Institute of Applied Physics Nizhny Novgorod - SGS Berlin & Potsdam Joint Field Camp 2014
|Location||Nizhny Novgorod, Russia|
|Student Chapter||Institute of Applied Physics Geophysical Society, Student Geoscientific Society Berlin & Potsdam|
|Project lead||Andrey Konkov, Lennart Brüning|
|Methods||Geomagnetics, Geoelectrics, Ground-penetrating radar, Near-surface seismics|
|Student Chapters • Field camps|
The cooperation between the Student Chapters Student Geoscientific Society Berlin & Potsdam and the Institute of Applied Physics Geophysical Society is one of the results of the 4th International Geosciences Student Conference 2013 in Berlin. This event brought many students from all over the world together and initialized the idea of organizing a SEG Joint Field Camp by the SEG Student Chapters from Berlin and Nizhny Novgorod. Beside the project leads Lennart Brüning and Andrey Konkov, who established the cooperation four more students were involved in pre-organising the Field Camp. One important issue was to figure out an appropriate research target. Therefore contacts to local archaeologists from the Lobachevsky State University of Nizhny Novgorod (UNN) have been made. The Institute of Applied Physics and a geophysics instrument company provided the technical equipment. On the German side scientists from Freie Universitaet Berlin supported the students in their practical preparation. At the end of July 2014 three German students travelled to Nizhny Novgorod to implement the Russian-German Joint Field Camp along with their Russian fellow students.
The target of the field camp was the Podvyaze burial site close to Nizhny Novgorod. It is a monument of the 4th to 6th century, which was discovered in 1959. The first grave of the complex was found and studied in 2010. Two years later, the first research excavation was carried out by local archaeologists. Nowadays, Podvyaze is listed in the state register of cultural heritage of the Russian Federation and is recognized as a monument of federal importance. At the beginning of 2014, the excavated area of the monument had a size of about 120 sqm. Along with the 28 discovered graves with human and animal remains, archaeologists have found 584 objects. Amongst others, these are costume parts, belts, adornments of shoes, coins, elements of weapons, items of horse-gear and antique pottery. Most of these materials turned out as evidences of Finish and Baltic tribes which settled during the period of mass migration. So far, the most valuable artifact was a short single-blazed sword, which usually appeared in Roman provinces in the Danube river region. The aim of our field camp was to reveal buried structures and to support archeological work in the sense of pinpointing proper excavation sites. Knowing where to search for subsurface artifacts does not only help archeologists save time, but more importantly also helps them to prevent object damages during future excavations. We reached this goal by applying and comparing different geophysical methods, which were ground-penetrating radar, geomagnetics, geoelectrics, and acoustic methods based on surface waves. Besides a better understanding of the local history of Nizhny Novgorod and its surroundings, we wanted to highlight the educational aspect. Each member of our team brought advanced experience in at least one of the conducted methods to the project. The basic idea of the joint field camp was that each participant shared his or her expertise with the others.
Nizhny Novgorod is the fifth biggest city in Russia and located about 420 km to the east of Moscow. The Podviaze burial site itself is situated about 30 km to the west of Nizhny Novgorod nearby the Oka River. The underground on-site was of quartery deposits such as loess. We agreed to define a survey grid which measured 9 m x 15 m next to a forest edge. Since we decided an array step of 25 cm for some applications (e.g. geomagnetics), the measuring campaigns comprised partly 2257 measurement points distributed over 37 profiles. Due to the problem of grave robbery, further information about the detailed location should not be provided publicly. This issue ought always to be considered in relation to archaeological research.
Each field camp participant was aware of at least one geophysical method and prepared to share his or her expertise with the others.
Geomagnetics - It is one of the most efficient methods within archaeogeophysical studies. The method is based on material-dependent magnetic properties, which cause slight local disturbances in the Earth’s magnetic field. Especially steel and iron as well as burned soil are easy to detect with the most common type of magnetometers. Moreover, evidences of cultural activities, such as agricultural work and infrastructures can be found too. Due to the size of the measurement grid and its spacing the geomagnetics survey comprised 2257 measurement points within 37 profiles and took one and a half day. Before the measurements started the operator had to get rid of all magnetic materials like belt buckles and glasses, since they would have affected the results. Since we expected geomagnetics to be the most promising method in terms of revealing particularly small anomalies and due to its sensitivity to the magnetic environment we started with that application and continued afterwards and simultaneously with the others. For the survey a proton precession magnetometer (PPM) with two vertical adjacent sensors has been used. It measures the magnetic gradient between both sensors. To have an exact reference, a fixed base station in a distance of about 50 m recorded the strength of the total Earth’s magnetic field (here: +- 53 000 nT).
