- ACTIVIDADES DEL PROGRAMA

• If we compare the short proles P1 and P3 with the long prole P4, we can see that on the one hand the resolution of the long prole is not good enough to represent the topmost thin low velocity layer. On the other hand, the penetration depth of the short proles is not deep enough to reach bedrock with high velocities of 4000 m/s.

6.6 Electric resistivity tomography (ERT)

On the Pont Bourquin landslide, ve short and one long ERT proles were recorded with a Syscal R1 Plus device, using 48 and 72 electrodes respectively (see Table 6.4 for prole details and Figure 6.11 for prole locations). Electrode spacing was 1.5 and 2 m for the transversal short proles, 3 m for the longitudinal short prole and 5 m for the 355 m longitudinal long prole.

The Wenner-Schlumberger and the Schlumberger-VES method were chosen for the long and the short proles, respectively. See Section 4.3 for descriptions of the methods.

Prol Number of Spacing, Horizontal Orientation Electrode conguration

name electrodes m length, m (Array)

Table 6.4: Characteristics of the performed ERT proles. Sch-VES: Schlumberger-Vertical electrical sounding, W-Sch: Wenner-Schlumberger.

On Figure 6.17, the longitudinal ERT prole E5 is displayed. The absolute error of the prole after 11 iterations is 1.4. A low resistivity top layer with < 100 Ωm which is up to 10 m deep and gets thinner towards the top of the slope is observed. Below this layer, several units which are dipping into the slope are distinguished. These are from bottom to top: A unit of 500 Ωm with a high resistivity block of >3000 Ωm close to the top, a unit of 2500 Ωm, a unit of 500 Ωm, a unit of very low resistivity around 100 Ωm, and again a unit of 500 Ωm.

The 5 short proles are displayed on Figure 6.18. 6 to 9 iterations were performed and absolute error between 1.4 and 2.1 was obtained. Higher resistivity around 500 Ωm are observed in the western part of the transversal proles from the head scarp area E1, E2 and E4, in the the upper part of E3, and in the lower part of E6. Two dierent surface layers can be distinguished: a low resistivity layer (10 - 80 Ωm) and an irregular high resistivity layer (200 - up to several 1000 Ωm). Further, a deep low resistivity unit (70-100 Ωm) can be observed in the middle/eastern part of E1 and E2 and in the lower part of E3. An intermediate low resistivity layer in less than 5 m depth is seen in the eastern part of prole E1 and E4.

The low resistivity top layer observed in all proles except E4 is interpreted as the landslide.

It becomes thinner toward the top of the slope and disappears below the chalet. The bedrock below the landslide can be divided in dierent lithologies (see Figure 6.19). The high resistivity block at the bottom of the prole is interpreted as cellular dolomite associated with gypsum. The middle part with resistivity between 400 and 2000 Ωm could be Flysch or again cellular dolomite associated with gypsum. The low resistivity area in the upper part of the landslide is interpreted as shale lithology, overlaid by high resistive cellular dolomite. The lithological boundaries dip steep into the slope with about 45°. The high resistivity areas in the western part of E1, E2 and

6.6. ELECTRIC RESISTIVITY TOMOGRAPHY (ERT)

Figure 6.17: ERT prole E5 recorded along the Pont Bourquin landslide compared with seimic reraction prole P4 from the same location. The slip surface observed in both proles coincides well.

6.6. ELECTRIC RESISTIVITY TOMOGRAPHY (ERT)

Figure 6.18: Short ERT proles across (E1, E2, E4, E5) and along (E3) the Pont Bourquin landslide.

6.6. ELECTRIC RESISTIVITY TOMOGRAPHY (ERT)

E4 most likely are cellular dolomite overlaid by saturated moraine. In the middle part of E1 and E2 black shale is overlaid by the landslide. In the western part it is overlaid by moraine. The boundary between black shale and cornieule can also be seen in the middle on prole E3. Prole E6 shows the landslide mass in the middle overlaying Flysch or cellular dolomite rock. In this area, the landslide is much thicker due to the accumulation of debris.

Figure 6.19: Geological interpretation of the geophysical proles.

6.6.1 Summary ERT and comparison with seismic tomography

The best results were obtained with the robust inversion method. With the ERT proles three important information about the Pont Bourquin landslide were obtained:

• The low resistivity top layer most likely represents the landslide (see Figure 6.19). It is up to 14 m deep, the average depth is less than 10 m and it becomes thinner towards the top of the slope. The slip surface detected in the seismic refraction and ERT are consistent.

• The bedrock below the landslide is divided in dierent lithologies: The high resistivity areas in the lower part of the landslide and at the top west of the landslide is interpreted as cellular dolomite (associated with gypsum), the low resistivity area in the upper and eastern part is the black shale formation and the high resistivity area in the middle could correspond to Flysch.

• A last feature are the intermediate low resistivity layers in the east of the landslide. The low resistivity of these layers could indicate saturation of either moraine or fractured black shale bedrock. The second explanation seems more likely because of the depth of these layers (see Figure 6.19).