Ocean Bottom Interface Research Articles

Waveform modelling of hydroacoustic teleseismic earthquake records from autonomous Mermaid floats

SUMMARY We present a computational technique to model hydroacoustic waveforms from teleseismic earthquakes recorded by mid-column Mermaid floats deployed in the Pacific, taking into consideration bathymetric effects that modify seismo-acoustic conversions at the ocean bottom and acoustic wave propagation in the ocean layer, including reverberations. Our approach couples axisymmetric spectral-element simulations performed for moment-tensor earthquakes in a 1-D solid Earth to a 2-D Cartesian fluid–solid coupled spectral-element simulation that captures the conversion from displacement to acoustic pressure at an ocean-bottom interface with accurate bathymetry. We applied our workflow to 1129 seismograms for 682 earthquakes from 16 Mermaids (short for Mobile Earthquake Recording in Marine Areas by Independent Divers) owned by Princeton University that were deployed in the Southern Pacific as part of the South Pacific Plume Imaging and Modeling (SPPIM) project. We compare the modelled synthetic waveforms to the observed records in individually selected frequency bands aimed at reducing local noise levels while maximizing earthquake-generated signal content. The modelled waveforms match the observations very well, with a median correlation coefficient of 0.72, and some as high as 0.95. We compare our correlation-based traveltime measurements to measurements made on the same data set determined by automated arrival-time picking and ray- traced traveltime predictions, with the aim of opening up the use of Mermaid records for global seismic tomography via full-waveform inversion.

Model Study of the Effects of Climate Change on the Methane Emissions on the Arctic Shelves

Based on a regional ice-ocean model, we simulated the state of the water masses of the Arctic Ocean to analyze the transport of dissolved methane on the Arctic shelves. From 1970 to 2019, we obtained estimates of methane emissions at the Arctic seas due to the degradation of submarine permafrost and gas release at the ocean–bottom interface. The calculated annual methane flux from the Arctic shelf seas into the atmosphere did not exceed 2 Tg CH4 year−1. We have shown that the East Siberian shelf seas make the main contribution to the total methane emissions of the region. The spatial variability of the methane fluxes into the atmosphere is primarily due to the peculiarities of the water circulation and ice conditions. Only 7% of the dissolved methane originating from sediment enters the atmosphere within the study area. Most of it appears to be transported below the surface and oxidized by microbial activity. We found that increasing periods and areas of ice-free water and decreasing ice concentration have contributed to a steady increase in methane emissions since the middle of the first decade of the current century.

Tank experiment and simulation of sunken hazardous and noxious substances detection using high frequency active sonar

Hazardous and Noxious Substances are defined as substances that are likely to create hazards to the marine ecosystem and human health when they are released into the sea. Because even a small amount of leakage may cause a fatal effect, it is important to develop a technology to handle effectively the HNS spill incidents. Especially, it is difficult to detect or monitor the HNS deposited on the seabed after HNS spill incident using a direct observation method. In this paper, the remote sensing technique using underwater acoustic method is suggested as an alternative method to observe effectively the HNS sunk to ocean bottom. First, to examine the detection possibility of underwater acoustic method, tank experiment was carried out using 200-kHz active sonar. The target HNS was chloroform, which is one of major sunken HNS. However, castor oil having a similar acoustic impedance with the chloroform was used as an alternative substance due to the toxicity of chloroform. Second, a computer simulation to investigate the availability of side scan sonar to detect the sunken HNS was carried out. It is assumed that the chloroform and tetrachloride, which is another sunken HNS, are randomly distributed on the several types of seabed. The simulation algorithm is based on the difference of backscattering strengths from the ocean bottom interface and the sunken HNS.

A Method for Estimating the Distance of Near-Ocean-Bottom Sources by Combining VLF Underwater Acoustic Field and Scholte Wave Field

In order to solve the problems of passive detection of very-low-frequency (VLF) underwater sound sources, this paper proposes a method of estimating sound source distance based on combining underwater sound field and Scholte wave field. VLF sources near the ocean bottom generate the sound field in the water, then seismic waves are excited by the of near-field effect. The surface wave propagating along the ocean bottom interface is the Scholte wave. By deploying a hydroacoustic&seismic-wave joint detection system on the ocean bottom, the acoustic field and the Scholte wave field excited by the source can be collected. Due to the different physical properties of water acoustics and Scholte waves, the Scholte waves can be distinguished by analysing the polarization characteristics of the captured signals. Due to the different velocity between the underwater acoustics and Scholte waves, the propagation delays to the joint detection system are also different, and the delay difference can be obtained by the correlation method. Combined with the surrounding parameters, the distance of the target source can be determined. Theoretical analysis and finite element simulation experiments show that this method can estimate the position information of near-ocean-bottom VLF underwater sources in the ideal case.

