Seabed Surface Research Articles

3D UHR seismic and back-scattering analysis for seabed and ultra-shallow subsurface classification

AbstractRecently, the seabed classification method based on back-scattering data of multi-beam echo-sounder (MBES) is widely used to analyze the distribution of seabed sediment. Although various analysis methods for seabed classification using multi-spectral MBES have been developed, they are limited in securing penetration depth to consider the characteristics of the shallow subsurface structure. In this study, the seabed and ultra-shallow subsurface classification was performed by comparative analysis of box corer sampling, back-scattering, and 2D/3D ultra-high-resolution (UHR) seismic data obtained from Yeongil Bay, South Korea. We proposed a process for seismic ultra-shallow subsurface classification by the segmentation of the primary seabed reflection wavelet and the amplitude analysis. The seabed-reflected amplitude and back-scattering intensity showed similar mapping trends in the relatively homogeneous and thick surface sediment. On the other hand, it was confirmed that back-scattering data and seabed-reflected amplitude show different patterns when the subsurface structure is related to the seabed surface. It is presumed that because seismic data containing relatively low-frequency components have a deeper penetration depth than MBES, they contain more characteristics of the ultra-shallow subsurface than back-scattering data. These were determined that back-scattering has advantages in representing acoustic anomaly distribution by surface sediment type, and seabed-reflected amplitude is advantageous for representing sediment type by ultra-shallow subsurface. In particular, these results were well shown when the surface sediment thinly covered the rocky bottom. Therefore, it is necessary not only to analyze the back-scattering of MBES but also the ultra-shallow subsurface features through seismic data for valid seabed classification.

Experimental study on flow around horizontal multiple pipelines laid on the erodible seabed surface

Experimental study on flow around horizontal multiple pipelines laid on the erodible seabed surface

Underwater half-space analysis to oblique 3D seismic waves based on exact free-field response

The seismic behavior of underwater half-space under arbitrary 3D incidence of P- or SV-waves is analyzed by the 2.5D finite/infinite element method. Firstly, the 3D responses of solid and liquid in free field are newly derived exactly, which cover both the sub- and super-critical conditions. Then, the equivalent seismic forces acting on the solid boundary of the 2.5D finite/infinite element method are merely computed from the free-field displacement for given earthquakes. Subsequently, the equation of motions for the underwater half-space under arbitrary 3D incident seismic waves are established in the 2.5D finite/infinite element system, with the key parameters specified. The proposed method was verified against the previous 2D solutions and inversely computed free-field solutions in frequency domain. Major conclusions drawn from the present analysis include: (1) The effects of liquid, concerning enlargement, reduction or disturbance, on the seabed responses become stronger for increasing liquid depth under sub-critical condition; (2) The seabed interior responses are less sensitive to the variation in liquid depth under super-critical condition; and (3) Under the super- and sub-critical conditions, respectively, more attention should be given to the interior response for P-waves and to the seabed surface response for SV-waves.

A Computational Investigation of the Influence of Seafloor Conditions on the Turbulent Flow Characteristics of an Autonomous Underwater Vehicle

The effect of the seabed on the hydrodynamics of three-dimensional autonomous underwater vehicles (AUVs) varies according to the physical conditions of the place where AUVs interact with the environmental conditions. This study examines the hydrodynamics of an AUV resembling a torpedo model while taking the influence of the seabed surface as a function of the dimensionless distances (G/D) between the torpedo and the seabed. Reynolds numbers, varying from 1 × 104 to 8 × 104, were considered. These Reynolds numbers were associated with various seabed distances falling within 0.25 ≤ G/D ≤ 1.5. To perform the simulations, governing equations were utilized and incorporated with the k–ω SST turbulence model. It has been observed that when AUVs or torpedo models operate in close proximity to the seabed surface, several key hydrodynamic parameters and flow characteristics are affected. These include the pressure coefficient (Cp), drag coefficient (CD), overall flow structures, maneuverability, and performance of the torpedo model. As the AUV or torpedo model approaches the seabed surface, the symmetrical flow pattern deteriorates. This deterioration is associated with changes in vortical flow structures under the influence of seabed surfaces. Additionally, the intensity of the shear stress (τ) near the seabed surface gradually increases as the AUV or torpedo model gets closer to it. In summary, the proximity of AUVs or torpedo models to the seabed surface causes disruptions in the flow patterns, increased shear stress, and alterations in key hydrodynamic parameters, ultimately affecting the system's performance and behavior.

