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Structural study of the Ballandaz landslide (French Alps) using geophysical imagery
Journal of Applied Geophysics 75 (2011) 531–542
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Structural study of the Ballandaz landslide (French Alps) using geophysical imagery G. Grandjean a,⁎, J.C. Gourry a, O. Sanchez a, A. Bitri a, S. Garambois b a b
BRGM, Orléans, France LGIT, Grenoble, France
a r t i c l e
i n f o
Article history: Received 16 April 2010 Accepted 11 July 2011 Available online 24 August 2011 Keywords: Seismics ERT Landslide French Alps
a b s t r a c t This study of the Ballandaz landslide (Savoie, French Alps) was carried out as part of the ANR/PGCU-SIGMA research project. Its aim was to characterize the slope by different methods of geophysical imagery, and then use this characterization to provide a combined interpretation of the morpho-structures in order to simulate the geomechanical behavior of the sliding mass. First, electromagnetic mapping was used to identify the variations of the surface lithology and moisture from which one can more precisely locate the active zone of the slope. Then, electrical and seismic 2D imagery methods were used along several transverse and longitudinal profiles in order to produce electrical resistivity and seismic-velocity depth sections showing up the slope's structures. The H/V method was also tested locally to determine the depth to basement so as to complement the profiles: it reveals potential seismic site effects from the deeper structures. Helped by drilling and inclinometer surveys, the geophysical interpretations have revealed the various units structuring the landslide: (i) the active, very heterogeneous shallow level overlying (ii) a more rigid, less porous, and probably stable shallow bedrock, (iii) boulders, and (iv) the sound geological basement of quartzite and gypsiferous facies. The study has shown the usefulness of combining such different sounding techniques for studying complex environments like landslides; it has also revealed the limitations of each method when used for studying very heterogeneous environments. © 2011 Elsevier B.V. All rights reserved.
1. Introduction To understand the dynamics of a landslide, it is essential (i) to determine the relevant internal structural units (Bruno and Martillier, 2000; Hack, 2000; Meric et al., 2005), (ii) to characterize the major hydrological processes (Malet et al., 2005) and (iii) to make an inventory of the external conditions, mainly of a climatic (Jaboyedoff et al., 2003; Polemio and Trizzino, 1999; Schmidt and Glade, 2003) or seismic (Keefer, 1984) nature. Geophysical methods, like seismic and electrical imagery, are well suited for carrying out the structural study of landslides and understanding their internal mechanisms. In particular, seismic and electrical imagery give a direct and non-intrusive measurement of acoustic-wave (P) or shear-wave (S) velocities and of electrical resistivity, three parameters considered as essential for defining the properties of the remobilized geological material (Grandjean et al., 2005; Jongmans and Garambois, 2007). The H/V method, based on passive seismics (Konno and Ohmachi, 1998; Nakamura, 1989; Nogoshi and Igarashi, 1971) can also be used to identify the resonance frequencies of the sliding mass in order to determine the thickness of the unconsolidated layer overlying the rigid basement (Meric et al.,
⁎ Corresponding author. E-mail address: [email protected] (G. Grandjean). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.07.008
2007) and the potential amplification effects of seismic waves (Guéguen et al., 2007). Finally, electromagnetic (EM) mapping is particularly effective for revealing spatial variations in the electrical resistivity, and thus the lithology—and/or water content—at the surface of the massif (Cummings, 2000). The present study concerns the Ballandaz landslide (Savoie, French Alps) and was carried out as part of the ANR/PGCU-SIGMA research project (monitoring the kinematic regimes of slow recurrent landslides in relation to climatic change). One of the project aims was to test various geophysical methods, including seismic and electrical imagery, passive seismics and EM mapping, in order to characterize the sliding mass, i.e., to obtain a combined interpretation of the landslide's morpho-structures, that is intended to be used in future geomechanical models. We present here the results of this study, from the geophysical data acquisition to the combined interpretation of the massif. The Ballandaz landslide (Fig. 1), known since the beginning of the 20th Century, lies at the edge of the Vanoise Massif, near Pralognan. It is located in a small narrow glacial valley, 300 m wide, bounded to the north by the schistose Tour du Merle cliff and to the south by the quartzite spur of Notre Dame de Salette at the base of which flows the Ballandaz River. It lies between altitudes 1050 m (Doron-de-Pralognon River) and 1650 m, at which level one still finds signs of surface movement in the houses (hamlet of Mollard) and civil engineering structures
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Fig. 1. Location of the profiles (white solid lines) . The Ballandaz landslide is contoured with white dashed line.
(Fenetre 11: EDF inspection tunnel located on the Pélapoët road). The average slope is 40%, with intervals in excess of 60%. The downstream part of the slope between the 1050 and 1250 m levels is most active; it threatens the departmental road D915 and an EDF pressure pipeline located along the road, and risks damming the Doron-de-Pralognan River in a section where the river is very boxed in. The Ballandaz slope is located in a complex geological zone (Fig. 2a) characterized
by a tectonic contact on a quartzitic and gypseous sole. This geological pattern is found on many of the valley slopes, which explains the many landslides in the area. The stable bedrock of the studied slope consists of sericite schist, quartzite, cargneule, schist, and gypsum in the low part of the slope. This bedrock is covered by a heterogeneous cover of basement debris and also of morainic material of a priori unknown thickness. Although the Ballandaz landslide is downstream of
Fig. 2. a: Geological map of the studied area (1: stephanian sandstones and schists, 2: permian schists, 3: triasic quartzites, 4: triasic gypsum, 5: quaternary moraines, 6: scree, 7: faults; b: interpreted SC1/I2b borehole.
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a large exposure of gypsum, this has never been clearly seen within the landslide. Another exposure of gypsum occurs to the southwest of the landslide, on the opposite slope. The landslide was already active 100 years ago according to the archives which record that drainage work had been carried at Ballandaz River level. Following this work, no event was recorded on the slope for several decades. In 1993, however, the landslide seems to become alive again; signs of movement were once again observed. Then, in 1996, movement of the landslide resulted in flows that reached Doron-de-Pralognan and the road D915 with, in 2000, an abrupt acceleration of surface movement that led to road subsidence over a distance of 100 m. Major stabilization work was undertaken with the construction of a micro-Berlin wall founded on 12-m-deep piles and anchored to the ground by 15 m subhorizontal tie rods. In spite of this structure, however, the site inclinometer survey that was begun in 2003 has shown that two sliding planes are still active, one at 8 m and the other at 14 m depth, i.e. below the base of the micropiles. This phenomenon is gradually undercutting the heads the micropiles and threatens the long-term survival of the wall. Many geotechnical, hydrogeological and geophysical studies have been carried out on the Ballandaz site since 1996. Drainage and reinforcement work has been carried out. A dozen boreholes have been drilled in the downstream part of the landslide where the risk is highest. The installation of piezometers and inclinometer tubes in some of the boreholes enables monthly monitoring of the site. The slope's hydrogeology is complex: the moisture balance is controlled by the snowmelt in April–June, but one finds sinks and resurgences within the scree and moraines on the upper part of the slope above 1400 m. The groundwater mineralization increases gradually from upstream to downstream as already observed by Binet et al. (2009) on several alpine landslides, although one notes an abrupt increase in conductivity and sulphate content downstream of road D915, probably because of the influence of underlying gypsum formations. Such formations were moreover found in the drilling cuttings of the SC1 cored borehole (Fig. 2b). This borehole, located at the end of the profile P17, allowed us to confirm that the nature of the sedimentary overlap to a depth of more than 45 m is a heterogeneous amalgam of schist, mica schist and quartzite boulders caught up in a sandy–clayey matrix; within this overlap one can distinguish a more weathered surface layer, up to 20 m deep, overlying a more rigid and less permeable layer which is considered here as shallow bedrock. The inclinometer measurements carried out close to road D915 reveal two sliding planes at about 8 and 15 m depth. The internal structure of the landslide is thus still poorly known, particularly concerning the depth to the basement supporting the sedimentary overlap, the spatial delimitation of the active zone and its geometry at slope scale, and the role of the water flows in the onset of the active periods. So, we deployed our geophysical experiments on profiles P17, P18 and P19 (Fig. 2a), particularly well suited to sound the central parts of the slope and to show in what way geophysical methods can provide information about the landslide's structure or the presence of water-saturated zones which can play a key role in slope activity. 2. Aquisitions and geophysical methods To begin with, the entire slope was characterized so as to determine the major lithological and hydrogeological units. Using the electromagnetic technique EM34, the electrical resistivity was measured to a depth defined by the spacing between the transmitting and receiving reels of the electromagnetic field induced in the environment (McNeil, 1985). Two configurations were used with a spacing of 20 m: the Horizontal Coil Parall (HCP) and Vertical Coil Parall (VCP) orientations with maximum sensitivities of around 20 m in HCP mode and between 0 and 10 m in VCP mode. The resistivity maps were drawn up by interpolating the resistivity values measured in a same mode. Two field programmes were carried out: the first in June 2006 over the entire slope between 1100 and 1700 m altitude (an area of 1400 m E–W by 1200 m N–S) in
