SM5.3

EDI
Shallow shear-wave and multi-component seismic techniques – methodical capability, technical developments, data processing, and case studies

In recent years, the application of shear-wave seismic methods for shallow investigations (< 500 m depth) has become more and more popular. Shear waves are utilized for structural imaging, geotechnical investigations, and elastic parameter analysis. Methods using shear waves comprise, e.g., reflection imaging, tomography, and full waveform inversion.
Shear-wave imaging has great potential for shallow studies. For instance, near-surface resolution profits from low shear-wave velocities. Especially, shear wave reflection signals can be detected at small offsets compared to P-waves, which makes shear-wave reflection surveying cost efficient.
Shear-wave surveys can profit from sealed ground conditions due to the suppression of Lovewaves, and, thus, are predesignated for urban areas. But shallow shear-wave and multicomponent seismic requires a continuous technical development of specialized sources and customized equipment and makes innovative concepts for acquisition and data processing necessary (e.g. interferometry, full waveform inversion, converted waves).
Exciting as well as recording several components of the ground motion simultaneously is further beneficial, since it allows separating vertically (SV) and horizontally (SH) polarized shear wavefields, which is mandatory for 3-D surveys. Wave conversion and scattering effects can be distinguished, and differently polarised shear waves simplify the detection of seismic anisotropy. This session promotes the exchange of experience using shear waves in shallow applications and triggers discussions about their potential in seismic imaging. Combined studies using P- and shear waves are a plus. With the focus on shear waves, we invite, but do not restrict, contributions to technical development, data analysis, seismic processing, and case studies. The latter may comprise, e.g., (a) geotechnical studies, such as examination of soil rigidity, (b) exploration of structures, such as volcanic craters or groundwater resources, (c) analysis of neotectonics, active faults, quick clays, landslides, sinkholes, and subrosion structures, and (d) more exotic applications, such as the exploration of glacier ice thickness.

Convener: Sonja WadasECSECS | Co-conveners: Thomas BurschilECSECS, Barbara Dietiker, Pier Vittorio Radogna
vPICO presentations
| Thu, 29 Apr, 09:00–09:45 (CEST)

Session assets

Session materials

vPICO presentations: Thu, 29 Apr

Chairpersons: Sonja Wadas, Thomas Burschil, Barbara Dietiker
09:00–09:05
09:05–09:10
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EGU21-10170
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solicited
Andre Pugin, Barbara Dietiker, Kevin Brewer, and Timothy Cartwright

In the vicinity of Ottawa, Ontario, Canada, we have recorded many multicomponent seismic data sets using an in-house multicom­ponent vibrator source named Microvibe and a landstreamer receiver array with 48 3-C 28-Hz geophones at 0.75-m intervals. The receiver spread length was 35.25 m, and the near-offset was 1.50 m. We used one, two or three source and three receiver orientations — vertical (V), inline-horizontal (H1), and transverse-horizontal (H2). We identified several reflection wave modes in the field records — PP, PS, SP, and SS, in addition to refracted waves, and Rayleigh-mode and Love-mode surface waves. We computed the semblance spectra of the selected shot records and ascertained the wave modes based on the semblance peaks. We then performed CMP stacking of each of the 9-C data sets using the PP and SS stacking velocities to compute PP and SS reflection profiles.

Despite the fact that any source type can generate any combination of wave modes — PP, PS, SP, and SS, partitioning of the source energy depends on the source orientation and VP/VS ratio. Our examples demonstrate that the most prominent PP reflection energy is recorded by the VV source-receiver orientation, whereas the most prominent SS reflection energy is recorded by the H2H2 source-receiver orientation with possibility to obtain decent shear wave near surface data in all other vibrating and receiving directions.

Pugin, Andre and Yilmaz, Öz, 2019. Optimum source-receiver orientations to capture PP, PS, SP, and SS reflected wave modes. The Leading Edge, vol. 38/1, p. 45-52. https://doi.org/10.1190/tle38010045.1

How to cite: Pugin, A., Dietiker, B., Brewer, K., and Cartwright, T.: Shear and compressive wave data acquisition using multicomponent vibrating source and landstreamer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10170, https://doi.org/10.5194/egusphere-egu21-10170, 2021.

09:10–09:12
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EGU21-2916
Barbara Dietiker, André J.-M. Pugin, Matthew P. Griffiths, Kevin Brewer, and Timothy Cartwright

Based on our experience, one of the most important steps in processing shear-wave seismic reflection data is the velocity analysis. In unconsolidated materials a very fine velocity analysis is more essential for S-waves than for P-waves because shear-wave velocities vary over several orders of magnitude and can change very quickly laterally and with depth. Velocities between 100m/s in glaciolacustine/marine deposits (clay-sized silts) and 1200m/s in stiff diamicton (till) were encountered in recent surveys. Shear-wave velocities have the large advantage of not being changed by the phase of the pore content such as the groundwater table.

