Seafloor Roughness Research Articles

Seafloor roughness reduces melting of East Antarctic ice shelves

Heat delivered by the ocean circulation is melting the Antarctic ice sheet from below. This melt is largest where warm Circumpolar Deep Water accesses the continental shelf and reaches the ice shelf cavities. Future melt rate projections are based on ocean thermal forcing derived from climate models, which tend to be biased warm around Antarctica. The bias has been attributed to unresolved ocean processes that occur at scales poorly represented in models. Using a high-resolution model of the Denman Glacier region we show that seafloor roughness unresolved in climate models suppresses the impact of warm water on ice sheet melting. Seafloor roughness slows down the shelf circulation, reducing the presence of warm water over the shelf and the heat transport towards the ice cavities. As a result, the total meltwater discharge drops by 4 Gt year−1. Our results suggest a mechanism missing in global ocean and climate models that could reduce the spread in climate projections.

Bottom Multi-Parameter Bayesian Inversion Based on an Acoustic Backscattering Model

The geoacoustic and physical properties of the bottom, as well as spatial distribution, are crucial factors in analyzing the underwater acoustic field structure and establishing a geoacoustic model. Acoustic inversion has been widely used as an economical and effective method to obtain multi-parameters of the bottom. Compared with traditional inversion methods based on acoustic propagation models, acoustic backscattering models are more suitable for multi-parameter inversion, because they contain more bottom information. In this study, a Bayesian inversion method based on an acoustic backscattering model is proposed to obtain bottom multi-parameters, including geoacoustic parameters (the sound speed and loss parameter), partial physical parameters of the sediment, and statistical parameters of the seafloor roughness and sediment heterogeneity. The bottom was viewed as a kind of fluid medium. A high-frequency backscattering model based on fluid theory was adopted as the forward model to fit the scattering strength between the model prediction and the measured data. The Bayesian inversion method was used to obtain the posterior probability density (PPD) of the inversion parameters. Parameter estimation, uncertainty, and correlation were acquired by calculating the maximum a posterior (MAP), the mean values, the one-dimensional marginal distributions of the PPD, and the covariance matrix. Finally, the high-frequency bottom backscattering strength from the Quinault Range site was employed for inversion tests. The estimated values and uncertainties of various bottom parameters are presented and compared with the directly measured bottom parameters. The comparison results demonstrate that the method proposed herein can be used to estimate the sediment/water sound speed ratio, the sediment/water density ratio, and the spectral exponent of the roughness spectrum effectively and reliably.

Geomorphology and mechanisms of subduction erosion in the sediment-starved Mariana convergent margin

Subduction erosion, commonly occurring in convergent margins, serves as a key mechanism for effective recycling of materials into the deep subduction. Subduction of bathymetric highs is widely recognized as a critical factor of subduction erosion of the forearc. The Mariana margin is an often-cited example of a tectonically erosive zone, with numerous topographic features such as seamounts, ridges, horsts, and grabens. However, few studies have attempted to determine the regional subduction erosion regime. By combining high-resolution bathymetric data and multichannel seismic profiles, this study analyzed the seafloor roughness, forearc taper angle, accretionary prism, and forearc topography to reveal evidence of subduction erosion at the Mariana margin. The subduction of rough seafloor (seamounts, horsts, and grabens) has accelerated the erosion process of the sediment-starved (<1 km) Mariana margin. In addition to causing depressions and landslides on the inner trench slope, this erosive process resulted in a flat forearc topography, which in turn may have induced a steeper forearc slope and taper angle. The arc-trench distance also shows a strong negative correlation with subduction erosion along the Mariana margin. The involvement of fluid inhibits the erosive process by favoring the development of a smooth surface along the plate interface. Our results suggest that tectonic erosion at the Mariana margin is jointly due to three mechanisms: seamount subduction, horst and graben subduction, and hydrofracturing. This integrated erosive process corresponds to a long-term subduction erosion rate of the submerged forearc of 7–8 km3/km/Myr, significantly lower than those calculated along the other global convergent margins.

