Testing the effectiveness of an inverse Wenner-Schlumberger array for geoelectrical karst void reconnaissance, on the Swabian Alb high plain, new line Wendlingen-Ulm, southwestern Germany

摘要

On the Swabian Alb high plain, southwestern Germany, the new high-speed railway line Wendlingen-Ulm of the German Railway Company (Deutsche Bahn AG) is currently under construction. 2.5D electrical resistivity tomography (ERT) measurements (meaning the use of parallel 2D lines) were performed at the excavation bottom level in addition to the investigation program of the construction company including microgravimetric and seismic methods. The aim was to test the effectiveness of an inverse Wenner-Schlumberger array for exploring geological defective karst voids and other karstic features in a highly heterogeneous karstic environment. A complete 3D ERT survey (meaning the use of additional orthogonal cross-lines) was not carried out due to restrictive circumstances (terrain effect, anisotropic effect). The 2.5D ERT survey (ABEM Terrameter SAS 4000, inverse Wenner-Schlumberger array, six lines, 60 m line length, 0.75 m electrode spacing, lines oriented perpendicular to the long axis of an oval-shaped collapse doline and the most prominent voids, 1.5 m line spacing, smoothness-constrained least-squares/robust inversion, RMS error: 1.60-3.20% for a maximum of 5 iterations) provided resistivity values of marls and marlstones, a limestone bedrock, a collapse doline and of an air-filled cavity. A clear demarcation between the marls and marlstones and the limestone bedrock was only possible outside the area of influence of the collapse doline; the latter could not be exactly delineated from the adjoining geological units. In addition, one of three air-filled voids (open fractures, cavities) could be derived accurately in location and roughly in size and shape from the 2.5D ERT survey; the other two were not detected by means of ERT because of (i) its location beneath a high-resistive anomaly and (ii) its comparatively small maximum horizontal opening width of only 1 time the used inter-electrode spacing. Extensive direct probing investigations served for calibration and discussion. The 2.5D ERT measurements conducted with an inverse Wenner-Schlumberger array complemented the microgravimetric and seismic results of the previously conducted investigation program of the construction company and provided a contribution to karst void reconnaissance in a highly heterogeneous karstic environment like the Swabian Alb high plain, on the new line Wendlingen-Ulm.