Gerald Gabriel

Estimation of depth to the bottom of magnetic sources by a modified centroid method for fractal distribution of sources: An application to aeromagnetic data in Germany

We propose a modified centroid method to compute the depth to the bottom of magnetic sources (DBMS) based on a fractal source distribution. This approach provides better estimates than the assumption of an uncorrelated source distribution. We apply our approach to a recently compiled homogeneous set of aeromagnetic data from Germany. The deepest DBMS values are found for some large basin areas, i.e., the Molasse basin and parts of the North German basin. Smaller DBMS were estimated for the Moldanubian region in southern Germany and the northern part of the North German basin. A comparison of DBMS with heat-flow data, crustal temperatures at 3-km depths, and Moho depth indicates that DBMS is controlled by the geothermal condition of the earth’s crust in Germany and lithologic changes. Although the Upper Rhine graben and the Moldanubian region are characterized by small DBMS, a change in DBMS values in northern Germany seems to be related to the Elbe lineament.

The Heidelberg Basin drilling project: Geophysical pre-site surveys

Abstract. Currently, the Heidelberg Basin is under investigation by new cored research boreholes to enhance the understanding concerning the control on Pliocene and Quaternary sedimentation by (neo)tectonics and climate. The Heidelberg Basin is expected to serve as a key location for an improved correlation of parameters that characterise the climate evolution in North Europe and the Alpine region. The recovery of sediment successions of high temporal resolution that are complete with respect to the deposition of Pleistocene glacials and interglacials in superposition is of special importance. Prior to the new research boreholes in Viernheim and Heidelberg geophysical pre-site surveys were performed to identify borehole locations that best achieve these requirements. In the area of the Heidelberg Basin the strongest negative gravity anomaly of the entire Upper Rhine Graben is observed (apart from the Alps), hinting at anomalously thick sediment deposits. However, especially reflection seismic profiles contributed significantly to the decision about the borehole locations. In the city of Heidelberg for the first time, the depocentre of the Heidelberg Basin, as indicated by additional subsidence compared to its surroundings, was mapped. In this area, sediments dip towards the eastern margin of the Upper Rhine Graben. This is interpreted to represent a rollover structure related to the maximum subsidence of the Upper Rhine Graben in this region. At the Viernheim borehole location the seismic survey revealed several faults. Although these faults are mainly restricted to depths greater than 225 m, the borehole location was fi nally adjusted with respect to this information.

Gravity Observations in the Western Rhenish Massif and Forward Modelling of the Eifel Plume Bouguer Anomaly

We study the gravity field of the western Rhenish Massif, Germany, to search for relationships to the mantle source of the recent Eifel volcanism. First we compile the available gravity data from the region and present them as Bouguer gravity map. This map does hardly show any anomaly univocally related to the Eifel plume, even after careful wavelength filtering the data. Forward modelling of the Bouguer gravity field with the seismological low-velocity anomaly in the upper mantle as input anomaly finds that the seismological model corresponds to an approximately -10 mGal anomaly (1 mGal is 10−5 m s−2). Such a tiny signal can be hardly recognized in the real data: Intrusions of magmatic high-density dykes and sills may even hide or reverse the expected negative mantle gravity signal which makes the situation even worse. As conclusion we find that a gravity signal of the Eifel plume cannot be seen in the available Bouguer data, but that this negative result is at least consistent with the seismological mantle models. Furthermore the forward modelling results suggest that small-scale upper mantle plumes may not be recognized as a Bouguer anomaly underneath a typical heterogeneous continental lithosphere.

Geodetic monitoring of subrosion-induced subsidence processes in urban areas

Abstract The research project SIMULTAN applies an advanced combination of geophysical, geodetic, and modelling techniques to gain a better understanding of the evolution and characteristics of sinkholes. Sinkholes are inherently related to surface deformation and, thus, of increasing societal relevance, especially in dense populated urban areas. One work package of SIMULTAN investigates an integrated approach to monitor sinkhole-related mass translations and surface deformations induced by salt dissolution. Datasets from identical and adjacent points are used for a consistent combination of geodetic and geophysical techniques. Monitoring networks are established in Hamburg and Bad Frankenhausen (Thuringia). Levelling surveys indicate subsidence rates of about 4–5 mm per year in the main subsidence areas of Bad Frankenhausen with a local maximum of 10 mm per year around the leaning church tower. Here, the concept of combining geodetic and gravimetric techniques to monitor and characterise geological processes on and below the Earths surface is exemplary discussed for the focus area Bad Frankenhausen. For the different methods (levelling, GNSS, relative/absolute gravimetry) stable network results at identical points are obtained by the first campaigns, i.e., the results are generally in agreement.