Effect of seawater on incident plane P and SV waves at ocean bottom and engineering characteristics of offshore ground motion records off the coast of southern California, USA

The effect of seawater on vertical ground motions is studied via a theoretical method and then actual offshore ground motion records are analyzed using a statistical method. A theoretical analysis of the effect of seawater on incident plane P and SV waves at ocean bottom indicate that on one hand, the affected frequency range of vertical ground motions is prominent due to P wave resonance in the water layer if the impedance ratio between the seawater and the underlying medium is large, but it is greatly suppressed if the impedance ratio is small; on the other hand, for the ocean bottom interface model selected herein, vertical ground motions consisting of mostly P waves are more easily affected by seawater than those dominated by SV waves. The statistical analysis of engineering parameters of offshore ground motion records indicate that: (1) Under the influence of softer surface soil at the seafloor, both horizontal and vertical spectral accelerations of offshore motions are exaggerated at long period components, which leads to the peak spectral values moving to a longer period. (2) The spectral ratios (V/H) of offshore ground motions are much smaller than onshore ground motions near the P wave resonant frequencies in the water layer; and as the period becomes larger, the effect of seawater becomes smaller, which leads to a similar V/H at intermediate periods (near 2 s). These results are consistent with the conclusions of Boore and Smith (1999), but the V/H of offshore motion may be smaller than the onshore ground motions at longer periods (more than 5 s).

Measuring seismic waves using a towed underwater acoustic array

This paper discusses the possibility of detecting shear and interface (Scholte) wave effects in the ocean using a towed hydrophone array. The shear field will be evanescent in the water and so may only be detected near to the ocean bottom interface. A benefit of measuring the acoustic field with a towed array is that a Hankel transform can be used to construct the horizontal wavenumber spectrum. If a shear or interface wave is measured, then it will be visible in the horizontal spectrum. The possibility of detecting the shear field will be strongly dependent on the shear wave speed in the sediment and this will also affect the detection of the more difficult to detect Scholte wave, which travels at about 90 % the shear wave velocity. The Scholte wave has a circular polarization where the shear wave may be vertical, horizontal, or a combination of the two polarities and may not be detectable for all frequency and source depth configurations. [Work supported by ONR.]

Nonuniform depth grids in parabolic equation solutions

The parabolic wave equation is solved using a finite-difference solution in depth that involves a nonuniform grid. The depth operator is discretized using Galerkin's method with asymmetric hat functions. Examples are presented to illustrate that this approach can be used to improve efficiency for problems in ocean acoustics and seismo-acoustics. For shallow water problems, accuracy is sensitive to the precise placement of the ocean bottom interface. This issue is often addressed with the inefficient approach of using a fine grid spacing over all depth. Efficiency may be improved by using a relatively coarse grid with nonuniform sampling to precisely position the interface. Efficiency may also be improved by reducing the sampling in the sediment and in an absorbing layer that is used to truncate the computational domain. Nonuniform sampling may also be used to improve the implementation of a single-scattering approximation for sloping fluid-solid interfaces.

A single-scattering correction for the seismo-acoustic parabolic equation

An efficient single-scattering correction that does not require iterations is derived and tested for the seismo-acoustic parabolic equation. The approach is applicable to problems involving gradual range dependence in a waveguide with fluid and solid layers, including the key case of a sloping fluid-solid interface. The single-scattering correction is asymptotically equivalent to a special case of a single-scattering correction for problems that only have solid layers [Küsel et al., J. Acoust. Soc. Am. 121, 808-813 (2007)]. The single-scattering correction has a simple interpretation (conservation of interface conditions in an average sense) that facilitated its generalization to problems involving fluid layers. Promising results are obtained for problems in which the ocean bottom interface has a small slope.

