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We have evaluated the results of a receiver decimation study in a deepwater context using separated wavefield imaging (SWIM) algorithms to provide extended illumination for imaging without ocean-bottom node (OBN) positioning constraints. We carried out subsurface imaging using the SWIM imaging technique with a reduced OBN layout, and we compared the results with those from conventional one-way wave-equation migration. We found from the results that the SWIM algorithm makes it possible to reduce the OBN layout while obtaining a similar subsurface image with the same shot geometry, which allows a reduced receiver acquisition effort, offers more geometry flexibility without affecting the image quality, with a potentially significant reduction of acquisition cost and 4D processing turnaround time.

The study of slow earthquake activity, which occurs in the shallow and deep sides of seismogenic zone, is crucial for understanding subduction zones, including variations in frictional properties with depth and interplate coupling. Observations at the seafloor are necessary, particularly for shallow slow earthquakes occurring in offshore areas; however, few observations of such activity have been made. We conducted long-term seismic observations on the seafloor in the Hyuga-nada region, located at the western end of the Nankai Trough, to characterize shallow low-frequency tremor activity from 2014 to 2017. Although these observations lasted for only a few years, the occurrence frequency of shallow tremors in Hyuga-nada was lower than that of deep tremors in the Nankai Trough, and major activity involving migration occurred only once every two or more years. In contrast, minor activity with a duration of a few days occurred several times a year. Major activities in 2015 were accompanied by migration similar to those in 2013. The tremors in 2013 were characterized by south to north migration at a rate of 30–60 km/day. However, the tremors in 2015 were characterized by west to east migration, and the activity area extended further to the east. The migration rates were also much slower (several to 20 km/day) than in 2013. These different migration properties likely reflect the state of interplate coupling in the down-dip side of shallow slow earthquake area. Minor activity was identified, including tremors triggered by the 2015 Nepal and 2016 Kumamoto earthquakes. Activity occurred mainly in the focal regions of major activities. Very-low-frequency earthquakes (VLFEs) occurred concurrently with tremors, and their epicenters coincided within the margin of error. However, the VLFEs were mostly peripheral to the shallow tremor concentration zones. This indicates that minor heterogeneities in frictional properties are present along the shallow plate boundary.

The first example of artificial intelligence (AI) geological interpretation on a large scale, densely sampled ocean bottom node (OBN) exploration dataset, Utsira OBN, is presented here. This entirely new suite of derivative seismic products provides enhanced exploration insights through AI, especially in areas where infrastructure led exploration (ILX) is drawing increased focus from the EP a combined analysis can extract added value and reduce uncertainties. Such analysis is presented here.

Horizontal records of ocean-bottom seismographs are usually noisy at low frequencies (< 0.1 Hz). The noise source is believed to be associated with ocean-bottom currents that may tilt the instrument. Currently horizontal records are mainly used to remove the coherent noise in vertical records, and there has been little literature that quantitatively discusses the mechanism and characteristics of low-frequency horizontal noise. In this article, we analyze in situ ocean-bottom measurements by rotating the data horizontally and evaluating the coherency between different channels. Results suggest that the horizontal noise consists of two components, random noise and principle noise whose direction barely changes in time. The amplitude and the direction of the latter are possibly related to the intensity and direction of ocean-bottom currents. Rotating the horizontal records to the direction of the principle noise can largely suppress the principle noise in the orthogonal horizontal channel. In addition, the horizontal noise is incoherent with pressure, indicating that the noise source is not ocean surface water waves (infragravity waves). At some stations in shallow waters (

We have developed an integrated workflow for estimating elastic parameters within the Late Triassic Skagerrak Formation, the Middle Jurassic Sleipner and Hugin Formations, the Paleocene Heimdal Formation, and the Eocene Grid Formation in the Utsira High area of the Norwegian North Sea. Our workflow begins with petrophysical analysis carried out at the available wells. Then, model-based prestack simultaneous impedance inversion outputs were derived, and attempts were made to estimate the petrophysical parameters (the volume of shale, porosity, and water saturation) from seismic data using extended elastic impedance. On not obtaining convincing results, we switched over to multiattribute regression analysis for estimating them, which yielded encouraging results. Finally, the Bayesian classification approach was used for defining different facies in the intervals of interest.

Based on a simple physical model, we derive a formula for the energy radiated into a tsunami by a dynamic deformation of the ocean floor. We use the analytical solutions developed by J.L. Hammack, Jr. (1972, Tsunamis – A model of their generation and propagation, Ph.D. Dissertation, 261 pp., California Institute of Technology, Pasadena) to show that, in addition to its expected dependence on the source rise time, and even in the limiting case of an instantaneous source, the energy of the tsunami is controlled by the ratio of water depth to source size. For very large earthquakes, these results have no impact on the time-honored approximation which uses the static deformation of the ocean floor as a set of initial conditions of the surface displacements for numerical simulations, but they provide insight into the theoretical limits of this practice.

