Seismic Amplification

Relative seismic amplification analysis determines how much ground motion is amplified at a measurement site compared to a nearby stable reference station (site-to-reference spectral ratio, SRSR). Amplification scales with the degree of rock fragmentation, making seismic amplification analysis valuable for assessing the condition of rock slopes and other geological features, as well as for mapping the spatial extent of an instability.

• Recording of ambient vibrations using portable seismometers during at least 20 minutes.
• Amplification factors indicate the condition of the rock mass, with higher values indicating more pronounced rock fragmentation and thus greater risk of instability.
• Useful for rapid mapping and characterization of slope instabilities.

Publications

• Häusler et al. (2022): Monitoring the changing seismic site response of a fast-moving rockslide (Brienz/Brinzauls, Switzerland)

Modal Analysis

Modal analysis is a technique commonly used in civil engineering to determine resonance frequencies, damping, and mode shapes. Using Frequency Domain Decomposition (FDD), these properties can be calculated from recordings of ambient vibrations. When applied to geological structures such as landslides, rock walls, or freestanding rock towers, modal analysis provides valuable information about the dynamic characteristics of an instability. Since separate compartments of an instability exhibit different mode shapes, the directions of natural oscillations can be used to distinguish different compartments. In addition, the direction of the vibrations is approximately perpendicular to the dominant fractures, making it possible to identify hidden fractures based on ambient vibrations.

• Recording of ambient vibrations using portable seismometers during at least 20 minutes.
• Instabilities often exhibit characteristic dynamic properties that can be used to distinguish different compartments and identify hidden fractures based on ambient vibration recordings.
• Useful for assessing the internal compartmentalization of an instability.

Publications

• Häusler et al. (2019): Fracture Network Imaging on Rock Slope Instabilities Using Resonance Mode Analysis

Resonance Frequency and Polarization

Slope instabilities in rock walls or freestanding rock towers exhibit distinct resonance frequencies and highly polarized seismic waves. These properties can be tracked over time using modal analysis techniques, providing a valuable tool for monitoring mechanical changes within an instability. Since the resonance frequency is a function of stiffness, a decreasing resonance frequency indicates a reduction in rock stiffness and thus progressive desintegration. Changes in wave polarization can be used to identify stress redistribution within a slope.

• Repeated measurements with portable (periodic monitoring) or permanently installed seismometers (continuous monitoring).
• Increasing rock fragmentation leads to a decrease in rock stiffness, resulting in a decrease in resonance frequency. Changes in seismic wave polarization can also be used to detect redistribution of internal stresses.
• Useful for long-term stability assessment and early-warning systems for instabilities with distinct resonance frequencies (e.g., rock walls, freestanding rock towers).

Publications

• Lévy et al. (2010): Dynamic response of the Chamousset rock column (Western Alps, France)
• Häusler et al. (2021): Monitoring the Preonzo Rock Slope Instability Using Resonance Mode Analysis
• Häusler et al. (2022): Monitoring the changing seismic site response of a fast-moving rockslide (Brienz/Brinzauls, Switzerland)

Seismic Interferometry

By cross-correlating the ambient vibration signals between two sensors, the relative shear wave velocity changes (dv/v) can be calculated with respect to a reference period. Since the shear wave velocity depends on subsurface material properties (e.g., rigidity, pore water pressure), seismic interferometry is a powerful tool for monitoring the physical properties of unstable slopes and identifying potential precursors to failure.

• One or more permanently installed seismometers that continuously record ambient vibrations.
• A decrease in shear wave velocity indicates a decrease in rigidity, which is directly related to failure.
• Useful for long-term stability assessment and early-warning systems for instabilities in unconsolidated rock and permafrost bodies.

Publications

• Mainsant et al. (2012): Ambient seismic noise monitoring of a clay landslide: Toward failure prediction
• Le Breton et al. (2021): Landslide monitoring using seismic ambient noise correlation: challenges and applications

Seismic Refraction Tomography (SRT)

Seismic refraction tomography (SRT) uses seismic wave traveltimes to map subsurface velocity variations. By analyzing seismic waves that refract along material boundaries, SRT provides valuable insight into subsurface velocity distributions and material properties. SRT is widely used in geotechnical investigations, engineering applications, and natural hazard assessments.

• Seismic waves are generated (e.g., with a hammer or an explosion) and recorded by geophones placed in a linear array.
• SRT provides detailed information about the subsurface, including bedrock depth, faults, and groundwater levels.
• Especially useful for obtaining information on material contrasts (e.g., basal detachment, bedrock depth).

Multichannel Analysis of Surface Waves (MASW)

Multichannel Analysis of Surface Waves (MASW) is used to determine near-surface shear wave velocity profiles. By analyzing the propagation of surface waves generated by a seismic source, MASW provides critical information about subsurface stiffness and material properties. MASW is widely used in geotechnical, environmental, and engineering studies.

• Surface waves are generated (e.g., with a hammer or an explosion) and recorded by a linear array of geophones to extract phase velocity information.
• MASW provides shear wave velocity profiles that indicate subsurface stiffness and stratigraphy.
• Especially useful for obtaining information on material contrasts (e.g., basal detachment, bedrock depth) and the shallow shear wave velocities (e.g., Vs30 for microzonation, elastic moduli).