A scaled mapping approach for treating sloping interfaces in parabolic equation solutions

An area of research in ocean acoustics is on solution techniques for environments with sloping seafloors. In these range-dependent environments the oceanic waveguide varies in the direction of propagation and analytic solutions do not exist. In order to enforce continuity conditions at the seafloor the environment is approximated as a series of range-independent regions. An alternative to this approach, the mapping solution [M. D. Collins et al., JASA 107] applies a change of variables that results in a vertical translation of the environment and makes it easier to enforce ocean bottom interface conditions. An alternative mapping approach that uses a characteristic depth length scale is considered here. In contrast to the earlier mapping solution, this approach uses a change of variables in which the depth variable is scaled relative to the depth of the bottom interface. Along with the simplification of bottom interface conditions in both approaches, extra terms arising from the mapping are neglected, and...

Seismo‐acoustic propagation near low‐shear speed poroelastic ocean sediments using a hybrid parabolic equation solution.

Accurate and efficient parabolic equation solutions exist for complex propagation environments featuring elastic and porous elastic sediment types. An area of concern has been low‐shear wave speed sediments that become singular as their shear modulus tends toward zero. A historic approach for treating sediments of this type has been to assume that it is a fluid, and effects due to elasticity are negligible. This approach is limited in accuracy unless shear is accounted for. In this presentation, the ocean bottom sediment interface layer is treated as a porous elastic layer in which poroelastic momentum equations are solved and combined with an existing elastic parabolic equation implementation. Appropriate boundary conditions are enforced at the fluid‐poroelastic and poroelastic‐elastic interfaces. The new solution is tested on problems with a low‐shear ocean bottom interface layer.

Low-frequency seismo-acoustic propagation near thin and low-shear speed ocean sediment layers.

Accurate and efficient prediction of propagation over realistic models of elastic ocean sediments has been achieved recently using parabolic equations. A concern has been low‐shear wave speed sediments that can become singular as the wave speed tends toward zero. A historic approach for treating a sediment of this type has been to assume that it is a fluid and that effects due to elasticity are negligible. This approach does not account for second order effects such as energy loss due to frequency‐dependent attenuation. In addition, thin sediment layers typical of that found at the ocean bottom interface have been difficult to treat numerically. At low frequencies, layers of this type can be treated as a massive interface between the water and higher‐shear speed sediment basement layers. To satisfy interface conditions across the layer, Rayleigh jump conditions are imposed [F. Gilbert, Ann. Geophys. 40, 1211 (1997)]. A consequence of this approximation is that interface and other wave types become dispersive where they were not previously. In this presentation, the massive elastic interface is benchmarked with an elastic parabolic equation and the effects of resultant errors are quantified. In addition, the energy partition between compressional and shear energies is determined.

Propagation in an elastic wedge using the virtual source technique

The virtual source technique, which is based on the boundary integral method, provides the means to impose boundary conditions on arbitrarily shaped boundaries by replacing them by a collection of sources whose amplitudes are determined from the boundary conditions. In this paper the virtual source technique is used to model propagation of waves in a range-dependent ocean overlying an elastic bottom with arbitrarily shaped ocean-bottom interface. The method is applied to propagation in an elastic Pekeris waveguide, an acoustic wedge, and an elastic wedge. In the case of propagation in an elastic Pekeris waveguide, the results agree very well with those obtained from the wavenumber integral technique, as they do with the solution of the parabolic equation (PE) technique in the case of propagation in an acoustic wedge. The results for propagation in an elastic wedge qualitatively agree with those obtained from an elastic PE solution.

The physical Properties of marine sediments as inferred from Wavelet Correlation Analysis

Seismic waves traveling in the water/sediment or sub-bottom sediment interface have been the subject of considerable interest in underwater acoustics in recent years. Some progress has been made in understanding the propagation and attenuation characteristics of interface waves in different geological environments. However, the generating mechanisms are poorly understood. In particular, what is the acoustic-seismic energy conversion process? As seismic waves involve both time and space parameters it should be able to relate directly the propagation characteristics of the ocean bottom interface waves to the shear properties of the sediments over the propagation area. To address these problems we have applied the wavelet correlation method (WCR) to examine the variations of bottom characteristics and their role in coupling waterborne sound into the sea bottom. To confirm the validity of the developed modeling technique, we applied the wavelet correlation analysis for synthetic seismograms and field data. In this discussion paper we present images of the first and second shear modes and the interface wave component as a function of arrival time and frequency. We also discuss the possibility of inverting phase and group velocity information directly from the wavelet cross-correlation function and propose the way of using the WCR method to predict physical dynamic parameters of marine sediments.

