Surface Wave Dispersion Curves Research Articles

Ambient noise wavefield decomposition and shear-wave velocity retrieval in the south of Tehran, Iran and in the Colfiorito basin, Italy

Knowledge about ambient noise wavefield, through decomposition into different participant waves and determination of precise singularities in ellipticity curves, could be very beneficial to improve the obtained subsurface velocity structures. In this paper, two array processing methods; WaveDec and high-resolution Rayleigh three-component beamforming (RTBF) and four single-station methods; Horizontal-to-Vertical Spectral Ratio (HVSR) or H/V, HVTFA (H/V using time frequency analysis), RayDec (Random Decrement Technique) and DOP-E (Degree Of Polarization) are applied to decompose ambient noise wavefield and to extract dispersion and ellipticity curves of surface waves. Two sites with different geological and geophysical conditions are chosen to examine the results of the measurements. The first selected site is in the south of Tehran, Iran, as a site with a low impedance contrast structure. The second one is in the Colfiorito basin, Italy, as a major impedance contrast structure site. Wavefield decomposition resulted in Rayleigh and Love wave separation and also mode distinction. The decomposed modes of surface waves and ellipticity curves of Rayleigh waves are used in a joint inversion process. The retrieved shear-wave velocity profiles are in close agreement with previous studies in the mentioned sites.

Retrieval of shallow S-wave profiles from seismic reflection surveying and traffic-induced noise

In urban subsurface exploration, seismic surveys are mostly conducted along roads where seismic vibrators can be extensively used to generate strong seismic energy due to economic and environmental constraints. Generally, Rayleigh waves also are excited by the compressional wave profiling process. Shear-wave (S-wave) velocities can be inferred using Rayleigh waves to complement near-surface characterization. Most vibrators cannot excite seismic energy at lower frequencies (<5 Hz) to map greater depths during surface-wave analysis in areas with low S-wave velocities, but low-frequency surface waves ([Formula: see text]) can be extracted from traffic-induced noise, which can be easily obtained at marginal additional cost. We have implemented synthetic tests to evaluate the velocity deviation caused by offline sources, finding a reasonably small relative bias of surface-wave dispersion curves due to vehicle sources on roads. Using a 2D reflection survey and traffic-induced noise from the central North China Plain, we apply seismic interferometry to a series of 10.0 s segments of passive data. Then, each segment is selectively stacked on the acausal-to-causal ratio of the mean signal-to-noise ratio to generate virtual shot gathers with better dispersion energy images. We next use the dispersion curves derived by combining controlled source surveying with vehicle noise to retrieve the shallow S-wave velocity structure. A maximum exploration depth of 90 m is achieved, and the inverted S-wave profile and interval S-wave velocity model obtained from reflection processing appear consistent. The data set demonstrates that using surface waves derived from seismic reflection surveying and traffic-induced noise provides an efficient supplementary technique for delineating shallow structures in areas featuring thick Quaternary overburden. Additionally, the field test indicates that traffic noise can be created using vehicles or vibrators to capture surface waves within a reliable frequency band of 2–25 Hz if no vehicles are moving along the survey line.

Lithospheric thermal and compositional structure of South China jointly inverted from multiple geophysical observations

The detailed lithospheric structure of South China is the basis for the understanding of tectonic processes of eastern China. Specifically, two essential issues in the study of lithospheric structure are the thermal and compositional structures, which are usually derived from either geophysical or geochemical observations. However, inversions from single geophysical or geochemical datasets have certain limitations, making it necessary to develop joint inversions of geophysical, geochemical and petrological datasets. In this paper, through thermodynamic simulation and probabilistic inversion, we inverted multiple datasets including topography, geoid height, surface heat flow and surface wave dispersion curves for the 3D lithospheric thermal and compositional structure of South China. The results reveal a thin ( 200 km), suggesting that the thick and cold craton lithospheric roots are retained. There may exist a discontinuous interface beneath the Sichuan Basin, with the saturated lower layer thicker than the refractory upper layer. As a result, the lithospheric mantle of the Sichuan Basin and surrounding regions is mainly composed of saturated and transitional peridotite.

