Seafloor Reflection Research Articles

Reflected acoustic energy from geological layers during seismic reflection surveys.

Acoustic propagation is significantly impacted by seabed characteristics, which play a large role in propagation modeling. Shallow seabed characteristics comprise a notable area of research due to their impacts on bottom loss, but deep seabed characteristics are often ignored. At low frequencies (several hundred Hertz, particularly below 100 Hz) and at ranges less than that corresponding to the seafloor critical angle, these deep layer characteristics have non-negligible effects. Those effects are explored here using a subset of data from a marine seismic reflection survey, MGL2104, in an environment with a nearly constant ∼2.6 km bathymetry. The source is a 5700 in.3 airgun array and reflections are measured by a 1200 channel, ∼15 km streamer, with both arrays at 12 m depth. The results show that in one-third-octave bands below 100 Hz, a significant fraction of the reflected energy (sometimes >50%) at certain ranges in the water column is attributable to sub-seabed layers, and the seafloor reflections only become the dominant source at ranges where the reflection path approaches a critical angle. The analysis also considers the effects of layer depths on reflected energy, demonstrating that increased depth does not necessarily correlate with decreased energy reflected in the water column.

Imaging the shallow velocity structure of the slow‐spreading ridge of the South China Sea with downward continued multichannel seismic data

AbstractHigh‐resolution shallow oceanic crust velocity models provide crucial information on the tectonothermal history of the oceanic crust. The ocean bottom seismometers record wide‐angle seismic reflection and refraction data to image deeper structures compared with streamer data set. However, most ocean bottom seismometers experiments produce low‐resolution velocity models with limited shallow crustal structure due to sparse ocean bottom seismometers spacing. Multichannel seismic data recorded by towed streamers provide complementary seismic images of the oceanic crust but yield little information on subseafloor velocity because most subseafloor refractions are masked by seafloor reflections. Therefore, it is difficult to obtain fine‐scale velocity structure of shallow upper oceanic crust with both ocean bottom seismometers and multichannel seismic data. Downward continuation technique redatumed the shots and receivers to the seafloor to collapse the seafloor reflections to the zero offset and extract refractions as first arrivals from nearly zero offset, enabling dense ray coverage at the shallow crust. We applied the downward continuation and traveltime tomography methods to two synthetic models, Marmousi and SEAM Phase I Salt models, to demonstrate the performance of the strategy in the situations of flat seafloor and rough seafloor topography. We conducted the first‐arrival traveltime tomography on downward continued towed‐streamer multichannel seismic data across a slow‐spreading ridge of the South China Sea, providing unprecedented details of shallow velocity structure in the sediments. The low velocity sediments revealed by traveltime tomography match well with the prestack depth migration profile.

A robust array geometry inversion method for a deep-towed multichannel seismic system with a complex seafloor

Deep-towed multichannel seismic exploration technology has better applicability and more development potential when utilized to invert the geoacoustic properties of deep-sea sediment. The accurate geometric inversion results of the receiving array are crucial for fine submarine sediment imaging and physical property parameter inversion based on deep-towed multichannel seismic data. Thus, this study presents an array geometry inversion method suitable for complex seafloors to address the challenge of precise source-receiver positioning. The objective function of the deep-towed seismic array geometry inversion is built using the shortest path algorithm according to the traveltimes of direct waves and seafloor reflections, and the particle swarm optimization algorithm is used to achieve high-precision inversion of the source-receiver position. The results showed that the proposed method is shown to have incomparable applicability and effectiveness in obtaining exact source-receiver positions for deep-towed multichannel seismic systems. Regardless of the complexity of the seabed morphology, seismic image processing techniques using the source-receiver position data obtained by the suggested method produce fine seismic imaging profiles that clearly and accurately reflect the structural characteristics of sediments. These findings provide insights for the accuracy and reliability of the proposed geometric shape inversion method for deep-towed seismic arrays in practical applications to meet the requirements of near-bottom acoustic detection for fine imaging of deep-sea seabed strata and precise inversion of geoacoustic parameters.