Multitemporal Acoustic Backscatter Data Analysis to Monitor the Dynamics of Seabed Surface Sediments

Backscatter strength is a product of underwater acoustic remote sensing. This study used a multibeam echosounder as an acoustic remote sensing instrument to collect backscatter strength data. These data are then used to classify the surficial sediment distribution. To monitor the difference in seabed sediment distribution, a time-series survey was performed to obtain multitemporal acoustic backscatter data. An EM 304 multibeam system from Kongsberg was mounted on the Research Vessel Baruna Jaya III from the Indonesia National Research and Innovation Agency (BRIN). It was used to collect backscatter data in the waters of Raja Ampat, Indonesia. The data were collected at different times, April and July 2021. Geometric and radiometric corrections were applied to these backscatter data. Based on the angular response curve analysis from acoustic backscatter strength, the research area can be classified into four seabed sediment types: boulder, gravel, sand, and mud. A comparison of both time series backscatter data shows that the boulder and gravel areas increased by 13.6% and 19.0%, respectively. Elsewhere, the area with sediment types of sand and mud diminished by 30.5% and 2.0%. The change in the sediment type area occurred as much as 50.5%, while the remaining 49.5% area remained unchanged. This resulting value is apparently derived from the steep topography that rapidly changes sediment distribution. One such suggestion is that sediment sampling should be performed to confirm the model from angular response curve analysis.

Multitemporal Acoustic Backscatter Data Analysis to Monitor the Dynamics of Seabed Surface Sediments

Backscatter strength is a product of underwater acoustic remote sensing. This study used a multibeam echosounder as an acoustic remote sensing instrument to collect backscatter strength data. These data are then used to classify the surficial sediment distribution. To monitor the difference in seabed sediment distribution, a time-series survey was performed to obtain multitemporal acoustic backscatter data. An EM 304 multibeam system from Kongsberg was mounted on the Research Vessel Baruna Jaya III from the Indonesia National Research and Innovation Agency (BRIN). It was used to collect backscatter data in the waters of Raja Ampat, Indonesia. The data were collected at different times, April and July 2021. Geometric and radiometric corrections were applied to these backscatter data. Based on the angular response curve analysis from acoustic backscatter strength, the research area can be classified into four seabed sediment types: boulder, gravel, sand, and mud. A comparison of both time series backscatter data shows that the boulder and gravel areas increased by 13.6% and 19.0%, respectively. Elsewhere, the area with sediment types of sand and mud diminished by 30.5% and 2.0%. The change in the sediment type area occurred as much as 50.5%, while the remaining 49.5% area remained unchanged. This resulting value is apparently derived from the steep topography that rapidly changes sediment distribution. One such suggestion is that sediment sampling should be performed to confirm the model from angular response curve analysis.

Improved predictive model for the strength of fluidized seabed sediments with rate effect characteristics by full-scale spherical penetrometer tests

Sampling fluidized seabed surface sediments with viscous fluid characteristics is highly challenging, making in situ penetration tests essential to evaluate their strength. Unfortunately, an in situ full-flow spherical penetrometer (i.e., ball) is typically used to assess the strength via deep penetration, and research on surface penetration, extensive viscous fluid shear behavior of fluidized surface sediments, and ball-ambient water-surface sediment interaction is lacking. To address this gap, this study utilizes a computational fluid dynamics (CFD) model validated through centrifugal tests to examine the penetration of balls with free and no-slip in fluidized seabed surface sediments with the shear rate effect. This study analyzes the stress characteristics on a ball under complex conditions and proposes three phases of the penetration resistance coefficient of the ball in fluidized surface sediments. Additionally, this study quantifies the effects of various factors, such as initial undrained shear strength, rate effect, ambient water above the ball, and contact relationship between the ball and fluidized seabed surface sediment, on sediment strength evaluation. Finally, this study establishes a three-phase method to evaluate the undrained shear strength of fluidized seabed surface sediments by full-scale spherical penetrometer tests. This method calculates the surface penetration resistance coefficients as functions of the dimensionless penetration depth, providing a vital basis for acquiring the strength of fluidized seabed surface sediments.

Numerical Investigations of Undrained Shear Strength of Sensitive Clay Using Miniature Vane Shear Tests

The laboratory miniature vane shear test (MVST) has been widely used to measure the undrained shear strength of marine sediments in offshore engineering. However, the transfer of the soil sample in tube samplers from the seabed to the laboratory releases the in situ confining stress acting on the soil and will decrease the soil strength. In this research, in order to investigate the effects of confining stress on the undrained shear strength of marine sediments, the Coupled Eulerian-Lagrangian (CEL) approach in ABAQUS is used to model the three-dimensional standard and miniature vane shear tests to estimate the undrained shear strength of sensitive clay with different sensitivities under various stress conditions. Based on the numerical simulation results, a linear strength model that not only considers confining stress effects but also can eliminate the size effects caused by vane blades of MVST is proposed. The proposed model can be used to estimate the undrained shear strength of the sensitive clay under shallow seabed surfaces.