Deep Learning for Extracting Dispersion Curves

High-frequency surface-wave methods have been widely used for surveying near-surface shear-wave velocities. A key step in high-frequency surface-wave methods is to acquire dispersion curves in the frequency–velocity domain. The traditional way to acquire the dispersion curves is to identify the dispersion energy and manually pick phase velocities by following energy peaks at different frequencies. A large number of dispersion curves need to be extracted for inversion, especially for surveys with long two-dimensional sections or large three-dimensional (3D) coverages. Human–machine interaction-based dispersion curves extraction, however, is still common, which is time-consuming. We developed a deep learning model, termed Dispersion Curves Network (DCNet), that can rapidly extract dispersion curves from dispersion images by treating dispersion curves extraction as an instance segmentation task. The accuracy of the dispersion curves extracted by our DCNet model is demonstrated by theoretical data. We used a 3D field application of ambient seismic noise to demonstrate the effectiveness and robustness of our method. The real-world results showed that the accuracy of the dispersion curves extracted from the field data using our method can achieve human-level performance and our method can meet the requirement of geoengineering surveys in rapidly extracting massive dispersion curves of surface waves.

The Frequency‐Bessel Spectrograms of Multicomponent Cross‐Correlation Functions From Seismic Ambient Noise

AbstractThe recently developed frequency‐Bessel transformation (F‐J) method is effective to extract multimode surface wave dispersion curves from ambient noise cross‐correlation functions (CCFs). However, this method is currently limited to the vertical‐vertical component CCFs, and only Rayleigh wave dispersion curves can be obtained. In this study, we first relate the F‐J spectrogram to the spatial autocorrelation coefficients; we then extend the F‐J method to the full multicomponent CCFs tensor, including the radial‐radial, transverse‐transverse, and the mixed‐component CCFs. Using the newly derived formulation, not only the signal of higher‐mode Rayleigh wave phase velocity dispersion can be enhanced, but also the multimode Love wave phase velocity dispersion curves and the higher‐mode Rayleigh wave ellipticity can be extracted. The formulation is tested in several numerical examples and is applied to field data in North America. Our derivation and formulation provide an extension to the current F‐J method and help to take usage of multicomponent CCFs. The resulting higher‐mode surface wave dispersions and Rayleigh wave ellipticity provide complementary constraint on the Earth structures.

3D S-wave velocity modelling with surface waves in oil seismic prospecting

It is a convenient and effective way to infer near-surface S-wave velocity () structures by using seismic surface waves. In spite of many successful applications on the geotechnical or engineering scale, surface-wave analysis and inversion methods are still not widely used in oil seismic exploration. Particularly, there are few researches reported on the three-dimensional (3D) structure modelling with the surface wave methods on this exploration scale. In this paper, we proposed a seismic surface wave data processing and inversion scheme for 3D near-surface modelling, and applied it to a field seismic data acquired for oil prospecting in Eastern China. Firstly, we analysed and adjusted the acquisition geometry to suit surface-wave analysis. Next, the interpolation and stacking processing was applied to the seismic data to eliminate spatial aliasing and improve the quality of dispersion images. In term of phase velocity dispersion imaging method, we adopted the cross-correlation and phase-shifting (CCPS) method to acquire accurate dispersion images. Simultaneous linearisation inversion of and layer thickness was used to inverse the surface wave dispersion curves. This inversion method reduces dependence of the initial models and has ability to detect the top interface of high-velocity layer. At last, the 3D near-surface structure was constructed by interpolating the all of 360 1D structures. We contrasted the surface wave inversion result with the first-arrival tomography inversion result, and the geological stratification results of both were coincident. Within the near-surface range, the surface wave inversion result has a higher resolution. This confirms that 3D modelling with surface waves in oil seismic prospecting is effective and practical.

Site Characterization at Downhole Arrays by Joint Inversion of Dispersion Data and Acceleration Time Series

ABSTRACTWe present a sequential data assimilation algorithm based on the ensemble Kalman inversion to estimate the near-surface shear-wave velocity profile and damping; this is applicable when heterogeneous data and a priori information that can be represented in forms of (physical) equality and inequality constraints in the inverse problem are available. Although noninvasive methods, such as surface-wave testing, are efficient and cost-effective methods for inferring an VS profile, one should acknowledge that site characterization using inverse analyses can yield erroneous results associated with the lack of inverse problem uniqueness. One viable solution to alleviate the unsuitability of the inverse problem is to enrich the prior knowledge and/or the data space with complementary observations. In the case of noninvasive methods, the pertinent data are the dispersion curve of surface waves, typically resolved by means of active source methods at high frequencies and passive methods at low frequencies. To improve the inverse problem suitability, horizontal-to-vertical spectral ratio data are commonly used jointly with the dispersion data in the inversion. In this article, we show that the joint inversion of dispersion and strong-motion downhole array data can also reduce the margins of uncertainty in the VS profile estimation. This is because acceleration time series recorded at downhole arrays include both body and surface waves and therefore can enrich the observational data space in the inverse problem setting. We also show how the proposed algorithm can be modified to systematically incorporate physical constraints that further enhance its suitability. We use both synthetic and real data to examine the performance of the proposed framework in estimation of the VS profile and damping at the Garner Valley downhole array and compare them against the VS estimations in previous studies.

