Estimation of shear wave velocity in shallow marine sediments

The objectives of this paper are to review and study selected measurements of the velocity of shear waves at various depths in some principal types of unlithified, water‐saturated sediments, and to discuss probable variations of shear velocity as a function of pressure and depth in the sea floor. Because of the lack of data for the full range of marine sediments, data from measurements on land were used, and the study was confined to the two “end‐member” sediment types (sand and silt‐clays) and turbidites. The shear velocity data in sands included 29 selected in‐situ measurements at depths to 12 m. The regression equation for these data is: [Formula: see text], where [Formula: see text] is shear‐wave velocity in m/sec, and D is depth in meters. The data from field and laboratory studies indicate that shear‐wave velocity is proportional to the 1/3 to 1/6 power of pressure or depth in sands; that the 1/6 power is not reached until very high pressures are applied; and that in most sand bodies the velocity of shear waves is proportional to the 3/10 to 1/4 power of depth or pressure. The use of a depth exponent of 0.25 is recommended for prediction of shear velocity versus depth in sands. The shear velocity data in silt‐clays and turbidites include 47 selected in‐situ measurements at depths to 650 m. Three linear equations are used to characterize the data. The equation for the 0 to 40 m interval [Formula: see text] indicates the gradient [Formula: see text] to be 4 to 5 times greater than is the compressional velocity gradient in this interval in comparable sediments. At deeper depths, shear velocity gradients are [Formula: see text] from 40 to 120 m, and [Formula: see text] from 120 to 650 m. These deeper gradients are comparable to those of compressional wave velocities. These shear velocity gradients can be used as a basis for predicting shear velocity versus depth.

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Converted waves in a shallow marine environment: Experimental and modeling studies

Geophysics ◽

10.1190/1.3511524 ◽

2011 ◽

Vol 76 (1) ◽

pp. T1-T11 ◽

Cited By ~ 8

Author(s):

Nihed Allouche ◽

Guy G. Drijkoningen ◽

Willem Versteeg ◽

Ranajit Ghose

Keyword(s):

Shallow Water ◽

Shear Wave ◽

Wave Velocity ◽

Shear Wave Velocity ◽

Shear Waves ◽

Seismic Survey ◽

Wave Source ◽

Shallow Marine ◽

Modeling Studies ◽

Water Bottom

Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear-wave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of good-quality converted-wave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A high-resolution seismic survey was conducted in a shallow-water canal using different types of seismic sources; data were recorded with a four-component water-bottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bot-tom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shear-wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in air-gun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/water-bottom coupled system, kinematically behave as if they were generated by an S-wave source placed at the water bottom. In a shallow-water environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shear-wave velocity in the sediments.

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1.5D inversion of lateral variation of Scholte‐wave dispersion

Geophysics ◽

10.1190/1.1707052 ◽

2004 ◽

Vol 69 (2) ◽

pp. 330-344 ◽

Cited By ~ 69

Author(s):

Thomas Bohlen ◽

Simone Kugler ◽

Gerald Klein ◽

Friedrich Theilen

Keyword(s):

Shear Wave ◽

Wave Velocity ◽

Marine Sediments ◽

Shear Wave Velocity ◽

Wave Dispersion ◽

Lateral Variation ◽

Sea Floor ◽

Air Gun ◽

Scholte Wave ◽

The Common

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.

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