Geophysical investigation of a Celtic burial mound

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Geophysical investigation of a Celtic burial mound HARALD VON DER OSTEN-WOLDENBURG, Regierungspraesidium Stuttgart, Landesamt für Denkmalpflege

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agnetic methods have become common tools in the archaeological disciplines. They are generally used to localize and classify archaeological features relatively close to the ground surface. Within a short time onsite, these methods can scan large areas with high spatial resolution and distinguish anthropogenic anomalies within native soils. However, a comprehensive inventory and documentation of archaeological features requires measuring several physical properties, something that is possible only through a combination of several different prospecting methods. The example described in this article details a situation in which three different geophysical methods perfectly complement one another and therefore can enable a comprehensive inventory and documentation of the archaeological feature lying below the surface. The object A small mound rising approximately 0.4 m above its native surroundings is evident on an undeveloped site within the city limits of Ludwigsburg, Germany. It is generally thought

that this uninvestigated surface feature is the remains of a large Celtic burial mound (from approximately 800 to 50 BCE). With planning at the site underway, we undertook an extensive geophysical investigation of the site in April 2011. Geophysical investigation Our approach was to use geomagnetic mapping techniques to establish the exact position and dimensions of the burial mound, and then make a three-dimensional image and description of the structure using ground-penetrating radar. In addition, we evaluated a new device designed to record the conductivity of materials as well as the susceptibility of the subsurface strata through electromagnetic induction. Results Geomagnetic mapping. The magnetic survey of an area of 126 × 120 m was performed on a regular 0.05 × 0.25-m grid using FEREX, an instrument manufactured in Germany by Dr. Foerster, employing a design that includes an array of four differential fluxgate magnetometers recording the

Figure 1. Magnetogram of the Celtic burial mound in Ludwigsburg, Germany. Features from the central burial as well as the enclosing ring ditch with its diameter of approximately 65 m have been recorded. 958

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Figure 2. Results from the radar prospection at depths of 0.4–1.2 m. Features from the central burial mound and a surrounding circular area of gathered stones were documented.

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vertical gradient of the Earth’s magnetic field. The magnetogram (Figure 1) shows a previously unknown ring ditch with a diameter of approximately 65 m and a width varying between 3 and 7 m, probably due to different degrees of erosion. The eastern part of the ditch lies behind a modern metal fence in the area isolating a drinking-water reservoir and therefore could not be completely examined. Disturbances to the Earth’s magnetic field caused by the metal fence appear as relatively extensive white and black anomalies at the eastern edge of the magnetogram. Geomagnetic anomalies with minor amplitude properties are recognizable within the center of the ring ditch and can be attributed to the central burial volume. The faintly recognizable, bow-shaped anomalies that lie mainly east and south of center could be the result of extensive, modern agricultural activities on this site. Linear and rectangular features, however, are probably the results of archaeologiFigure 3. The Celtic burial mound clearly sets itself apart from its cally relevant features (secondary burials, internal structure surroundings in the distribution of the in-phase component. of the mound). The disturbance of the magnetic field that lies parallel to the road at the southwestern edge of the area under investigation is caused by a water main. Singular anomalies in the magnetogram show the position of modern boundary markers (black, circular anomalies with white central areas) and metallic trash, respectively (small black/white anomalies spread across the entire area). Ground-penetrating radar survey. The radar investigation was conducted with a 500-MHz antenna and the SIR-3000 radar system made by GSSI. An area 80 × 80 m including the mound was chosen for a closer examination based upon the results from the geomagnetic mapping. Within this area, we acquired profile lines lying half a meter apart with a resolution of 20 measurements per profile-meter. The velocity of propagation of the electric impulses at the time of the measurement was established at v = 0.078 m/ns. The time-slices separated by a vertical distance of 0.1 m show that archaeological features exist Figure 4. The superimposing of the results from the geomagnetic mapping with a partial result from the ground-penetrating radar combines the archaeological only at a depth of 0.4–1.0 m below the surface of features recognized by the different methods. the site (Figure 2). The larger ring ditch evident on the magnetogram was not recognizable in the images from the radar survey. Within the confines of the mapped an area of 80 × 72 m with a profile and measurring ditch, the radar data have captured an area of stones ing distance of 0.5 m. The distribution of the electric congathered from the surroundings which are arranged in a cir- ductivity at depths of 0.4 and 0.8 m is shown in Figure 3. cular manner (marked by a in Figure 2). Toward the middle The surrounding ditch enclosure and the area of the cenof this area is a wide circular clearing with a diameter of tral burial are clearly recognizable. The inner area of the approximately 12 m (b). The significance of this feature is burial mound is especially set apart from the rest of the currently unknown. As far as we can tell, this particular surroundings by its conductivity values at a depth of 0.8 m. feature is not commonly found in Celtic burial mounds and At this depth, we also can clearly see a rectangular strucmay in fact be the desirable remnants of an attempted grave ture (Figure 3, marked by the arrow) of approximately 8 robbery. At the center of this area a rectangular structure × 3 m to the north of the central burial that was recorded can be seen lying some 0.8–0.9 m below the surface (Figure by ground-penetrating radar at a depth of only about 0.5 2, marked by c) and which probably related to the central m. This feature is probably a secondary burial. The occurburial chamber. rence of secondary burials following the central burial was a Electromagnetic induction mapping. With the CMD widespread tradition. This led to the burial mound growing Mini Explorer, manufactured by GF Instruments, we in area over time. 960

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Final remarks The magnetogram clearly shows the ring ditch as the outer enclosure of the princely grave, while the ground-penetrating radar method yields clear results regarding the inner area of the burial mound. The best results can be obtained clearly by combining the individual results from these two complementary methods. To verify correlation in the results between the geomagnetic survey and the ground-penetrating radar study, the magnetogram was superimposed with a GPR time-slice from a depth of 0.7 m, a depth interpreted to be consistent with both data sets (Figure 4). The anomalies from the center of the burial mound, interpretable on the magnetogram become considerably more apparent in the ground-penetrating radar image with excellent correlation in regard to their position and size. Conclusions Based upon this investigation of a Celtic burial mound, we conclude that, when using geophysical tools for studying archaeological objects, it is key to acquire data with the ne-

cessity of a high-resolution mapping potential. Combining different geophysical methods provides the best possible advantage for addressing, interpreting, and documenting an archaeological feature. In most cases, geomagnetic surveys will be the first choice to localize and delimit the area of the suspected archaeological feature. Use of other methods, especially three-dimensional (or pseudo three-dimensional) recording of underground structures by ground-penetrating radar, will provide enhanced indications to detailed features and enable an estimation of the state of preservation and exact footprint of the archaeological objects. Acknowledgment: We thank the municipal administration of Ludwigsburg for granting permission to conduct the measurements. We also thank the volunteers from the Landesamt für Denkmalpflege (the State Office for the Preservation of Monuments) in Esslingen for its support and assistance during the recording of the measurements. Corresponding author: [email protected]

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