Full-waveform inversion (FWI) can be applied to time-lapse (4D) seismic data for subsurface reservoir monitoring. However, nonrepeatability (NR) issues can contaminate the data and cause artifacts in the estimation of 4D rock and fluid property changes. Therefore, evaluating and studying the NR effects on the 4D data and FWI results can help, for instance, discriminate inversion artifacts from true changes and guide seismic survey design and processing workflows. Using realistic reservoir models, data, and field measurements of NR, we find the effects of NR source-receiver position and seawater velocity changes on the data and the 4D FWI results. We show that ignoring these NR effects in the inversion can cause strong artifacts in the estimated velocity change models and thus should be addressed before or during inversion. The NR source-receiver positioning issue can be successfully addressed by 4D FWI, whereas the NR water velocity issue requires measurements or estimations of water velocities. Furthermore, we compare the accuracy and robustness of the parallel, double-difference, and central-difference 4D FWI methods to realistic NR ocean-bottom node data in a quantitative way. Parallel 4D FWI fails to capture geomechanical changes and also overestimates the aquifer layer changes with NR data. Double-difference 4D FWI is capable of recovering the geomechanical changes, but it is also sensitive to NR noises, generating more artifacts in the overburden. By averaging the forward and reverse bootstrap 4D estimates, central-difference 4D FWI is more robust to NR noises and also produces the most accurate 4D estimates.

A large-scale permanent ocean-bottom seismograph network, named S-net, has been established in the Japan Trench area and consists of 150 observatories equipped with seismometers and tsunamimeters. Most stations at water depths

About 70% of Earth’s surface is covered by ocean, for which seismic observations are challenging. Seafloor seismology overcame this fundamental difficulty and radically transformed the earth sciences, as it expanded the coverage of seismic networks and revealed otherwise inaccessible features. At the same time, there has been a recent increase in the number of studies on cryoseismology. These have yielded multiple discoveries but are limited primarily to land and ice-surface receivers. Near ice calving fronts, such surface stations are noisy, primarily due to crevassing and wind, are hazardous to maintain, and can be lost due to iceberg calving. To circumvent these issues, we have applied ocean-bottom seismology to the calving front of a tidewater glacier in northwest Greenland. We present details of this experiment, and describe the technical challenges, noise analysis, and examples of recorded data. This includes tide-modulated seismicity with thousands of icequakes per day and the first near-source (∼200–640 m) underwater record of a major kilometer-scale calving event in Greenland, which generated a glacial earthquake that was detectable ∼420 km away. We also identified a decrease in bottom-water temperature, presumably due to modified water stratification driven by extreme Greenland glacial melting, at the end of July 2019. Importantly, we identify glacial sediments as the key reason for the anomalously long (∼9.7 hr) delay in the sensor release from the fjord seafloor. Our study demonstrates a methodology to undertake innovative, interdisciplinary, near-source studies on glacier basal sliding, calving, and marine-mammal vocalizations.

Orientation of the Ocean-Bottom Seismograph (OBS) devices is an important task that must complete before using these data. While the OBS direction is determined correctly, we can correct the rotation angle of the coordinate system so that we obtain the maximum amplitude seismic signals for different seismic waves. In this article, we present the method using seismic waves to determine the direction of the OBS. The results obtained from 11 OBSs in the East Sea show that these stations have misdirected from different angles. This method has advantage with high reliability. Specially, we can widely apply for the OBS devices in other oceans.