An analytical solution for the dynamic response of a seawater‐sloping seabed‐bedrock system under an oblique incident P wave

AbstractMany offshore marine structures are built on seabeds with slight or considerable slopes, while most analytical studies have focused on the dynamic responses of horizontal seabeds. Based on the theory of porous media, an analytical solution for the response of a sloping seabed to earthquakes is developed in this paper. Stress and displacement responses can be shown at any location of the seabed. Using numerical examples, we analyze the influence of the slope angle, frequency and angle of incident waves, and thicknesses of the seawater and sediment layers on the dynamic response of the seabed. The results show that the solid displacement amplification factor of the surface of the horizontal seabed is always constant, while that of the sloping seabed changes periodically along the seabed surface. Even at a small site, the difference between a horizontal seabed and a sloping seabed is still significant. The slope angle has different effects on the seabed at different depths, and the inclination plays an amplification role at some depths and a reduction role at other depths. With an increase in slope angle, the difference between a horizontal seabed and a sloping seabed becomes more significant. Especially at natural frequencies, the amplification factor becomes more significant with increasing slope angle. Therefore, even though the same seawater depth is used in the model, a horizontal seabed model cannot be used to analyze the dynamic response of a sloping seabed or significant errors will occur.

Simulation of mooring Lines in complex bathymetries using a finite element method

This paper presents a novel method for the modeling of the seabed interaction of mooring lines in complex bathymetries, known as “continuous projection method”. This method is able to calculate ground normal and friction forces for any seafloor surface. This provides an improvement in the mooring systems simulation, as it captures additional non-linearities on the mooring line performance due to seabed interaction.The method is based on constructing a triangulation for the seabed and projecting mooring line nodes by using the vertex normal vectors of each triangle, ensuring the continuity of the projection. For the sake of the computational cost reduction, the line nodes are first projected into the closest triangles. Also, whenever the floor is flat, inclined or horizontal, a point-to-plane projection expression is used instead. The projection method described has been implemented to a finite element model. The initial condition problem was solved with a static approach, based on finding the static equilibrium with Newton–Armijo algorithm. This improves other static approaches which use the catenary equation, and that are only valid for a flat seabed.The model was successfully verified against the analytical solution of an inextensible catenary line in a slope. Furthermore, the simulation results were validated against experimental scale tests on a single chain mooring line with three different seafloor structures: one flat floor and two different sloped steps. For each of them, static and dynamic regular tests were performed. Moreover, high and low frequency fairlead movements were imposed in the dynamic tests, aiming to validate the model both in cases with and without snapping loads. Overall, the obtained results were coherent and allow to validate the accuracy of the proposed model. Finally, a mooring line over an irregular seabed surface was studied, comparing the results obtained by directly applying the developed method for complex bathymetries, by interpolating the surface by an inclined plane and using the constructed projection algorithm for that case or, by last, approximating by a flat seafloor. The comparison of the results among the different approaches illustrates the importance of considering the seabed slope and irregularities for the fairlead tension prediction. Also, this flat seafloor was evaluated with two different projection methods: the one specific for horizontal seafloors and the one developed for general seabed surfaces. This allows to compare the computational time required in both of them.

Geomorphological characteristics of the Southwest waters of Vietnam

Based on the principle of origin, the southwestern coastal topography of Vietnam is divided into 32 geomorphological units. Coastal and island topography consists of 9 geomorphological units belonging to 4 topographic groups: (i) terrain with the common erosive origin, (ii) plants of marine origin, (iii) mixed river and sea origin, and (iv) waste of biological origin - river and sea. The seabed topography consists of 23 geomorphological units belonging to 3 topographic groups (i) topography of marine origin; (ii) economics of river and sea origin; (iii) marsh-sea accumulation terrain. The topography of a small part of the coastal continent and most islands were formed in the Quaternary period, and the rest of the continental coastal topography and the entire seabed surface of Southwest Vietnam was developed in the Quaternary period, from Late Pleistocene to Holocene.

