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Journal ArticleDOI

Shallow water sediment properties derived from high‐frequency shear and interface waves

10 Apr 1992-Journal of Geophysical Research (John Wiley & Sons, Ltd)-Vol. 97, Iss: 4, pp 4739-4762
TL;DR: In this paper, bottom-mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high-resolution reflection profiles give substantial supporting geologic information about the subsurface.
Abstract: Low-frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom. Thus, as well as being important for geophysical description of the seabed, subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom-mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high-resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the “ground truth” properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30–50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1-m intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the Quaternary. When the seabed structure is relatively uniform over the length of the profiles, the shear wave fields are well ordered, and the matching of the data with full waveform synthetics has been successful, producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and interface waves have been modeled for velocity/attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques. Some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5–10 m of sediments and of extremely high velocity gradients in the topmost 1–2 m, possibly exceeding 30 s−1.

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Journal ArticleDOI
TL;DR: In this paper, the authors proposed to estimate the size and recurrence interval of submarine landslides from the size of earthquakes in the near vicinity of the said landslides, and found that the calculated distance and failure areas from the slope stability analysis is similar or slightly smaller than the maximum triggering distances and failure area in subaerial observations.

138 citations


Cites background from "Shallow water sediment properties d..."

  • ...Shallow (b100 m) Pliocene and Quaternary marine sediments on the New Jersey shelf have a shear wave velocity of 200–400 m/s (Ewing et al., 1992), and the thickness of the sliding layer is typically 20–100 m....

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Journal ArticleDOI
TL;DR: In this paper, 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, and a 2D shear-wave velocity section is generated.
Abstract: 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.

111 citations

Journal ArticleDOI
TL;DR: In this article, the authors suggest that the only reliable way of obtaining high fidelity particle motion data from the ocean floor is to bury the sensors below the bottom in a package with density close to that of the sediment.
Abstract: The often poor quality of ocean bottom seismic data, particularly that observed on horizontal seismometers, is shown to be the result of instruments responding to motions in ways not intended. Instruments designed to obtain the particle motion of the ocean bottom are found to also respond to motions of the water. The shear discontinuity across the ocean floor boundary results in torques that cause package rotation, rather than rectilinear motion, in response to horizontal ground or water motion. The problems are exacerbated by bottom currents and soft sediments. The theory and data presented in this paper suggest that the only reliable way of obtaining high fidelity particle motion data from the ocean floor is to bury the sensors below the bottom in a package with density close to that of the sediment. Long period signals couple well to ocean bottom seismometers, but torques generated by bottom currents can cause noise at both long and short periods. The predicted effects are illustrated using parameters appropriate for the operational OBS developed for the U. S. Office of Naval Research. Examples of data from ocean bottom and buried sensors are also presented.

71 citations


Cites background from "Shallow water sediment properties d..."

  • ...Ewing et al. (1992) observed 37 m/s; Schreiner and Dorman (1990) obtained 40 m/s; Tuthill et aI. (1981) obtain 15-20 rrds in the upper few meters....

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Journal ArticleDOI
TL;DR: The increasing availability of long-term records of ocean sound will provide new opportunities for a deeper understanding of natural and anthropogenic sound sources and potential interactions between them.
Abstract: Very-low-frequency sounds between 1 and 100 Hz propagate large distances in the ocean sound channel. Weather conditions, earthquakes, marine mammals, and anthropogenic activities influence sound levels in this band. Weather-related sounds result from interactions between waves, bubbles entrained by breaking waves, and the deformation of sea ice. Earthquakes generate sound in geologically active regions, and earthquake T waves propagate throughout the oceans. Blue and fin whales generate long bouts of sounds near 20 Hz that can dominate regional ambient noise levels seasonally. Anthropogenic sound sources include ship propellers, energy extraction, and seismic air guns and have been growing steadily. The increasing availability of long-term records of ocean sound will provide new opportunities for a deeper understanding of natural and anthropogenic sound sources and potential interactions between them.

58 citations


Cites background from "Shallow water sediment properties d..."

  • ...However, in the VLF band, the accuracy of these equations is limited even in areas of uniform seafloor, because wave-front curvature can be significant and the primary (P) and shear (S) wave velocities increase significantly with depth on the scale of the seismic wavelength (Ewing et al. 1992)....

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Journal ArticleDOI
TL;DR: In this article, the 3D in situ shear-wave velocities of shallow-water marine sediments were determined by extending the method of surface wave tomography to Scholte-wave records acquired in shallow waters.
Abstract: 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.

56 citations


Cites background from "Shallow water sediment properties d..."

  • ...Especially in the first ten’s of meters beneath the seafloor significant changes in S-wave velocities over small distances are commonly observed (Ewing et al. 1992; Stoll et al. 1994; Bohlen et al. 2004) which strongly effect processing algorithms for multicomponent seismic data like static…...

