Today, surface-wave analysis is widely adopted for building near-surface S-wave velocity models. The surface-wave method is under continuous and rapid evolution, also thanks to the lively scientific debate among different disciplines, and interest in the technique has increased significantly during the last decade. A comprehensive review of the literature in the main scientific journals provides historical perspective, methodological issues, applications, and most-promising recent approaches. Higher modes in the inversion and retrieval of lateral variations are dealt with in great detail, and the current scientific debate on these topics is reported. A best-practices guideline is also outlined.

/pdf/surface-wave-analysis-for-building-near-surface-velocity-39ze2co0if.pdf

Surface-wave analysis for building near-surface velocity models — Established approaches and new perspectives

Surface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages. Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this topic.

/pdf/application-of-surface-wave-methods-for-seismic-site-3j7j41wf74.pdf

Application of Surface-Wave Methods for Seismic Site Characterization

SUMMARY

To study the applicability of the passive seismic interferometry technique to near-surface geological studies, seismic noise recordings from a small scale 2-D array of seismic stations were performed in the test site of Nauen (Germany). Rayleigh wave Green's functions were estimated for different frequencies. A tomographic inversion of the traveltimes estimated for each frequency from the Green's functions is then performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved at engineering–geotechnical scales. Furthermore, a 2-D S-wave velocity cross-section is obtained by combining 1-D velocity structures derived from the inversion of the dispersion curves extracted at several points along a profile where other geophysical analyses were performed. It is shown that the cross-section from passive seismic interferometry provides a clear image of the local structural heterogeneities that are in excellent agreement with georadar and geoelectrical results. Such findings indicate that the interferometry analysis of seismic noise is potentially of great interest for deriving the shallow 3-D velocity structure in urban areas.

/pdf/characterization-of-shallow-geology-by-high-frequency-o11l7c1vz0.pdf

Characterization of shallow geology by high‐frequency seismic noise tomography

Structure of young East Pacific Rise lithosphere from ambient noise correlation analysis of fundamental- and higher-mode Scholte-Rayleigh waves

Dominant higher modes of surface waves are known to be generated at sites with large stiffness contrasts and/or reversals between layers. At many engineering sites, quite often all modal energy is transferred to successively higher modes with increasing frequency. These ‘jumps’ may occur at low (20 Hz) to high (60 Hz) frequencies, and, more than once over the recorded bandwidth, but the common features are that the jumps are in amounts exceeding 50% of the phase velocity, and dispersion does not return to the fundamental mode at high frequency. A nonlinear geophone spacing helps to resolve these modes more uniquely. Full-wavefield synthetic seismogram modelling reinforces the effect to be due to a steep, nonlinear stiffness gradient in the uppermost 1–2 meters of soil. If a high-velocity substrate at depth is not present, the ‘effective’ phase velocity of the higher modes can exceed that of the maximum shear-wave velocity of the model, where leaky modes persist. When this happens, conventional modal dispersion modelling cannot be applied in the inversion. If an artificial stiff layer is added at depth, jumps across several modal boundaries can occur, which is also generated in a Gibson half-space, both at very low and high frequencies. This is a major pitfall, where mode-misidentification, especially at low frequency, can lead to errors in the estimated shear wave velocity models of over 50%. Guided P-waves also manifest as large dispersion discontinuities, but usually at higher frequency and phase velocities, with buried explosive sources. Although Poisson9s ratio has a strong influence of the generation of—and frequencies of transitions to—dominant higher modes, the higher-mode phase velocities themselves are relatively independent of shallow P-wave velocity.

Dominant Higher Surface-wave Modes and Possible Inversion Pitfalls

Reliable models of in-situ shear-wave velocities of shallow-water marine sediments are important for geotechnical applications, lithological sediment characterization, and seismic exploration studies. We infer the 2D shear-wave velocity structure of shallow-water marine sediments from the lateral variation of Scholte-wave dispersion. Scholte waves are recorded in a common receiver gather generated by an air gun towed behind a ship away from a single stationary ocean-bottom seismometer. An offset window moves along the common receiver gather to pick up a local wavefield. A slant stack produces a slowness-frequency spectrum of the local wavefield, which contains all modes excited by the air gun. Amplitude maxima (dispersion curves) in the local spectrum are picked and inverted for the shear-wave velocity depth profile located at the center of the window. As the window continuously moves along the common receiver gather, a 2D shear-wave velocity section is generated. In a synthetic example the smooth lateral variation of surficial shear-wave velocity is well reconstructed. The method is applied to two orthogonal common receiver gathers acquired in the Baltic Sea (northern Germany). The inverted 2D models show a strong vertical gradient of shear-wave velocity at the sea floor. Along one profile significant lateral variation near the sea floor is observed.

