Propagation of dispersed compressional and Rayleigh waves on the Texas coastal plain

Geophysics ◽  
1983 ◽  
Vol 48 (1) ◽  
pp. 27-35 ◽  
Author(s):  
Joseph Ebeniro ◽  
Clark R. Wilson ◽  
James Dorman
Keyword(s):  
Coastal Plain ◽  
Rayleigh Waves ◽  
Poisson Ratio ◽  
Normal Modes ◽  
Compressional Wave ◽  
Unconsolidated Sediments ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Strong Surface ◽  
Group Velocities

During a refraction profile on the Texas coastal plain, a strong surface wave with predominant frequencies between 2 and 10 Hz and group velocities near 1.9 km/sec was observed to ranges as great as 64 km. This dispersed wave, with velocities near the compressional wave speed of near‐surface sediments, corresponds to the “leaky” compressional or PL wave. PL dispersion can be predicted from the theory of the normal modes of a layered liquid medium. Efficient propagation of the PL wave is related to the high Poisson ratio of the unconsolidated sediments in the shallow subsurface, and additional examples from the published literature show that the PL wave is commonly excited by shallow sources both on land and offshore. In addition to the PL waves, dispersed waves with group velocities between 0.3 and 0.7 km/sec were observed at ranges less than 10 km. These are identified as Rayleigh waves (LR). Smoothly varying P and S velocity structures for the upper 1 km are obtained by fitting theoretical dispersion curves to the observed PL and LR data.

DEPENDENCE OF IN‐SITU COMPRESSIONAL‐WAVE VELOCITY ON POROSITY IN UNSATURATED ROCKS

Geophysics ◽  
1972 ◽  
Vol 37 (1) ◽  
pp. 29-35 ◽  
Author(s):  
Joel S. Watkins ◽  
Lawrence A. Walters ◽  
Richard H. Godson
Keyword(s):  
Wave Velocity ◽  
Seismic Refraction ◽  
Compressional Wave ◽  
Unconsolidated Sediments ◽  
Compressional Wave Velocity ◽  
Near Surface ◽  
Least Squares Fit ◽  
Water Saturated ◽  
Porosity Range

The relation of in‐situ compressional‐wave velocities to porosities, determined by seismic refraction for unsaturated near‐surface rocks from different areas in Arizona, New Mexico, and California, is grossly similar to relations determined by other investigators for water‐saturated rock and unconsolidated sediments. The principal difference is that in the porosity range 0.0–0.2, compressional waves travel somewhat more slowly in unsaturated rocks than in water‐saturated rocks, and much more slowly, in the porosity range 0.2–0.8. The function, ϕ=−0.175 ln (α)+1.56, where ϕ is the fractional porosity and α is the compressional‐wave velocity, was obtained as a least squares fit to the experimental data. Bulk densities are reported for all samples; moisture contents are reported in some instances.

scholarly journals Acoustic-to-seismic ground coupling: coupling efficiency and inferring near-surface properties

2020 ◽  
Vol 223 (1) ◽  
pp. 144-160
Author(s):  
Artemii Novoselov ◽  
Florian Fuchs ◽  
Goetz Bokelmann
Keyword(s):  
Ground Motion ◽  
Poisson Ratio ◽  
Coupling Efficiency ◽  
Seismic Energy ◽  
Acoustic Energy ◽  
Transfer Coefficients ◽  
Frequency Dependent ◽  
Low Frequencies ◽  
Near Surface ◽  
Shallow Subsurface

