Investigation of a Tertiary maar structure using three-dimensional resistivity imaging

This paper presents the application of the electrical resistivity tomograph y (ERT) method to the investigation of the Tertiary maar structure of Barut h (Germany) known from previous gravimetric surveys. ERT was applied to sup port the optimum location for a palaeoclimatological drill hole. Special modifications of data acquisition, signal processing and inversion are introduced to adapt the method of ERT to the special requirements for t he 3-D investigation of structures with horizontal extensions of 1 km or mo re. More than 5000 dipole-dipole combinations were recorded at three concen tric circular electrode arrangements using stand-alone transient data acqui sition systems (RefTek). We present a fast approximate imaging technique based on the simultaneous i terative reconstruction technique (SIRT). As the complete calculation of th e inverse Frechet matrix is avoided, the algorithm is especially suitable f or large data and model spaces, where complete inversion is beyond the limi ts of available computing hardware. The single-step method is applicable to arbitrary irregular electrode layouts. Synthetic tests show that the imagi ng procedure reconstructs the main features of the subsurface. A low-resistivity body could be interpreted as limnic sediments filling the interior of the Tertiary maar crater. Considering the horizontal resistivi ty gradient, estimates for the lateral and depth extents of the structure w ere made. An optimum position for a palaeoclimatological borehole was found , and was in good agreement with the gravimetric minimum.