Analytical Method for High-Precision Seabed Surface Modelling Combining B-Spline Functions and Fourier Series

High-accuracy seabed surface modelling provides multi-source high-precision fundamental geographic datasets for marine visual computing, seabed topography detection, marine biology, marine engineering and other fields. Proposed in this paper is a high-precision seabed surface model, which combines B-spline functions and Fourier-series, referred to as the Spline-Fourier-series (S-FS) method. Firstly, the mathematical relationship between the B-spline functions and Fourier-series in the modelling process is explored in depth, deducing the non-recursive basis functions of the Spline-Fourier-series model and the specific representation of the two dimensional Spline-Fourier-series model. Furthermore, using a publicly available Large-area bathymetric dataset, extensive experiments are conducted for comparisons with traditional methods (nearest-neighbor, bilinear, bicubic) and traditional Fourier-series, which generally shows the S-FS method has higher accuracy, better convergence and stronger robustness. Finally, based on its mathematically theoretical model, three characteristics (dimensionality reduction, multi-resolution expression and multi-scale visualization) of the S-FS method for constructing high-precision seabed surface are analyzed visually and deeply. Compared with B-spline function, the basic functions of the S-FS method inherit its prioritized compactly-supported performance and do not need to be recursively calculated anymore, thereby further showing its feasibility and extensibility in the field of high-precision seabed surface modelling.

Analysis of morphological features and chemical properties of the dried seabed soils of the North – Eastern Aral Sea region

On the dried-out surface of the Aral Sea, peculiar soil-formation processes take place. First, littoral soils whose development proceeds according to the zonal desert type. The general pattern is weak development of soil-forming processes on the background of strong initial salinity and carbonation. As a result of conducted field researches the basic types of soils and their morphological, chemical properties, content of heavy metals (gross and mobile) and specific features of salinization of soils of north-eastern Aral region are determined. The following soil types have been identified: typical solonchak, takyr-like and takyr solonchak, meadow solonchak, seaside solonchak, desert sand, brown desert non-saline soils. A comparative assessment of soil changes in the Eastern Aral Sea region as a result of the Aral Sea desiccation has been carried out. The leading factor of soil cover formation and development on the exposed seabed surfaces is the desert climate, which determines the development of salinization process after the sea receded, followed by rapid aridization of the area.

Using seabed acoustic imagery to characterize and classify seabed sediment types in the pockmark area of the North Yellow Sea, China

Pockmarks are circular or elliptical depressions in the seabed that are geomorphologically similar but otherwise distinct from volcanic craters. Pockmarks are found at various depths in the ocean including on the continental shelf, the continental slope and the deep-sea plain. This paper presents data covering a large-scale pockmark group in the middle part of the shallow sea sedimentary plain of the North Yellow Sea at a water depth of 51–58 m. The distribution of pockmarks within this group is very dense, and pockmark diameters range from several hundred meters to several kilometers. Based on multibeam echo sounder data, a high-resolution image of the pockmark area is obtained by efficient image processing methods. The textural features of the seabed surface layer are then described and analyzed. Multibeam echo sounder backscatter data are combined with seabed sediment sampling data to develop a statistical model linking seabed backscatter sonar strength and sediment grain-size characteristics. Additionally, an improved BP neural network is used to realize rapid and automatic classification and identification of seabed sediment types. Fine-scale processing analysis of sonar images and acoustic classification of seabed sediments as presented in this paper are useful for determining seabed surface textures and local sediment distribution laws. This study thus provides a basis for further study of the formation and evolution of seabed pockmarks.

Seabed modelling with a least-squares support-vector machine and sample cross-validation

This paper reports on a study to model seabed surfaces using the least-squares support-vector machine algorithm with a sample cross-validation (CV) method. It starts with a brief overview of the sample selection method of the algorithm and gives two important characteristics of the algorithm. It then focuses on the theory of sample CV and the steps of sample selection using this theory. Finally, to verify the validity of the sample CV method in sample selection for the algorithm, the measured multi-beam bathymetric data are selected to calculate and analyse. It is concluded that the sample CV method can reasonably screen out the sounding training samples with a large contribution to the function model, making the constructed function model more reasonable.

Practical Large-Deformation Finite-Element Method for 3D Geotechnical Problems Involving Free Surface Deformations

The remeshing and interpolation technique with small-strain (RITSS) finite-element method has been widely adopted to address a broad range of two-dimensional (2D) large deformation geotechnical problems. However, its three-dimensional (3D) applications remain extremely limited due to the requirement of user-defined coding and the challenges in accurate tracking of free surfaces. This paper introduces a recently developed 3D RITSS method to tackle geotechnical problems that involve free surface deformations with only minimal programming effort required. Both the automated mesh regeneration and the Lagrangian finite-element increments are realized using available commercial packages. The entire simulation can be controlled and executed with a single piece of Python script. The reliability of the proposed method was verified by three validation examples. A further application example modeling the reinstallation of a circular footing near an existing footprint is also presented to demonstrate the advantage of the proposed method in coping with dramatic seabed surface deformations and hence illustrates its potential application to 3D geotechnical problems.

