Experimental Investigation of Seismic Surface Waves in the Seafloor

Stokoe, Kenneth H.; Gauer, Robert C.; Bay, James A.

doi:10.1007/978-94-011-3568-9_6

Cited by 4 publications

(1 citation statement)

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“…Propagation of interface waves in shallow water over different types of seabed was studied by Rauch () and Chapman and Staal (). Stokoe et al () used physical modelling to investigate the influence of the water layer on the properties of surface waves. Collins and Kuperman () performed numerical modelling of Scholte waves using an elastic parabolic equation.…”

Section: Introductionmentioning

confidence: 99%

Estimating seismic velocities below the sea‐bed using surface waves

Shtivelman

2004

Near Surface Geophysics

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simulation of a land survey (i.e. application of horizontal sources and receivers) at the sea-bottom does not seem to be practical, even in very shallow water. An additional problem may arise when shallow subwater layers are composed of relatively soft material. In this case, the lack of sharp velocity contrasts in water-saturated rocks may hamper the use of P-wave refraction surveys for estimating V P below the sea-bed.In recent years, seismic methods based on the use of surface waves have been gaining a growing popularity, mainly for estimating near-surface V S distribution. The main advantage of the surface-wave methods is that they do not require a special acquisition technique since they utilize the data contained in the conventional (P-wave) seismic records. The methods are based on the analysis of the dispersion properties of surface waves, i.e. the dependence of the propagation velocity of different spectral components of the waves upon their frequency. This dependence (shown as dispersion curves) is closely related to the vertical distribution of V S in the subsurface. Thus, estimates of shear-wave velocities can be obtained by analysing the dispersion relationships derived from the surface waves contained in P-wave seismic records. Various modifications of the method have been successfully used for estimating V S in a variety of applications (Gabriels et al. ABSTRACTSeismic data acquired in shallow offshore surveys often display well-defined dispersion patterns related to two types of surface wave, propagating in shallow subwater layers. The waves of the first type propagate as normal modes and are represented by a low-velocity, low-frequency wavetrain identified with Scholte waves. The phase velocities of Scholte waves are related to the shear-wave velocities V S below the water-bottom and can be inverted to estimate V S in the subwater layers. The waves of the second type propagate as leaking modes and are characterized by a number of distinctive features: their dispersion patterns have a resonant frequency-tuned appearance, they have relatively high cut-off frequencies and their phase velocities exceed the velocity in the water. When the subwater layers are composed of relatively soft saturated rocks with high Poisson's ratio, the leaking modes can be closely approximated by acoustic waves. By inverting the approximating dispersion curves, the vertical distribution of the compressional-wave velocity V P in the shallow subwater layers can be estimated.Estimation of V S and V P below the sea-bottom using the two types of surface wave is illustrated by examples from shallow offshore surveys conducted at several sites in the eastern Mediterranean area. * vladi@gii.co.il

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Section: Introductionmentioning

confidence: 99%

Estimating seismic velocities below the sea‐bed using surface waves

Shtivelman

2004

Near Surface Geophysics

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Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Klein