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HCP mode; the second in June 2007 over the small valley around the Ballandaz river between 1100 and 1250 m altitude (an area of 400 m E–W by 200 m N–S) in both VCP and HCP mode. These experiments were scheduled just after the spring period, when the water saturation due to raining and snow melting is maximum; this situation favoring strong contrasts of resistivity within the slope. Profiles P17, P18 and P19 were each used for joint electrical and seismic acquisitions. Profile P17, respectively 1435 m and 1515 m long for the electrical and seismic acquisitions, begins slightly upstream of road D915, crosses the downstream part of the landslide, then rises to the upper parts of the slope at around level 1650 m. This profile should thus enable a more precise determination of the extent of the active part of the slope in the longitudinal direction. Profile P18, respectively 396 m and 288 m long for the electrical and seismic acquisitions, begins downstream of the road (10 m upstream of Doron-de-Pralognan), crosses the low part of the landslide, and reaches the level of the gypsum exposed in the north of the study area. The major interest of this profile was to determine whether gypsum is present below the landslide and at what depth, since geochemical measurements reveal an increase of sulphate concentration in springs located in this area. Profile P19, respectively 285 m and 235 m long for the electrical and seismic acquisitions, generally follows the road D915 in the lower part of the landslide and passes close to the drilled boreholes. The electrical data were obtained using multi-gradient (Dahlin and Zhou, 2006) and pole–dipole arrays for profiles P18 and P19, and only the multi-gradient array for Profile P17. The multi-gradient array is an asymmetrical array that is well suited for multi-receiver acquisition, an acquisition mode that enables to significantly increase the speed of data acquisition and, at the same time, to obtain a high density of data. The multi-gradient configuration offers the same resolution compromise—vertical and lateral—as a dipole–dipole configuration (Dahlin and Zhou, 2004), but with a better signal-to-noise ratio for the measurements (Dahlin and Zhou, 2006). Similarly, the pole–dipole array gives a greater depth of investigation and a better signal-to-noise ratio than the Wenner and dipole–dipole arrays. This array requires an electrode to be placed at “infinity”, i.e. a long way from the measurement point. A 96-electrode Syscal Pro 10 resistivity meter was used for the measurements along the three profiles, with an electrode spacing of 5 m (P17) and 3 m (P18 and P19). The data were inverted with RES2DINV software (Loke, 2006). Processing of the seismic data, from the pick of the first arrivals to the inversion step of corresponding traveltimes, was done with JaTS software (Grandjean and Sage, 2004; Sage et al., 2003). This software uses a Fresnel volumes approach to calculate the wave traveltimes and a SIRT (Simultaneous Iterative Reconstruction Technical) type reconstruction technique reformulated within a probabilistic formulation. With this procedure, the propagation is affected by the velocity variations within the Fresnel zone along the wave path. The traveltimes are calculated by numerical resolution of the Eikonal equation which governs the wave path in the medium. The seismic shooting was done with an acquisition array of 48 geophones at 5 m (Profile 17) and 2 m (Profiles 18 and 19) intervals. The shots were from every third geophone by detonating 100 grams of explosives, and the signals were recorded by 10 Hz geophones on a Geometrics Stratvizor type acquisition station. Spectral analysis of surface waves (SASW) is a method of processing seismic shootings that is often used for characterizing the subsurface from a geotechnical standpoint (Nazarian and Stokoe, 1984; Park et al., 1998, 2000). The analysis enables one to estimate shear wave (Vs) velocity variations with depth. The dispersion diagrams (Bitri et al., 1998), calculated at regular intervals along the acquisition profile, show how the seismic energy is distributed during the propagation according to the frequency and the phase velocity. By picking the amplitude maximum of each frequency, one defines the dispersion curve of the surface waves which can then be inverted. Considering the very heterogeneous medium composing the slope, local dispersion curves
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Fig. 3. EM34 electrical resitivity map, a) for the entire slope, b) for the conductive downstream area.
(Grandjean and Bitri, 2006) were calculated every 10 m, which corresponds to an average measurement every three receivers. Where the passive seismic measurements are concerned, two profiles (H/V1 and H/V2) were run with respectively 14 and 6 measurement points (Fig. 2a). These measurements benefited from a 200 Hz sampling frequency with a 15 min monitoring time. The data were processed with GEOPSY software (Wathelet et al., 2004) in considering 25 s windows selected according to an anti-trigger filter based on the STA/LTA (Short Time Average/Long Time Average) ratio.
3. Processing and interpreting the results In the following section, we respectively analyse the contribution of a) the EM34 mapping, b) the electrical and seismic tomography,
c) the SASW and d) the H/V method. The combined interpretation of the results derived from these various methods is then discussed. 3.1. EM34 resistivity mapping The EM34 resistivity map of the entire slope (Fig. 3a) reveals tendencies due to the slope's different geological contexts: i) resistant basement outcrops at Chambéranger, on the Notre Dame de la Salette spur and at Fenetre 11 (quartzite), and ii) drier environments made up of scree to the east of Pelapoët. The area between N.D. de la Salette and Chambéranger corresponds to a generally wetter ground where a mudflow occurred in 1988 downstream of Plan Bois. The upstream area of Vers le Pré is also known as being continuously very wet and many resurgences are found in the scree. On descending the Ballandaz river one finds a 200-m-wide more conductive band (approximately
Fig. 4. Electrical resistivity section along Profile P17 using the multi-gradient array (RMS = 12%).
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300 ohm.m) surrounded by highly resistive ground (N1000 ohm.m). The area farthest to the west has a resistivity of less than 100 ohm.m. In order to characterize this zone in more detail, a second EM34 campaign was run on the downside of the slope (Fig. 3b); this confirmed the existence of a clear boundary between the ground with a resistivity N300 ohm.m and that with a resistivity b150 ohm.m. It is this downstream part that is the most unstable. Although the shallow ground shows no lithological difference, the variation in resistivity between 10 and 30 m depth could be due either to a more clayey layer, or to a wetter horizon due to a high porosity, or to a change in pore-water conductivity. The electrical resistivity tomography (ERT), detailed in the following section, will enable one to determine the origin of this phenomenon along with its geometry. 3.2. Electrical resistivity tomography The electrical resistivity profiles (Figs. 4, 5 and 6) obtained from the Ballandaz site show a high degree of lateral heterogeneity in the electrical resistivity of the shallow layers. The profiles were run in morainic materials, identified by drilling as being an amalgam of sericite schist boulders in a sandy–clayey matrix. The infiltration of water through this channelized structure appears to be responsible for certain
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observed resistivity anomalies. In fact, the simple location of the electrodes on the ground (electrode positioned above a boulder as opposed to an electrode positioned between 2 boulders, for example) can generate strong resistivity contrasts. The electrical resistivity profiles nevertheless enable one to distinguish conductive zones at shallow depths of investigation. Indeed, the spatial variability of the resistivity is interpreted as a channelization of tills controlled by the presence of boulders and that drive the water flows within the landslide. As it can be observed in Fig.6a, these contrasts essentially appear in the central part of the profile, where the landslide is mainly developed. The RMS errors arising from the data inversion vary from 4% to 15% after 5 iterations, the convergence being assured for error variations of less than 1% between 2 iterations. The relatively high values of the RMS errors are probably due to surface lithology heterogeneities giving rise to strong variations of resistivity resulting from variable contact resistance between electrodes and ground (Ritz et al., 1999). Profile P17 reveals this high environmental heterogeneity at shallow depths of investigation, and this over a large part of the slope (Fig. 4). In the downstream part of the profile, near the 1550 m level, one notes a highly conductive bed under a resistive cover; this is located in the Vers le Pré area which is considered as very conductive (EM34) and interpreted as wet: the moisture increase would be due to the
Fig. 5. Electrical resistivity section along Profile P18 using the multi-gradient (RMS = 8%) and pole–dipole (RMS = 10%) arrays.
