Large-scale DC resistivity measurements

Large-scale resistivity investigations are a specialty of the institute. For more than 30 years numerous Schlumberger soundings have been measured in order to reveal the electrical properties of the deep subsurface for several geological problems. For the large current bipoles a hardware was developed, which is able to inject up to 40A into the ground.

As alternative to classical soundings, large-scale dipole-dipole experiments are able to give informations about the three-dimensional subsurface characteristics. Both current injection and potential measurements are realized by dipoles of limited length whose distance can be spread by several kms. The signals are registered continuously by using self-sufficient data loggers such as the Geolore, which was developed by the university of Frankfurt. In order to evaluate the resistances from current and voltage time series', powerful algorithms had to be developed.

In order to create 3D subsurface models we use and develop efficient modelling and inversion methods on the basis of finite differences and finite elements. Synthetic models and sensitivity analyses are able to give insight into the causes of anomalies and their resolution. Special attention is paid to the determination of the apparent resistivity tensor by using each two current injections and voltage. Their invariants can delineate boundaries and preferred directions.

The institute is closely cooperating with the universities of Leipzig and Frankfurt that have valuable experience in large-scale resistivity.

Measuring campaigns

Cuxhaven-Bremerhaven buried valley

The fist measurements were made in spring 2006 in the Cuxhaven area within the scope of the PhD thesis of Jörn Schünemann. Main aim was to delineate the sediment structure within and outside of the present buried valley, which was investigated in the BurVal project. We used 20 stations with three measuring directions and injected almost 30 current injections. As a central point of the work we developed a time domain method for the determination of the resistance on basis of a harmonic least squares approach. The analysis is mainly done and gives hints to the distribution of clays, salt water and fresh water in a depth scale that complements the relatively shallow electromagnetic measurements. The thesis is almost completed and results of the methodical work as well as of the geoloical interpretation are currently published. Details

Kinzig valley project

A comparable experimental design as in the latter project was realised in autumn 2006 together with the university of Frankfurt in the Kinzig valley. Aim was the investigation of faults in the area between the Vogelsberg and Spessart ranges. For the first time underwater stations have been successfully operated. This was a pre-survey for a possible project at lake Balaton, Hungary. The interpretation in the scope of a PhD thesis by Thomas Agricola is not yet completed. Differently from the Cuxhaven project he concetrates on the development of frequency domain methods. Details

Vogelsberg campaign

The Vogelsberg range is the only stratovolcano and the biggest basalt massiv in Europe. Aim of the campaign in the summer 2008 was to discover a possible endogene lava dome and the southern caldera boundary. 21 receiver stations with two or three measuring dipoles were installed. At 13 positions we injected currents of up to 30A in two preferably perpendicular directions. For the first time we used a borehole, the research borehole Eschenwald, for injection. The analysis by Thomas Agricola is still under process. However, first interesting results have been already presented. Details