SGS Berlin & Potsdam High School Field Camp 2011
A principle of the Student Geoscientific Society Berlin & Potsdam is to pass on knowledge in the field of geology and geophysics to interested and inexperienced people. Therefore, the members of our SEG student chapter organize field work projects which provide the opportunity to gain hands-on experience as well as theoretical knowledge to process the acquired data. In 2011, Aurelian Röser had the idea to set up a summer field camp, which allowed high school kids to get an idea about what is possible in geophysics. In cooperation with a local secondary school and the Berliner Unterwelten e.V. (Berlin Underworlds), which is a society for exploration and documentation of subterranean architecture in Berlin, an object of research and an appropriate syllabus have been worked out. All in all, nine SGS members, three representatives of Berlin Underworlds and 22 high school students as well as their physics teacher participated in this project, implemented on three days in August 2011.
The main focus of the project was on the educational aspect. The collaboration with the high school students represented a special motivation for the SGS members, because they had to present their field of study to young people without any geophysical background in an interesting, comprehensible and also scientifically correct way. During the course of studies as well as after graduation either in academia or industry, presentation of scientific topics or results belongs to the basics of any geoscientist's tasks. For many geoscientists, this becomes an especially heavy challenge if they have to present potentially complex scientific topics to people without any background in those topics. At the same time there was the occasion to introduce the world of geophysics to potentially future scientists. The prospective target of the geophysical measurements was a tunnel of a bunker system which has been constructed during World War II under the Humboldthain Public Park in the center of Berlin. This underground system was destroyed after the war and has been later only partly revealed. The applications which has been used to detect the tunnel were geoelectrics, geomagnetics and near-surface seimics (with a hammer as source). According to the time schedule the first two days were spent on the survey site. The processing and interpretation of the data were done on the third day of the summer field camp at the Institute of Geological Sciences on GeoCampus of Freie Universitaet Berlin.
Humboldthain Public Park was created between 1869 and 1876 in honor of German naturalist and explorer Alexander von Humboldt. On the 10th of September 1940, as a result of a personal order from Hitler, the construction of a flak tower in Humboldthain Public Park began, which was composed of the actual flak tower in the northern part of the park and a second tower in the southern part. This additional tower, the so-called “Leitturm” (control tower) helped to aim the anti-aircraft cannons. The anti-aircraft towers were intended to protect the city center of Berlin from aerial bombardment. Inside the flak tower, which was considered at that time to be completely bomb-proof, there was also space for 15.000 civilians. While the southern tower was completely destroyed by the Allies after World War II, they decided to destroy only the southern half of the flak tower because of the nearby railway routes in the north. From the bunker debris there emerged the Humboldthain hill. Nowadays, the northern half of the flak tower serves as a viewing platform for locals and tourists as well as a starting point for guided historic tours of Berliner Unterwelten e.V.. Through their efforts parts of the destroyed turret have been made accessible. On top of the Humboldthain hill, there is a vineyard, from whose annual harvest about 200 bottles of wine are produced. The target of the geophysical measurements was an escape tunnel which connected the two towers. While the southern part of this tunnel was mapped by former measurement campaigns, there are still many open questions about the course of the northern part as well as about its condition and accessibility.
Due to the number of participants we decided to split the crowd into 3 groups of 7-8 high school students. Each group implemented alternately one of the three geophysical methods. The geophysical devices have been provided by the Institute of Geological Sciences of Freie Universitaet Berlin.
Geomagnetics - Geomagnetics is an appropriate application to explore different objects (e.g. metal barrels), structures (tunnel, cavity) or geological formations (ore bodies, layers of lava) in the subsurface. For that, the method uses the material-dependent magnetic properties which cause slight local disturbances in the Earth’s magnetic field. 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. For the survey a proton precession magnetometer (PPM) with two vertical adjacent sensors has been used. It measures the magnetic gradient between both sensors. An additional fixed base station in a certain distance to record the strength of the total Earth’s magnetic field has not been considered as necessary. The actual focus of the survey was on the handling and understanding of the magnetometer, since the high school kids have never used such a device before.
Geoelectrics - In contrast to geomagnetical measurements, working with the natural potential field, geoelectrics methods are active methods, measuring the potential difference of an artificial electrical field. This field is generated by inducing current into the underground via two electrodes. The underground can be interpreted as a resistor and its resistance depends upon geometry and material properties. Thus, the specific resistivity of the material in the underground is what is interesting to know, because it directly reflects if there is a good (low resistivity) or a bad (high resistivity) conductor. Geoelectrics is applied, for example, for deposits exploration, detecting leaks in dumping grounds, finding ground water tables and in the field of archaeology. Since, it was believed that the tunnel as prospection target contains significant air-filled gaps this method was considered to be very efficient. Air has a much higher specific electrical resistivity than most other underground materials, as we could also see at the results of the survey. Beside that, it was regarded as to be important to introduce this major geophysical application to the high school students.
Seismics - The source for the near-surface seismics method is a sledge hammer which is struck on a metal plate. The detonation generates seismic waves which propagate in the subsurface and are reflected or refracted on layer boundaries or material transitions. Geophones ordered in particular array steps record the waves which arrive after a certain time again the surface. While running through the underground the seismic waves are split into different components. By picking the arrival times of the waves and their components on the resulting seismogram it is possible to determine their transit time and to draw a conclusion about the structure of the underground. Seismics methods are convenient to image the subsurface and are applied in the fields of oil and gas explorations as well as, for example, to detect geological faults. The most efficient way for using seismics is in conjunction with other geophysical methods by comparing their results.
The SGS Field Camp 2011 was a memorable experience for all its participants because beside the scientific and didactic aspects we wanted to achieve, it was a lot of fun. Geoelectrics turned out to be the most convenient geophysical application to detect the tunnel. The inverse model shows clearly a spot which might be interpreted as the tunnel. In contrast, even though geomagnetics and seimics could have been useful ways to detect the structure of the tunnel the surveys unfortunately didn't deliver results we hoped for. An interesting side note: Due to a light rain during the seismics survey the effect of how rain drops affect the seismogram could be observed. However, it was important to explain the geophysical principles of both methods to the high school students. According to their feedback most students found it interesting to get an insight into this new domain of nature science. Especially, swinging the sledge hammer was very popular among the kids. And even if we were able to convince only a few high school students taking the geophysics pathway, the efforts for the project were worth it.
- Röser, A. et al.(2011): SGS Berlin & Potsdam Sommerprojekt 2011 im Volkspark Humboldthain. (German language)
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