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Fig. 6. Electrical resistivity section along Profile P19 using the multi-gradient (RMS = 5%) and pole–dipole (RMS = 15%) arrays.
infiltration of runoff waters into the surface scree. In the western part of the profile (0b distance b 400 m), which is the part concerned by the landslide, one finds in particular the presence of three conductive zones which could reflect a higher moisture content. They could correspond to the active part of the landslide, stimulated by the presence of groundwater generating instabilities, as was indicated previously with the EM34 method. The results obtained for Profile P18 with the multi-gradient and pole–dipole arrays (Fig. 5) show the same overall tendencies. The results are particularly close for the very uppermost metres investigated. They enable one to locate conductive surfaces
possibly corresponding to relatively water-saturated zones. The gypsum exposed at the northeastern end of the profile appears to be highly resistive. The sections of Profile P19 measured with the multi-gradient and pole–dipole arrays (Fig. 6) also show the same tendencies, in particular for the shallow depths of investigation. At the level of borehole I2bis, inclinometric monitoring has revealed a sliding surface at around 13/14 m depth. Of note here is that the base of the conductive zone, well seen on Fig. 6, appears to correspond to the sliding surface revealed by the inclinometer. In theory, the great depth of investigation of the pole– dipole array enables one to study the deep stable part of the landslide
Fig. 7. Examples of adjustment for Profile P17 after 30 iterations.
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(shallow bedrock). For the rest of the study we refer only to the electrical profiles acquired with the pole–dipole array. 3.3. Seismic tomography For each of the presented results, the adjustments obtained between the measured and calculated traveltimes appear satisfactory apart from the short wavelength variations (Fig. 7. Because of the environment's complexity and the relative length of the profile, 30 iterations were necessary to adjust the traveltimes of P17, whereas 10 iterations were sufficient for the two other shorter profiles. The P-wave velocity profiles obtained after inversion clearly reveal a high velocity level (about 3000 m/s) whose depth varies along the profiles Figs. 8a, 9; dashed line). Although the northeastern end of Profile P18 reached the gypsum outcrop during the acquisition, the tomogram obtained after inversion (Fig. 9a) does not show a very high surface velocity at this location. This is explained by the fact that at the method's resolution at the profile edge is not as good as in the profile centre because of low seismic ray coverage. The southwestern end of the profile is just upstream of the road D915, at the level of the bridge spanning the Ballandaz River. This end of the profile shows a clear high-velocity anomaly which could be due to the presence of a shallow, relatively massive boulder. Considering the site geology, this could possibly be a boulder of gypsum or quartzite. At the level of the I2bis inclinometer, the upper boundary of the shallow bedrock, estimated at 14–15 m depth, would be located
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at the limit between the weak (blue) and strong (red) velocity domains, represented on the three seismic profiles, P17, P18 and P19 (Figs. 8 and 9b) by the black dashed line. The shallow bedrock would thus correspond to a rapid velocity level of N3000 m/s. For the subsurface layers, the seismic tomography tends rather to smooth the shallow velocity anomalies and thus seems fairly insensitive to the strong subsurface heterogeneities, unlike the electrical tomography.
3.4. SASW The spectral analysis of surface waves was tested on Profile P17 (Fig. 8b). The observed dispersion on each seismic shooting was inverted and the resulting vertical Vs soundings interpolated along the profile to obtain a 2D section. Due to the low signal to noise ratio observed on dispersion images of seismic shots, the results do not provide any significant information on the environment's structure. Such problems have several origins: (i) the summation of the signals used for calculating the dispersion curves does not take sufficient account of the medium's lateral heterogeneity; (ii) the picking of the dispersion curves is made difficult by the presence of higher modes that are difficult to invert properly; (iii) the inverse problem established within the used inversion code (Herrmann and Al-Eqabi, 1991) being limited to a 1D approach, the results obtained are not easily exploitable within the context of studying 2D, if not 3D, structures.
Fig. 8. Vp (a) and Vs (b) seismic section of Profile P17 obtained after 30 iterations for a likelyhood function of 75%.
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Fig. 9. Seismic section of Profile P18 (a) and P19 (b) obtained after 10 iterations for a likelyhood function of 85%.
3.5. H/V method The heterogeneity discussed in relation to the surface wave analysis of Profile 17 also has negative repercussions on the seismic noise analysis, which also again a lack of continuity marked by relatively low-frequency peaks of less than 1 Hz. These difficulties can be explained as resulting from the presence of wind during the measurements, as well as from the anticipated strong surface-wave diffraction due to the many heterogeneities, and also the marked 2D, even 3D, effects. Stable sections of the H/V response, however, do exist in the centre of Profile H/V1 and these can be interpreted. The central point (Fig. 10a) shows a clear resonance peak around 1.7 Hz with an average amplification of more than 7.5. This could accredit the hypothesis of a more contrasted interface being detected at this possibly less heterogeneous location. We could suppose here that such a resonance frequency corresponds to the typical depth of the deep geological substratum. To establish the depth H of the interfaces corresponding to resonance frequencies f, we used the well-known formula of Nakamura (1989) f = VS/4H, Vs being the average velocity of the shear waves. Taking the average velocity determined from the SASW analyses of Profile P17 (~400 m/s), we find an interface at about 60 m depth. Fig. 10b also shows the azimuthal dependence of this resonance peak, which is maximum at 120° in relation to the N–S direction (N10× amplification)
and minimum at azimuth 30° (b2× amplification). This reveals the strong directivity effect of the movement whose direction is along the same azimuth. One should note the absence of resonance frequency associated with the movement (expected at a few Hz), which is doubtless because of the lack of seismic contrast. The amplification noted around 35 Hz is very shallow (less than 2 m). Other than an interest in terms of reconnaissance, this strong amplification and its directivity would have dramatic consequences in the event of an earthquake considering that the amplification of the seismic waves within the upper layers (uppermost 60 m) is a potentially aggravating factor for zone's stability. 3.6. Combined interpretation The EM34 measurements enabled us to delimit the major lithohydrogeological units for optimal positioning of the electrical and seismic tomography profiles. The combined interpretation of the electrical resistivity and seismic velocity tomograms from these profiles helps improve the interpretation of the slope structures. Here we concentrated on profiles P18 and P19, taking account of the major north-south and east-west structures. With Profile P18 (Fig. 11), we find a fairly good correlation at the northeastern end of the profile between a fairly high level of
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Fig. 10. (a) Evolution of the amplitude of the H/V ratio with frequency for the middle part of Profile H/V1. The coloured lines are associated with the calculations of the spectral ratio for each asociated temporal window. The average amplitude and standard de.
