Abstract
The detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history. Objective evidence for the existence of such structures can only be provided by geophysics and boreholes. The combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures. Their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density, and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character. Seismic measurements provide the most detailed information about the internal structure of the maar sediments. Zones of low seismic reflectivity and very low density represent sediments of the late maar-lake period. The early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors. The seismic sections clearly reveal the bowl-like structure of the maar. Outside this bowl-like structure, there are only a few reflections, which represent the basement. Taking into account the shape of the gravity anomaly, seismic information allows geometrical modelling of the maar structure. Optimal drilling sites can be selected based on the results of geophysical surveying. Comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern.
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Acknowledgements
We thank P. Suhr, K. Goth (both LfUG Freiberg), H.-J. Harms, M. Felder (both FIS Messel), and V. Lorenz (Würzburg University) for their excellent collaboration and their patience in explaining to us the geology of maar structures. Potential field data in the Messel area were provided by W. Jacoby, H. Wallner and their colleagues and students (Mainz University); we are much obliged to them, also for many helpful discussions. We owe the high quality of the field data, processing and figures to our colleagues in the engineering staff of the GGA-Institute. Reviews by M. Roach and D.B. Stone improved the paper considerably.
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Schulz, R., Buness, H., Gabriel, G. et al. Detailed investigation of preserved maar structures by combined geophysical surveys. Bull Volcanol 68, 95–106 (2005). https://doi.org/10.1007/s00445-005-0424-8
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DOI: https://doi.org/10.1007/s00445-005-0424-8