SUMMARY A fraction of the acoustic wave energy (from the atmosphere) may couple into the ground, and it can thus be recorded as ground motion using seismometers. We have investigated this coupling, with two questions in mind, (i) how strong it is for small explosive sources and offsets up to a few tens of meters and (ii) what we can learn about the shallow subsurface from this coupling. 25 firecracker explosions and five rocket explosions were analysed using colocated seismic and infrasound sensors; we find that around 2 per cent of the acoustic energy is admitted into the ground (converted to seismic energy). Transfer coefficients are in the range of 2.85–4.06 nm Pa–1 for displacement, 1.99–2.74 μm s–1 Pa–1 for velocity, and 2.2–2.86 mm s−2 Pa–1 for acceleration. Recording dynamic air pressure together with ground motion at the same site allows identification of different waves propagating in the shallow underground, notably the seismic expression of the direct airwave, and the later air-coupled Rayleigh wave. We can reliably infer shallow ground properties from the direct airwave, in particular the two Lamé constants (λ and μ) and the Poisson ratio. Firecrackers as pressure sources allow constraining elastic parameters in the top-most layer. In this study, they provide frequency-dependent values of λ decreasing from 119 MPa for low frequencies (48 Hz) to 4.2 MPa for high frequencies (341 Hz), and μ values decreasing from 33 to 1.8 MPa. Frequency-dependent Poisson ratios ν are in the range of 0.336–0.366.

scholarly journals Detecting clandestine tunnels by using near-surface seismic techniques

2021 ◽  
Author(s):  
Steven Sloan ◽  
Shelby Peterie ◽  
Richard Miller ◽  
Julian Ivanov ◽  
J. Schwenk ◽  
...  
Keyword(s):  
Surface Wave ◽  
Wave Diffraction ◽  
Seismic Data ◽  
Body Wave ◽  
Test Site ◽  
Compressional Wave ◽  
Complex Problem ◽  
Unconsolidated Sediments ◽  
Near Surface ◽  
Reliable Solution

Geophysical detection of clandestine tunnels is a complex problem that has been met with limited success. Multiple methods have been applied spanning several decades, but a reliable solution has yet to be found. This report presents shallow seismic data collected at a tunnel test site representative of geologic settings found along the southwestern U.S. border. Results demonstrate the capability of using compressional wave diffraction and surface-wave backscatter techniques to detect a purpose-built subterranean tunnel. Near-surface seismic data were also collected at multiple sites in Afghanistan to detect and locate subsurface anomalies (e.g., data collected over an escape tunnel discovered in 2011 at the Sarposa Prison in Kandahar, Afghanistan, which allowed more than 480 prisoners to escape, and data from another shallow tunnel recently discovered at an undisclosed location). The final example from Afghanistan is the first time surface-based seismic methods have detected a tunnel whose presence and location were not previously known. Seismic results directly led to the discovery of the tunnel. Interpreted tunnel locations for all examples were less than 2 m of the actual location. Seismic surface wave backscatter and body-wave diffraction methods show promise for efficient data acquisition and processing for locating purposefully hidden tunnels within unconsolidated sediments.

scholarly journals Microanchored borehole fiber optics allows strain profiling of the shallow subsurface

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Cheng-Cheng Zhang ◽  
Bin Shi ◽  
Song Zhang ◽  
Kai Gu ◽  
Su-Ping Liu ◽  
...  
Keyword(s):  
Fiber Optics ◽  
Field Experiments ◽  
Fiber Optic ◽  
Strain Sensing ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Buried Cables ◽  
Capacity Theory ◽  
Confining Pressures ◽  
Strain Determination

AbstractVertical deformation profiles of subterranean geological formations are conventionally measured by borehole extensometry. Distributed strain sensing (DSS) paired with fiber-optic cables installed in the ground opens up possibilities for acquiring high-resolution static and quasistatic strain profiles of deforming strata, but it is currently limited by reduced data quality due to complicated patterns of interaction between the buried cables and their surroundings, especially in upper soil layers under low confining pressures. Extending recent DSS studies, we present an improved approach using microanchored fiber-optic cables—designed to optimize ground-to-cable coupling at the near surface—for strain determination along entire lengths of vertical boreholes. We proposed a novel criterion for soil–cable coupling evaluation based on the geotechnical bearing capacity theory. We applied this enhanced methodology to monitor groundwater-related vertical motions in both laboratory and field experiments. Corroborating extensometer recordings, acquired simultaneously, validated fiber optically determined displacements, suggesting microanchored DSS as an improved means for detecting and monitoring shallow subsurface strain profiles.