Temporal change of seafloor scattering and its dependence on environmental parameters in shallow-water sandy sites

In the ocean, the performance of active sonar systems used for object detection and seafloor characterization can be affected when the acoustic properties of the seafloor change due to near-bottom hydrodynamics and biological activity. Determining the dominant environmental mechanisms and corresponding time scales that regulate seafloor scattering will increase our understanding of the performance of these remote-sensing applications. To this end, a high-frequency active acoustic system (operating at 38 kHz, 70 kHz and 200 kHz), a wave-sensing CTD, and a stereo camera were deployed on the seafloor in a series of experiments lasting from two weeks to five months. Seafloor scattering measurements were obtained in two shallow water locations in New Hampshire, USA: a wave-dominated site and a tidal current dominated site. Daily and weekly trends in mean scattered levels and the mechanisms causing their temporal variability are discussed. The temporal change in scattering as a function of angle is compared to the small-slope approximation model where seafloor roughness estimates were obtained using stereophotogrammetry.

Acoustic scattering modeling and sound field characteristics of rough seafloor in shallow sea

Acoustic scattering is an important part of ocean acoustics, and the acoustic scattering caused by the unevenness of the seafloor surface is one of the reasons for the fluctuation of acoustic propagation in the ocean. In order to solve the acoustic scattering problem of sea bottom surface roughness, normal wave theory is used to model the acoustic field. To simplify the problem, Lambert’s law is used to establish the seafloor rough scattering model in horizontal layered shallow sea waveguides, and the scattering field is assumed to be isotropic in the horizontal direction. Based on this model, the amplitude distribution and the phase distribution of the scattered sound pressure are obtained, and the intensity of the scattered sound field and its spatial correlation coefficient are simulated numerically. The prediction of the scattered sound field under rough interface conditions is realized, and the variation of the spatial characteristics of the scattered sound field with the roughness of the seafloor is revealed. The results show that when Lambert’s law is used to describe the rough interface acoustic scattering and when the seafloor roughness is smaller than the wavelength, the spatial correlation coefficient of the scattered sound field at two different positions in space has a change rule of periodic oscillation attenuation with the increase of spatial distance, and in the vertical direction, the oscillation period is larger and the attenuation is slower. When the roughness increases, the oscillation amplitude of the horizontal and the vertical correlation coefficient gradually increase, the oscillation period of the horizontal correlation coefficient gradually decreases, and the vertical correlation coefficient no longer attenuates in the direction near the seafloor, which is the result of the weakening of the seafloor acoustic scattering. The model theory in this paper can also be extended to the acoustic scattering modeling of rough sea surface. For the case of non-horizontal seabed, the scattered sound field of the rough interface in the waveguide can be obtained by using coupled normal wave or adiabatic normal wave theory.

The sandpaper theory of flow–topography interaction for homogeneous shallow-water systems

Recent studies reveal the dramatic impact of seafloor roughness on the dynamics and stability of broad oceanic flows. These findings motivate the development of parameterizations that concisely represent the effects of small-scale bathymetric patterns in theoretical and coarse-resolution numerical circulation models. The previously reported quasi-geostrophic ‘sandpaper’ theory of flow–topography interaction a priori assumes gentle topographic slopes and weak flows with low Rossby numbers. Since such conditions are often violated in the ocean, we now proceed to formulate a more general model based on shallow-water equations. The new version of the sandpaper model is validated by comparing roughness-resolving and parametric simulations of the flow over a corrugated seamount.

Characterizing the porosity structure and gas hydrate distribution at the southern Hikurangi Margin, New Zealand from offshore electromagnetic data.