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References
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Journal ArticleDOI
TL;DR: In this article, the authors extended the theory of acoustic propagation in porous media to include anisotropy, viscoelasticity, and solid dissipation, and a more refined analysis of the relative motion of the fluid in the pores is also developed by introducing the concept of viscodynamic operational tensor.
Abstract: The theory of acoustic propagation in porous media is extended to include anisotropy,viscoelasticity, and solid dissipation. A more refined analysis of the relative motion of the fluid in the pores is also developed by introducing the concept of viscodynamic operational tensor. The nature of this operator is analyzed by applying variational and Lagrangian methods. Viscoelasticity and solid dissipation are introduced by applying the correspondence principle as derived from thermodynamics in earlier work by the author. Various dissipative models are discussed and the corresponding operators and relaxation spectra are derived. The physical chemistry of the multiphase porous medium including surface effects lies within the scope of the thermodynamic theory. The nature of thermoelastic dissipation and electrokinetic effects in relation to the thermodynamic theory is also brought out.

978 citations

Journal ArticleDOI
TL;DR: Geoacoustic models of the sea floor are basic to underwater acoustics and to marine geological and geophysical studies of the earth's crust, including stratigraphy, sedimentology, geomorphology, structural and gravity studies, geologic history, and many others as mentioned in this paper.
Abstract: Geoacoustic models of the sea floor are basic to underwater acoustics and to marine geological and geophysical studies of the earth’s crust, including stratigraphy, sedimentology, geomorphology, structural and gravity studies, geologic history, and many others A ’’geoacoustic model’’ is defined as a model of the real sea floor with emphasis on measured, extrapolated, and predicted values of those properties important in underwater acoustics and those aspects of geophysics involving sound transmission In general, a geoacoustic model details the true thicknesses and properties of sediment and rock layers in the sea floor A complete model includes water‐mass data, a detailed bathymetric chart, and profiles of the sea floor (to obtain relief and slopes) At higher sound frequencies, the investigator may be interested in only the first few meters or tens of meters of sediments At lower frequencies information must be provided on the whole sediment column and on properties of the underlying rocks Complete geoacoustic models are especially important to the acoustician studying sound interactions with the sea floor in several critical aspects: they guide theoretical studies, help reconcile experiments at sea with theory, and aid in predicting the effects of the sea floor on sound propagation The information required for a complete geoacoustic model should include the following for each sediment and rock layer In some cases, the state‐of‐the‐art allows only rough estimates, in others information may be nonexistent (1) Identification of sediment and rock types at the sea floor and in the underlying layers (2) True thicknesses and shapes of layers, and locations of significant reflectors (which may vary with sound frequencies) For the following properties, information is required in the surface of the sea floor, in the surface of the acoustic basement, and values of the property as a function of depth in the sea floor (3) Compressional wave (sound) velocity (4) Shear wave velocity (5) Attenuation of compressional waves (6) Attenuation of shear waves (7) Density (8) Additional elastic properties (eg, dynamic rigidity and Lame’s constant); given compressional and shear wave velocities and density, these and other elastic properties can be computed There is an almost infinite variety of geoacoustic models; consequently, the floor of the world’s ocean cannot be defined by any single model or even a small number of models; therefore, it is important that acoustic and geophysical experiments at sea be supported by a particular model, or models, of the area However, it is possible to use geological and geophysical judgement to extrapolate models over wider areas within geomorphic provinces To extrapolate models requires water‐mass data (such as from Nansen casts and velocimeter lowerings), good bathymetric charts, sediment and rock information from charts, cores, and the Deep Sea Drilling Project, echo‐sounder profiles, reflection and refraction records (which show detailed and general layering and the location of the acoustic basement), sound velocities in the layers, and geological and geophysical judgement Recent studies have provided much new information which, with older data, yield general values and restrictive parameters for many properties of marine sediments and rocks These general values and parameters, and methods for their derivation, are the main subjects of this paper

885 citations

Book
01 Jan 1940

568 citations

Journal ArticleDOI
TL;DR: In this article, the dispersion curves for the mode overtones and fundamental are directly observed in the transformed wave field, where the data wave field is linearly transformed from the time-distance domain into the slowness-time intercept (p − τ) domain.
Abstract: The dispersive waves in a common‐shot wave field can be transformed into images of the dispersion curves of each mode in the data. The procedure consists of two linear transformations: a slant stack of the data produces a wave field in the phase slowness‐time intercept (p — τ) plane in which phase velocities are separated. The spectral peak of the one‐dimensional (1-D) Fourier transform of the p — τ wave field then gives the frequency associated with each phase velocity. Thus, the data wave field is linearly transformed from the time‐distance domain into the slowness‐frequency (p — ω) domain, where dispersion curves are imaged. All the data are present throughout the transformations. Dispersion curves for the mode overtones as well as the fundamental are directly observed in the transformed wave field. In the p — ω domain, each mode is separated from the others even when its presence is not visually detectable in the untransformed data. The resolution achieved in the result is indicated in the p — ω wave ...

517 citations

Journal ArticleDOI
TL;DR: In this article, the polarizations of three-component shear wavetrains carry unique information about the internal structure of the rock through which they pass: specifically, commonly observed shear-wave splitting may contain information about orientation of crack distributions.
Abstract: The polarizations of three‐component shear wavetrains carry unique information about the internal structure of the rock through which they pass: specifically, commonly observed shear‐wave splitting may contain information about the orientation of crack distributions. This information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface, even for nearly vertical incidence. However, shear‐wave splitting in synthetic three‐component vertical seismic profiles, in some cases, may be interpreted directly in terms of the direction of strike of vertical cracks. Because the human eye is not skilled at identifying the phase relationships between three‐component seismograms played out conventionally as parallel time‐series, the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear‐wave splitting. Estimating the orientations of cracks, an...

419 citations