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

On the Swabian Alb high plain, new line Wendlingen–Ulm, southwestern Germany, 2D electrical resistivity tomography (ERT) measurements were performed during construction at the excavation bottom level in addition to the standard investigation program (including microgravimetric and seismic methods) 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. The 2D ERT survey (ABEM Terrameter SAS 4000, inverse Wenner-Schlumberger array, 2.0 m electrode spacing, roll-along technique, robust inversion, RMS error: 1.61% for a maximum of 5 iterations) provided a high-resolution image of the subsurface, revealed the size, shape and spatial distribution of conductive clayey/loamy deposits ( 1000 Ωm) and air-filled voids (>2000 Ωm). These observations were confirmed by extensive direct probing investigations (1 exploration drilling, 4 core drillings, 19 destructive drillings, 2 trial pits). Moreover, the 2D ERT data coincided with the microgravimetric and seismic results of the previously conducted standard investigation program to a high degree. The exact spatial distribution with depth as well as the internal structure of the detected anomalies (clayey/loamy deposits, moderately fractured and/or medium weathered limestone bedrock) could, however, only be obtained from the 2D ERT data. The 2D ERT measurements conducted with an inverse Wenner-Schlumberger array provided a decisive contribution to karst reconnaissance in a highly heterogeneous karstic environment like the Swabian Alb high plain, on the new line Wendlingen–Ulm.