The dynamics of accretionary prisms and the processes that take place along subduction interfaces are controlled, in part, by the porosity and fluid overpressure of both the forearc wedge and the sediments transported to the system by the subducting plate. The Hikurangi Margin, located offshore the North Island of New Zealand, is a particularly relevant area to investigate the interplay between the consolidation state of incoming plate sediments, dewatering and fluid flow in the accretionary wedge and observed geodetic coupling and megathrust slip behaviour along the plate interface. In its short geographic extent, the margin hosts a diversity of properties that impact subduction processes and that transition from north to south. Its southernmost limit is characterized by frontal accretion, thick sediment subduction, the absence of seafloor roughness, strong interseismic coupling and deep slow slip events. Here we use seafloor magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data collected along a profile through the southern Hikurangi Margin to image the electrical resistivity of the forearc and incoming plate. Resistive anomalies in the shallow forearc likely indicate the presence of gas hydrates, and we relate deeper forerarc resistors to thrust faulting imaged in colocated seismic reflection data. Because MT and CSEM data are highly sensitive to fluid phases in the pore spaces of seafloor sediments and oceanic crust, we convert resistivity to porosity to obtain a representation of fluid distribution along the profile. We show that porosity predicted by the resistivity data can be well fit by an exponential sediment compaction model. By removing this compaction trend from the porosity model, we are able to evaluate the second-order, lateral changes in porosity, an approach that can be applied to EM data sets from other sedimentary basins. Using this porosity anomaly model, we examine the consolidation state of the incoming plate and accretionary wedge sediments. A decrease in porosity observed in the sediments approaching the trench suggests that a protothrust zone is developing ∼25km seaward of the frontal thrust. Our data also imply that sediments deeper in the accretionary wedge are slightly underconsolidated, which may indicate incomplete drainage and elevated fluid overpressures of the deep wedge.

A generalized theory of flow forcing by rough topography

An analytical model is developed which explores the impact of irregular sea-floor roughness on large-scale oceanic flows. The previously reported asymptotic ‘sandpaper’ theory of flow-topography interaction represents relatively swift currents and exhibits singular behaviour in the weak flow limit. The present investigation systematically spans a wider parameter space and identifies the principal dissimilarities in the topographic regulation of slow and fast currents. The fast flows are controlled by the Reynolds stresses produced by topographically generated eddies. In contrast, relatively weak flows are more affected by the eddy-induced bottom form drag. The asymptotic models for fast and slow currents are then combined to arrive at a concise description of flow forcing by small-scale topography in homogeneous and multilayer models. The proposed closure is validated by comparing corresponding topography-resolving and parametric simulations.

The effect of seafloor roughness on passive estimates of the seabed reflection coefficient.

In this work, a model is developed for the effect of seafloor interface roughness on passive estimates of the reflection coefficient. The main result is an expression for the total intensity reflection coefficient, with separate coherent and incoherent contributions. Assumptions of this model include constant sound speed in the ocean, stationary and Gaussian seafloor roughness, and ambient noise. Numerical examples for the coherent, incoherent, and total contributions to the intensity reflection coefficient are presented for halfspace and layered environments-all using the small slope approximation. To illustrate the potential parameter errors that results from using a flat interface wave model when roughness is present, a geoacoustic inversion is performed using the proposed model as input data. A joint roughness-geoacoustic inversion of simulated data using the proposed model was also performed. It was found that the true roughness and geoacoustic parameters can be inverted using this model, but the sensitivity to the outer scale of the rough surface has the highest error compared to the other parameters.

Dating seafloor spreading of the southwest sub-basin in the South China Sea

Marine magnetic lineations play important role in interpreting the age and spreading processes of the oceanic crust. However, it is difficult to identify magnetic lineations from sea surface observations in a deep and narrow low-latitude slow-spreading inactive ocean basin for many reasons, such as the highly filtered low-amplitude magnetic anomalies, highly oblique magnetization, contamination from narrow stripes, etc., which may hamper accurate lineation discrimination and identification. The spreading history of the Southwestern Sub-basin of the South China Sea (SW-SCS), which is located in such a low-latitude tectonic setting, has been debated for a long time. Near-seafloor deep-tow magnetic measurements provide high-resolution reversal records, making accurate lineation discrimination feasible. Corrections, including diurnal variation correction, regional field correction, and de-skewing, are explored. The results, calibrated with drill sites from the International Ocean Discovery Program (IODP) Expedition 349, show that the spreading ages of the SW-SCS range from ∼ 21 to 15 Ma (from C6A to C5B) and the full spreading rate ranges from 26 to 46 mm/yr, with an average rate of 37 mm/yr. Spreading was initially symmetric but later became asymmetric, with a rate differential reaching 10%. Combined with two seismic profiles, it is found that the roughness of the oceanic basement to the south of the extinct ridge is mainly influenced by faulted blocks, whereas to the north it is mainly influenced by magmatism. The RMS seafloor roughness is estimated to be 211 m in the south and 191 m in the north respectively. This indicates that SW-SCS seafloor spreading tends to be a transition type between axial high and axial low at the intermediate-slow spreading rate. There is a large valley in the spreading center, but the roughness far away from the center is low. the RMS basement roughness is obviously lower than that of the typical slow spreading oceanic basin. we conjecture that the larger than normal syn-spreading magma may be the reason for the low roughness. A long history of being subducted might be the deep-seated reason for all these behaviors where, and the mantle is rich in water and carbon.