We present two fundamentally different methods for velocity determination: 1) velocity semblance analysis based on hyperbolic reflection move-out on common midpoint (cmp) gathers and 2) Local Phase – Local Shift (LPLS) method which automatically estimates the reflection slope (local static shift) in the time-frequency domain of cmp gathers. Published in 2020, the latter method can be used for automated processing and substantially saves processing time.

Processing steps in preparation for velocity analysis (independent of the chosen method) include frequency filtering, trace equalizing and muting. We show velocity semblance images from different geological settings (glacial, postglacial) and from different shear components and discuss differences. Information gained besides shear velocities include mapped reflectors and located diffractions. Using those examples, we demonstrate how combining all information using visualisation techniques enhances interpretation of such data sets.

How to cite: Dietiker, B., Pugin, A. J.-M., Griffiths, M. P., Brewer, K., and Cartwright, T.: Shear-wave seismic reflection processing - the importance of velocity analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2916, https://doi.org/10.5194/egusphere-egu21-2916, 2021.

09:12–09:14
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EGU21-11685
Ulrich Polom, Rebekka Mecking, Phillip Leineweber, and Andreas Omlin

In the North German Basin salt tectonics generated a wide range of evaporite structures since the Upper Triassic, resulting in e.g. extended salt walls, salt diapirs, and salt pillows in the depth range up to 8 km. Due to their trap and seal properties these structures were in the focus of hydrocarbon exploration over many decades, leading to an excellent mapping of their geometries below 300 m in depth. During salt rise Rotliegend formations were partly involved as a constituent. Some structures penetrated the salt table, some also the former surface. Dissolution (subrosion) and erosion of the salt cap rock by meteoric water took place, combined with several glacial and intraglacial overprints. Finally the salt structures were covered by pleistocene and holocene sediments. This situation partly resulted in proneness for ongoing karstification of the salt cap rock, leading to e.g. local subsidence and sinkhole occurrence at the surface. The geometry, structure and internal lithology of these shallow salt cap rocks are widely unknown. Expanding urban and industrial development, water resources management and increasing climate change effects enhance the demands for shallow mapping and characterization of these structures regarding save building grounds and sustainable water resources.

Results of shallow drilling investigations of the salt cap rock and the overburden show unexpectedly heterogenous subsurface conditions, yielding to limited success towards mapping and characterization. Thus, shallow high-resolution geophysical methods are in demand to close the gaps with preferred focus of applicability in urban and industrial environments. Method evaluations starting in 2010 geared towards shallow high-resolution reflection seismic to meet the requirements of both depth penetration and structure resolution. Since 2017 a combination of S-wave and P-wave seismic methods including depth calibrations by Vertical Seismic Profiling (VSP) enabled 2.5D subsurface imaging starting few meters below the surface up to several hundred meters depth in 0.5-5 m resolution range, respectively. The resulting profiles image strong variations along the boundaries and on top of the salt cap rock. Beside improved mapping capabilities, aim of research is the development of characteristic data features to differentiate save and non-save areas.

How to cite: Polom, U., Mecking, R., Leineweber, P., and Omlin, A.: Reflection seismic imaging of buried shallow salt structures – examples from ongoing case studies in Northern Germany, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11685, https://doi.org/10.5194/egusphere-egu21-11685, 2021.

09:14–09:16
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EGU21-12202
Ranajit Ghose

Shear waves are uniquely informative because of their vector nature – with both polarization and propagation of shear waves being useful sources of information, their sensitivity to in-situ stress and grain-to-grain contact, and also because of the low velocity of shear waves in relatively soft formations - offering short wavelength and hence high resolution. Decimetre-scale resolution found in shear-wave reflection data in soft soil has resulted in new application possibilities. Medium anisotropy extracted from multi-component shear-wave data has provided information on natural symmetries in small-strain rigidity and/or stress in the shallow subsurface, which are caused by factors that are of great interest to the engineers. AVO response of shear waves at near-surface soil-layer boundaries has also proven to be useful for extracting local information in the subsoil.

In the present research we have looked at the sensitivity of shear-wave velocity and the underlying physics in both saturated and unsaturated near-surface soils, and if these can practically be used for monitoring soil dynamics and soil stability. Time-lapse changes in shear-wave velocity could be used to monitor changes in in-situ stress in the saturated sands. More recently, we have developed methodologies to invert time-lapse shear-wave velocity information together with geo-electrical information to obtain in-situ values of water saturation and suction in different partially saturated soil units. Incorporation of this information in a spatially varying sense is imperative in order to make assessment of stability of unsaturated soil slopes subjected to rainfall, modelling flooding and sediment flows due to increased surface runoff and erosion, sustainable agriculture through in-situ water moisture monitoring, and modelling pollutant transport through soils.