resistivity and the shallow bedrock boundary determined from the seismic profile. The same type of correlation is seen at the intersection of profiles P18 and P19. In addition, 2 resistive anomalies occur at the level of the high-velocity anomaly noted in the seismic profile. These anomalies may thus confirm the presence of a shallow boulder in this part of the landslide. With Profile P19 (Fig. 12), we find a good correlation between the seismic and electrical sections both at the level of the I2bis inclinometer and at the intersection of profiles P18 and P19. In each of these zones, the shallow bedrock boundary seems to correspond to the base of a conductive zone. The shallow bedrock, as defined in our study, is situated in same material but corresponds to a threshold where compaction has led to an abrupt decrease in porosity, with a resultant reduction in conductivity and increase in seismic velocity. In general, the strong lithological contrasts identified in the shallow zones show up very clearly on the electrical and electromagnetic images, but are not located by the seismics. This lack of resolution is due to seismic signal's wavelength being too great in relation to the size of the local subsurface lithological heterogeneities. The seismic measurements nevertheless enable reconstruction of the highvelocity level marking the shallow bedrock: with the seismic wave paths concentrating preferentially in the high velocity zones, this reconstruction is facilitated by the information redundancy. This level, which is deeper and more resistive than the weathered surface zone, is scarcely hardly picked up by electrical tomography because of the lower sensitivity generated during the inversion process. So as to be able to geometrically identify the active zone of the landslide, we undertook 3D modelling (Fig. 13a and b) that integrated the information derived from each of the geophysical techniques. The
data provided by inclinometers I2bis (sliding surface at 14 m depth), I3 (sliding surface at 15 m depth) and I4 (sliding surface at 23 m depth) were also taken into account, followed by a kriging interpolation was so as to model the surface's geometry. This surface, represented in gridded and coloured form, thus corresponds to the upper boundary of the shallow bedrock within the entire slope area. The black dotted surface represents the area where this surface has been identified as being the major sliding surface. Integration of geophysical information in terms of geological ones is proposed for the profile P18 (Fig. 14). This synthesis shows that the slope is mainly composed by morainic filling materials of around 40 to 60 m thick in its western part lying on a geological substratum of triasic quarzites, shists with some gypsum levels. Inside the moraines, a low porosity compacted layer of around 10 to 20 m thick is identified as the shallow bedrock. This layer is generally stable. Just above, the same material, but highly weathered and channelized, compose the subsurface layer. This layer is subject to many water flows that take place around the numerous boulders. The Ballandaz landslide is located in the western part of the slope, where the water flows accumulate a high degree of moisture and the possible presence of gypsum levels that could favor the slope instability. 4. Conclusion The geophysical sections obtained at the Ballandaz site show a high degree of heterogeneity in the spatial distribution of the inverted parameters, particularly where the electrical resistivity is concerned. This property comes manifestly from the composition of material
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Fig. 11. Combined interpretation of Profile P18; a) electrical profile (pole–dipole); b) seismic profile.
Fig. 12. Combined interpretation of Profile P19; a) electrical profile (pole–dipole); b) seismic profile.
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Fig. 13. Modelling the sliding surface from seismic (a) and electrical (b) profiles. Inclinometer measurements are also indicated as welle as 3D representation of seismic (a) and electrical (b) profiles P18 and P19.
making up the landslide, identified by drilling as being an amalgam of scree and sericite schist boulders caught up in a sandy–clayey matrix. The infiltration of water through this structure appears to be responsible for the observed resistivity anomalies, as far as this could be confirmed by the EM34 method, in particular in the uphill sector of the slope where many water sinks are visible. The electrical profiles enable one to distinguish conductive zones within the shallow depths of investigation. It is also of note that the base of the conductive zone visible at the level of inclinometer I2bis in Profile P19, actually appears to correspond to the sliding surface revealed by the inclinometric monitoring. The seismics, which are less sensitive to the high subsurface heterogeneity, tend rather to smooth the shallow velocity anomalies. On the other hand, the seismic tomograms enable one to clearly assess the depth to the shallow bedrock, whose upper boundary corresponds to sliding surfaces noted at from the boreholes. Profile P18 also reveals the presence of a highvelocity anomaly within the active part of the landslide, just upstream of road D915. It could possibly represent a quartzite or gypsum
boulder; the latter would well explain the instability in this part of the landslide. As for the spectral analysis of surface waves, this was not exploitable because of the high heterogeneity of the landslide material. The relative performance of each method at this site indicates that the electrical imagery gives a better identification of the shallow heterogeneities, whilst the seismic imagery is more effective in identifying the shallow bedrock boundary. These results are certainly due to the strong resistivity contrasts of the shallow heterogeneities which are more easily measured by electrical methods, but which are too small for the seismic signal wavelength. Conversely, reconstruction of the high-velocity level forming the shallow bedrock is possible with seismic methods, whereas the sensitivity of the electrical signal at these depths is too low to ensure reconstruction. Lastly, the methods based on surface waves (SASW, seismic background noise) proved to be disappointing in such a heterogeneous environment for detecting the thickness of the deformed zone, although the seismic background noise was nevertheless able to provide constraints
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Fig. 14. Synthetic interpretation of profile 18. 1: weathered morainic materials including water drains and boulders; 2: shallow bedrock; 3: geological substratum (quarzite).
on the depth to the deep bedrock and the expected amplification of the waves in the event of an earthquake. Acknowledgements This work was financed by BRGM and the ANR/PGCU-SIGMA project. The authors would particularly like to thank the Savoie Conseil général (Departmental Council), which encouraged the work on the site, and all the project partners who contributed to these results. References Binet, S., Spadini, L., Bertrand, C., Guglielmi, Y., Mudry, J., Scavia, C., 2009. Variability of the groundwater sulfate concentration in fractured rock slopes: a tool to identify active unstable areas. Hydrology and Earth System Sciences Discussions 6, 5415–5444. Bitri, A., LeBégat, S., Baltassat, J.M., 1998. Shear wave velocity determination of soils from in-situ Rayleigh wave measurements. Proc. 4th Meeting, EEGS, Barcelona, Spain, pp. 503–506. Bruno, F., Martillier, F., 2000. Test of high-resolution seismic refection and other geophysical techniques on the Boup landslide in the Swiss Alps. Surveys in Geophysics, 21, pp. 335–350. 4. Cummings, D., 2000. Transient electromagnetic survey of a landslide and fault, Santa Susanna Mountains, Southern California. Environmental and Engineering Geoscience 6 (3), 247–254. Dahlin, T., Zhou, B., 2004. A numerical comparison of 2D resistivity imaging using ten electrode arrays. Geophysical Prospecting 52, 379–398. Dahlin, T., Zhou, B., 2006. Multiple gradient array measurements for multi-channel 2D resistivity imaging. Near Surface Geophysics 4, 13–123. Grandjean, G., Bitri, A., 2006. 2M-SASW: inversion of local Rayleigh waves dispersion in laterally heterogeneous subsurface. Application on a landslide. Near Surface Geophysics 4 (6), 367–375. Grandjean, G., Sage, S., 2004. JaTS: a fully portable seismic tomography software based on Fresnel wavepaths and a probabilistic reconstruction approach. Computers and Geosciences 30, 925–935. Grandjean, G., Malet, J.-P., Bitri, A., Meric, O., 2005. Geophysical data fusion by fuzzy logic for imaging earthflow mechanical behaviour. EAGE near Surface, Palermo, Italy. Guéguen, P., Cornou, C., Garambois, S., Banton, J., 2007. On the limitation of the H/V spectral ratio using seismic noise as an exploration tool: application to the Grenoble valley (France), a small apex ratio basin. PAGEOPH 164 (1), 115–134. Hack, R., 2000. Geophysics for slope stability. Surveys in Geophysics, 21. Kluwer Academic Ed, pp. 423–448. Herrmann, R.B., Al-Eqabi, G., 1991. Surface waves: inversion for shear wave velocity. In: Hovem, et al. (Ed.), Shear Waves in Marine Sediments. Kluwer, Dordrecht, pp. 545–556. Hovem et al (Eds.). Jaboyedoff, M., Bardou, E., Baillifard, F., 2003. Incipient weathering and crushing as a potentially important mechanical effect for landslide behaviour. Geophysical Research Abstracts 5, 03449 EGS-EUG-AGU joint assembly Nice 2003..