Seismic velocity structure beneath the Western Solomon Islands from the joint inversion of receiver functions and surface-wave dispersion curves

An MW 8.1 earthquake occurred in 2007 and induced a tsunami that hit the Western Solomon Islands and caused casualties. This motivated us to deploy a seismic network around the rupture zone of the 2007 event. To investigate the seismic velocity structure of the Western Solomon Islands, we select seismograms of teleseismic events recorded by our seismic network. Joint inversion of P-wave receiver functions and surface-wave group velocity dispersion curves is used to estimate station-based 1-D velocity models. The resulting velocity models show a highly variable crustal structure across the region. The Moho depths beneath the stations range from 25 to 40 km. A low-velocity zone (LVZ) is observed at most seismic stations in this work. Our study provides preliminary station-based seismic velocity models for the study region, and more stations will be deployed in the continuing project. An integrated 3-D velocity model will be determined in the future.

Technique for Combining Surface Wave Tomography with Receiver Function Results for Studying Upper Mantle Velocity Structure

Abstract—A technique is proposed for joint inversion of the surface-wave velocity dispersion data from long travelpaths and the results of the receiver function method. The technique is based on a modification of a surface-wave tomography procedure and allows for the use of the data not only from travelpaths but also obtained at individual isolated points. Initially, the data are reduced to a single type: either to surface wave velocity by calculating the dispersion curve from the vertical velocity section obtained by the receiver function method or to shear wave velocity by one-dimensional inversion of surface-wave dispersion curves from travelpaths. Thus, from the computational point of view, the problem reduces to the following one: to obtain a smoothed estimate of a two-dimensional function from the values ​​of this function at individual points and its average values along lines (travelpaths) which can be considered as straight. A direct application of a surface-wave tomography procedure under the assumption that the points are lines of zero length is impossible because the solution turns out to be singular at the points. To get rid of this obstacle, it is proposed to replace the points by circles of a small radius surrounding these points. Then, the problem of tomography is generalized by using data not only from linear travelpaths, but also from circular travelpaths. A numerical study has shown that an addition of point data results in an increase in the resolution.

A Feasibility Study on Monitoring Crustal Structure Variations by Direct Comparison of Surface Wave Dispersion Curves from Ambient Seismic Noise

This work assesses the feasibility of the direct use of surface-wave dispersion curves from seismic ambient noise to gain insight into the crustal structure of Bransfield Strait and detect seasonal seismic velocity changes. We cross-correlated four years of vertical component ambient noise data recorded by a seismic array in West Antarctica. To estimate fundamental mode Rayleigh wave Green’s functions, the correlations are computed in 4-hr segments, stacked over 1-year time windows and moving windows of 3 months. Rayleigh wave group dispersion curves are then measured on two spectral bands—primary (10–30 s) and secondary (5–10 s) microseisms—using frequency-time analysis. We analyze the temporal evolution of seismic velocity by comparing dispersion curves for the successive annual and 3-month correlation stacks. Our main assumption was that the Green’s functions from the cross-correlations, and thus the dispersion curves, remain invariant if the crustal structure remains unchanged. Maximum amplitudes of secondary microseisms were observed during local winter when the Southern Ocean experiences winter storms. The Rayleigh wave group velocity ranges between 2.1 and 3.7 km/s, considering our period range studied. Interannual velocity variations are not much evident. We observe a slight velocity decrease in summer and increase in winter, which could be attributed to the pressure melting of ice and an increase in ice mass, respectively. The velocity anomalies observed within the crust and upper mantle structure correlate with the major crustal and upper mantle features known from previous studies in the area. Our results demonstrate that the direct comparison of surface wave dispersion curves extracted from ambient noise might be a useful tool in monitoring crustal structure variations.