Ground-penetrating radar - GPR is a near-surface geophysical method which is used in a broad range of applications, including geological, environmental and archaeological investigations. The technique utilizes electromagnetic wave propagation: An antenna emits a short pulse which travels through the subsurface, is reflected at inhomogeneity and travels back to a receiver antenna. Due to because of physical constraints, penetration depth might be limited. Regarding the expected location of the targets, however, we did not consider this as an issue. In the GPR survey data are being recorded continuously at relatively high resolution which yields a high acquisition speed, accuracy and immediate data visibility in the field. While pulling the GPR device along the profiles a wheel counts the distance between the measurement points, which was 25 cm.
Spectral analysis of surface waves - SASW is a method widely used in engineering seismic surveys. In its standard realization for soil parameter reconstruction the Rayleigh surface wave dispersion characteristic is used solely, which allows determining only the shear wave velocity profile. However, it is known that in homogeneous half space the ratio between amplitudes of horizontal projection of ground displacement and vertical one is a function of Poisson’s ratio and monotonically increases when decreasing the Poisson’s ratio. Therefore, the dependence of projections ratio on frequency can provide sufficient knowledge about the distribution of Poisson’s ratio in a vertically layered medium. The corresponding inverse problem solution in the case of considering variations of this parameter becomes thus more correct. Based on this fact, we decided to implement the modification of the standard SASW technique by taking into account the frequency dependence of the projections ratio in the Rayleigh wave along with the analysis of its dispersion characteristic. The possibilities of such a method for some specific problems (e.g. the assessment of water saturation effects on the soil’s structure) were proved in a number of papers, written by students from the Nizhny Novgorod team. However, it seemed interesting to test the modified SASW technique for the tasks of locating not only layers, but also of spatially distributed inhomogeneities such as our archaeological targets. The seismic waves were launched by a vertical vibrator mounted on the surface of the ground and creating a vertical force directed downwards.
Geoelectrics - Electrical resistivity tomography (ERT) is a common method in archaeogeophysics research. The principle is to induce current into the underground by electrodes to determine the electric resistivity of the material in the subsurface. This enables to reveal covered or buried structures. For our main task, which was locating possible grave goods in the near subsurface, it did not seem to be the most suitable method because in theory the grave goods might be too small in size to reduce the apparent resistivity enough in order to measure the resistivity difference when large arrays (to cover a large area) are used. Despite this restriction we still intended to use the geomagnetics at survey spots where the other measurement techniques indicated an anomaly to prove if our theoretical understanding was right or wrong. We also performed a 3D-survey, which should improve the resolution of the measurement and we were able to use a very small electrode spacing at spots of anomalies detected by the other methods. Besides, no participant had ever carried out a 3D-ERT measurement. So, it was a new challenge for us.
In this case study several spots showing significant anomalies could be detected by implementing the four geophysical applications and comparing their results. The geomagnetics survey revealed two relatively prominent anomalies and based on that information we focused following on these two spots when implementing the other three methods. Especially, the geoelectrics survey took a long time, which why we couldn’t cover the entire grid. Nevertheless, the ERT delivered noticeable results of electric resistivity in a shallow depth, which correspond with the magnetic anomaly spots. Unfortunately, we were not able to process the data of the 3D-survey as proper as intended, since we only used a demo processing program. A regular version was not affordable. The results of the GPR survey didn’t fulfill our expectations entirely. We started the survey in the morning after a rainy night. So, the underground was still wet, which caused unsatisfying results. It is more convenient for GPR measurements if the soil is dry since water influences the propagation velocity of the electromagnetic waves. The seismic method which has been developed by the Institute of Applied Physics in Nizhny Novgorod was carried out the first time in the context of an archaeogeophysical survey. Based on the recorded travel time of the seismic waves the results of the SASW application showed inhomogenities in the subsurface on several spots within the grid. In the following, excavations shall clarify if the anomalies we detected by using the four geophysical applications are archaeologically relevant. In summary, the Russian-German Joint Field Camp has been successful in terms of educational, scientific and social teamwork. Each participant broadened her or his horizon and gained theoretical and practical experience in the field of geophysics. Moreover, the project participant Andrey Konkov submitted successfully an abstract about the work we did at the SEG and presents it in the SEG Annual Meeting 2015 in New Orleans.
Further detailed informations are available via the Student Geoscientific Society Berlin & Potsdam and the Institute of Applied Physics Geophysical Society.
- ↑ Konkov, A., Brüning, L., Handel, J., Prasche, M., Shalimova, E., Zakluchnov, I. et al. (2014): Integration of geophysics and archaeology: exploring a burial site of the 4th to 7th century near Nizhny Novgorod, Russia. // SEG International Exhibition and Annual Meeting 2015 (New Orleans, USA), Extended abstract, 2015. P. 2369-2373.
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Personal experience written by a Field Camp participant