f Fault, karst cavity detection with simultaneous radio magnetotellurics (RMT) and continuously recording very low frequency - electromagnetic (VLF-EM) survey. A case study in the swiss Jura.

Engineers expect that new highway tunnel construction in the Jura-mountains (Switzerland), in the karstified limestone region of Ajoie, will probably intersect water bearing fractures. The estimated quantity, under high water level conditions could be 1500 l/min. We wanted to look for any permeable fracture zone adjacent to the highway area that could release this high volume of drainage water. We did geophysical surveys simultaneously using Radio Magnetotellurics (RMT, 12-240 kHz - Turberg, 1994) and Very Low Frequency Electromagnetics (VLF-EM 15 –30 kHz – Stiefelhagen, 1998). The Hydrogeology Center, University of Neuchâtel has developed fast and precise geophysical survey techniques with prototype instrumentation to locate and map high-permeability fracture zones. The planned highway will cross a flat, layered and karstified jurassic limestone area in which there are major north – south faults. In the vicinity of the proposed tunnel, in a typical small “dry valley”, several parallel profiles were performed with the RMT survey, using 183, 77.5 and 16 kHz radio emitters. Every ten meter vertical soundings were done along the profiles and apparent resistivity and phase shift were measured with all three frequencies. Each profile detected, at all frequencies, a low resistivity anomaly, with limited extension, centered in the axis of the valley. Along the same profiles with a 1 Hz datalogger, our VLF-EM device measured continuously without any ground contact, both “in phase “ and “out of phase” components of the secondary magnetic field produced by the primary field of the 16 kHz radio broadcasting antenna. Again, we obtained a good anomaly of the “out of phase” at the same locations already observed with RMT. Excavation found more than a six-meter thick clay and silt deposit at the precise site where the geophysics located the highest anomaly. Water-infiltration testing was successful and the fast infiltration of water also opened a karst cavity where a mammoth tusks with a skull and several other bones were discovered. This is the first adult mammoth found in Switzerland. At the moment archeologists are cleaning up the site. This may provide a 3-D visualization of the geological setting. We will see the vertical limestone sinkhole wall generated by two types of karst fracture families. A rather complex loose sedimentary deposit is also to be seen, resulting from the area’s complex periglacial history. Speleologists already mapped the accessible part of the cavity, about 40 m long and 10 m deep. The hydraulic conductivity of this karst-channel is high enough to evacuate all drainage water from the planed highway tunnel. Our conclusions are that both RMT and VLF-EM are rapid, cost-effective and high-resolution techniques easily applied by hydrogeologists and engineering-geologists. A two-man crew can map large areas at several depths in a short time and survey permeable geologic heterogeneity like faults or cavities. In the present case study, there was an excellent correlation between the measured geophysical anomalies and the obtained field results.