Development and Application of an in Situ Penetrator for Rapid Strength Testing of Submarine Sediment

In marine engineering, the strength of a submarine sediment is an indispensable parameter for assessment of construction. In this study, a free-fall cone penetrator named IPen was developed to realize a rapid and efficient measurement of sediment strength. The equipment is characterized by modular design and self-contained data acquisition. It is equipped with an acceleration sensor, a water pressure sensor, and a piezocone penetration test (CPTu) probe. It is designed to be released from near seabed surface with a releaser and then fall freely to provide a higher penetration velocity. Its maximum working depth is approximately 2500 m and maximum penetration depth is approximately 3 m. To derive the correlation between penetration resistance and sediment strength, a calibrator was devised to determine the penetration-rate factor. In addition, the factor applicable to in situ test points was determined in laboratory experiments. In June 2016, the IPen was tested in situ in the South Yellow Sea, China, during a shared voyage funded by the National Science Foundation. Meanwhile, undisturbed column samples were collected for laboratory tests. Based on the in situ test results, it was demonstrated that the IPen could accurately record the working states of various sensors during the freely falling course. IPen test results reliably reflected the sediment strength at all the testing points when compared with laboratory calibration tests, in situ vane tests and penetration tests, laboratory penetration tests, and unconsolidated and undrained triaxial compression tests.

Effect of Touchdown Trench Modelling on SCR Fatigue Life

Touchdown zone (TDZ) is one of the most critical fatigue hot spot for a Steel Catenary Riser (SCR). In current industry practice, a flat profile is always used when modelling the seabed surface. However, survey data shows that trenches are formed at SCR touch down zone after riser installation. It is important to understand the impact of trench modelling for fatigue life at TDZ. This paper presents a SCR TDZ fatigue life comparison between a flat and trenched seabed profile using Finite Element Analysis (FEA) models based on linear soil stiffness. The trenched seabed is modelled in its assumed equilibrium condition using an analytical equation derived from non-linear hysteretic analysis in the literature. Further analyses are conducted for more parameters. The objective of this study is to determine if a linear seabed model with trenched profile can improve TDZ fatigue life of an SCR when compared to an SCR on a flat seabed surface.

Side-Scan Sonar Image Mosaic Using Couple Feature Points with Constraint of Track Line Positions

To obtain large-scale seabed surface image, this paper proposes a side-scan sonar (SSS) image mosaic method using couple feature points (CFPs) with constraint of track line positions. The SSS geocoded images are firstly used to form a coarsely mosaicked one and the overlapping areas between adjacent strip images can be determined based on geographic information. Inside the overlapping areas, the feature point (FP) detection and registration operation are adopted for both strips. According to the detected CFPs and track line positions, an adjustment model is established to accommodate complex local distortions as well as ensure the global stability. This proposed method effectively solves the problem of target ghosting or dislocation and no accumulated errors arise in the mosaicking process. Experimental results show that the finally mosaicked image correctly reflects the object distribution, which is meaningful for understanding and interpreting seabed topography.

Evaluating satellite-derived bathymetry accuracy from Sentinel-2A high-resolution multispectral imageries for shallow water hydrographic mapping

The wide-ranging evolution of remote sensing technology has offered a promising application to the hydrographic surveying, the Satellite-Derived Bathymetry (SDB). SDB is a contemporary acquisition technique which derives bathymetric data from multispectral satellite imagery for various hydrographic purposes. The introduction and availability of the chargeless Sentinel-2A multispectral satellite data in the region, has inspired the National Hydrographic Centre (NHC) and Universiti Teknologi Malaysia (UTM) to collaborate in assessing the level of accuracy provided by Sentinel-2A imageries as a data sources for shallow water hydrographic mapping. The assessment is concentrating on both empirical algorithm methods, Lyzenga and Stumpf models in two different seabed topology conditions. Both Lyzenga and Stumpf model produced almost identical results for the area with normal gradient seabed. Conversely, for the rough and irregular seabed surface, results from Lyzenga model was much better compared to Stumpf’s. Lyzenga model has delivered better results where a total of 166,747 (36.23%) of depths samples achieved the IHO minimum survey order standard compared to 103,418 (22.47%) by Stumpf model. Thus, this paper would deliberate the detail findings from the study primarily on the accuracy level of the Sentinel-2A data for SDB hydrographic mapping application over the tropical environmental setting in Malaysia