Assessment of time–spatial changes of shear wave velocities of flysch formation prone to mass movements by seismic interferometry with the use of ambient noise
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This paper presents the results of monitoring flysch formation shear wave velocities at varying conditions of water content by seismic interferometry. Studies were conducted on a section of the active Just-Tegoborze landslide in the southern region of Poland. The landslide is intersected by the state road no. 75, which has a high traffic density. Measurements of the seismic interferometry were made on the basis of the registration of a local high-frequency noise generated mostly by passing heavy vehicles. Measurements were carried out in three series on January, March and July in 2015. Seismic noise vibration was registered using Güralp CMG-6TD broadband seismometers. One of the seismic profiles was located along a geological engineering cross-section prepared from research boreholes data. One of the main results of the work was the sections of changes of shear wave velocity. The interpreted seismic boundaries made it possible to correlate them with the lithological ones. The trough-like shape of the bedrock boundary may be an effect of weathering processes developed in the zone consisting of weak hieroglyphic beds. It is also supposed to be one of the slip surfaces. The seismic sections of shear wave velocity allowed to analyse velocity variations between three measurement series and to compare them with expected water content in colluvium deposits. The presented results of the study confirmed the utility of seismic interferometry based on ambient noise registration in evaluating the behaviour of landslides in the Carpathian flysch formation.
KeywordsLandslide Seismic interferometry Ambient noise Carpathian flysch
The recognition of landslide behaviour in various geological conditions is crucial in landslide hazard evaluation (Aleotti and Chowdhury 1999; Anbarasu et al. 2010; Guzzetti et al. 1999; Dai et al. 2002; Barla et al. 2013; Bonnard and Corominas 2005; Cascini et al. 2005; Fukuoka et al. 2005; Mansour et al. 2011; Sassa et al. 2004, Song and Kim 2014). In this sense, geophysical methods may be helpful in monitoring landslides and, consequently, in assessing the landslide hazard (Bichler et al. 2004; Bièvre et al. 2015; Brückl et al. 2013; Bogoslovsky and Ogilvy 1977; Frasheri et al. 1998; Hack 2000; Méric et al. 2005; Jongmans and Garambois 2007; Jongmans et al. 2009; Lundström et al. 2009; Hibert et al. 2011; Reynolds 2011, Salas-Romero et al. 2015; Singh et al. 2013; Travelleti et al. 2013; Zarroca et al. 2014). In recent years, a new seismic method based on ambient noise has been intensively developed, known as seismic interferometry, which may be demonstrated to be useful in monitoring mass movements (Jongmans and Garambois 2007; Méric et al. 2007; Jongmans et al. 2009; Renalier et al. 2010a, 2010b; Mainsant et al. 2012b; Del Gaudio et al. 2013; Pilz et al. 2013).
Seismic interferometry is a method enabling the use of local seismic noise for analysis of changes of elastic properties within the geological medium. It is originated from the phenomenon of interference of seismic signals (Curtis et al. 2006; Schuster 2009). The principles and applications of seismic interferometry were broadly explained by Schuster (2009) and Wapenaar et al. (2010a, 2010b).
Seismic interferometry consists of retrieving the impulse response of the medium, the so-called Green’s function, between a pair of receivers using the operations of cross-correlation, deconvolution or cross-coherence of seismic signals registered at the receivers. The method assumes a Rayleigh surface wave, which is dispersive, i.e., its velocity strictly depends on the frequency. Rayleigh waves are described by dispersion curves, which demonstrate the dependence of a phase velocity on wave frequency. Shear wave (S-wave) velocities have the dominant influence on the dispersion curve in a high-frequency range (>5 Hz) (Xia et al. 1999). As a result of inversion of a Rayleigh wave’s phase velocity, an image of the changes in S-wave velocities is obtained. The S-wave velocity is sensitive to changes in properties of the ground medium, especially to changes in its rigidity, which could be also the consequence of changes in water content in the medium.
Seismic interferometry has increasing geo-engineering applications. Larose et al. (2015) presented recent developments in ambient seismic noise methods to monitor the variations in the mechanical properties of the subsurface and to identify new sources of seismic signals that occur outside the solid Earth. The authors demonstrated the use of seismic noise in marking the thermal variations in the subsoil, in buildings or in rock columns, the temporal and spatial evolution of a water table and the changes of the rigidity of the soil constituting a slope prone to mass movements.
