Experimental Study on the Identification of Scholte Waves Based on Acoustic Pressure Field Measurement

Scholte waves at the seafloor interface are generally identified by their velocity features and seismic fields,which are measured using ocean bottom seismometers and geophones.These methods are effective in cases where there is a considerable difference between the velocities of Scholte and acoustic waves in water.However,they are ineffective when the velocities of these two types of waves are close to each other.Thus,in this paper,a method based on acoustic pressure field measurement for identifying Scholte waves is proposed according to their excitation and propagation characteristics.The proposed method can overcome the limitations on the velocities of two types of waves.A tank experiment is designed and conducted according to the proposed method,and an ocean environment is scaled down to the laboratory size.Acoustic measurements are obtained along virtual arrays in the water column using a robotic apparatus.Experiments show that changes in Scholte wave amplitudes,depending on different source depths and propagation distances,are consistent with the theoretical results.This means that Scholte waves generated at the seafloor interface are successfully measured and identified in the acoustic pressure field.

Scholte wave dispersion and particle motion mode in ocean and ocean crust

The dispersion equation of the Scholte wave was reviewed using the homogeneous elastic half-space covered by a liquid layer,and the range of the Scholte wave propagation velocity was examined using the dispersion equation.The displacement expressions of the Scholte waves in liquid and solid were derived.Additionally,the mode of motion of Scholte waves in liquid and solid and their variation with depth was studied.The following results were obtained:The dispersion equation shows that the propagation velocity of the fundamental Scholte wave was greater than the P-wave in liquid and less than that of the Scholte wave in homogeneous elastic half-space.In contrast,the velocity of higher-order Scholte waves was greater than that of P waves in liquid and S-waves in solid.Only the fundamental Scholte wave has no cutoff frequency.The Scholte wave at the liquid surface moved only vertically,while the particles inside the liquid medium moved elliptically.The amplitude variation with depth in the solid medium caused the particle motion to change from a retrograde ellipse to a prograde ellipse.The above results imply the study of Scholte waves in the ocean and oceanic crust and help estimate ocean depths.

Dispersion characteristics of seabed Scholte waves with variable velocity seawater in deep water

Acoustic velocity varies in deep-water environments.To obtain accurate inversion interpretations,it is necessary to develop a horizontally layered seawater–seabed(HLSS)model with continuously varying velocities.In this work,we used an HLSS model based on wave theory to deduce the Scholte wave dispersion equations and established an HLSS model based on the acoustic velocity profile and the submarine medium parameters of the South China Sea.We studied the dispersion characteristics of Scholte waves and theoretically calculated the amplitude–depth distribution.We also examined the influence of deep-water environments on the dispersion characteristics of Scholte waves.Using the real geological parameters of the Dongsha Islands in the South China Sea,we exploited the spectral element method to simulate seismic wave propagation in the fluid–solid interface and extracted the Scholte wave dispersion curves using multichannel analysis of surface waves(MASW).The consistent theoretical and extracted dispersion curve results verified the accuracy of our method.Numerical experiments showed that the dispersion characteristics of Scholte waves in deep water are weaker than those in shallow water.In addition to the seawater depth and the physical parameters of seabed sediments,the seawater’s variable velocity also influences Scholte wave dispersion characteristics.

Analytical Solution for the Transient Response of A Sloping Seabed Induced by A P-Wave Line Source

Many offshore marine structures are built on the seabed that are slightly or considerably sloping.To study the sloping seabed transient response during marine earthquakes,an analytical solution induced by a P-wave line source embedded in the solid is presented.During the derivation,the wave fields in the fluid layer and the semi-infinite solid are firstly constructed by using the generalized ray method and the fluid-solid interface reflection and transmission coefficients.Then,the analytical solution in the transformed domain is obtained by superposing these wave fields,and the analytical solution in the time domain by applying the analytical inverse Laplace transform method.The the head wave generation conditions and arrival times at the fluid-solid interface are derived through this solution.Through the use of numerical examples,the analytical solution is proved right and the impacts of the sloping angle on the hydrodynamic pressure in the sea,the seismic wave propagation in the seabed,the head wave,and the Scholte wave at the seawater-seabed interface are also addressed.

Theoretical dispersion curves for borehole real-valued wave modes in vertically transverse isotropic formations

The dispersion curves of real-valued modes in a fluid-filled borehole are widely used in acoustic well logging.The accurate dispersion curves are the precondition of theoretical analysis and inversion process.Generally,these curves can be obtained by solving the conventional dispersion equation for isotropic formations and most vertically transverse isotropy(VTI)formations.However,if the real-valued solutions exist when the radial wavenumbers for the formation quasi-P and quasi-S equals to each other,the existed methods based on the conventional dispersion equation could lead to incorrect results for some VTI formations.Few studies have focused on the influence of these real-valued solutions on dispersion curve extraction.To remove these real-valued solutions,we have proposed a modified dispersion equation and its corresponding solving process.When solving the dispersion equation,the Scholte wave velocity of VTI formation at high frequency is used as the initial guess.The two synthetic examples including fast and slow VTI formations validate that these real-valued solutions do not contribute to the wavefield,and the new dispersion curve extraction method is suitable for all kinds of VTI formations.Consequently,the method can provide reliable dispersion curves for both theoretical analysis and anisotropic parameters inversion in VTI formations.