Jongmans, D., Garambois, S., 2007. Geophysical investigation of landslides: a review. Bulletin de la Societe Geologique de France 178 (2), 101–112. Keefer, D.K., 1984. Landslides caused by earthquakes. Bulletin of the Geological Society of America 95 (4), 406–421. Konno, K., Ohmachi, T., 1998. Ground-motion characteristics estimated from spectral ratio between horizontal and vertical components of microtremor. Bulletin of the Seismological Society of America 88 (1), 228–241. Loke, M.H., 2006. RES2DINV ver. 3.55, Rapid 2D resistivity & IP inversion using the least-squares method. Software Manual. 139pp. Malet, J.-P., Laigle, D., Remaître, A., Maquaire, O., 2005. Triggering conditions and mobility of debris flows associated to complex earthflows. Geomorphology 66 (1–4), 215–235. McNeil, J.D., 1985. Technical Note TN-6, Electromagnetic Terrain Conductivity Measurement at Low Induction Numbers: Technical Note TN-6. Geonics Ltd., Mississauga, Ontario, Canada. Meric, O., Garambois, S., Jongmans, D., Vengeon, J.-M., Chatelain, J.-L., 2005. Application of geophysical methods for the investigation of the large gravitational mass movement of Séchilienne (France). Canadian Geotechnical Journal 42, 1105–1115. Meric, O., Garambois, S., Malet, J.-P., Cadet, H., Guéguen, P., Jongmans, D., 2007. Seismicnoise based methods for soft-rock landslides characterization. Bulletin de la Societe Geologique de France 2, 137–148. Nakamura, Y., 1989. A method for dynamic characteristics estimation of subsurface using microtremor on ground surface. Quarterly Report of Railway Technical Research Institute 30, 25–33. Nazarian, S., Stokoe II, K.H., 1984. In situ shear wave velocities from spectral analysis of surface waves. Proc. 8th Conf. on Earthquake Eng, 3, pp. 31–38. San Francisco. Nogoshi, M., Igarashi, T., 1971. On the amplitude characteristics of microtremor (Part 2). Journal of Seismological Society of Japan 24, 26–40. Park, C.B., Miller, R.D., Xia, J., 1998. Imaging Dispersion Curves of Surface Waves on Multi-Channel Record. Soc. Exploration Geophys, New Orleans, LA, USA. Park, C.B., Miller, R.D., Xia, J., Ivanov, J., 2000. Multichannel seismic surface-wave methods for geotechnical applications. Proceedings of the First International Conference on the Application of Geophysical Methodologies to Transportation Facilities and Infrastructure, St. Louis, December, pp. 11–15. Polemio, M., Trizzino, R., 1999. Hydrogeological, kinematic, and stability characterisation of the 1993 Senerchia landslide (Southern Italy). Landslides News, Prevention Research Institute, Kyoto, Japan 12, 12–16. Ritz, M., Robain, H., Pervago, E., Albouy, Y., Camerlynck, C., Descloitres, M., Mariko, A., 1999. Improvement to resistivisty pseudosection modelling by removal of nearsurface inhomogeneity effects: application to a soil system in South Cameroon. Geophysical Prospecting 47, 85–101. Sage, S., Grandjean, G., Verly, J., 2003. Seismic traveltime tomography using Fresnel volumes and a Fast Marching Eikonal solver. Proc. 9th EEGS, Prague, Czech Republic. Schmidt, M., Glade, T., 2003. Modelling climate change impacts for landslide activity: case studies from New Zealand. Climate Research 25, 135–150. Wathelet, M., Jongmans, D., Ohrnberger, M., 2004. Surface wave inversion using a direct search algorithm and its application to ambient vibration measurements. Near Surface Geophysics 2, 221–231.
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Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Structural study of the Ballandaz landslide (French Alps) using geophysical imagery G. Grandjean a,⁎, J.C. Gourry a, O. Sanchez a, A. Bitri a, S. Garambois b a b
BRGM, Orléans, France LGIT, Grenoble, France
a r t i c l e
i n f o
Article history: Received 16 April 2010 Accepted 11 July 2011 Available online 24 August 2011 Keywords: Seismics ERT Landslide French Alps
a b s t r a c t This study of the Ballandaz landslide (Savoie, French Alps) was carried out as part of the ANR/PGCU-SIGMA research project. Its aim was to characterize the slope by different methods of geophysical imagery, and then use this characterization to provide a combined interpretation of the morpho-structures in order to simulate the geomechanical behavior of the sliding mass. First, electromagnetic mapping was used to identify the variations of the surface lithology and moisture from which one can more precisely locate the active zone of the slope. Then, electrical and seismic 2D imagery methods were used along several transverse and longitudinal profiles in order to produce electrical resistivity and seismic-velocity depth sections showing up the slope's structures. The H/V method was also tested locally to determine the depth to basement so as to complement the profiles: it reveals potential seismic site effects from the deeper structures. Helped by drilling and inclinometer surveys, the geophysical interpretations have revealed the various units structuring the landslide: (i) the active, very heterogeneous shallow level overlying (ii) a more rigid, less porous, and probably stable shallow bedrock, (iii) boulders, and (iv) the sound geological basement of quartzite and gypsiferous facies. The study has shown the usefulness of combining such different sounding techniques for studying complex environments like landslides; it has also revealed the limitations of each method when used for studying very heterogeneous environments. © 2011 Elsevier B.V. All rights reserved.
1. Introduction To understand the dynamics of a landslide, it is essential (i) to determine the relevant internal structural units (Bruno and Martillier, 2000; Hack, 2000; Meric et al., 2005), (ii) to characterize the major hydrological processes (Malet et al., 2005) and (iii) to make an inventory of the external conditions, mainly of a climatic (Jaboyedoff et al., 2003; Polemio and Trizzino, 1999; Schmidt and Glade, 2003) or seismic (Keefer, 1984) nature. Geophysical methods, like seismic and electrical imagery, are well suited for carrying out the structural study of landslides and understanding their internal mechanisms. In particular, seismic and electrical imagery give a direct and non-intrusive measurement of acoustic-wave (P) or shear-wave (S) velocities and of electrical resistivity, three parameters considered as essential for defining the properties of the remobilized geological material (Grandjean et al., 2005; Jongmans and Garambois, 2007). The H/V method, based on passive seismics (Konno and Ohmachi, 1998; Nakamura, 1989; Nogoshi and Igarashi, 1971) can also be used to identify the resonance frequencies of the sliding mass in order to determine the thickness of the unconsolidated layer overlying the rigid basement (Meric et al.,
⁎ Corresponding author. E-mail address: [email protected] (G. Grandjean). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.07.008
2007) and the potential amplification effects of seismic waves (Guéguen et al., 2007). Finally, electromagnetic (EM) mapping is particularly effective for revealing spatial variations in the electrical resistivity, and thus the lithology—and/or water content—at the surface of the massif (Cummings, 2000). The present study concerns the Ballandaz landslide (Savoie, French Alps) and was carried out as part of the ANR/PGCU-SIGMA research project (monitoring the kinematic regimes of slow recurrent landslides in relation to climatic change). One of the project aims was to test various geophysical methods, including seismic and electrical imagery, passive seismics and EM mapping, in order to characterize the sliding mass, i.e., to obtain a combined interpretation of the landslide's morpho-structures, that is intended to be used in future geomechanical models. We present here the results of this study, from the geophysical data acquisition to the combined interpretation of the massif. The Ballandaz landslide (Fig. 1), known since the beginning of the 20th Century, lies at the edge of the Vanoise Massif, near Pralognan. It is located in a small narrow glacial valley, 300 m wide, bounded to the north by the schistose Tour du Merle cliff and to the south by the quartzite spur of Notre Dame de Salette at the base of which flows the Ballandaz River. It lies between altitudes 1050 m (Doron-de-Pralognon River) and 1650 m, at which level one still finds signs of surface movement in the houses (hamlet of Mollard) and civil engineering structures
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Fig. 1. Location of the profiles (white solid lines) . The Ballandaz landslide is contoured with white dashed line.