A study on bubble suppression for deep marine reflection data acquired by a small airgun array

AbstractThe oscillatory bubble pulses generated by airguns in seawater are known to produce artefacts in seismic images. Although such artefacts are suppressible by employing larger airgun arrays in acquisition, small airgun arrays are used more often now to minimize the environmental impacts, thus raising the need for further suppressing bubble pulses at data processing stage. For a deep marine reflection dataset recently acquired by a small airgun array, we compare the effectiveness of three popular debubbling methods that estimate the far‐field source signature based on theoretical simulation and wavelets extracted from seafloor reflections and direct arrivals, respectively. In this case, due to the lack of near‐field measurements for calibration, the debubbling via simulation underperforms the two wavelet extraction methods. Overlapping events in the noisy response of seafloor sediments lead to the failure of the wavelet extraction from primary seafloor reflections. The estimated source signature based on direct arrivals achieves the best bubble suppression result, indicating the importance of signal‐to‐noise ratio and a low level of directionality of the small airgun array source.

Classification of Marine Sediment in the Northern Slope of the South China Sea Based on Improved U-Net and K-Means Clustering Analysis

The classification of marine sediment based on acoustic data is crucial for various applications such as marine resource exploitation, marine engineering construction, and marine ecological environment maintenance. It serves as a valuable alternative to limited geological sampling. However, the accuracy of sediment classification is limited due to constraints in acoustic data detection methods, data quality, and classification techniques. To address this issue, this study proposes an automatic classification method for marine sediment using an improved U-convolutional neural network and K-means clustering algorithm. In the coding part, a spatial pyramid pool layer is introduced to fuse low-dimensional feature data of different scales with the features of each level of the corresponding coding layer. This fusion method enhances the accuracy of the constructed relationship between the physical property parameters of the seabed bottom. The K-means clustering algorithm is optimized through selecting the point at the density center as the initial clustering center during the initial clustering center selection stage. This approach solves the sensitivity problem of the initial clustering center of K-means, improves the edge extraction effect of sediment types, and enhances the classification accuracy of sediment types. To validate the proposed method, an application test is conducted in the Northern Slope area of the South China Sea. The mean grain size of sediments in the study area is predicted using the improved U-Net neural network and the seafloor reflection intensity of the sub-bottom profile. Compared to the standard U-Net network results, the mean grain size prediction results show an increase of 4.9% and 2.8%, respectively. The sediment with the predicted mean grain size is then classified using the K-means clustering algorithm, resulting in the division of five sediment types: gravelly sand, sand, silty sand, sandy silt, and clayey silt. These classifications align well with the South China Sea sediment type map. The findings of this study not only provide an important supplement to existing marine sediment classification methods but also contribute significantly to understanding the sedimentary environment and processes in the Northern Slope of the South China Sea.

Characteristics of the far-field source signature and its ghost effect of a marine single air gun

The estimation of the air gun source signature is a crucial topic in marine seismic. Typical approaches such as numerical modelling or near-field measurement-based inversion normally work in practice, but suffer from lack of knowledge on the varying sea surface reflection coefficient, the signature directional consistency and the dynamically changed source geometry. For better understanding of the above uncertainties, real far-field air gun source signatures were acquired. A customized approach combined with sparse f-k transformation and adaptive subtraction is applied to deblend the source signature from the sea-floor reflections. Characteristic of the ghost notch distribution and bubble oscillation period indicate an overall good match to the simulated results, except the latter part due to the unexpected air-leakage. Amplitude decay with incidence-angle increase is observed, indicating that the point-source spherical theory for directional signature estimation may be questionable. The sea-surface reflection coefficient appears time-variant and frequency-dependent which challenges the assumption of flat sea surface in signature estimation. An innovative notional source inversion workflow is adapted to the frequency-dependent reflection coefficient. The notional signature inversed through the adapted approach results in less ghost residuals and weaker spectral notch effect comparing with the normal inversion result, i.e., based on a constant reflection coefficient.

