S-wave Velocity Field Research Articles

Analyzing the Stability of Rock Surrounding Deep Cross-Tunnels Using a Dynamic Velocity Field

With the increasing number of deep rock engineering projects, many different types of tunnels have emerged, such as cross-tunnels. These tunnels intersect with each other in rock, which causes potential safety hazards. We must analyze the stability of the surrounding rock, to ensure worker safety. This article presents a method for dynamically assessing the stability of the surrounding rock in deep-buried cross-tunnels. The method consists of two main analysis steps: (1) P-wave velocity field inversion; and (2) Stability analysis of the surrounding rock. The P-wave velocity field inversion involves inverting the S-wave velocity field by Rayleigh wave and inverting the P-wave velocity field by adjoint state traveltime tomography. Then, a method of stability analysis is proposed which is used to update the mechanical properties of the rock (based on the continuously updated wave velocity field). The elastic modulus of the surrounding rock is approximated throughout the excavation process. CASRock V1.0 (Cellular Automation Software for engineering Rockmass fracturing processes) is used to assess rock damage via the equivalent plastic shear strain and local energy release rate. The new method is used to analyze the stability of a new tunnel excavated in Jinping (in China). The results reveal the severity and spatial distribution of the damage caused. The yield depth is concentrated near the sidewalls, while the top and bottom of the tunnel exhibit a smaller depth. The yield depths present a particular pattern of change (high–low–high–low) with increasing distance from tunnel #2. Finally, this research enriches our understanding of excavating deep cross-tunnels and makes an important contribution to improving worker safety in deep cross-tunnels.

Structure of the Kathmandu basin inferred from microtremor measurements: a preliminary study

SUMMARY This research aims at characterizing the deep sedimentary basin of Kathmandu (Nepal) from microtremor measurements, by using two surveys carried out in November 2015 and November 2018. The data collected are sufficient for a qualitative investigation of the seismic response of such a complex basin. The measurements were undertaken in the framework of a collaboration between the NSET (National Society for Earthquake Technology, Nepal) and the OGS (National Institute of Oceanography and Applied Geophysics, Italy). During the two campaigns, a series of passive seismic surveys were made, taking into consideration both single broad-band sensors and arrays. The 2015 study initially focused on the Lalitpur area (southern part of Kathmandu) and was later extended to the northern and more urbanized area of the capital city of Nepal. The 2018 survey focused on estimating the S-wave velocity field of the entire stratigraphic sequence by array and a repetition of some Horizontal to Vertical Spectral Ratio (HVSR) measurements close to two boreholes with reliable stratigraphy. Two inversion methods, using the data from the seismic arrays, were useful to infer the S-wave velocity profiles of the shallower layers and to interpret some HVSRs. The outcome is a map of the periods of single station data representing the seismic response of the basin. Comparing these results with the strong motion data of the Gorkha 2015 main shock, we confirm that the longer periods are not only the effects of the earthquake source, but also of the local response of the basin itself.

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.

Detailed Structure of Attenuation Field in the Western Tien Shan Based on Short-Period Coda Waves

The seismic coda waves attenuation characteristic in the western Tien Shan region were studied. The short-period coda waves of the local earthquakes, recorded by the frequency-selective seismic stations in 1960–1984, and digital seismogram data of 150 earthquakes that had been occurring in the western Tien Shan region since 2005 until 2017, recorded at the AAK station (KNET network), were analyzed. The attenuation field is represented by the zones (blocks) having high Q-factor and being close to isometric in plan and by the linear zones of strong attenuation (weakened zones), which coincide with faults. It was revealed that the structure of attenuation field agrees with the structure of S-wave velocity field and the velocity anomalies of P-waves: low-velocity anomalies correspond to low-Q zones. The most expressed conducting zones, revealed from magnetotelluric data, coincided with the low-Q zones, indicating their significant fluid saturation. Sources of the strongest earthquakes with K > 16.0 were confined to the zones of maximum attenuation contrast, to the boundaries of blocks and weakened zones, where the mobility of blocks was determined by reduced viscosity of low-Q layers and small thickness of high-Q ones. The relationship between the features of seismic processes and spatial inhomogeneities of the attenuation field in the background of variations in the Earth’s rotation speed was also considered.