Bohlen²,

Theilen³

et al. 2005

Mar Geophys Res

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ZusammenfassungDie Kenntnis der elastischen Eigenschaften mariner Sedimente, insbesondere die Scherwellengeschwindigkeit, ist eine wichtige Information für die Untersuchung der Stabilität von Meeresböden. Dies ist insbesondere für geotechnische Anwendungen im flachmarinen Bereich von Bedeutung, aber auch für die Untersuchung von Hangstabilitäten der Kontinentalrän-der und für die Gashydratforschung von Interesse.Seismische Experimente in der Ostsee sowie die Analyse eines Datensatzes aus der Nordsibirischen Laptevsee konnten zeigen, dass dispersive seismische Wellenfelder mit Hilfe einer nahe der Meeresoberfläche geschleppten seismischen Quelle (Airgun) angeregt werden können. AbstractThe knowledge of the shear wave velocity structure of shallow marine sediments is an important information for the assessment of sediment stability. This is of interest for marine geotechnical applications as well as investigations of sea-floor stability of continental shelves and margins and in gashydrate research. The derivation of seismic velocities with special interest on the shear wave velocity from the analysis and inversion of dispersive seismic waves is investigated in this work.Dispersive waves are excited by surface towed airguns and the acquisition was achieved with two different configurations. Firstly, the stationary-receiver method comprises of an ocean bottom seismometer station (OBS) and excitation with a surface towed airgun. Secondly, a towed-acquisition system includes a streamer towed at ten meters above the sea bed and excitation with an airgun. Two different wave types could be observed with either of the two acquisition systems. Interface waves, namely of the Scholte wave type, propagating along the sea floor and guided waves in the water column, denoted as acoustic guided waves despite of their sediment interaction at the sea bed. The both wave types were observed in certain environments indicating the limitations in the acquisition of these wave types depending on the sediment properties. These limitations as well as the difference in dispersion sensitivity to variations of the seismic properties give rise for separate treatment of both wave types. While the dispersion sensitivity of Scholte waves as well as other interface waves is dominant for shear wave velocity variations, the variation of the dispersion of acoustic guided waves is affected by variations of compressional and shear wave velocity as well as density to a smaller extend. The difference in velocity and frequency range of interest for both wave types, i.e. the ranges of most variations in the dispersion characteristics, also affect the required acquisition configuration and parameters. While a long offset recording (exceeding 800 m) is required for acoustic guided waves, the fulfillment of the spatial sample criteria sometimes limits the feasibility to adequately acquire the dispersive wavefield. This is most dominant for the Scholte wave measurements in very soft marine sediments.The derivation of the shear wave velocity structure was inve...

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Detailed S-wave velocity structure of sediment and crust off Sanriku, Japan by a new analysis method for distributed acoustic sensing data using a seafloor cable and seismic interferometry

Fukushima

Shinohara

Nishida

et al. 2022

Earth Planets Space

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The S-wave velocity (Vs) structure of sediments and the uppermost crust in the landward slope of a subduction zone are important for determining the dynamics of the overriding plate. Although distributed acoustic sensing (DAS) measurements have improved the horizontal resolution of Vs structure in marine areas, the estimations have been limited to the uppermost sedimentary layers. In the present study, we applied seismic interferometry to DAS data of 13 h duration to image the sedimentary and crustal structure offshore of Sanriku, Japan with a spatial horizontal resolution of 2.5 km and > 3.0 km depth. We grouped the DAS data into 10 km long subarrays with 75% overlaps. We first applied a frequency-wavenumber filter to the DAS data to remove DAS instrumental noise and to allow effective extraction of surface waves from short-time records. We then applied a seismic interferometry method and estimated the phase velocities at each subarray. The estimated phase velocities of the fundamental-mode and first higher-mode Rayleigh waves were then used to determine one-dimensional Vs structures for each subarray. The resultant 2-D Vs structure was interpreted as representing sediments and crust. The upper sedimentary layers thicken seaward, while the entire sedimentary unit shows complex lateral variations in depth. The boundary between the sedimentary layers and the uppermost crust varies in depth from 1.8 to 6.8 km and is the deepest in the middle of the profile. Combining this result with the P-wave velocity (Vp) structure along the nearest survey line, determined in previous studies, allowed us to estimate Vp/Vs = 3.12, on average, for the lower sedimentary layers. Our method of applying seismic interferometry to marine DAS data broadens the techniques for estimating Vs and Vp/Vs structure of sedimentary layers and the upper crust across subduction zones. These results show that application of the frequency-wavenumber filtering and seismic interferometry to marine DAS data can estimate the Vs structure and the Vp/Vs structure, together with standard marine geophysical surveys of sedimentary layers and the upper crust across subduction zones. Graphical Abstract

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Experimental Investigation of Seismic Surface Waves in the Seafloor

Cited by 4 publications

References 1 publication

Estimating seismic velocities below the sea‐bed using surface waves

Estimating seismic velocities below the sea‐bed using surface waves

Acquisition and Inversion of Dispersive Seismic Waves in Shallow Marine Environments

Detailed S-wave velocity structure of sediment and crust off Sanriku, Japan by a new analysis method for distributed acoustic sensing data using a seafloor cable and seismic interferometry

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