Compressional‐wave velocities in attenuating media: A laboratory physical model study

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1162-1167 ◽  
Author(s):  
Joseph B. Molyneux ◽  
Douglas R. Schmitt
Keyword(s):  
Phase Velocity ◽  
Group Velocity ◽  
Wave Velocity ◽  
Propagation Distance ◽  
Compressional Wave ◽  
Arrival Times ◽  
Wave Velocities ◽  
Amplitude Maximum ◽  
Phase And Group Velocities ◽  
Group Velocities

Elastic‐wave velocities are often determined by picking the time of a certain feature of a propagating pulse, such as the first amplitude maximum. However, attenuation and dispersion conspire to change the shape of a propagating wave, making determination of a physically meaningful velocity problematic. As a consequence, the velocities so determined are not necessarily representative of the material’s intrinsic wave phase and group velocities. These phase and group velocities are found experimentally in a highly attenuating medium consisting of glycerol‐saturated, unconsolidated, random packs of glass beads and quartz sand. Our results show that the quality factor Q varies between 2 and 6 over the useful frequency band in these experiments from ∼200 to 600 kHz. The fundamental velocities are compared to more common and simple velocity estimates. In general, the simpler methods estimate the group velocity at the predominant frequency with a 3% discrepancy but are in poor agreement with the corresponding phase velocity. Wave velocities determined from the time at which the pulse is first detected (signal velocity) differ from the predominant group velocity by up to 12%. At best, the onset wave velocity arguably provides a lower bound for the high‐frequency limit of the phase velocity in a material where wave velocity increases with frequency. Each method of time picking, however, is self‐consistent, as indicated by the high quality of linear regressions of observed arrival times versus propagation distance.

Surface-Wave Simulation for the Continuously Moving Seismic Sources

2021 ◽  
Author(s):  
Yuefeng Yan ◽  
Chengyu Sun ◽  
Tengfei Lin ◽  
Jiao Wang ◽  
Jidong Yang ◽  
...  
Keyword(s):  
Rayleigh Waves ◽  
Frequency Dispersion ◽  
Dispersion Curves ◽  
Velocity Estimation ◽  
Surface Velocity ◽  
Seismic Sources ◽  
Near Surface ◽  
Moving Source ◽  
Source Data ◽  
Doppler Effects

Abstract In exploration and earthquake seismology, most sources used in subsurface structure imaging and rock property estimation are fixed in certain positions. Continuously moving seismic sources, such as vehicles and the metro, are one kind of important passive sources in ambient noise research. Commonly, seismic data acquisition and processing for moving sources are based on the assumption of simple point passive sources, and the dispersion curve inversion is applied to constrain near-surface velocity. This workflow neglects the Doppler effects. Considering the continuously moving properties of the sources, we first derive the analytical solution for the Rayleigh waves excited by heavy vehicles and then analyze their Doppler effects and dispersion curves. We observe that the moving source data have the Doppler effect when compared with the changes in the frequency of the source intensity, but this effect does not affect the frequency dispersion of Rayleigh waves. The dispersion curves computed for moving source records are consistent with the analytical dispersion solutions, which provide a theoretical foundation for velocity estimation using moving source data.

scholarly journals Automated detection and association of surface waves

1994 ◽  
Vol 37 (3) ◽  
Author(s):  
R. G. North ◽  
C. R. D. Woodgold
Keyword(s):  
Surface Waves ◽  
Group Velocity ◽  
Rayleigh Waves ◽  
Arrival Time ◽  
Narrow Band ◽  
Broad Band ◽  
Detection Algorithm ◽  
Automated Detection ◽  
Short Period ◽  
Group Velocities