(Fenetre 11: EDF inspection tunnel located on the Pélapoët road). The average slope is 40%, with intervals in excess of 60%. The downstream part of the slope between the 1050 and 1250 m levels is most active; it threatens the departmental road D915 and an EDF pressure pipeline located along the road, and risks damming the Doron-de-Pralognan River in a section where the river is very boxed in. The Ballandaz slope is located in a complex geological zone (Fig. 2a) characterized
by a tectonic contact on a quartzitic and gypseous sole. This geological pattern is found on many of the valley slopes, which explains the many landslides in the area. The stable bedrock of the studied slope consists of sericite schist, quartzite, cargneule, schist, and gypsum in the low part of the slope. This bedrock is covered by a heterogeneous cover of basement debris and also of morainic material of a priori unknown thickness. Although the Ballandaz landslide is downstream of
Fig. 2. a: Geological map of the studied area (1: stephanian sandstones and schists, 2: permian schists, 3: triasic quartzites, 4: triasic gypsum, 5: quaternary moraines, 6: scree, 7: faults; b: interpreted SC1/I2b borehole.
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a large exposure of gypsum, this has never been clearly seen within the landslide. Another exposure of gypsum occurs to the southwest of the landslide, on the opposite slope. The landslide was already active 100 years ago according to the archives which record that drainage work had been carried at Ballandaz River level. Following this work, no event was recorded on the slope for several decades. In 1993, however, the landslide seems to become alive again; signs of movement were once again observed. Then, in 1996, movement of the landslide resulted in flows that reached Doron-de-Pralognan and the road D915 with, in 2000, an abrupt acceleration of surface movement that led to road subsidence over a distance of 100 m. Major stabilization work was undertaken with the construction of a micro-Berlin wall founded on 12-m-deep piles and anchored to the ground by 15 m subhorizontal tie rods. In spite of this structure, however, the site inclinometer survey that was begun in 2003 has shown that two sliding planes are still active, one at 8 m and the other at 14 m depth, i.e. below the base of the micropiles. This phenomenon is gradually undercutting the heads the micropiles and threatens the long-term survival of the wall. Many geotechnical, hydrogeological and geophysical studies have been carried out on the Ballandaz site since 1996. Drainage and reinforcement work has been carried out. A dozen boreholes have been drilled in the downstream part of the landslide where the risk is highest. The installation of piezometers and inclinometer tubes in some of the boreholes enables monthly monitoring of the site. The slope's hydrogeology is complex: the moisture balance is controlled by the snowmelt in April–June, but one finds sinks and resurgences within the scree and moraines on the upper part of the slope above 1400 m. The groundwater mineralization increases gradually from upstream to downstream as already observed by Binet et al. (2009) on several alpine landslides, although one notes an abrupt increase in conductivity and sulphate content downstream of road D915, probably because of the influence of underlying gypsum formations. Such formations were moreover found in the drilling cuttings of the SC1 cored borehole (Fig. 2b). This borehole, located at the end of the profile P17, allowed us to confirm that the nature of the sedimentary overlap to a depth of more than 45 m is a heterogeneous amalgam of schist, mica schist and quartzite boulders caught up in a sandy–clayey matrix; within this overlap one can distinguish a more weathered surface layer, up to 20 m deep, overlying a more rigid and less permeable layer which is considered here as shallow bedrock. The inclinometer measurements carried out close to road D915 reveal two sliding planes at about 8 and 15 m depth. The internal structure of the landslide is thus still poorly known, particularly concerning the depth to the basement supporting the sedimentary overlap, the spatial delimitation of the active zone and its geometry at slope scale, and the role of the water flows in the onset of the active periods. So, we deployed our geophysical experiments on profiles P17, P18 and P19 (Fig. 2a), particularly well suited to sound the central parts of the slope and to show in what way geophysical methods can provide information about the landslide's structure or the presence of water-saturated zones which can play a key role in slope activity. 2. Aquisitions and geophysical methods To begin with, the entire slope was characterized so as to determine the major lithological and hydrogeological units. Using the electromagnetic technique EM34, the electrical resistivity was measured to a depth defined by the spacing between the transmitting and receiving reels of the electromagnetic field induced in the environment (McNeil, 1985). Two configurations were used with a spacing of 20 m: the Horizontal Coil Parall (HCP) and Vertical Coil Parall (VCP) orientations with maximum sensitivities of around 20 m in HCP mode and between 0 and 10 m in VCP mode. The resistivity maps were drawn up by interpolating the resistivity values measured in a same mode. Two field programmes were carried out: the first in June 2006 over the entire slope between 1100 and 1700 m altitude (an area of 1400 m E–W by 1200 m N–S) in
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HCP mode; the second in June 2007 over the small valley around the Ballandaz river between 1100 and 1250 m altitude (an area of 400 m E–W by 200 m N–S) in both VCP and HCP mode. These experiments were scheduled just after the spring period, when the water saturation due to raining and snow melting is maximum; this situation favoring strong contrasts of resistivity within the slope. Profiles P17, P18 and P19 were each used for joint electrical and seismic acquisitions. Profile P17, respectively 1435 m and 1515 m long for the electrical and seismic acquisitions, begins slightly upstream of road D915, crosses the downstream part of the landslide, then rises to the upper parts of the slope at around level 1650 m. This profile should thus enable a more precise determination of the extent of the active part of the slope in the longitudinal direction. Profile P18, respectively 396 m and 288 m long for the electrical and seismic acquisitions, begins downstream of the road (10 m upstream of Doron-de-Pralognan), crosses the low part of the landslide, and reaches the level of the gypsum exposed in the north of the study area. The major interest of this profile was to determine whether gypsum is present below the landslide and at what depth, since geochemical measurements reveal an increase of sulphate concentration in springs located in this area. Profile P19, respectively 285 m and 235 m long for the electrical and seismic acquisitions, generally follows the road D915 in the lower part of the landslide and passes close to the drilled boreholes. The electrical data were obtained using multi-gradient (Dahlin and Zhou, 2006) and pole–dipole arrays for profiles P18 and P19, and only the multi-gradient array for Profile P17. The multi-gradient array is an asymmetrical array that is well suited for multi-receiver acquisition, an acquisition mode that enables to significantly increase the speed of data acquisition and, at the same time, to obtain a high density of data. The multi-gradient configuration offers the same resolution compromise—vertical and lateral—as a dipole–dipole configuration (Dahlin and Zhou, 2004), but with a better signal-to-noise ratio for the measurements (Dahlin and Zhou, 2006). Similarly, the pole–dipole array gives a greater depth of investigation and a better signal-to-noise ratio than the Wenner and dipole–dipole arrays. This array requires an electrode to be placed at “infinity”, i.e. a long way from the measurement point. A 96-electrode Syscal Pro 10 resistivity meter was used for the measurements along the three profiles, with an electrode spacing of 5 m (P17) and 3 m (P18 and P19). The data were inverted with RES2DINV software (Loke, 2006). Processing of the seismic data, from the pick of the first arrivals to the inversion step of corresponding traveltimes, was done with JaTS software (Grandjean and Sage, 2004; Sage et al., 2003). This software uses a Fresnel volumes approach to calculate the wave traveltimes and a SIRT (Simultaneous Iterative Reconstruction Technical) type reconstruction technique reformulated within a probabilistic formulation. With this procedure, the propagation is affected by the velocity variations within the Fresnel zone along the wave path. The traveltimes are calculated by numerical resolution of the Eikonal equation which governs the wave path in the medium. The seismic shooting was done with an acquisition array of 48 geophones at 5 m (Profile 17) and 2 m (Profiles 18 and 19) intervals. The shots were from every third geophone by detonating 100 grams of explosives, and the signals were recorded by 10 Hz geophones on a Geometrics Stratvizor type acquisition station. Spectral analysis of surface waves (SASW) is a method of processing seismic shootings that is often used for characterizing the subsurface from a geotechnical standpoint (Nazarian and Stokoe, 1984; Park et al., 1998, 2000). The analysis enables one to estimate shear wave (Vs) velocity variations with depth. The dispersion diagrams (Bitri et al., 1998), calculated at regular intervals along the acquisition profile, show how the seismic energy is distributed during the propagation according to the frequency and the phase velocity. By picking the amplitude maximum of each frequency, one defines the dispersion curve of the surface waves which can then be inverted. Considering the very heterogeneous medium composing the slope, local dispersion curves
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Fig. 3. EM34 electrical resitivity map, a) for the entire slope, b) for the conductive downstream area.