Research on the characteristics of fluid escape near the seabed in Bohai Bay

AbstractSeabed fluid flow happens due to the complex interactionamong the accumulation, migration and escape of hydrocarbon and/or fluids. As shallow gas accumulations are anomalously developed in Bohai Bay, studying fluid flow features provides insights for understanding environment fluid in the Bohai Sea. Therefore, in our study, we aimed to make practical and effective use of single‐channel seismic reflection data to delineate and interpret subsurface fluid flow features by analysing the variance, sweetness, instantaneous frequency, and root mean square amplitude of seismic attributes. Four types of submarine fluid escape features, including pockmarks, mounds, fluid vents, dimmed or chaotic seafloor reflections, and two shallow fluid migration pathways (gas chimneys and faults) are distinguished on the single‐channel seismic reflection profiles. Gas chimneys are divided into strict columnar and irregular columnar chimneys, fluid migration along faults is critically dependent on the activity of faults. Enhanced reflections and bright spots may be good indicators of lateral fluid migration and accumulation of fluids in porous formations. We propose a shallow fluid flow model in the study area that illustrates the dynamic fluid migration process in terms of possible fluid sources, shallow migration pathways, and submarine fluid escape features.

Imaging ocean water columns by acoustic contrast reflection signals in existing marine seismic data

Marine seismic surveys conducted for oil and gas exploration use a source-receiver system consisting of airgun sources and hydrophone receiver arrays. The collected data were commonly used to image subsurface beneath the seafloor. The surveys had also collected signals of acoustic reflections from ocean water columns due to acoustic impedance contrast by temperature and salinity. These signals can be processed to provide high-resolution images of the water-column structures such as eddies and internal waves, forming a discipline termed as seismic oceanography in the literature. Acoustic contrast reflection signals in a three-dimensional seismic survey, collected by multiple parallel receiver arrays, can even be used to explore movements of water columns {see our recent study in [Zou,etal., Ocean Sci. 17(4), 1053–1066 (2021)]}. Nevertheless, because of the low acoustic impedance contrast in oceans, these water-column reflection signals are extremely weak, e.g., about 2–3 orders weaker than the seafloor reflections. Meanwhile, these signals consist of acoustic multipath features that require an appropriate filtering of the signals (see our recent study in the study by Zou and Zhang [JASA 150(5), 3852–3860 (2021)]. Understanding of the signal features and appropriate applications of acoustic signal processing methods are inspired to improve the imaging quality.

Physics-based acoustic inversion of sound velocity and attenuation in low-velocity marine sediments

Using data from the Yellow Sea, arrival times of the direct wave and surface/bottom reflections from explosive sources to a vertical hydrophone array are used to precisely determine each explosive source’s location, the source energy spectral density (SESD), and the water depth. Long-range propagation waveforms reveal modal dispersion: the ground wave, water wave, Airy phase, etc. There are two high-frequency (HF) groups of water waves. One propagates with the sound speed in the water below the thermocline, the other with a speed close to the sound speed in the water above the thermocline. The HF group arrival times offer a time reference for dispersion analyses, including the ground wave speed at the cutoff frequency and the group velocity at Airy frequency. Associated with a top layer oflow-velocity sediments (LVS), seafloor reflections have two pulses: one from the water-sediment interface, one from the sediment-basement interface; Long-range transmission loss (TL) exhibits abnormal peaks at selected frequencies. Above-mentioned physics characteristics and the SESD-normalized TL(r) and TL(f) in 50–5000 Hz range up to 27.6 km are used for observationally driven inferences of seafloor geo-acoustic parameters, such as the sound velocity and attenuation in the LVS layer, its thickness, the sound velocity in the basement, etc.

Ranging to fin whale calls at full ocean depths using single ocean bottom seismometers

Fin whale calls are routinely recorded on ocean bottom seismometers (OBSs), but the networks are often too sparse for multi-station call localization. Single station ranging methods are thus important for efforts to utilize OBSs for population density studies. Ranging using the timing and amplitude of multipaths has been demonstrated in relatively shallow (∼2500 m) settings, but in sedimented regions it is challenging to detect the first multipath at small ranges because seafloor reflections have low amplitudes at high incidence angles and subseafloor reflections often have higher amplitudes. To enhance the identification of weak multipaths, we consider all calls in 10-min time windows together. Calls are detected using spectrogram cross-correlation to yield a time-dependent recognition score. The time difference between direct and first multipath arrivals is identified by (1) autocorrelation of the recognition score and (2) spectrogram cross-correlation of the sum of spectrograms for all detected calls. The range is estimated by comparing this time with the predictions of ray tracing. We have tested these methods at five deep (4200–6000 m) OBS sites with contrasting seafloor properties. The results are promising but show the importance of understanding the nature of subsurface reflections.