Two-station continuous wavelet transform cross-coherence analysis for surface-wave tomography using active-source seismic data

ABSTRACT Surface-wave tomography has great potential to improve the lateral resolution of near-surface characterization compared to 2D surface-wave analysis with multichannel analysis of surface waves (MASW). Surface-wave tomography has been widely applied to obtain high-resolution maps of phase or group velocity from dispersion curves between pairs of stations in seismological studies. However, very few studies have done surface-wave tomography with active-source (exploration) seismic data, probably because extracting surface-wave dispersion curves between two stations is difficult due to the complex wave propagation in heterogeneous near-surface structures. Here, we describe a method to estimate reliable phase-velocity dispersion curves between two stations from exploration seismic data. In our approach, we compute cross coherences between pairs of stations to extract phase information, stacking the cross coherences from different shot gathers to improve the signal-to-noise ratio. To further distinguish surface-wave signals from noise in the time domain, we perform a time-frequency analysis using the continuous wavelet transform (CWT) on the stacked cross coherences. We used modeling of the wavelet transform between station pairs to extract phase-velocity dispersion curves from the stacked cross coherences. We apply this two-station CWT cross-coherence method to synthetic and field data sets. Both applications demonstrate that our method can extract stable phase-velocity dispersion curves between two stations better than two-station or multistation analysis without time-domain filtering. In phase-velocity distributions constructed by surface-wave tomography from the dispersion curves between two stations, the horizontal resolution is improved over MASW-based analyses. Improvement of the horizontal resolution is also achieved in S-wave velocity structures derived by inversion of the phase-velocity distributions. Our method is effective in estimating reliable phase-velocity dispersion curves and may contribute to constructing high-resolution S-wave velocity models located with a laterally heterogeneous structure, by subsequent surface-wave tomography and S-wave velocity inversion.

Reliable workflow for inversion of seismic receiver function and surface wave dispersion data: a \u201c13 BB Star\u201d case study

Non-linear inverse problems arising in seismology are usually addressed either by linearization or by Monte Carlo methods. Neither approach is flawless. The former needs an accurate starting model; the latter is computationally intensive. Both require careful tuning of inversion parameters. An additional challenge is posed by joint inversion of data of different sensitivities and noise levels such as receiver functions and surface wave dispersion curves. We propose a generic workflow that combines advantages of both methods by endowing the linearized approach with an ensemble of homogeneous starting models. It successfully addresses several fundamental issues inherent in a wide range of inverse problems, such as trapping by local minima, exploitation of a priori knowledge, choice of a model depth, proper weighting of data sets characterized by different uncertainties, and credibility of final models. Some of them are tackled with the aid of novel 1D checkerboard tests—an intuitive and feasible addition to the resolution matrix. We applied our workflow to study the south-western margin of the East European Craton. Rayleigh wave phase velocity dispersion and P-wave receiver function data were gathered in the passive seismic experiment “13 BB Star” (2013–2016) in the area of the crust recognized by previous borehole and refraction surveys. Final models of S-wave velocity down to 300 km depth beneath the array are characterized by proximity in the parameter space and very good data fit. The maximum value in the mantle is higher by 0.1–0.2 km/s than reported for other cratons.

A MATLAB Package for Calculating Partial Derivatives of Surface-Wave Dispersion Curves by a Reduced Delta Matrix Method

Various surface-wave exploration methods have become increasingly important tools in investigating the properties of subsurface structures. Inversion of the experimental dispersion curves is generally an indispensable component of these methods. Accurate and reliable calculation of partial derivatives of surface-wave dispersion curves with respect to parameters of subsurface layers is critical to the success of these approaches if the linearized inversion strategies are adopted. Here we present an open-source MATLAB package, named SWPD (Surface Wave Partial Derivative), for modeling surface-wave (both Rayleigh- and Love-wave) dispersion curves (both phase and group velocity) and particularly for computing their partial derivatives with high precision. The package is able to compute partial derivatives of phase velocity and of Love-wave group velocity analytically based on the combined use of the reduced delta matrix theory and the implicit function theorem. For partial derivatives of Rayleigh-wave group velocity, a hemi-analytical method is presented, which analytically calculates all the first-order partial differentiations and approximates the mixed second-order partial differentiation term with a central difference scheme. We provide examples to demonstrate the effectiveness of this package, and demo scripts are also provided for users to reproduce all results of this paper and thus to become familiar with the package as quickly as possible.