3D S-wave velocity imaging of a subsurface disturbed by mining using ambient seismic noise

Seismic interferometry based on ambient seismic noise was used to image the 3D S-wave velocity field of a subsurface disturbed by mining under the geological conditions at the KGHM Rudna copper mine in southwestern Poland. We recorded ambient seismic noise data for 3 months using 11 surface-mounted broadband seismometers located across the mining area. We used ambient-noise cross-correlation techniques to create a three-dimensional S-wave velocity model of the Cenozoic and Triassic formations up to a depth of 500 m. Cross-correlation functions (CCFs) indicate the Rayleigh and Love waves; however, only the Love waves in the 0.19–0.4 Hz frequency range were reliable, based on the symmetry of the CCFs in the positive and negative conditions. Imaging using the Love waves indicated that the changes in the S-wave velocity field in the model may be explained by the subsidence processes occurring above the mined area. The S-wave velocity decreased above the areas of the current mining operations. However, the S-wave velocity increased above older mined areas, probably due to suppression of the subsidence process and progressive consolidation of the geological medium over time. We also observed low-velocity anomalies in the intersection zones of the larger faults, with high energy seismic events. This could be caused by fault activation by current mining operations and a reduction in the stiffness of the layers in the fault intersection zone. This study presents the possibility of imaging the subsurface layers in a mining area using ambient seismic noise. It also has the potential to give a broader insight into how a geological medium responds to mining in terms of protecting the terrain surface and developing mining technologies.

Comparison analysis of numerically calculated slip surfaces with measured S-wave velocity field for Just-Tęgoborze landslide in Carpathian flysch

The article presents the comparison analysis between deformation field from numerical model and shear wave (S-wave) velocity field obtained from seismic interferometry (SI). Tests were conducted on active Just-Tęgoborze landslide. Geologically, the study area lies in Magura Nappe in the Outer Carpathians. The landslide’s flysch bedrock is covered by Quaternary colluvium built of clays and weathered clayey-rock deposits. During geotechnical investigation, properties of landslide body were established and failure surfaces were distinguished. In order to obtain S-wave velocity models, one-hour of ambient seismic noise was recorded by 12 broadband seismometers. As a result of data processing with SI method, Rayleigh surface wave propagation was reconstructed. The analysis of dispersion curves allowed to estimate a two dimensional S-wave velocity field. The deformation field were calculated assuming an elastic-plastic Coulomb-Mohr strength criterion. Images of shear strain increment, and values of factor of safety of the slope were obtained as a result of calculation. The comparison of the results indicates the similar characteristic features in the S-wave velocity field and the field of deformation calculated numerically.

OBS Data Analysis to Quantify Gas Hydrate and Free Gas in the South Shetland Margin (Antarctica)

The presence of a gas hydrate reservoir and free gas layer along the South Shetland margin (offshore Antarctic Peninsula) has been well documented in recent years. In order to better characterize gas hydrate reservoirs, with a particular focus on the quantification of gas hydrate and free gas and the petrophysical properties of the subsurface, we performed travel time inversion of ocean-bottom seismometer data in order to obtain detailed P- and S-wave velocity estimates of the sediments. The P-wave velocity field is determined by the inversion of P-wave refractions and reflections, while the S-wave velocity field is obtained from converted-wave reflections received on the horizontal components of ocean-bottom seismometer data. The resulting velocity fields are used to estimate gas hydrate and free gas concentrations using a modified Biot-Geertsma-Smit theory. The results show that hydrate concentration ranges from 10% to 15% of total volume and free gas concentration is approximately 0.3% to 0.8% of total volume. The comparison of Poisson’s ratio with previous studies in this area indicates that the gas hydrate reservoir shows no significant regional variations.

