Seismic anisotropy of Opalinus Clay: tomographic investigations using the infrastructure of an underground rock laboratory (URL)

AbstractSeismic anisotropy and attenuation make claystone formations difficult to characterize. On the other hand, in many geotechnical environments, precise knowledge of structure and elastic properties of clay formations is needed. In crystalline and rock salt underground structures, high-resolution seismic tomography and reflection imaging have proven a useful tool for structural and mechanical characterization at the scale of underground infrastructure (several deca- to hundreds of meters). This study investigates the applicability of seismic tomography for the characterization of claystone formations from an underground rock laboratory under challenging on-site conditions including anisotropy, strong attenuation and restricted acquisition geometry. The seismic tomographic survey was part of a pilot experiment in the Opalinus Clay of the Mont Terri Rock Laboratory, using 3-component geophones and rock anchors, which are installed 2 m within the rock on two levels, thus suppressing effects caused by the excavation damage zone. As a source, a pneumatic impact source was used. The survey covers two different facies types (shaly and carbonate-rich sandy), for which the elliptical anisotropy is calculated for direct ray paths by fitting an ellipse to the separated data for each facies. The tomographic inversion was done with a code providing a good grid control and enabling to take the seismic anisotropy into account. A-priori anisotropy can be attributed to the grid points, taking various facies types or other heterogeneities into account. Tomographic results, compared to computations using an isotropic velocity model, show that results are significantly enhanced by considering the anisotropy and demonstrate the ability of the approach to characterize heterogeneities of geological structures between the galleries of the rock laboratory.

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Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography

Scientific Reports ◽

10.1038/s41598-021-84642-7 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Quan Sun ◽

Shunping Pei ◽

Zhongxiong Cui ◽

Yongshun John Chen ◽

Yanbing Liu ◽

...

Keyword(s):

High Resolution ◽

Seismic Tomography ◽

High Velocity ◽

Three Dimensional ◽

Large Earthquake ◽

Velocity Model ◽

Tectonic Stress ◽

Double Difference ◽

Source Regions ◽

Large Earthquakes

AbstractDetailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

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Passive seismic tomography using recorded microseismicity: Application to mining-induced seismicity

Geophysics ◽

10.1190/geo2018-0076.1 ◽

2019 ◽

Vol 84 (1) ◽

pp. B41-B57 ◽

Cited By ~ 4

Author(s):

Himanshu Barthwal ◽

Mirko van der Baan

Keyword(s):

Seismic Tomography ◽

Velocity Model ◽

P Wave ◽

Double Difference ◽

Lateral Velocity ◽

Study Region ◽

Arrival Times ◽

3D Velocity Model ◽

Passive Seismic ◽

Dip Angle

Microseismicity is recorded during an underground mine development by a network of seven boreholes. After an initial preprocessing, 488 events are identified with a minimum of 12 P-wave arrival-time picks per event. We have developed a three-step approach for P-wave passive seismic tomography: (1) a probabilistic grid search algorithm for locating the events, (2) joint inversion for a 1D velocity model and event locations using absolute arrival times, and (3) double-difference tomography using reliable differential arrival times obtained from waveform crosscorrelation. The originally diffusive microseismic-event cloud tightens after tomography between depths of 0.45 and 0.5 km toward the center of the tunnel network. The geometry of the event clusters suggests occurrence on a planar geologic fault. The best-fitting plane has a strike of 164.7° north and dip angle of 55.0° toward the west. The study region has known faults striking in the north-northwest–south-southeast direction with a dip angle of 60°, but the relocated event clusters do not fall along any mapped fault. Based on the cluster geometry and the waveform similarity, we hypothesize that the microseismic events occur due to slips along an unmapped fault facilitated by the mining activity. The 3D velocity model we obtained from double-difference tomography indicates lateral velocity contrasts between depths of 0.4 and 0.5 km. We interpret the lateral velocity contrasts in terms of the altered rock types due to ore deposition. The known geotechnical zones in the mine indicate a good correlation with the inverted velocities. Thus, we conclude that passive seismic tomography using microseismic data could provide information beyond the excavation damaged zones and can act as an effective tool to complement geotechnical evaluations.