How to cite: Ghose, R.: Physics of shear-waves propagating in saturated and unsaturated soils: new potential for monitoring soil dynamics and stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12202, https://doi.org/10.5194/egusphere-egu21-12202, 2021.

09:16–09:18
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EGU21-14727
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ECS
A shallow Vs model from full wave inversion for hydraulic fracturing site 
(withdrawn)
Guoliang Li and Min Chen
09:18–09:20
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EGU21-11287
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ECS
Prabhakar Kumar and Dibakar Ghosal

The continent-continent collision between the Indian and Asian Plate formed a series of major faults from north to south along the Himalayan belt. Among these Himalayan Frontal Thrust (HFT) is the southernmost and youngest one and is tectonically very active. Any information on the shear wave velocity distribution across the fault is therefore very important. In this study, we have used the Wide Angle Multichannel Analysis of Surface Wave (WAMASW) to estimate the subsurface shear wave velocity profiles across HFT at Pawalgarh in Uttarakhand, India, using widely used stochastic global search Particle Swarm Optimization (PSO) and Grey wolf Optimization (GWO) algorithms. To gain confidence on the accuracy of the inversion results, we first generated an elastic synthetic seismic shot gather with ground rolls by using the forward modelling scheme of SOFI2D for a two-layer velocity depth model overlying a half-space. The generated gather was then processed in MATLAB to generate the experimental dispersion curve using the Phase shift method. We then extracted the fundamental mode for the gather and inverted it using the standard PSO and GWO algorithms and estimated 1D shear wave velocity profile. After getting acceptable results for the synthetic dataset, we then applied the PSO algorithm to generate the 1D S-wave velocity (Vs) profile across the Himalayan Frontal Thrust (HFT). In the study area, the Rayleigh wave phase velocity for the first shot varies from 444 to 743 m/s. We then obtained the 1D shear wave velocity profiles and a jump in Vs is observed across the HFT indicating variation in the sediment stiffness across the fault.

Keywords: WAMASW, dispersion, Meta- Heuristic, PSO, GWO, 1D Shear wave velocity

 

How to cite: Kumar, P. and Ghosal, D.: Delineation of S-wave velocity profiles across the Himalayan Frontal Thrust (HFT) using metaheuristic approaches., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11287, https://doi.org/10.5194/egusphere-egu21-11287, 2021.

09:20–09:22
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EGU21-9243
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ECS
Sebastián Carrasco, Brigitte Knapmeyer-Endrun, Ludovic Margerin, Cédric Schmelzbach, John Clinton, Simon Stähler, Domenico Giardini, Sharon Kedar, Matthias Grott, Matthew Golombek, Philippe Lognonné, and Don Banfield

The InSight mission landed on Mars on November 26th, 2018 and its seismometer, the Seismic Experiment for Interior Structure (SEIS), has recorded continuous Martian seismic data since February 2019, consisting of mainly ambient seismic noise but also hundreds of seismic events.

We used the SEIS data to study the horizontal-to-vertical spectral ratios from both the ambient seismic noise (nHV) and the seismic events (eHV), for frequencies above 0.6 Hz, in order to get further constraints on the first tens of meters at the Insight landing site. The nHV curve was obtained by using data segments of 50 s over more than 400 Sols. The preferred nHV curve is observed during the northern spring and summer at low wind levels and it is a mostly flat curve with a prominent trough around ~2.4 Hz. Outside of these time periods, the nHV curve is contaminated with artificial peaks likely related to lander modes. On the other hand, the eHV curve was created using 336 seismic events with quality either A, B or C, as defined by the Marsquake Service. For each seismic event, we computed the signal-to-noise ratio (SNR) at each frequency and only frequencies with SNR>3 were used to obtain the final eHV curve. In addition to the 2.4 Hz trough, the final eHV curve shows a strong peak around 8 Hz, which is not observed from the ambient noise data possibly due to a lack of seismic energy in this frequency band able to excite it.

A preliminary inversion of the eHV curve, considering the fundamental mode of the Rayleigh wave only, shows that the 2.4 Hz trough and the 8 Hz peak can be explained by a shear-wave velocity model increasing from the surface to a depth of 5-8 m (likely the boundary between the regolith and coarse ejecta), in good agreement with previous analysis based on compliance observations, hammering measurements and satellite images. At this depth, a discontinuity leading to a higher velocity layer is observed, which is followed by a deeper low-velocity layer about 20 m thick. The modeling assuming body waves only or a full diffuse seismic wavefield is currently under investigation.

How to cite: Carrasco, S., Knapmeyer-Endrun, B., Margerin, L., Schmelzbach, C., Clinton, J., Stähler, S., Giardini, D., Kedar, S., Grott, M., Golombek, M., Lognonné, P., and Banfield, D.: H/V spectral ratios at the InSight landing site using ambient noise and Marsquake records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9243, https://doi.org/10.5194/egusphere-egu21-9243, 2021.

09:22–09:45