Comparison of an empirical S-wave velocity model and a calculated stress-strain model for a rock mass disturbed by mining

In the article a comparison analysis is presented between a numerical model of the stress and deformation state in a rock mass and an S-wave velocity model obtained as a result of in situ measurement. The research was conducted using data from the Jastrzebie and Moszczenica coal mines. The part of the rock mass examined was strongly disturbed by multi-seam exploitation of coal. To obtain the S-wave velocity model 6 hours of ambient seismic noise data were recorded using 11 seismometers. The propagation of the Rayleigh surface wave between the seismometers was reconstructed utilising the seismic interferometry and the cross correlation technique. Estimation of a two dimensional model of the Swave velocity field was performed on the basis of dispersion curves of the Rayleigh wave phase velocity. The stress and deformation field were calculated assuming a plane state of stress with the use of the elastic-plastic Coulomb-Mohr strength criterion. Images of the vertical stress, horizontal stress, vertical strain and horizontal strain as well as the subsidence profile on the model surface were obtained as a result of the calculation. Analysis of the results shows correlation between the field of S-wave velocity and the modelled field of stress and strain.

Near-surface velocity modeling using a combined inversion of surface and refracted P-waves

We propose an innovative workflow based on the complementary use of Rayleigh waves alongside standard P-wave refraction tomography, which better depicts the shallow part of the near-surface P-wave velocity model. Our surface-wave processing sequence led to an S-wave near-surface velocity field that can be used as a constraint for P-wave tomography and can improve P-wave statics determination. Rayleigh waves are processed in three steps. The first step consists of an accurate frequency-dependent traveltime measurement for each selected source-receiver pair in which the phase difference between two adjacent traces is used to derive the phase velocity. Then, a frequency-dependent surface-wave velocity tomography is performed from the picked traveltimes. Finally, after surface-wave tomography, the frequency-dependent phase velocity volume output by the tomography is inverted to deliver an S-wave near-surface velocity model. This model is used to constrain the first-arrival P-wave tomography. To illustrate our method, we use a 3D narrow-azimuth land data set, acquired along a river valley. Strong lateral velocity variations exist in the shallow part, with slow velocities around the unconsolidated sediments of the riverbed and faster velocities in the consolidated sediments of the surrounding hills. A combined first-arrival tomography using the S-wave velocity model, the initial unconstrained refracted P-wave velocity model, and the original first arrivals is used to obtain a more accurate near-surface P-wave velocity model. This new approach led to a constrained P-wave velocity model from which primary statics are derived and then applied, leading to an improved image with better focusing and continuity of thin layers in the shallowest part.

Wet and gassy zones in a municipal landfill from P- and S-wave velocity fields

The knowledge of the distribution of leachate and gas in a municipal landfill is of vital importance to the landfill operators performing improved landfill treatments and for environmental protection and efficient biogas extraction. We have explored the potential of using the velocity fields of seismic S- and P-waves to delineate the wet and gassy (relatively dry, gas/air-filled) zones inside a landfill. We have analyzed shallow S- and P-wave reflection data and seismic surface-wave data acquired at a very heterogeneous landfill site, where biogas was extracted. A joint interpretation of the independently estimated velocity fields from these various approaches has allowed us to localize anomalously low- and high-velocity zones in the landfill. From the complementary information provided by P- and S-wave velocity fields, we have inferred the leachate-bearing wet zones and the gassy zones inside the landfill. Independent measurements of gas flow and mechanical tip resistance to waste deformation validate our seismic interpretations.

Earthquake-faulting-related deformation in soil evidenced in S-wave shallow reflection data: Field results from Portugal

Expressions of ductile, soft-sediment deformations induced by ground movements due to past earthquakes are difficult to recognize in near-surface soils. We have carried out shallow S-wave reflection studies in a seismically active area located northeast of metropolitan Lisbon, Portugal. Identifying shallow disturbed zones and hidden fault segments in this area is important but quite difficult because of small vertical slips due to earthquakes, the Holocene alluvial cover hiding the fault segments, and a high rate of surficial sedimentation. We have performed S-wave reflection profiling at two sites — Vila Franca Xira and Castanheira de Ribatejo. We detected different but interrelated evidence of soft-sediment deformation in the seismic data. This evidence includes sharp lateral changes in the S-wave velocity field; changes in the reflection horizons in stacked sections; aligned diffractions in unmigrated sections; discontinuities in common-offset gathers; and discontinuities, backscattered, and diffracted arrivals in common-source gathers. Though not equally clear everywhere, this evidence is recognizable at many locations where earthquake-motion-induced disturbed zones are interpreted. To confirm these interpretations, we have performed synthetic modeling of a seismic wavefield using the same acquisition geometry as in the field experiments, and with multiple disturbed zones present as vertical emplacements through horizontally lying soil layers. The modeling results resemble the observations in field data. It is possible to confirm the signatures of soft-sediment deformation in the shallow S-wave reflection data. The approach that we used will be useful in many seismically active, soil-covered areas in the world.