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Influence of pre-existing discontinuities on ground behavior in the Opalinus Clay – a new mine-by project at the Mont Terri rock laboratory

Rock Mechanics in Civil and Environmental Engineering ◽

10.1201/b10550-186 ◽

2010 ◽

pp. 795-798

Keyword(s):

Opalinus Clay ◽

Rock Laboratory ◽

Mont Terri ◽

Mont Terri Rock Laboratory

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Multi-proxy facies analysis of the Opalinus Clay and depositional implications (Mont Terri rock laboratory, Switzerland)

Swiss Journal of Geosciences ◽

10.1007/s00015-018-0303-x ◽

2018 ◽

Vol 111 (3) ◽

pp. 383-398 ◽

Cited By ~ 2

Author(s):

Bruno Lauper ◽

David Jaeggi ◽

Gaudenz Deplazes ◽

Anneleen Foubert

Keyword(s):

Facies Analysis ◽

Opalinus Clay ◽

Rock Laboratory ◽

Mont Terri ◽

Mont Terri Rock Laboratory

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Results of the downhole microseismic monitoring at a pilot hydraulic fracturing site in Poland — Part 1: Event location and stimulation performance

Interpretation ◽

10.1190/int-2017-0205.1 ◽

2018 ◽

Vol 6 (3) ◽

pp. SH39-SH48 ◽

Cited By ~ 6

Author(s):

Wojciech Gajek ◽

Jacek Trojanowski ◽

Michał Malinowski ◽

Marek Jarosiński ◽

Marko Riedel

Keyword(s):

Hydraulic Fracturing ◽

Seismic Anisotropy ◽

Transverse Isotropy ◽

Velocity Model ◽

Microseismic Monitoring ◽

Baltic Basin ◽

Hydraulic Stimulation ◽

Lower Paleozoic ◽

Event Location ◽

Microseismic Events

A precise velocity model is necessary to obtain reliable locations of microseismic events, which provide information about the effectiveness of the hydraulic stimulation. Seismic anisotropy plays an important role in microseismic event location by imposing the dependency between wave velocities and its propagation direction. Building an anisotropic velocity model that accounts for that effect allows for more accurate location of microseismic events. We have used downhole microseismic records from a pilot hydraulic fracturing experiment in Lower-Paleozoic shale gas play in the Baltic Basin, Northern Poland, to obtain accurate microseismic events locations. We have developed a workflow for a vertical transverse isotropy velocity model construction when facing a challenging absence of horizontally polarized S-waves in perforation shot data, which carry information about Thomsen’s [Formula: see text] parameter and provide valuable constraints for locating microseismic events. We extract effective [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text] for each layer from the P- and SV-wave arrivals of perforation shots, whereas the unresolved [Formula: see text] is retrieved afterward from the SH-SV-wave delay time of selected microseismic events. An inverted velocity model provides more reliable location of microseismic events, which then becomes an essential input for evaluating the hydraulic stimulation job effectiveness in the geomechanical context. We evaluate the influence of the preexisting fracture sets and obliquity between the borehole trajectory and principal horizontal stress direction on the hydraulic treatment performance. The fracturing fluid migrates to previously fractured zones, while the growth of the microseismic volume in consecutive stages is caused by increased penetration of the above-lying lithologic formations.

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Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models

10.5194/egusphere-egu21-507 ◽

2021 ◽

Author(s):

John Keith Magali ◽

Thomas Bodin ◽

Navid Hedjazian ◽

Yanick Ricard ◽

Yann Capdeville

Keyword(s):

Seismic Tomography ◽

Large Scale ◽

Seismic Anisotropy ◽

Texture Evolution ◽

Scaling Law ◽

Preferred Orientation ◽

Elastic Model ◽

Effective Anisotropy ◽

Crystallographic Preferred Orientation ◽

Radial Anisotropy

Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy &#958;* is approximately just the product of the effective intrinsic radial anisotropy &#958;*CPO and the extrinsic radial anisotropy &#958;SPO:&#958;*&#160;= &#958;*CPO &#215; &#958;SPOBased on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.

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