuments and Sites (BLfD) in 2018, included a detailed description and documentation of the first geophysical prospection performed for archaeology in Bavaria. This prospection was conducted by Claus Colani (1921-2016). His field notebooks and electronically generated “spectrograms” were accompanied by a multitude of magnetic tapes with the original data, and an UHER tape recorder which enabled automatic data collection (Read out Unit) and served as equipment for automatic data display by means of a Hewlett-Packard X–Y plotter. This was developed early on when Irvin Scollar applied a punch card recorder system, mounted in a Volkswagen Bus, for digital data recording (published in Lericis Journal Prospezioni Archaeologiche, volume 1, 1966). State-of-the-art archaeo-geophysical data collection at the time called for manual writing in a field notebook. Therefore, Colani’s documents provide detailed insight into an extremely smart and elaborate geophysical prospecting system. A short (unpublished) field report by the archaeologist Franz Schubert (1973) mentions the test measurements by Claus Colani and his wife Ingrid performed at Manching in 1972 (Fig. 1). Other documents, as well as multiple spectrograms and inscriptions on data sheets, date from 1972 to 1974. It thus turns out that Colani was the first to design and perform such a “large-scale” archaeological prospection in Bavaria.
Figure 1. Archaeogeophysical measurements on a training ground in Manching in 1972. From left to right: Ingrid Colani operates the transmitter and receiver loops; unknown helper leads the connecting cable; Claus Colani monitors the measurements as they are sent and their plotting on the X–Y recorder. Unknown author of the photograph.
2It is a mystery why outstanding results, achieved on such a prominent archaeological site, have never been published or presented to the public so far. Müller-Scheeßel et al. (2002) reported on these measurements, but pictured Franz Schubert as the geophysical expert. All the more reason to take a closer look at Claus Colani’s achievements and his work with archaeologists, thus paying a tribute to his pioneering work not only at Manching.
3Colani was born on March 23, 1921. He worked as a research and development engineer at the Allgemeine Elektricitäts-Gesellschaft, better known as AEG, in Berlin and later as an employee of the Siemens & Halske Aktiengesellschaft in Berlin and Munich. His name as an inventor is inscribed on several patent specifications from the 1940s, 1950s and 1960s. At the same time, he published several articles on physical measurement methods in medical journals for people. A patent specification No. DE1180550 B 19641029 from 1961 is the first to be specifically aimed at applications in archaeological contexts. It could well have been a prelude to Colani’s interest in developing geophysical methods for archaeological prospection. Two years later, in one of the first issues of the journal Archaeometry, Colani presented a new type of locating device based on the pulse induction principle he had developed (Colani, 1966) and, together with Martin J. Aitken, he undertook field trials of this instrument on archaeological features in England (Colani & Aitken, 1966a and 1966b). Due to more extensive and stable possibilities of interference suppression, Colani’s new device achieved a correspondingly higher range, especially with regard to the detection depth, than the common-time sinusoidal alternating magnetic fields instruments. In his last known paper, which appeared in the journal Frequenz in 1968, Colani outlined the possibilities for deriving object information through an analysis of electromagnetic signals from his devices (Colani, 1968). His continuous activity in archaeogeophysics broke off suddenly in 1974. All that is known is that from 1975 Colani was a corresponding member of the DAI. He passed away on February 14, 2016, in Gröbenzell near Munich.
4Colani’s devices produce pulsed magnetic fields that induce secondary magnetic fields in metallic objects and magnetized soil. These secondary fields are measured and recorded by timed receivers. Nowadays, we would call such a principle Time Domain Electromagnetics (TDEM). According to Colani, the received signals from these devices are rich in information, so that the location and depth of the object, as well as its size and material, can be determined (Fig. 2) (Colani, 1968). Colani and Aitken observed that metal detectors operating on this pulse induction principle detect not only metallic objects, but also non-metallic features, such as refilled pits and buried hearths (Colani and Aitken, 1966). The reason is that signal detection depends on the magnetic viscosity of objects rather than their conductivity (Colani, 1966). This dependency resulted in the Colani-devices having higher operating depths than other metal detectors common at the time (Colani, 1968).
Figure 2. Colani’s notes on the calibration of his device in Manching. “Calibration” is written between the scales at the top. Colani expressed the measurement results in decibels (dB) (on the left side). The bottom line of the “calibration scale” marks the “Null-line”; next line above: “Null dB”; next line above “+10 dB” and so on. These dB-units are proportional to the induced voltage at the receiving coil and represent its decadic logarithm. Below this “calibration scale”, on the right side of the large bracket, Colani wrote “Comparison runs with known objects on gravel surface”. Bottom line resp. measurement: “without any object”; the next measurement above: five German Mark coin resp. “5 DM on the surface”; next measurement above: “5 DM–10 cm depth”; next measurement: “5 DM–20 cm depth”; next measurement: “5 DM–30 cm depth”; next measurement: “100 x 100 x 1 mm brass plate on the surface”.
5In the mid-1950s, the archaeological excavations at Manching represented the first German interest in researching the Celtic past at this European oppidum (Müller-Scheeßel et al., 2002). It proved possible to locate all of the areas measured by Colani in the western zone of the main settlement at this oppidum, while at the same time not all of the available spectrograms could be placed precisely. The documentation indicates that Colani carried out some measuring experiments in 1972, testing several visualisation techniques (Schubert, 1973).
6The numerous spectrograms from 1974 could not be relocated, unlike those from 1973 which were georeferenced and vectorized using QGIS, based on an aerial photograph taken by Irwin Scollar during test flights for aerial archaeology conducted for the Bavarian State Department for Monuments and Sites (BLfD); the photographs contained some reference points permitting a measurement grid to be produced for the purpose of georeferencing and vectorising (Fig. 3B). One of these scanned and georeferenced spectrograms from 1973 is shown in Fig. 3A. Voltages higher than the threshold value of 4dB are plotted on this particular spectrogram. In Fig. 3 C, vectorised spectrograms in the range from 2dB to 10dB are overlaid on Scollar’s aerial photo. The anomalies coincide with contrasting soil colours that indicate various archaeological features. These results show that Colani’s devices and approach allowed the contrast of varying apparent magnetic susceptibilities to be visualized, leading in turn to relevant archaeological features being predicted and traced, as well as their location at an operation depth of about 45 cm.
Figure 3. A) Scanned and georeferenced 4dB-spectrogram produced by Colani with a 1 m loop device at the level plane no 2 in early autumn 1973 (see also the southern part of the area in Fig. 3D). Arrows mark the presumed survey direction (traverse direction) during data acquisition; B) scanned and georeferenced aerial photograph of the excavation area produced by Irwin Scollar on 18 October 1973; C) vectorized spectrograms (range 2-10dB, level plane no 2) overlaid on the aerial photograph; D) oblique aerial view (to the north-west) of the entire excavation area taken by Irwin Scollar on 18 October 1973.