Multicomponent joint migration velocity analysis in the angle domain for PP-waves and PS-waves

Employing the vector processing of multicomponent seismic data, elastic Kirchhoff migration is used to conduct a multicomponent joint migration velocity analysis (MVA) of PP- and PS-waves in the angle domain. In vector-wavefield imaging, the elastic imaging condition has been extended to nonzero time and space shifts. We apply the extended imaging condition to elastic Kirchhoff migration to extract angle-domain common-image gathers (ADCIGs) of PP- and PS-waves. This method, which is derived from 3D wave propagation theory, directly operates on the vector wavefields and automatically resolves the problem of PS-wave polarity reversal in the migration sections. Based on the kinematic characteristics of PP- and PS-waves in the incident-angle domain, the velocity updating functions for P- and S-waves are derived. By combining the ADCIGs flatness criterion and the PP- and PS-wave image-depth consistency principle, the accuracy of P- and S-wave velocities can be assessed. The method was tested on a 2D thin-interbed model and the 2D Marmousi2 model. We found that the proposed method extracts the PP- and PS-wave ADCIGs effectively and produces appropriate P- and S-wave velocity fields. This method can be applied to time-delay gathers and to vertical or inline gathers in the 3D case.

Characterizing landslides through geophysical data fusion: Example of the La Valette landslide (France)

Many studies show that certain geophysical methods, such as seismic and electrical-resistivity imaging, appear to be well adapted for investigating the internal structures of landslides and understanding the related hydro-mechanical mechanisms. These are methods that allow the direct and non-intrusive measurement of acoustic (P) and shear (S) wave velocities and electrical resistivity (ρ), which are three physical parameters considered as essential for estimating the mechanical properties of moving reworked material. We applied these techniques to the La Valette landslide (Southern French Alps), a typical example of an intra-material landslide, carrying out measurements simultaneously along two profiles, 400m and 300m long and respectively perpendicular to and along the slide direction. We then used suitable inversion algorithms to estimate both the P- and S-wave velocity fields and the electrical resistivity field from the recorded data. The results, aided by field surface observations, show that a correlation exists between the state of the material and the seismic-velocity and/or electrical-resistivity data, thus confirming that the simultaneous use of the two methods provides complementary information on the geomechanical behavior of the landslide. More particularly, the seismic data provide information on fissure density variations and the presence of shear-bent material, whereas the electrical resistivity data provide information on the groundwater content. To enable a more integrated petrophysical interpretation, we applied a data-fusion strategy based on fuzzy subsets to the geophysical datasets. Through combining the tomograms we identified a surface layer of soft material along the two profiles; the bottom of this layer was also recognized in a borehole. From a methodological point of view, the results show the applicability of adopting geomechanical hypotheses as inputs of geophysical data fusion for identifying areas where sediment mobilization could occur.

Rayleigh-wave dispersive energy imaging and mode separating by high-resolution linear Radon transform

In recent years, multichannel analysis of surface waves (MASW) has been increasingly used for obtaining vertical shear-wave velocity profiles within near-surface materials. MASW uses a multichannel recording approach to capture the time-variant, full-seismic wavefield where dispersive surface waves can be used to estimate near-surface S-wave velocity. The technique consists of (1) acquisition of broadband, high-frequency ground roll using a multichannel recording system; (2) efficient and accurate algorithms that allow the extraction and analysis of 1D Rayleigh-wave dispersion curves; (3) stable and efficient inversion algorithms for estimating S-wave velocity profiles; and (4) construction of the 2D S-wave velocity field map.