Evidence of metamaterial physics at the geophysics scale: the METAFORET experiment

Abstract The METAFORET experiment was designed to demonstrate that complex wave physics phenomena classically observed at the meso- and micro-scales in acoustics and in optics also apply at the geophysics scale. In particular, the experiment shows that a dense forest of trees can behave as a locally resonant metamaterial for seismic surface waves. The dense arrangement of trees anchored into the ground creates anomalous dispersion curves for surface waves, which highlight a large frequency band-gap around one resonant frequency of the trees, at ∼45 Hz. This demonstration is carried out through the deployment of a dense seismic array of ∼1000 autonomous geophones providing seismic recordings under vibrating source excitation at the transition between an open field and a forest. Additional geophysical equipment was deployed (e.g. ground-penetrating radar, velocimeters on trees) to provide essential complementary measurements. Insights and interpretations on the observed seismic wavefield, including the attenuation length, the intensity ratio between the field and the forest and the surface wave polarization, are validated with 2D numerical simulations of trees over a layered halfspace.

Comparison of MASW and seismic interferometry with use of ambient noise for estimation of S-wave velocity field in landslide subsurface

This study presents a comparison of data acquisition, processing and interpretation between passive seismic interferometry (SI) and active multichannel analysis of surface waves (MASW) methods, both using surface waves for estimation S-wave velocity field. Measurements have been taken in the same geological engineering conditions on Just-Tegoborze landslide on the south of Poland. This comparison study has an important meaning from landslide hazard evaluation point of view. The landslide is located in Magura Nappe in Outer (Flysch) Carpathians. SI was based on registration of local seismic noise generated by high traffic on the state road which intersects the landslide. The main processing step was cross-correlation of seismic noise between every pair of receivers. It led to obtain series of empirical Green’s functions for Rayleigh surface wave. However, in MASW method, seismic energy was released by an impact of 5 kg sledgehammer in a metal plate. Both methods included analysis of dispersion curves of Rayleigh surface wave. The inversion of picked fundamental modes was applied using genetic algorithm and resulted in 1D S-wave velocity models. The last step of interpretation included model visualization as the 2D S-wave velocity sections for studied profiles. Both MASW and SI methods allowed to estimate S-wave velocity field in Just-Tegoborze landslide subsurface. Dispersion images obtained from both methods provided similar phase velocity and frequency ranges. On S-wave velocity sections, the greater depth range was observed for SI method; however, lateral resolution was better for MASW. Slip surfaces in colluvial layer were not observed on either SI or MASW S-wave velocity sections. Only results obtained from SI allowed to distinguish probable slip surface located deeper, i.e. on the contact with less weathered flysch bedrock.

Integrated use of ambient vibrations and geological methods for seismic microzonation

ABSTRACT Ambient vibrations spectral ratios at forty-five sites and shear wave velocity profiles obtained by inverting surface waves dispersion curves were used to assess seismic site response properties in the Floridia village located in southeastern Sicily. The results of the horizontal-to-vertical spectral ratios analysis often indicate fundamental in the frequency range 1.5–4.0 Hz. Conversely, where the bedrock outcrop, spectral ratios are flat. The calcarenites outcropping in the north-west sector of the village and the soft sediments in the south-east are characterized by shear wave velocity of about 700 m/s and ranging between 200 and 600 m/s, respectively. The geological substratum has instead shear wave velocity of about 1500 m/s. The achieved geophysical and geological data help to build a subsoil model of the studied area, classifying the main lithotypes above the bedrock and determining the hidden morphology. Moreover, amplification functions were computed for zones with homogenous geological and geophysical properties. Therefore, these site amplification functions were used to simulate ground motion scenarios corroborating the efficiency of the obtained results. The used strategy, involving geophysical and geological methods, represents an important step to derive ground motion scenarios and seismic risk maps. Moreover, the characterization of amplification due to local site effects could help to understand the pattern and the damage distribution of historical earthquakes.

Full-waveform matching of VP and VS models from surface waves

SUMMARYThe analysis of surface wave dispersion curves (DCs) is widely used for near-surface S-wave velocity (VS) reconstruction. However, a comprehensive characterization of the near-surface requires also the estimation of P-wave velocity (VP). We focus on the estimation of both VS and VP models from surface waves using a direct data transform approach. We estimate a relationship between the wavelength of the fundamental mode of surface waves and the investigation depth and we use it to directly transform the DCs into VS and VP models in laterally varying sites. We apply the workflow to a real data set acquired on a known test site. The accuracy of such reconstruction is validated by a waveform comparison between field data and synthetic data obtained by performing elastic numerical simulations on the estimated VP and VS models. The uncertainties on the estimated velocity models are also computed.