Numerical modeling of low-frequency scattering from targets buried under rough seafloors

The acoustic backscattering of elastic bodies lying across the seafloor is a problem of great interest in many underwater applications, such as detecting and classifying buried objects. In this work, the backscattered wavefield of meter-sized targets is investigated numerically in the 1–50 kHz frequency band. The simulator solves the three-dimensional linearized equations of continuum mechanics. The computations are performed through a multi-GPU solver based on a multi-grid staggered finite-difference time-domain method (cf. Sabatini et al., 181st ASA Meeting, Seattle, Washington, 2021). The effects of the seafloor roughness and the target burial depth on the spectrum of the scattered field are more specifically analyzed. This study demonstrates the usefulness of direct numerical simulations of acoustic scattering from realistic elastic objects in complex underwater environments to improve the capabilities of sonar systems in detecting and identifying buried objects.

Modeling the effect of random roughness on synthetic aperture sonar image statistics.

A model has been developed to predict the effect of random seafloor roughness on synthetic aperture sonar (SAS) image statistics, based on the composite roughness approximation-a physical scattering model. The continuous variation in scattering strength produced by a random slope field is treated as an intensity scaling on the image speckle produced by the coherent SAS imaging process. Changes in image statistics caused by roughness are quantified in terms of the scintillation index (SI). Factors influencing the SI include the seafloor slope variance, geo-acoustic properties of the seafloor, the probability density function describing the speckle, and the signal-to-noise ratio. Example model-data comparisons are shown for SAS images taken at three different sites using three different high-frequency SAS systems. Agreement between the modeled and measured SI show that it is possible to link range-dependent image statistics to measurable geo-acoustic properties, providing the foundation necessary for solving problems related to the detection of targets using high-frequency imaging sonars, including performance prediction or adaptation of automated detection algorithms. Additionally, this work illustrates the possible use of SAS systems for remote sensing of roughness parameters such as root mean square slope or height.

Global Mesoscale Ocean Variability from Multiyear Altimetry: An Analysis of the Influencing Factors

Abstract Sea surface slope (SSS) responds to oceanic processes and other environmental parameters. This study aims to identify the parameters that influence SSS variability. We use SSS calculated from multiyear satellite altimeter observations and focus on small resolvable scales in the 30–100-km wavelength band. First, we revisit the correlation of mesoscale ocean variability with seafloor roughness as a function of depth, as proposed by Gille et al. Our results confirm that in shallow water there is statistically significant positive correlation between rough bathymetry and surface variability, whereas the opposite is true in the deep ocean. In the next step, we assemble 27 features as input variables to fit the SSS with a linear regression model and a boosted trees regression model, and then we make predictions. Model performance metrics for the linear regression model are R2 = 0.381 and mean square error = 0.010 μrad2. For the boosted trees model, R2 = 0.563 and mean square error = 0.007 μrad2. Using the hold-out data, we identify the most important influencing factors to be the distance to the nearest thermocline boundary, significant wave height, mean dynamic topography gradient, and M2 tidal speed. However, there are individual regions, that is, the Amazon outflow, that cannot be predicted by our model, suggesting that these regions are governed by processes that are not represented in our input features. The results highlight both the value of machine learning and its shortcomings in identifying mechanisms governing oceanic phenomena.