Numerical modeling of impact-induced modifications of the deep-sea floor

Abstract The mechanics of oceanic impact events differ in several respects from the process that accompanies the strike of an asteroid on land. In order to explore the cratering process on a water-covered target, a series of 2D hydrocode simulations for impact velocities of 10, 15, 20 and 30 km/s and water depth/diameter ratios between 5 and 7 have been carried out. In particular, these simulations were aimed to examine the effects of the asteroid on the deep-sea floor. The interaction of the impacting body and the water column causes loss of the body's kinetic energy both through oceanic drag and through mass loss due to hydrodynamic heating, prior to hitting the ocean bottom. Regardless of whether the impactor actually hits the ocean floor, the shock wave generated in the water column by the impact of the body on the water surface will leave behind unique traces in the marine sediments. For example an asteroid of 1 km in diameter arriving at the ocean surface with a velocity of 20 km/s induces a shock pressure of 6 GPa in the oceanic crust at a water depth of 5 km . The passage of the shock wave along the water–ocean bottom interface causes significant disturbance in the sediment layer. Moreover, the collapse of the transient cavity that results from the passage of the projectile through the water column causes a strong surge of water that generates a displacement and redeposition of sediments on the ocean floor. In order to examine sediment transport processes, we introduce computationally a large amount of massless particles within the projectile and the target material, and follow the movement of each individual particle. Tracer particles represent only forced material movement and do not account for any frictional forces. Thus, comparisions with real sediment disturbances may be qualitative only. For an impact velocity of 20 km/s and a water depth/diameter ratio of 5, this procedure reveals significant particle movements within a distance of 15 km from the center of the impact. The only known deep-sea impact structure so far, the Eltanin impact in the Bellingshausen Sea, Antarctica, is characterized by a zone of chaotically mixed sediments that most probably originated from impact-induced turbulent water currents. Based on the present observations, which do not allow the identification of an impact structure at the ocean bottom, our model yields estimates of a projectile diameter of 1 km and an impact velocity of 20 km/s .

New and improved parabolic equation models

The stability of the self starter and the accuracy of the elastic parabolic equation have been improved. The self starter is an initial condition that is generated by applying an operator to the source term [IEEE J. Ocean. Eng. 22, 102–109 (1997)]. Although the self starter is continuous, numerical difficulties can arise for problems involving relatively deep water and/or low bottom attenuation and/or high frequency. This problem has been eliminated by placing the pole of the smoothing operator far from the eigenvalues of the depth separated wave equation. The improved self starter has been incorporated into Version 1.1 of RAM (range-dependent acoustic model), which is available by anonymous ftp from ram.nrl.navy.mil. The elastic parabolic equation accounts for compressional and shear waves in the sediment. Handling a sloping ocean bottom is complicated by the change in the number of dependent variables across the interface. Gradual slopes can be handled accurately by applying a coordinate translation to flatten the ocean bottom interface. Energy is conserved and the adiabatic mode solution is invariant under this mapping. This approach has been incorporated into Version 1.0 of RAMS, which is also available by ftp. [Work supported by ONR.]

Coupling of land-based signals into the nearshore underwater acoustic field

Small, controlled land detonations on and near the beach recorded by land geophones and by offshore seismoacoustic sensors are used to characterize the coupling between land seismic and underwater acoustic fields. These data were collected in the Marine Physical Laboratory’s Adaptive Beach Monitoring program. The predominant arrival on land is an elastic surface wave. Although an ocean bottom interface wave can be seen clearly in seismoacoustic recordings just outside the surf zone, it attenuates to nearly background noise levels by the time it reaches a bottom hydrophone array 1.5 km offshore in 12-m water. Travel-time and frequency/wave-number analyses indicate that the hydrophone array’s received energy is composed mostly of dispersive body waves propagating as the lowest water-borne mode, with frequency-dependent phase and group velocities around 2.0 and 1.8 km/s, respectively. The frequency at which this lowest mode cuts off is the corner frequency of the high-pass filter representing the propagation from land to the underwater acoustic field. These results and numerical modeling, guided by insights obtained from classic work on acoustic propagation in a fluid wedge, are used to explain offshore underwater recordings of land vehicle activity. [Work supported by ONR, Code 32.]