(Grandjean and Bitri, 2006) were calculated every 10 m, which corresponds to an average measurement every three receivers. Where the passive seismic measurements are concerned, two profiles (H/V1 and H/V2) were run with respectively 14 and 6 measurement points (Fig. 2a). These measurements benefited from a 200 Hz sampling frequency with a 15 min monitoring time. The data were processed with GEOPSY software (Wathelet et al., 2004) in considering 25 s windows selected according to an anti-trigger filter based on the STA/LTA (Short Time Average/Long Time Average) ratio.
3. Processing and interpreting the results In the following section, we respectively analyse the contribution of a) the EM34 mapping, b) the electrical and seismic tomography,
c) the SASW and d) the H/V method. The combined interpretation of the results derived from these various methods is then discussed. 3.1. EM34 resistivity mapping The EM34 resistivity map of the entire slope (Fig. 3a) reveals tendencies due to the slope's different geological contexts: i) resistant basement outcrops at Chambéranger, on the Notre Dame de la Salette spur and at Fenetre 11 (quartzite), and ii) drier environments made up of scree to the east of Pelapoët. The area between N.D. de la Salette and Chambéranger corresponds to a generally wetter ground where a mudflow occurred in 1988 downstream of Plan Bois. The upstream area of Vers le Pré is also known as being continuously very wet and many resurgences are found in the scree. On descending the Ballandaz river one finds a 200-m-wide more conductive band (approximately
Fig. 4. Electrical resistivity section along Profile P17 using the multi-gradient array (RMS = 12%).
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300 ohm.m) surrounded by highly resistive ground (N1000 ohm.m). The area farthest to the west has a resistivity of less than 100 ohm.m. In order to characterize this zone in more detail, a second EM34 campaign was run on the downside of the slope (Fig. 3b); this confirmed the existence of a clear boundary between the ground with a resistivity N300 ohm.m and that with a resistivity b150 ohm.m. It is this downstream part that is the most unstable. Although the shallow ground shows no lithological difference, the variation in resistivity between 10 and 30 m depth could be due either to a more clayey layer, or to a wetter horizon due to a high porosity, or to a change in pore-water conductivity. The electrical resistivity tomography (ERT), detailed in the following section, will enable one to determine the origin of this phenomenon along with its geometry. 3.2. Electrical resistivity tomography The electrical resistivity profiles (Figs. 4, 5 and 6) obtained from the Ballandaz site show a high degree of lateral heterogeneity in the electrical resistivity of the shallow layers. The profiles were run in morainic materials, identified by drilling as being an amalgam of sericite schist boulders in a sandy–clayey matrix. The infiltration of water through this channelized structure appears to be responsible for certain
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observed resistivity anomalies. In fact, the simple location of the electrodes on the ground (electrode positioned above a boulder as opposed to an electrode positioned between 2 boulders, for example) can generate strong resistivity contrasts. The electrical resistivity profiles nevertheless enable one to distinguish conductive zones at shallow depths of investigation. Indeed, the spatial variability of the resistivity is interpreted as a channelization of tills controlled by the presence of boulders and that drive the water flows within the landslide. As it can be observed in Fig.6a, these contrasts essentially appear in the central part of the profile, where the landslide is mainly developed. The RMS errors arising from the data inversion vary from 4% to 15% after 5 iterations, the convergence being assured for error variations of less than 1% between 2 iterations. The relatively high values of the RMS errors are probably due to surface lithology heterogeneities giving rise to strong variations of resistivity resulting from variable contact resistance between electrodes and ground (Ritz et al., 1999). Profile P17 reveals this high environmental heterogeneity at shallow depths of investigation, and this over a large part of the slope (Fig. 4). In the downstream part of the profile, near the 1550 m level, one notes a highly conductive bed under a resistive cover; this is located in the Vers le Pré area which is considered as very conductive (EM34) and interpreted as wet: the moisture increase would be due to the
Fig. 5. Electrical resistivity section along Profile P18 using the multi-gradient (RMS = 8%) and pole–dipole (RMS = 10%) arrays.
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Fig. 6. Electrical resistivity section along Profile P19 using the multi-gradient (RMS = 5%) and pole–dipole (RMS = 15%) arrays.
infiltration of runoff waters into the surface scree. In the western part of the profile (0b distance b 400 m), which is the part concerned by the landslide, one finds in particular the presence of three conductive zones which could reflect a higher moisture content. They could correspond to the active part of the landslide, stimulated by the presence of groundwater generating instabilities, as was indicated previously with the EM34 method. The results obtained for Profile P18 with the multi-gradient and pole–dipole arrays (Fig. 5) show the same overall tendencies. The results are particularly close for the very uppermost metres investigated. They enable one to locate conductive surfaces
possibly corresponding to relatively water-saturated zones. The gypsum exposed at the northeastern end of the profile appears to be highly resistive. The sections of Profile P19 measured with the multi-gradient and pole–dipole arrays (Fig. 6) also show the same tendencies, in particular for the shallow depths of investigation. At the level of borehole I2bis, inclinometric monitoring has revealed a sliding surface at around 13/14 m depth. Of note here is that the base of the conductive zone, well seen on Fig. 6, appears to correspond to the sliding surface revealed by the inclinometer. In theory, the great depth of investigation of the pole– dipole array enables one to study the deep stable part of the landslide
Fig. 7. Examples of adjustment for Profile P17 after 30 iterations.
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(shallow bedrock). For the rest of the study we refer only to the electrical profiles acquired with the pole–dipole array. 3.3. Seismic tomography For each of the presented results, the adjustments obtained between the measured and calculated traveltimes appear satisfactory apart from the short wavelength variations (Fig. 7. Because of the environment's complexity and the relative length of the profile, 30 iterations were necessary to adjust the traveltimes of P17, whereas 10 iterations were sufficient for the two other shorter profiles. The P-wave velocity profiles obtained after inversion clearly reveal a high velocity level (about 3000 m/s) whose depth varies along the profiles Figs. 8a, 9; dashed line). Although the northeastern end of Profile P18 reached the gypsum outcrop during the acquisition, the tomogram obtained after inversion (Fig. 9a) does not show a very high surface velocity at this location. This is explained by the fact that at the method's resolution at the profile edge is not as good as in the profile centre because of low seismic ray coverage. The southwestern end of the profile is just upstream of the road D915, at the level of the bridge spanning the Ballandaz River. This end of the profile shows a clear high-velocity anomaly which could be due to the presence of a shallow, relatively massive boulder. Considering the site geology, this could possibly be a boulder of gypsum or quartzite. At the level of the I2bis inclinometer, the upper boundary of the shallow bedrock, estimated at 14–15 m depth, would be located
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at the limit between the weak (blue) and strong (red) velocity domains, represented on the three seismic profiles, P17, P18 and P19 (Figs. 8 and 9b) by the black dashed line. The shallow bedrock would thus correspond to a rapid velocity level of N3000 m/s. For the subsurface layers, the seismic tomography tends rather to smooth the shallow velocity anomalies and thus seems fairly insensitive to the strong subsurface heterogeneities, unlike the electrical tomography.
3.4. SASW The spectral analysis of surface waves was tested on Profile P17 (Fig. 8b). The observed dispersion on each seismic shooting was inverted and the resulting vertical Vs soundings interpolated along the profile to obtain a 2D section. Due to the low signal to noise ratio observed on dispersion images of seismic shots, the results do not provide any significant information on the environment's structure. Such problems have several origins: (i) the summation of the signals used for calculating the dispersion curves does not take sufficient account of the medium's lateral heterogeneity; (ii) the picking of the dispersion curves is made difficult by the presence of higher modes that are difficult to invert properly; (iii) the inverse problem established within the used inversion code (Herrmann and Al-Eqabi, 1991) being limited to a 1D approach, the results obtained are not easily exploitable within the context of studying 2D, if not 3D, structures.