Seismic interferometry studies analysing changes in the S-wave velocity preceding major landslide movements were broadly undertaken by Mainsant et al. (2012b). These researchers concluded that the observed changes in the S-wave velocity decreased two or even threefold before mass movement. The drop in S-wave velocities was correlated with the measurements of displacement performed on the terrain surface. Moreover, Mainsant et al. (2012a) studied the S-wave velocity in terms of rheological changes in clay-rich geological formations, which are responsible for many landslides. These researchers conducted rheometrical laboratory experiments to investigate the solid-fluid transition, which affects derangement of the particle network and is associated with a loss in rigidity. The results indicated that the S-wave velocity significantly varies with rheological changes in the clay, supposedly caused by the disorganization of the particle network. According to the outcomes of their investigations, the authors concluded that measuring the S-wave velocity could be applicable to monitoring rheological changes in landslides in clayey deposits and that the time-dependent variation in this parameter could be a beneficial indicator for earthflows. The authors continued laboratory experiments (Mainsant et al. 2015) in order to examine the physical origin of the S-wave velocity decreases which occur prior to the onset of mass movements. Additional rheometrical studies indicated that it is probably caused by a complex transient rheological response of the clay. These researchers supposed that such response is connected to the existence of the so-called “delayed fluidization” phases in which the shear modulus quickly decreases while the apparent viscosity varies more slowly and requires more time to reach its fluid level. The results are comparable with a real in-field situation of a landslide movement.
Renalier et al. (2010a) applied ambient seismic measurements to determine the deepest slip surface of a landslide in clayey deposits, which had not been detected by active seismic methods. This surface was found by obtaining the dispersion curves at lower frequencies than 3 Hz. During a 30-month period of seismic noise observations, they detected a slight but significant decrease in the surface wave velocities. Moreover, authors observed both lateral and vertical variations of the S-wave velocity which have been related to landslide activity and the location of slip surfaces. Additionally, Renalier et al. (2010b) successfully applied an ambient noise cross-correlation to obtain a 3D model of the S-wave velocity of a landslide affecting clayey deposits. They computed cross-correlations between the vertical components of recording stations which allowed them to retrieve the Rayleigh wave Green’s functions and estimate their group velocity dispersion curves in the 1.7–5 Hz frequency range. The dispersion curves were then inverted with a neighbourhood algorithm which resulted in the 3D model of the S-wave velocity of the studied landslide.
The results of seismic studies described in this paper are a continuation of the work by Pilecki and Harba (2015) who presented the usefulness of the seismic interferometry method in mapping the structure and properties of the near-surface geological medium in the area of the active Just-Tegoborze landslide (southern Poland). Their preliminary results are based on the measurements carried out along single seismic profile in two measurement series. In this paper, we present the results developed with one additional series. Moreover, the results were extended with data from transverse seismic profiles. We also describe the geological conditions of the Just-Tegoborze landslide and the measurement methodology, as well as the methodology of seismic data processing and interpretation. The changes of elastic properties of geological medium are characterized by the changes in the S-wave velocity. The recognized seismic boundaries are correlated to lithological ones shown on the geological engineering cross-section.
Geological and geomorphological setting
Several failure surfaces of the landslide were distinguished as a result of archival geological works (Chowaniec and Wojcik 2012). Failure surfaces were investigated on the study area within colluvial deposits at approximately 10 and 12 m depth and between colluvial deposits and the flysch bedrock at approximately 14 to 17 m depth (Fig. 4).
The Just-Tegoborze landslide developed on the contact between shale and shale-sandstone complexes of Sub-Magura and hieroglyphic beds, and Magura sandstones. Shale and shale-sandstone complexes are less resistant and more susceptible to weathering than Magura sandstones. Magura sandstones have uniaxial compressive strength (UCS) from 19 to 91 MPa, whereas shales from hieroglyphic beds have UCS from 0.25 to 2.9 MPa (Czudec 2012, unpublished data). Other factors, which have an influence on the development of the Just-Tegoborze landslide, include the occurrence of weak variegated shales prone to swelling and complicated tectonics between Magura, Sub-Magura and hieroglyphic beds.
The Just-Tegoborze landslide has southeastern exposure and descends to the Roznow Lake (Fig. 1). The azimuth of its movement is 130°, and its average slope angle is 11.8° (Chowaniec and Wojcik 2012). From a geomorphological point of view, it consists of a number of mass movement areas, however, with no clear boundaries between them. Its slopes are of convex-concave type, and the terrain relief is strongly diversified. Slopes are mostly cut by small coomb valleys, dells and gullies. There are numerous scarps and landslide niches on the upper part of the landslide. On the study area, levels of water filtration were examined on 0.7, 2.0, 4.7 and 5.8 m. Water seepages are observed on the terrain surface close to the 3I/P (Fig. 3). Water level in a dug well, which is located on the study area, is 1.2 m deep, however, it reaches terrain surface during intense rainfalls and spring thaw. It leads to the conclusion that there are weakly permeable deposits which limit the infiltration of water deep into the geological medium. The analysis of the core from 3I/P borehole indicates the presence of low permeable, weakly and medium fractured shales and variegated shales at the depth of 13 m. Above them, there are very fractured shales and previously described colluvial deposits which have a water content up to 35% (Czudec 2012, unpublished data).