espanolSe aplico un esquema de inversion iterativo, inicialmente desarrollado por Hashida y Shimazaki (1984) y posteriormente modificado por Joshi et al., (2010), para estimar el factor de calidad de onda cortatridimensional, Qs (f), del centro-sur del Golfode California, Mexico. Es un area de 230 x 288 km quese divide en 108 bloques rectangulares de diferentes Qs (f). Se utilizo25 terremotos bien ubicados registrados en tres estaciones de banda ancha de la red regional RESBAN operada por CICESE (Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Baja California) y tres Sismografos de Fondo Oceanico (OBS, por sus siglas en ingles) deun conjunto del Experimento del Fondo Oceanico del Mar de Cortes (SCOOBA, por sus siglas en ingles). Este conjunto dedatos permitio obtener estimaciones de Qs (f)de diferentes bloques,utilizando el algoritmo de inversion modificado.ElQs (f)se obtuvoa varias frecuencias en el rango de 0,16 ~ 8,0 Hz. Se encontroque la estructura Qsestimada se correlaciona con los modelos geologicos y tectonicos de la region propuestos en estudios previos. Se obtuvounarelacion regional dependiente de la frecuencia que utiliza todos los valores de 1944 del factor de calidad de la onda de corte a 18frecuencias diferentes en todos los bloques y se puede aproximar mediante una funcion de la forma Qs (f) = 20 f 1.2. Esta relacion es tipica en una region tectonicamente activa con alta atenuacion de onda Sy es similar a las relaciones de atenuacion reportadas por otros autores para la region del Valle Imperial, California. EnglishWe apply an iterative inversion scheme, initially developed by Hashida and Shimazaki (1984) and later modified by Joshi et al.,(2010), to estimate three -dimensional shear -wave quality factor,Qs(f),of south-central Gulf of California, Mexico. An area of 230 km x 288 km in this region is divided into 108 rectangular blocks of different Qs(f). We use 25 well-located earthquakes recorded at three broadband stations of the regional network RESBAN operated by CICESE (Centro de Investigacion Cientifica y de Educacion Superior de Ensenada, Baja California) and three Ocean Bottom Seismographs (OBS) of the Sea of Cortez Ocean Bottom Array (SCOOBA) experiment. This dataset permits us to obtain Qs(f) estimates of different blocks using the modified inversion algorithm. Qs(f) is obtained at various frequencies in 0.16 ~8.0 Hz range. We found that the estimated Qsstructure correlates with geological and tectonic models of the region proposed in previous studies. Aregional frequency-dependent relation using all 1944 values of shear-wave quality factor is obtained at 18 different frequencies in all blocks can be approximated by a function of the form Qs(f) = 20 f 1.2. This relation is typical in a tectonically active region with high S-wave attenuation and is similar to attenuation relations reported by other authors for the Imperial Valley, California region.

SUMMARYOcean-bottom seismic acquisition systems deployed on the seabed give access to three-component geophone data and hydrophone data. Compared with conventional streamer acquisitions, the separation of sources and receivers makes it possible to increase the maximum offset and azimuth coverage for improving the illumination at depth. Furthermore, the three-component geophones naturally capture elastic wave propagation effects. While this information is mostly overlooked up to now, reconstructing jointly P- and S-wave velocities would significantly improve the subsurface characterization. To achieve a 3-D high-resolution multiparameter reconstruction, we design an efficient 3-D fluid–solid coupled full waveform modelling and inversion engine. In this engine, fluid and solid domains are divided explicitly and handled with the acoustic and elastic wave equations, respectively. The numerical implementation is based on a time-domain spectral-element method (SEM) with a flexible 3-D Cartesian-based hexahedral mesh, which contributes to an accurate coupling of the acoustic and elastic wave equations and high computational efficiency through domain-decomposition based parallelization. We select the best acoustic–elastic coupled formulations among 4 possibilities with criteria based on numerical accuracy and implementation efficiency. Moreover, we propose a specific hybrid approach for the misfit gradient building so as to use a similar modelling solver for both forward and adjoint simulations. Synthetic case studies on a 3-D extended Marmousi-II model and a 3-D deep-water crustal-scale model illustrate how our modelling and inversion engine can efficiently extract information from ocean-bottom seismic data to simultaneously reconstruct both P- and S-wave velocities within a full waveform inversion framework.

Full-waveform inversion (FWI), which is among the most powerful seismic data processing techniques for imaging subsurface geological structures, has a huge computational cost in proportion to the n...

An effective approach was developed for identifying and correcting ocean-bottom seismometer (OBS) time errors through improving ambient noise cross-correlation function (NCCF) analysis and combination with other methods. Significant improvements were illustrated through analyzing data from a passive-source seismic experiment in the southwestern sub-basin of the South China Sea. A novel method was first developed that can effectively identify errors in the sampling frequency of the OBS instruments. The traditional NCCF method was then expanded by increasing the analyzed data spectrum from a single-frequency band to dual-frequency band pairs, thus doubling the number of available data points and substantially improving the time correction quality. For data with relatively low signal-to-noise ratios, the average time errors were reduced from the original average values of 60–80 ms by the conventional methods to

A nonstationary problem of diffraction of acoustic waves excited by a point source by an interface between two acoustic media with positive effective curvature is considered. This is a model problem for wave phenomena arising near the ocean bottom in the approximation of the “liquid bottom”. Using the complex analysis techniques, a method for finding the exact solution to the studied class of problems is proposed. A method for constructing exact solutions in the form of an integral representation of the problem stated is given.

The obligations of state sponsorship in the exploration and exploitation of the resources in the international seabed area require states parties to the 1982 United Nations Convention on the Law of...