Fig. 8. Vp (a) and Vs (b) seismic section of Profile P17 obtained after 30 iterations for a likelyhood function of 75%.
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Fig. 9. Seismic section of Profile P18 (a) and P19 (b) obtained after 10 iterations for a likelyhood function of 85%.
3.5. H/V method The heterogeneity discussed in relation to the surface wave analysis of Profile 17 also has negative repercussions on the seismic noise analysis, which also again a lack of continuity marked by relatively low-frequency peaks of less than 1 Hz. These difficulties can be explained as resulting from the presence of wind during the measurements, as well as from the anticipated strong surface-wave diffraction due to the many heterogeneities, and also the marked 2D, even 3D, effects. Stable sections of the H/V response, however, do exist in the centre of Profile H/V1 and these can be interpreted. The central point (Fig. 10a) shows a clear resonance peak around 1.7 Hz with an average amplification of more than 7.5. This could accredit the hypothesis of a more contrasted interface being detected at this possibly less heterogeneous location. We could suppose here that such a resonance frequency corresponds to the typical depth of the deep geological substratum. To establish the depth H of the interfaces corresponding to resonance frequencies f, we used the well-known formula of Nakamura (1989) f = VS/4H, Vs being the average velocity of the shear waves. Taking the average velocity determined from the SASW analyses of Profile P17 (~400 m/s), we find an interface at about 60 m depth. Fig. 10b also shows the azimuthal dependence of this resonance peak, which is maximum at 120° in relation to the N–S direction (N10× amplification)
and minimum at azimuth 30° (b2× amplification). This reveals the strong directivity effect of the movement whose direction is along the same azimuth. One should note the absence of resonance frequency associated with the movement (expected at a few Hz), which is doubtless because of the lack of seismic contrast. The amplification noted around 35 Hz is very shallow (less than 2 m). Other than an interest in terms of reconnaissance, this strong amplification and its directivity would have dramatic consequences in the event of an earthquake considering that the amplification of the seismic waves within the upper layers (uppermost 60 m) is a potentially aggravating factor for zone's stability. 3.6. Combined interpretation The EM34 measurements enabled us to delimit the major lithohydrogeological units for optimal positioning of the electrical and seismic tomography profiles. The combined interpretation of the electrical resistivity and seismic velocity tomograms from these profiles helps improve the interpretation of the slope structures. Here we concentrated on profiles P18 and P19, taking account of the major north-south and east-west structures. With Profile P18 (Fig. 11), we find a fairly good correlation at the northeastern end of the profile between a fairly high level of
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Fig. 10. (a) Evolution of the amplitude of the H/V ratio with frequency for the middle part of Profile H/V1. The coloured lines are associated with the calculations of the spectral ratio for each asociated temporal window. The average amplitude and standard de.
resistivity and the shallow bedrock boundary determined from the seismic profile. The same type of correlation is seen at the intersection of profiles P18 and P19. In addition, 2 resistive anomalies occur at the level of the high-velocity anomaly noted in the seismic profile. These anomalies may thus confirm the presence of a shallow boulder in this part of the landslide. With Profile P19 (Fig. 12), we find a good correlation between the seismic and electrical sections both at the level of the I2bis inclinometer and at the intersection of profiles P18 and P19. In each of these zones, the shallow bedrock boundary seems to correspond to the base of a conductive zone. The shallow bedrock, as defined in our study, is situated in same material but corresponds to a threshold where compaction has led to an abrupt decrease in porosity, with a resultant reduction in conductivity and increase in seismic velocity. In general, the strong lithological contrasts identified in the shallow zones show up very clearly on the electrical and electromagnetic images, but are not located by the seismics. This lack of resolution is due to seismic signal's wavelength being too great in relation to the size of the local subsurface lithological heterogeneities. The seismic measurements nevertheless enable reconstruction of the highvelocity level marking the shallow bedrock: with the seismic wave paths concentrating preferentially in the high velocity zones, this reconstruction is facilitated by the information redundancy. This level, which is deeper and more resistive than the weathered surface zone, is scarcely hardly picked up by electrical tomography because of the lower sensitivity generated during the inversion process. So as to be able to geometrically identify the active zone of the landslide, we undertook 3D modelling (Fig. 13a and b) that integrated the information derived from each of the geophysical techniques. The
data provided by inclinometers I2bis (sliding surface at 14 m depth), I3 (sliding surface at 15 m depth) and I4 (sliding surface at 23 m depth) were also taken into account, followed by a kriging interpolation was so as to model the surface's geometry. This surface, represented in gridded and coloured form, thus corresponds to the upper boundary of the shallow bedrock within the entire slope area. The black dotted surface represents the area where this surface has been identified as being the major sliding surface. Integration of geophysical information in terms of geological ones is proposed for the profile P18 (Fig. 14). This synthesis shows that the slope is mainly composed by morainic filling materials of around 40 to 60 m thick in its western part lying on a geological substratum of triasic quarzites, shists with some gypsum levels. Inside the moraines, a low porosity compacted layer of around 10 to 20 m thick is identified as the shallow bedrock. This layer is generally stable. Just above, the same material, but highly weathered and channelized, compose the subsurface layer. This layer is subject to many water flows that take place around the numerous boulders. The Ballandaz landslide is located in the western part of the slope, where the water flows accumulate a high degree of moisture and the possible presence of gypsum levels that could favor the slope instability. 4. Conclusion The geophysical sections obtained at the Ballandaz site show a high degree of heterogeneity in the spatial distribution of the inverted parameters, particularly where the electrical resistivity is concerned. This property comes manifestly from the composition of material
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Fig. 11. Combined interpretation of Profile P18; a) electrical profile (pole–dipole); b) seismic profile.
Fig. 12. Combined interpretation of Profile P19; a) electrical profile (pole–dipole); b) seismic profile.
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Fig. 13. Modelling the sliding surface from seismic (a) and electrical (b) profiles. Inclinometer measurements are also indicated as welle as 3D representation of seismic (a) and electrical (b) profiles P18 and P19.
making up the landslide, identified by drilling as being an amalgam of scree and sericite schist boulders caught up in a sandy–clayey matrix. The infiltration of water through this structure appears to be responsible for the observed resistivity anomalies, as far as this could be confirmed by the EM34 method, in particular in the uphill sector of the slope where many water sinks are visible. The electrical profiles enable one to distinguish conductive zones within the shallow depths of investigation. It is also of note that the base of the conductive zone visible at the level of inclinometer I2bis in Profile P19, actually appears to correspond to the sliding surface revealed by the inclinometric monitoring. The seismics, which are less sensitive to the high subsurface heterogeneity, tend rather to smooth the shallow velocity anomalies. On the other hand, the seismic tomograms enable one to clearly assess the depth to the shallow bedrock, whose upper boundary corresponds to sliding surfaces noted at from the boreholes. Profile P18 also reveals the presence of a highvelocity anomaly within the active part of the landslide, just upstream of road D915. It could possibly represent a quartzite or gypsum
boulder; the latter would well explain the instability in this part of the landslide. As for the spectral analysis of surface waves, this was not exploitable because of the high heterogeneity of the landslide material. The relative performance of each method at this site indicates that the electrical imagery gives a better identification of the shallow heterogeneities, whilst the seismic imagery is more effective in identifying the shallow bedrock boundary. These results are certainly due to the strong resistivity contrasts of the shallow heterogeneities which are more easily measured by electrical methods, but which are too small for the seismic signal wavelength. Conversely, reconstruction of the high-velocity level forming the shallow bedrock is possible with seismic methods, whereas the sensitivity of the electrical signal at these depths is too low to ensure reconstruction. Lastly, the methods based on surface waves (SASW, seismic background noise) proved to be disappointing in such a heterogeneous environment for detecting the thickness of the deformed zone, although the seismic background noise was nevertheless able to provide constraints
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Fig. 14. Synthetic interpretation of profile 18. 1: weathered morainic materials including water drains and boulders; 2: shallow bedrock; 3: geological substratum (quarzite).
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