2. Paleoseismological studies

2.1. The Feldbiss Fault in Belgium

2.1.4 Geophysical prospecting results

The selected sites were first investigated using different geophysical techniques, including:

• ground-penetrating radar;
• geo-electrical resistivity measurements;
• geo-electrical tomography;
• electromagnetic measurements;
• seismic refraction.

All these techniques allow to identify disturbances of the stratification in the upper few metres below the ground surface, thus aiding in the exact localisation of the presumed fault trace. Coincidence of several geophysical anomalies with the frontal escarpment can be taken as very strong evidence for the existence of a surface fault.

In addition, high-resolution seismic reflection profiles were acquired on site 1, and also in a canal crossing the Feldbiss Fault in Neeroeteren.

2.1.4.1 Site 1

Figure 6 shows the most important geophysical results for site 1. All techniques show a distinct change of physical properties near m 55, in alignment with the topographical scarp. The electrical curve shows a large increase of resistivity, from 100 Ohm.m on the elevated SW side, to about 250 Ohm.m on the lower-lying NE side. The time-distance curve of the refraction seismic waves show a lateral decrease of the apparent velocities, from c. 2000 m/s in the SW block to less than 1667 m/s in the NE block (this is most evident for the reverse shot). On the radar profile, a more or less coherent reflector can be discerned (marked with blue arrows) which appears to be displaced (downthrow of about 0.5 m) in the same spot. Finally, the most illustrative 2D image of the faulted zone was obtained by the electrical tomography method, which indicates sharp lateral resistivity variations at shallow depth near m 46.5 and m 52.5.

           SW                                                                                                                  NE

Fig. 6 - Geophysical profiles across site 1. Top: geo-electrical resistivity and seismic refraction; middle: ground-penetrating radar; bottom: electrical tomography (note the slightly different scale). Data were acquired by the University of Liège.

2.1.4.1 Site 2

Work in progress...

2.1.4.1 Site 3

Work in progress...