Site characterization using passive seismic techniques: A case of Suez city, Egypt

This article introduces a site investigation study using quick and low-cost passive seismic techniques at Suez city which located in the northeastern part of Egypt. The Spatial Auto Correlation (SPAC) method is applied, as well as the newly developed Centerless Circular Array (CCA) method for estimating the shear wave velocity (Vs) structures of the subsurface layers using microtremor array data. The Horizontal/Vertical Spectral Ratio (HVSR) approach is employed in determination of predominant frequency (f0) of a site and its equivalent peak amplitude of ground motion (A0) using a single station microtremor measurements.In this study, microtremor array measurements have been carried out at five sites distributed in Suez city. In order to cover a wide frequency band, different array sizes are carried out at each observation site. Measurements are made using high performance accelerometers with a sampling rate 100 sample per second. In both SPAC and CCA methods, only the vertical component of microtremor data is considered in the analysis. The analysis includes two main stages, i.e., calculation of surface wave dispersion curves and inversion process to infer the Vs structure of the ground. The obtained dispersion curves cover frequency band (2.0–21.0 Hz) and show a good fitting between the observed and the calculated ones at all measured sites, proving the reliability of the inversion process performed in this work. In HVSR analysis, the E-W, N-S, and vertical components of microtremors are considered, and the ratio between the horizontal and the vertical components spectra is calculated, then the spectral ratio curves are produced. The predominant frequency (f0) and the peak amplitude (A0) of HVSR curve are reported for each site of observation. The inferred Vs profiles are used for calculating the average velocity value of shear wave up to 30 m depth (AVS30) which in turn employed in site classification at the investigated area.The results demonstrate that the parameter AVS30 varies between 248 m/s and 310 m/s in Suez city. The created site classification incorporates only one class (Class D), appearing no significant variation in the characteristics of the superficial and shallow soils in Suez city. The site predominant frequency (f0) ranges between 0.35 Hz and 5.0 Hz with relevant amplification (A0) between 1.6 and 4.2.These outputs represent a crucial site response model for future seismic hazard assessment and risk decrease studies in Suez area which is currently witnesses large investments and rapid land use planning.

Surface-Wave Extraction Based on Morphological Diversity of Seismic Events

It is essential to extract high-fidelity surface waves in surface-wave surveys. Because reflections usually interfere with surface waves on X components in multicomponent seismic exploration, it is difficult to extract dispersion curves of surface waves. To make matters worse, the frequencies and velocities of higher-mode surface waves are close to those of PS-waves. A method for surface-wave extraction is proposed based on the morphological differences between surface waves and reflections. Frequency-domain high-resolution linear Radon transform (LRT) and time-domain high-resolution hyperbolic Radon transform (HRT) are used to represent surface waves and reflections, respectively. Then, a sparse representation problem based on morphological component analysis (MCA) is built and optimally solved to obtain high-fidelity surface waves. An advantage of our method is its ability to extract surface waves when their frequencies and velocities are close to those of reflections. Furthermore, the results of synthetic and field examples confirm that the proposed method can attenuate the distortion of surface-wave dispersive energy caused by reflections, which contributes to extraction of accurate dispersion curves.

Ambient vibration classification of unstable rock slopes: A systematic approach

In this paper, we are comparing the seismic response of 25 different rock slope instabilities with diverse geological properties, activity level, failure mechanism, fracturing and volumes. We classify them according to their dynamic behaviour. In the dataset, we found two main classes of unstable rock slopes: depth-controlled sites and volume-controlled sites.For depth-controlled instabilities, the seismic wavefield is controlled by horizontally propagating surface waves in the highly fractured and weathered material. At such sites, surface-wave dispersion curves can be retrieved and inverted into velocity profiles of the underground. The lateral borders of depth-controlled instabilities are not obvious and the seismic properties mainly change with depth.The seismic response of volume-controlled sites is dominated by the eigenvibrations of the rock mass itself. Such instabilities have clear lateral and vertical borders and show highly amplified ground motion in limited frequency bands. The observed polarisation of the ground motion is perpendicular to deep open fractures that do not allow surface waves to propagate. A special case of the volume-controlled sites is represented by tower-like rock masses showing strong amplification and directionality. Their dynamic behaviour might not only be related to the internal structure, but also to their geometry that is similar to high-rise buildings.Another type of rock instabilities is represented by block structures, which are difficult to identify by the proposed ambient-vibration methods.A clear relation between geological properties and seismic response was not found.