PE modal decomposition of VLA data

The first step in any matched-mode processing application is the decomposition of the measured acoustic field into its horizontal wave-number (modal) components. Previously, a PE-based method was described for carrying out the modal decomposition of the pressure field received on a vertical line array (VLA) [D. J. Thomson etal., J. Acoust. Soc. Am. 89, 2000 (1991)]. For that method, the VLA field was first interpolated between hydrophones and then extrapolated into the ocean bottom. This interpolation/extrapolation stage was needed to construct a suitable starting field on the fine computational grid used by the PE algorithm. In this paper, a modified PE model is presented that eliminates the computational grid in the bottom by applying an exact, nonlocal boundary condition along the ocean-bottom interface. Consequently, it is no longer necessary to extrapolate the VLA field prior to propagating it with PE. To demonstrate the effectiveness of the boundary condition, PE-based modal decomposition is carried out using simulated VLA data generated for a shallow-water environment.

A nonreflecting boundary condition for use in PE calculations of sound propagation in air

Although the parabolic equation (PE) method has long been used to model underwater sound propagation, its applicability to outdoor sound propagation has been realized only recently [e.g., K. E. Gilbert and M. J. White, J. Acoust. Soc. Am. 85, 630–637 (1989)]. In ocean acoustics, an absorbing layer is usually appended to the base of the computational grid to suppress unwanted reflections that arise from a Dirichlet boundary condition imposed at a finite distance below the ocean-bottom interface. For sound propagation in air, however, this local boundary condition must be applied at the top (z=H) of an absorbing layer that occupies the region h≤z≤H above the air–Earth interface. Herein, this absorbing layer is replaced with an exact boundary condition applied along the plane z=h itself. Although this boundary condition is nonlocal and is thus more computationally demanding than a local one, the extra computing effort is offset by requiring fewer grid points in air. To illustrate the numerical effectiveness of the new boundary condition, PE calculations are compared to reference solutions for several examples of propagation in air.

Low-frequency reverberation simulations from range-dependent shear-supporting ocean bottoms.

Low-frequency reverberation from range-dependent ocean bottoms can be very complex. As in the case of high-frequency reverberation, the impedance contrast between water and ocean bottom is a major determinant in the reverberation. However, unlike the high-frequency reverberation case, the low-frequency acoustic field incident on the ocean-bottom interface can be both scattered from and propagated through the interface into the ocean bottom and subbottom. Additionally, the compressional wave incident from the fluid (water) media can be partially converted to Scholte waves along the interface and shear waves within the ocean bottom. These conversions appear as increased attenuation, thereby reducing the backscattered field. The converted interface and shear waves can also radiate back into the water column, thus adding to the background reverberant field. Simulation modeling of low-frequency reverberation from range-dependent ocean bottoms should include these complex coupled processes. This has been attempted here using the cw range-dependent seismo-acoustic finite-element model, SAFE, coupled to a parabolic equation model. Isolation of the backscattered field (and its components) was accomplished by subtracting selected components from the total two-way full-wave coupled field. Examples will be presented of backscatter simulation from rough penetrable ocean bottoms that include interface waves and shear conversions. [Work supported by ONR/NOARL and by the ONR Acoustic Reverberation SRP.]

Ocean bottom seismic instrumentation

During the past several years the quality of seismic data obtained in the deep oceans has improved markedly due to the development of better instrument packages and recording systems. These improvements take advantage of improved understanding of the seismic coupling problem, the effects of ocean currents, and Stoneley waves. The coupling problem arises from the inability of an ocean bottom seismometer (OBS) to move with the ocean bottom in response to seismic waves when the bottom is soft. The transfer function between the motion of the bottom and the OBS is good at very low frequencies and poor at high frequencies, the corner depending on the softness of the bottom and the mechanical characteristics of the OBS. The buoyancy of the OBS also has a pronounced effect on the response. These coupling effects are not observed on hydrophones. Several solutions to the above problems are being proposed and tested. First, since many of the problems with collecting high quality seismic data on the ocean floor are related to the ocean bottom interface, one solution is to remove the sensors from the interface and place them in drill holes under the ocean floor. This operation is considerably more expensive than emplacing OBS's but preliminary results suggest that the increase in data quality is significant. Solutions to the coupling problem include sensor burial, increasing the surface area in contact with the ocean bottom, and making sensor packages neutrally buoyant in the sediments. Current noise problems are greatly improved by decreasing the OBS cross section to the water. Stoneley wave generated noise is truly seismic and can be discriminated against by using multiple sensors or hydrophones that are much less sensitive to these signals.