Estimation of seafloor reflectivity in shallow water based on seismic data of sparker sources

Estimation of seafloor reflectivity in shallow water based on seismic data of sparker sources

Inversion of the physical properties of seafloor surface sediments based on AUV sub-bottom profile data in the northern slope of the South China Sea

Based on the seafloor reflection coefficient obtained from autonomous underwater vehicle (AUV) sub-bottom profile survey data of the northern slope of the South China Sea, combined with the sample test data of seafloor surface sediments, we use the Biot–Stoll model to establish the equations relating the seafloor reflection coefficient to the porosity, density, and mean grain size of the sediments at the dominant frequency of 5 kHz (the dominant frequency of the AUV sub-bottom profiler). The physical property parameters such as the porosity, density, and mean grain size of seafloor surface sediments are further inverted. Comparison of inversion results with measured results shows that the overall deviation ratios of the inverted mean grain size, porosity, and density of the surface sediments are in the ranges of − 13.56 to 14.44%, − 6.15 to 8.06%, and − 10.85 to 0.46%, respectively. Among them, the mean grain size directly reflects the size of seafloor sediment particles, and the particles are finer in deeper water. Overall, the inversion results are basically consistent with the measured values and thus can well reflect the variation characteristics of the physical properties of seafloor surface sediments.

Practical considerations in the implementation of time-domain acoustic full waveform inversion

Full waveform inversion (FWI) plays a major role in the oil and gas industry as a state-of-the-art technique that produces quantitative subsurface structures with high-fidelity images. Various FWI studies have been conducted, and these suggest that FWI is a promising inversion method. Recently, many attempts have been made toward three-dimensional (3D) and four-dimensional FWI applications (which were difficult to perform in the past) because of the progress made in computer science and the growth of computer resources. To manage the very large data requirement of 3D problems, a time-domain FWI that is relatively efficient in terms of memory demands must be implemented. However, it could encounter practical issues, leading to failure in its convergence. In this paper, we introduce these practical issues and several alternative methods for mitigating them. The first issue is the bandpass filtering of the observed seismograms. We suggest that the frequency-domain filter based on a reference wavelet would be optimal in terms of both bandpass filtering and source wavelet estimation. The second issue is related to acoustic approximation. We show that a simple density model comprising only water and solid layers is a reasonable option to address seafloor reflectivity properly. The last issue is the accumulation of round-off errors due to the massive computation of the objective function. We demonstrate that a simple modification of the error calculation can resolve this round-off error problem.

A reliable velocity estimation in a complex deep-water environment using downward continued long offset multi-channel seismic (MCS) data

The estimation of a reliable velocity–depth model from towed streamer marine seismic data recorded in deep water, especially with a complex seafloor environment, is challenging. The determination of interval velocities from the normal move-out (NMO) of the reflected seismic signals for shallow reflectors (<1 km below the seafloor) is compromised by the combination of a long wave path in the water column and the complex ray paths due to topography, leading to small move-out differences between reflectors. Furthermore, low sediment velocities and deep water produce refraction arrivals only at limited far offsets that contain information about deeper structures. Here, we present an innovative method where towed streamer seismic data are downward continued to the seafloor leading to the collapse of the seafloor reflection and the emergence of refraction events as first arrivals close to zero offset, which are used to determine a high-resolution near surface velocity–depth model using an efficient tomographic method. These velocities are then used to perform pre-stack depth migration. We found that the velocity–depth model derived from tomography of downward continued towed streamer data provides a far superior pre-stack depth migrated image than those produced from velocity–depth models derived from conventional velocity estimation techniques.