Exploring Ocean Floor Geodiversity in Relation to Mineral Resources in the Southwest Pacific Ocean

The future extraction of mineral resources may irreversibly damage ocean floor geodiversity in the Southwest Pacific Ocean. Therefore, understanding of the spatial distribution of ocean floor geodiversity in relation to mineral resources is important. For that purpose, we first developed a geodiversity index map of the western Pacific Ocean including spatial information of openly available digital layers of four components: seafloor geomorphology, sediment thickness, bathymetry and seafloor roughness. Second, we analysed how these components contributed to the geodiversity index. Finally, correlations between three mineral resources (seafloor massive sulphides, polymetallic nodules and cobalt-rich crusts) and the geodiversity index, its components, and the ocean floor age were calculated. The results showed that the ocean floor environment and the time necessary for the formation of the three mineral resources were predominantly related to the bathymetry component and the age of the ocean floor, and to a lesser extent to the seafloor roughness, geomorphology and sediment thickness components. We conclude that the ocean floor geodiversity index and its components contribute to an improved understanding of the spatial distribution of abiotic seafloor diversity and can be optimized by using higher resolution data. We suggest that ocean floor geodiversity could be considered in future resource extraction to support responsible mining and help limit environmental damage.

A numerical model for geometrically nonlinear analysis of long free spanning offshore pipelines enhanced with buoyancy modules

Despite a detailed oil and gas pipeline route assessment, the emergence of highly long free spans induced by seafloor roughness in deep waters, accompanied by significant gaps between the pipeline and the seafloor, can sometimes be unavoidable. In those cases, cost-efficient span correction can be achieved by attaching buoyancy modules along certain sections of the free span.This paper provides a simple numerical technique for static analysis of submarine pipeline free spans enhanced with buoyancy modules and resting on span shoulders of nonequal elevations.The entire pipeline is modeled as a single continuous segment considering geometrical nonlinearity induced by large deformations. The solution technique is based on consistent minimization of the total potential energy of the system discretized as a Riemann sum and the solution of the resulting algebraic system of nonlinear finite difference equations with a Newton-Raphson technique.The feasibility of the proposed technique is demonstrated through several pipeline free span scenarios. The effects of buoyancy module section length, its distribution, effective axial tension, span shoulders soil stiffness, and span shoulders elevation gap are considered.The proposed technique provides a fast and accurate alternative to special-purpose finite element commercial software, particularly throughout the intermediate stages of the design process.

Relating fine-grained sediment consolidation to acoustic seabed surveying

The process of fine-grained seabed sediment consolidation is governed by a variety of factors including grain size distributions, organics, sediment dynamics, and anthropogenic activities (e.g., marine traffic and dredging). Poorly consolidated sediments are typically softer and more susceptible to mobility from shear, while over consolidated sediments are typically stiffer with a higher threshold for mobility from shear. Knowledge of the state of consolidation of seabed sediments is important to assess and maintain navigable depth, predict length scales of seafloor roughness, and assess seabed-object interaction (e.g., anchors, unexploded ordnance). For example, the porosity varies significantly for the same type of sediment for different states of consolidation. Variability is expected and has been qualitatively demonstrated in acoustic seabed surveying including high- and low-frequency techniques. In this study, seabed sediments were characterized using a portable free fall penetrometer along with geotechnical and sedimentological laboratory testing of fine-grained seabed sediments. Testing results quantified both sediment textural (e.g., bulk density, water content, void ratio, and porosity) and strength properties (e.g., undrained shear strength and consolidation state and behavior) and were related to the backscatter intensity observed with different high- and low-frequency off-the-shelf acoustic surveying devices.

Error modeling of Doppler velocity logs using a point-based scattering model

As Doppler Velocity Log (DVL) systems become smaller, cheaper, and increasingly capable, they are becoming standard options on even low-end undersea vehicles. DVLs are the principal sensor that provide ground-referenced velocity measurements to compensate for the bias and drift errors found in inertial navigation systems. Therefore, DVLs fill an essential role in achieving an accurate navigation solution for unmanned undersea vehicles. Although there are many modeling and simulation techniques for comparing a fused-sensor inertial navigation solution to earth-reference, there is a sparsity in the literature for models and simulation environments that are suited for estimating the performance of a standalone DVL system. A practical model must incorporate sufficient environmental fidelity so as to match with empirical results, while still remaining computationally efficient. The point-based scattering model described by Brown et al. [JASA Elect. Lett. i(2017)] may be a useful tool for modeling DVL performance. The implemented model incorporates seafloor roughness, scatterer density, textures, and is well-suited for integration into Monte Carlo simulations. This presentation will compare the DVL performance predicted by this point-based scattering model to several other error models and discuss the utility for predicting performance in natural marine environments.