Geophysical data fusion by fuzzy logic for imaging the mechanical behaviour of mudslides

Abstract Geophysical methods such as seismic surveying or electrical resistivity imaging appear to be well adapted to investigate landslide structure and understand related mechanisms. They allow direct and non-intrusive measurements of acoustic (P), shear (S) waves velocity and electrical resistivity, three physical parameters considered as essential to define the properties of reworked moving materials. Both methods were applied at the “Super-Sauze” site, in the French South Alps, where a typical example of an intra-material mudslide can be observed. Measurements were taken simultaneously along a profile of 325 m in length, perpendicularly to the axis of the mudslide. The P and S-wave velocity fields, as well as the electrical resistivity field, were inverted from recorded data according to suitable algorithms. P and S-wave velocities as well as resistivity tomographies are presented and discussed in term of reliability. Preliminary interpreted results show a correlation between the seismic velocities and electrical resistivity data, confirming that the simultaneous use of both methods gives complementary information on the geomechanical behaviour of the landslide. The seismic data provide information on the variations of fissure density and on the presence of deformed material whereas the electrical resistivity data provide information on the variations of water content within the mudslide. In order to go deeper into the interpretation of the geomechanical behaviour of the mudslide from geophysical data, a data fusion strategy based on fuzzy subsets theory is developed. The computed fuzzy cross-sections show the possibility of geomechanical hypotheses to be realized in specific areas of the tomographic cross-sections highlighting the places where plastic or solid-body deformations could occur. This information is consistent with the geotechnical data and the borehole inclinometer measurements available for the mudslide.

Delineation of a collapse feature in a noisy environment using a multichannel surface wave technique

A collapse developed at Calvert Cliffs Nuclear Power Plant, Maryland, in early 2001. The location of the collapse was over a groundwater drainage system pipe buried at an elevation of +0·9 m (reference is to Chesapeake Bay level). The cause of the collapse was a subsurface drain pipe that collapsed because of saltwater corrosion of the corrugated metal pipe. The inflow/outflow of sea water and groundwater flow caused soil to be removed from the area where the pipe collapsed. To prevent damage to nearby structures, the collapse was quickly filled with uncompacted sand and gravel (∼36 000 kg). However, the plant had an immediate need to determine whether more underground voids existed. A high-frequency multichannel surface-wave survey technique was conducted to define the zone affected by the collapse. Although the surface-wave survey at Calvert Cliffs Nuclear Power Plant was conducted at a noise level 50–100 times higher than the normal environment for a shallow seismic survey, the shear (S)-wave velocity field calculated from surface-wave data delineated a possible zone affected by the collapse. The S-wave velocity field showed chimney-shaped low-velocity anomalies that were directly related to the collapse. Based on S-wave velocity field maps, a potential zone affected by the collapse was tentatively defined.

Delineation of a collapse feature in a noisy environment using a multichannel surface wave technique

A collapse developed at Calvert Cliffs Nuclear Power Plant, Maryland, in early 2001. The location of the collapse was over a groundwater drainage system pipe buried at an elevation of +0·9 m (reference is to Chesapeake Bay level). The cause of the collapse was a subsurface drain pipe that collapsed because of saltwater corrosion of the corrugated metal pipe. The inflow/outflow of sea water and groundwater flow caused soil to be removed from the area where the pipe collapsed. To prevent damage to nearby structures, the collapse was quickly filled with uncompacted sand and gravel (∼36 000 kg). However, the plant had an immediate need to determine whether more underground voids existed. A high-frequency multichannel surface-wave survey technique was conducted to define the zone affected by the collapse. Although the surface-wave survey at Calvert Cliffs Nuclear Power Plant was conducted at a noise level 50–100 times higher than the normal environment for a shallow seismic survey, the shear (S)-wave velocity field calculated from surface-wave data delineated a possible zone affected by the collapse. The S-wave velocity field showed chimney-shaped low-velocity anomalies that were directly related to the collapse. Based on S-wave velocity field maps, a potential zone affected by the collapse was tentatively defined.