The Bathymetry of Moray Sinus at Titan's Kraken Mare

AbstractMoray Sinus is an estuary located at the northern end of Titan's Kraken Mare. The Cassini RADAR altimeter acquired three segments over this mare during the T104 flyby of Titan, on August 21, 2014. Herein, we present a detailed analysis of the received echoes. Some of these waveforms exhibit a reflection from the seafloor, from up to m of depth (1σ error). Monte Carlo simulations have been performed in order to assess the most probable values and estimation errors for the seafloor depth. Insights from this study, featuring the synergic use of the synthetic aperture radar images coupled to the altimetry and passive radiometry datasets, have been used to constrain the dielectric properties (i.e., absorptivity of the liquid) and roughness of this region of Kraken Mare. The resulting Ku‐band specific attenuation of the liquid is dB/μs, corresponding to a loss tangent of , which is very similar to the loss tangent estimated at Ligeia Mare. The data in hand do not permit us to discern the most likely explanation for the lack of a seafloor reflection from the main body of Kraken Mare: either a very deep sea or a more absorbing liquid composition. However, if the main body of Kraken Mare is characterized by an absorption similar to Moray Sinus, then based on models of the response to altimetry mode observations we can conclude that it exceeds 100 m of depth, which is also compatible with radiometry observations.

Utilization of airgun source signatures to improve seismic imaging of ocean water columns

Seismic oceanography is a new interdiscipline which uses “legacy” marine seismic reflection data collected by the oil industry to image ocean water columns like eddies and internal waves. Unlike seafloor reflections, ocean water-column reflections are typically weak due to the low acoustic impedance contrast, which challenges seismic oceanography. Utilization of the acoustic characteristics of low-frequency, broadband airgun source can enhance water-column signals for better ocean seismic imaging. Acoustics characteristics of received direct-path, water-column, and seafloor signals are analyzed and applied to design adaptive filtering process to enhance water-column signals. The processing increases the signal-to-noise ratio of water-column signal and attenuates the direct waves without changing the spectral content of the airgun source. The results can be useful to improve seismic oceanography by providing better imaging of ocean water columns.

Mass wasting on Alpha Ridge in the Arctic Ocean: new insights from multibeam bathymetry and sub-bottom profiler data

Abstract Marine geological and geophysical data from Alpha Ridge in the Arctic Ocean are sparse because of thick perennial sea-ice cover, which prevents access by most surface vessels. Rare seismic data in this area, acquired largely from drifting ice-camps, had shown the hemipelagic drape that covers most of the ridge is highly disrupted within a large (&gt;90 000 km 2 ) south central region. Here, evidence of pronounced seafloor erosion and debris flows infilling seafloor lows was previously interpreted to be the result of a possible bolide impact. In recent years, several icebreaker expeditions have successfully acquired multibeam bathymetry and sub-bottom profiler data in the western segment of this region. Analysis of these data reveals a complex seafloor morphology characterized by ridges and troughs, angular blocks and escarpments as well as seismic facies characterized by hyperbolic seafloor reflections, and convoluted to incoherent and transparent sub-bottom reflectivity. These features are interpreted as evidence of sediment mass movement with varying degrees of lateral transport deformation. At least two episodes of failure are interpreted based on the presence of both buried and surficial mass-transport features. As multiple events are interpreted, seismicity is the most plausible trigger mechanism rather than bolide impact.

Study on the integrated calculation method of fluid–structure interaction vibration, acoustic radiation, and propagation from an elastic spherical shell in ocean acoustic environments

A modeling method based on the wave superposition method is studied in this paper. Different underwater acoustic propagation models are used in the near and far field. Taking the acoustic radiation field calculation of an elastic spherical shell as an example, the near and far field can be analyzed as a unified system, using this method. Many studies use FEM/BEM (finite element method/boundary element method) to calculate acoustic radiation fields of elastic structures, but only a few of these methods can be applied when considering finite ocean depth, seafloor reflection loss, and sound velocity profile. In this study, the Green function is calculated differently in the near and far field to significantly simplify calculations. The virtual source method is used to calculate the acoustic radiation field in the near field and source strength, while the normal mode method is used in the far field. The results are compared with those obtained using the COMSOL finite element software, which showed that this method was both computationally efficient and accurate. Based on numerical examples, the influence of sea surface, seafloor, and sound velocity on the acoustic radiation field of elastic structures in an ocean acoustic environment is quantitatively analyzed.