An algorithm for the automatic detection and association of surface waves has been developed and tested over an 18 month interval on broad band data from the Yellowknife array (YKA). The detection algorithm uses a conventional STA/LTA scheme on data that have been narrow band filtered at 20 s periods and a test is then applied to identify dispersion. An average of 9 surface waves are detected daily using this technique. Beamforming is applied to determine the arrival azimuth; at a nonarray station this could be provided by poIarization analysis. The detected surface waves are associated daily with the events located by the short period array at Yellowknife, and later with the events listed in the USGS NEIC Monthly Summaries. Association requires matching both arrival time and azimuth of the Rayleigh waves. Regional calibration of group velocity and azimuth is required. . Large variations in both group velocity and azimuth corrections were found, as an example, signals from events in Fiji Tonga arrive with apparent group velocities of 2.9 3.5 krn/s and azimuths from 5 to + 40 degrees clockwise from true (great circle) azimuth, whereas signals from Kuriles Kamchatka have velocities of 2.4 2.9 km/s and azimuths off by 35 to 0 degrees. After applying the regional corrections, surface waves are considered associated if the arrival time matches to within 0.25 km/s in apparent group velocity and the azimuth is within 30 degrees of the median expected. Over the 18 month period studied, 32% of the automatically detected surface waves were associated with events located by the Yellowknife short period array, and 34% (1591) with NEIC events; there is about 70% overlap between the two sets of events. Had the automatic detections been reported to the USGS, YKA would have ranked second (after LZH) in terms of numbers of associated surface waves for the study period of April 1991 to September 1992.

A laboratory study of NMR relaxation times in unconsolidated heterogeneous sediments

Geophysics ◽  
2011 ◽  
Vol 76 (4) ◽  
pp. G73-G83 ◽  
Author(s):  
Elliot Grunewald ◽  
Rosemary Knight
Keyword(s):  
Grain Size ◽  
Relaxation Time ◽  
Time Distribution ◽  
Relaxation Times ◽  
Nmr Relaxation ◽  
Unconsolidated Sediments ◽  
Relaxation Response ◽  
Relaxation Time Distribution ◽  
Near Surface ◽  
Heterogeneous Sediments

Nuclear magnetic resonance (NMR) relaxation-time measurements can provide critical information about the physiochemical properties of water-saturated media and are used often to characterize geologic materials. In unconsolidated sediments, the link between measured relaxation times and pore-scale properties can be complicated when diffusing water molecules couple the relaxation response of heterogeneous regions within a well-connected pore space. Controlled laboratory experiments have allowed us to investigate what factors control the extent of diffusional coupling in unconsolidated sediments and what information is conveyed by the relaxation-time distribution under varied conditions. A range of sediment samples exhibiting heterogeneity in the form of a bimodal mineralogy of quartz and hematite were mixed with varied mineral concentration and grain size. NMR relaxation measurements and geometric analysis of these mixtures demonstrate the importance of two critical length scales controlling the relaxation response: the diffusion length ℓD, describing the distance a water molecule diffuses during the NMR measurement, and the separation length ℓS, describing the scale at which heterogeneity occurs. For the condition of ℓS > ℓD, which prevails for samples with low hematite concentrations and coarser grain size, coupling is weak and the bimodal relaxation-time distribution independently reflects the relaxation properties of the two mineral constituents in the heterogeneous mixtures. For the condition of ℓS < ℓD, which prevails at higher hematite concentrations and finer grain size, the relaxation-time distribution no longer reflects the presence of a bimodal mineralogy but instead conveys a more complex averaging of the heterogeneous relaxation environments. This study has shown the potential extent and influence of diffusional coupling in unconsolidated heterogeneous sediments, and can serve to inform the interpretation of NMR measurements in near-surface environments where unconsolidated sediments are commonly encountered.

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