1.5D inversion of lateral variation of Scholte-wave dispersion

SUMMARY
We determine the 3-D in situ shear-wave velocities of shallow-water marine sediments by extending the method of surface wave tomography to Scholte-wave records acquired in shallow waters. Scholte waves are excited by air-gun shots in the water column and recorded at the seafloor by ocean-bottom seismometers as well as buried geophones. Our new method comprises three steps: (1) We determine local phase-slowness values from slowness-frequency spectra calculated by a local wavefield transformation of common-receiver gathers. Areal phase-slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra. (2) We infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental Scholte-wave mode. (3) The phase-slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area. From those dispersion curves we determine a model of the depth-dependency of shear-wave velocities for every location. We apply this method to a 1 km2 investigation area in the Baltic Sea (northern Germany). The phase-slowness maps obtained in step (2) show lateral variation of up to 150 per cent. The shear-wave velocity models derived in the third step typically have very low values (60–80 m s−1) in the top four meters where fine muddy sands can be observed, and values exceeding 170 m s−1 for the silts and sands below that level. The upper edge of glacial till with shear-wave velocities of 300–400 m s−1 is situated approximately 20 m below sea bottom. A sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom, and that density may be an important parameter, best resolvable with multimode inversion.

/pdf/scholte-wave-tomography-for-shallow-water-marine-sediments-4104rskzwa.pdf

Scholte-wave tomography for shallow-water marine sediments

Dispersive Scholte waves as well as acoustic guided waves have been excited by surface towed airguns and recorded with stationary receivers (OBS) and a towed hydrophone array. The inversion of the recorded wavefield spectra was adapted to the data from shallow marine environments. The dispersive interface wave travelling along the sea floor (Scholte Wave) is sensitive to the shear wave velocity (Vs) of the sediment and could be sucessfully recorded and inverted at different sites in Kiel Bay and the south-east rim of the Arkona Basin in the Baltic Sea. Limitations remain for very soft sediments with Vs < 50m/s. The acoustic guided wave is sensitive to density and compressional velocity (Vp) as well as Vs. Shear wave velocity and high resolution Vp information could be obtained from the inversion of this wave type in the central Arkona Basin, where very soft sediments (10-20m) overlaid the till and chalk layers, and no interface wave was observed. The lateral variation of the dispersion of Scholte waves was observed with the towed acquisition system, which could be suitable to effciently locate areas with a high shear wave velocity basement at shallow (0-100m) depth.

/pdf/acquisition-and-inversion-of-dispersive-seismic-waves-in-2dh9iyms4e.pdf

Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Models of in situ shear-wave velocities of shallow-water marine sediments are of importance for geotechnical applications, sediment characterization, and seismic exploration studies. Here pseudo-2D shear-wave velocity models are inferred from the lateral variation of Scholte-wave dispersion at five different geological sites in the Baltic Sea (northern Germany). To explore Scholte-wave dispersion and the lateral variability of shear-wave velocities, Scholte waves were excited by air gun shots in the water layer and recorded by stationary ocean-bottom-seismometers or buried geophones. We analyze the recorded seismograms in a common-receiver-gather using offset-windowed, multichannel dispersion analysis. The observed local slowness-frequency spectra for the different study sites vary significantly with respect to excitation amplitudes and phase slownesses of different modes, as well as the excited frequency range. The excitation amplitudes are influenced by the local shear-wave velocity structure, absorption, length of Scholte-wave travel path, and the elevation of the source above the sea floor. The inverted shear-wave velocities range from 50 m/s to 600 m/s. Directly at the sea bottom, shear-wave velocities of 50 m/s for fine muddy sand and 300 m/s for glacial till were inferred. The maximum vertical gradient was +/- 80% (mean 250 m/s) within a depth range of 40 m, and horizontally +/- 33% (mean 350 m/s) within 300 m distance. The layer boundaries in the inverted shear-wave velocity models are in good agreement with high-frequency, zero-offset compressional-wave reflections. However, it was not possible to acquire the fundamental Scholte mode above very soft, unconsolidated sediment with shear-wave velocities smaller 50 m/s. The analysis of synthetic data shows that this is due to the elevation of the source and the receiver response function.

Simone Kugler

Papers

1.5D inversion of lateral variation of Scholte-wave dispersion

Scholte-wave tomography for shallow-water marine sediments

Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Variability of Scholte-wave Dispersion in Shallow-water Marine Sediments

Recording Noise - Estimating Shear-Wave Velocities: Feasibility of Offshore Ambient-Noise Surface-Wave Tomography (ANSWT) On a Reservoir Scale