Characteristics of convergence zone formed by bottom reflection in deep water

There appears a convergence effect on the sound filed under the condition of sound channel in the deep sea due to the refraction effect of the sea water. For the deep water environment with an incomplete channel, sea bottom has an important influence on sound propagation. A long-range sound propagation experiment was conducted in the South China Sea in April 2018. Hyperbolic frequency modulated (HFM) signals with a frequency band of 250-350 Hz are transmitted by an acoustic source which is towed at a speed of 4 knots away from a vertical line array (VLA). The VLA consists of 20 hydrophones which are arranged from 85 m to 3400 m with an unequal depth space. Using the data collected in the experiment, the effects of bathymetry variation on sound propagation are studied. The physical causes of the seafloor reflection convergence phenomenon are explained by using the parabolic equation combined with ray theory. The observed phenomenon is different from the convergence phenomenon in the typical deep water environment, the spatial variation of bathymetry contributes to the formation of the seafloor reflection convergence zone in advance, and the sound intensity in part of shadow zone is significantly increased. Due to the reflection from the seabed, two obvious seafloor reflection convergence zones are observed near the range of 20 km and 40 km respectively, in which both gains increase up to 10 dB, and a high sound intensity area is formed in the shadow zone near the range of 11 km, where the gain is less than the gains in the two convergence zones. In addition, the grazing angle of the sound ray reaching the second convergence zone is smaller than that reaching the first convergence zone when the receiving depth is the same as the source depth, and the rays with smaller glancing angle have less reflection loss, which leads to a higher gain in the second convergence zone. As the water depth becomes gradually shallower with range increasing, the convergence zone near the range of 51 km under the SOFAR channel is destroyed, and the sound field energy in the corresponding range is much smaller since the number of arriving refracted sound rays is reduced. In the first convergence zone, the path of arriving rays is gradually increased as the receiver becomes deeper. Therefore, the arrival structure tends to be complicated, and the multi-path effect is more obvious. The study result is meaningful for the performance analysis of sonar in complex deep water environment.

Effect of incorrect sound velocity on synthetic aperture sonar resolution

Synthetic aperture sonar (SAS) is an imaging technique to produce centimeter resolution over hundreds-of-meter range on the sea floor, by constructing a virtual aperture whose length automatically adjusts itself for a given focusing range. SAS is near-field acoustic imaging, and this implies that the sound velocity should be accurately estimated for well focused imaging. Otherwise there will be image quality loss. However, sound velocity in the ocean varies with space and time, and there might also be measuring error of CTD (Conductivity, Temperature, and Depth) sensor, so sound velocity error has become one of the limiting factors to improve SAS resolution further. To characterize the effect of sound velocity error quantificationally, the practice SAS resolution is mode as the convolution of ideal seafloor reflectivity function and a phase error function in frequency domain, where the phase error is caused by incorrect sound velocity. Then the SAS resolution parameterized is calculated as a function of the sound velocity measuring error, or sound velocity gradient. It is shown that SAS azimuthally (along track) resolution loss, caused by sound velocity measurement error, increases linearly with detection range. Meanwhile the loss caused by sound velocity gradient increases squarely. It is simulated by considering the synthetic aperture data collection for a particular pixel, and results show that the point scatter response will defocus when the sound velocity measuring error is up to 1% at 200m range, or the sound velocity changes up to 2% over a typical gradient at 200m range, and be worse at a longer range. Furthermore, we demonstrate the influence of sound velocity errors on SAS imagery using a sea trial data and real CTD measurements at South China Sea. We evaluate the degradation in image quality with respect to sound velocity errors by using two plastic balls and a variable seafloor scene, and results also support the accuracy of theoretical conclusions above.