Processing and interpretation
Results and analysis
The 1st measurement series was carried out in the winter period in conditions which may be assumed to be dry (Fig. 6). Evident in this section are three layers with varied S-wave velocities (Fig. 10a). In the near-surface layer, with the lowest velocities ranging from 220 to 270 m/s, there is also a local minimum value in the central part. This layer is most likely connected with the increased water content in the medium, as demonstrated by the observed water table when executing borehole 3I/P and the visible dampness of the terrain surface while taking the measurements. The first layer is characterized by a thickness amounting to approximately 5 m along the entire length of the profile. The next, middle layer is characterized by the S-wave velocity ranging from 270 to 340 m/s. Its thickness is variable and primarily is approximately 8 to 10 m, locally decreasing to 5 m in the 70 to 75 m of the seismic profile (Fig. 10a). Thickness of the second layer is the largest in the central part of the profile. The deepest-located layer is characterized by the S-wave velocity values in the range of 340 to 400 m/s. The roof of this layer occurs at the depth of 10 to 15 m.
The trough-like shape of the seismic boundary between the second and third layers occurs in the zone of the weaker rock mass of the hieroglyphic beds made of clayey variegated shales and shales with instances of minor sandstone interbedding. A harder sandstone-shale rock mass of Sub-Magura beds lies in the vicinity (Fig. 4). The roof of the third layer likely constitutes the surface of a less weathered bedrock.
The 2nd measurement series was carried out in the spring period with a melting snow cover. The near-surface ground might have not yet been fully thawed (Fig. 6). The longitudinal (Fig. 10b) and transverse (Fig. 11a) sections are characterized by a significant decrease in the S-wave velocity in the two surface layers of the medium. In order to emphasize the differences, layer boundaries were marked in the longitudinal section according to contour lines interpreted in Fig. 10a. The zone of weakened elastic properties stretched toward the drop in the terrain surface of the area. The velocity value of the seismic boundary of the bedrock was very slightly changed. It should be noted that the depth of this boundary was maintained.
The 3rd measurement series was conducted in the summer period directly after storm rainfalls (Fig. 6, Fig. 10c, Fig. 11b). Increased infiltration into the medium caused a greater weakening of elastic properties, especially in the first layer, compared to the 2nd series. The velocity value of the seismic boundary of the bedrock again only slightly changed. The depth of this boundary was also maintained. A similar effect may be observed in the transverse sections (Fig. 11a, b).
In relation to the geological engineering cross-section of the research area (Fig. 4), one may notice that the first two near-surface seismic layers correspond to clayey colluvium formations. The seismic boundary occurring at a depth of approximately 10 to 15 m may be correlated to the roof of the less weathered rock mass of the shale and shale/sandstone series. In the seismic image, the bedrock (third layer) did not undergo significant changes as a result of infiltration, which confirms lesser disturbance by landslide processes in comparison with colluvium layers. Presumably, the bedrock roof boundary is one of the most important failure surfaces of the landslide. The trough-like shape of this seismic boundary in the central part of the seismic section may be connected with the greater range of weathering in the zone of weaker hieroglyphic beds.
This study has demonstrated that it is possible to monitor the changes of shear wave velocities of a geological compound medium, as it is monitored in the Just-Tegoborze landslide by the seismic interferometry method using a local high-frequency ambient noise. The results clearly show changes of the S-wave velocity which are most likely caused by various conditions of infiltration and water content in the geological medium. Generally, an increasing water content in the medium results in the decrease of the S-wave velocity value and also the medium’s rigidity.
The interpreted seismic boundaries may be correlated to lithological boundaries shown in a geological engineering cross-section. In the seismic image, the course of the bedrock boundary is more complex in comparison with the geological engineering cross-section. The trough-like shape of this seismic boundary in the central part of the seismic section may be related to the decrease of rigidity of weak hieroglyphic beds compared to the stronger surrounding rock mass, however, it requires further detailed studies. The investigated bedrock boundary is presumably one of the failure surfaces.
The complicated course of the bedrock boundary may also have an influence on the water flow paths in the colluvium layer affected by mass movements. In consequence, it has an influence on the stability of the state road structure.
The presented results of seismic interferometry may be helpful in evaluating the behaviour of the Just-Tegoborze landslide. These findings can be also used in designing protections against mass movements.
The article was prepared as a result of the LOFRES project No. PBS1/A2/13/2013 performed within the 1st call of the Applied Research Programme co-financed by the Polish National Centre for Research and Development. We thank the editor and anonymous reviewers for valuable comments and discussions.
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