Polar Region Bathymetry: Critical Knowledge for the Prediction of Global Sea Level Rise

The ocean and the marine parts of the cryosphere interact directly with, and are affected by, the seafloor and its primary properties of depth (bathymetry) and shape (morphology) in many ways. Bottom currents are largely constrained by undersea terrain with consequences for both regional and global heat transport. Deep ocean mixing is controlled by seafloor roughness, and the bathymetry directly influences where marine outlet glaciers are susceptible to the inflow relatively warm subsurface waters - an issue of great importance for ice-sheet discharge, i.e., the loss of mass from calving and undersea melting. Mass loss from glaciers and the Greenland and Antarctic ice sheets, is among the primary drivers of global sea-level rise, together now contributing more to sea-level rise than the thermal expansion of the ocean. Recent research suggests that the upper bounds of predicted sea-level rise by the year 2100 under the scenarios presented in IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCCC) likely are conservative because of the many unknowns regarding ice dynamics. In this paper we highlight the poorly mapped seafloor in the Polar regions as a critical knowledge gap that needs to be filled to move marine cryosphere science forward and produce improved understanding of the factors impacting ice-discharge and, with that, improved predictions of, among other things, global sea-level. We analyze the bathymetric data coverage in the Arctic Ocean specifically and use the results to discuss challenges that must be overcome to map the most remotely located areas in the Polar regions in general.

Experimental study of roughness spectrum of sandy seafloor with an underwater laser 3D scanning system

To measure and characterize the roughness spectrum of sea-bottom, which is an important input parameter of the bottom acoustic backscattering model, an underwater laser three-dimensional (3D) scanning system for measuring seafloor microtopography has been developed successfully. The proposed underwater laser 3D scanning system can provide 3D seafloor microtopography with the resolution of millimeter levels in both horizontal and vertical directions and obtain the roughness spectrum of sea-bottom further. Two sets of data, which were suitable to model low- and mid-frequency acoustic backscattering at 6–24 kHz, were collected in two sites of sandy bottom during the acoustic scattering experiment in the southern Yellow Sea. The one-dimensional (1D) and two-dimensional (2D) roughness spectrum of the seafloor have been calculated at the two sites named S1 and S2, respectively. The result shows that calculated 2D spectral indices and spectral intensities, respectively, were 3.45 and 2.17 × 10−4 m4 at S1 and 3.33 and 4.66 × 10−4 m4 at S2. 2 D roughness spectrum parameters were used to model the acoustic scattering at S1, and the result indicates that the measurement results of the seafloor roughness spectrum is suitable for modeling bottom acoustic scattering.

Inversion of bottom parameters using a backscattering model based on the effective density fluid approximation

An inversion method using backscattering strength is used to obtain bottom parameters. A backscattering model based on the effective density fluid approximation was established as the forward model. The objective function used to scale the mismatch between the model predictions and the measured data (obtained by employing two monostatic transducers with narrow directivity) was established on the assumption that the data errors follow a Gaussian distribution. The maximum a posteriori estimations of inversion parameters were obtained by using a two-step hybrid optimization algorithm combining the differential evolution algorithm and the particle swarm optimization algorithm. The uncertainty and the correlation of inversion parameters were presented using marginal probability distributions and a covariance matrix through Bayesian inversion. Using this method, we can acquire not only geoacoustic parameters (including sound speed and attenuation) but also partial physical parameters of marine sediments and statistical character parameters of seafloor roughness and sediment heterogeneity. Finally, the efficiency of this inversion method was verified through comparison between inversion results and sampling data from sand sediment at the bottom of a water tank.