Full-waveform inversion (FWI) applications on 3D ocean-bottom cable (OBC) data from the Valhall oil field in the North Sea have demonstrated the importance of appropriately accounting for attenuation. The Valhall field contains unconsolidated shallow sediments and a low-velocity anomaly in its center — indicative of gas clouds — which have a significant attenuation imprint on the data. Our challenge is to perform time-domain viscoacoustic 3D FWI, which requires more sophisticated tools than in the frequency domain wherein attenuation can be incorporated in a straightforward manner. The benefit of using a viscoacoustic, instead of a purely acoustic, modeling engine is illustrated. We have determined that, in the frequency band used (2.5–7.0 Hz), it is better to reconstruct the velocity only keeping the attenuation fixed because simultaneous inversion of the velocity and quality factor [Formula: see text] does not provide reliable [Formula: see text] updates. We develop an efficient time-domain workflow combining a random source decimation algorithm, modeling using standard linear solid mechanisms, and wavefield preconditioning. Our results are similar to those obtained from state-of-the-art frequency-domain algorithms, at a lower computational cost compared to conventional checkpointing techniques. We clearly illustrate the improvement in terms of imaging and data fit achieved when accounting for attenuation.

The article is dedicated to the memory of L.I. Moskalev – renowned bio-oceanographer, zoologist who spent his entire scientific career at the Laboratory of Ocean Bottom Fauna. L.I. Moskalev participated in more than 30 deep-sea voyages, spent 200 hours diving in manned submersibles “Pisces” and “Mir”, published about 100 scientific papers and a popular book «Masters of the Deep» (2005). Colleagues will remember Lev Moskalev – an extraordinary and deep person and a true patriot of the Laboratory and P.P. Shirshov Institute of Oceanology

To improve the computational efficiency of reverse-time migration (RTM) for vertically transverse isotropic (VTI) media, various acoustic approximations of the elastic wave equations have been pres...

In this article, the methods of geological research of hydrothermal fields on the ocean floor with the use of deep manned submersibles Mir-1 and Mir-2 are considered. A brief description of the Mir submersibles and their advantages over foreign counterparts is given. The results of geological studies of hydrothermal fields of various types in different regions of the World Ocean are presented. Quantitative estimations of ore formation and methane seeps in some areas of Mirs research are given.

The microseism is the strongest component of background seismic noise that masks seismic signals recorded by ocean-bottom seismographs (OBSs). Such undesired noise hampers the identification of critical seismic phases and sometimes even the entire waveform. Here, we introduce the data adaptive polarization filter (DAPF), an approach that suppresses random signals from the background seismic-noise significantly to overcome such difficulties. To automate this task, we have developed a self-contained software suite—DAPF-v1, supported by a MATLAB graphical user interface. The polarization filter is constructed from the data spectral density matrices of seismogram segments employing multitaper spectral analysis approach. We demonstrate a successful application of this technique to the OBSs deployed in the Indian Ocean. Our results confirm substantially enhanced signal-to-noise ratio after application of DAPF. The application of this technique has extensive implications for seismological studies particularly those aimed at understanding deep mantle dynamics, in which phase identification and qualitative waveform recovery are crucial yet challenging.

Analyses of the equilibrium conditions of an anchor and a cable of a ship at anchoring are presented. Three conditions are considered: (1) a part of the cable lies on the seafloor; (2) no ...

The oceans are a fundamental source for climate balance, sustainability of resources and life on Earth, therefore society has a strong and pressing interest in maintaining and, where possible, restoring the health of the marine ecosystems. Effective, integrated ocean observation is key to suggesting actions to reduce anthropogenic impact from coastal to deep-sea environments and address the main challenges of the 21st century, which are summarized in the UN Sustainable Development Goals and Blue Growth strategies. The European Multidisciplinary Seafloor and water column Observatory (EMSO), is a European Research Infrastructure Consortium (ERIC), with the aim of providing long-term observations via fixed-point ocean observatories in key environmental locations across European seas from the Arctic to the Black Sea. These may be supported by ship-based observations and autonomous systems such as gliders. In this paper, we present the EMSO Generic Instrument Module (EGIM), a deployment ready multi-sensor instrumentation module, designed to measure physical, biogeochemical, biological and ecosystem variables consistently, in a range of marine environments, over long periods of time. Here, we describe the system, features, configuration, operation and data management. We demonstrate, through a series of coastal and oceanic pilot experiments that the EGIM is a valuable standard ocean observation module, which can significantly improve the capacity of existing ocean observatories and provides the basis for new observatories. The diverse examples of use included the monitoring of fish activity response upon oceanographic variability, hydrothermal vent fluids and particle dispersion, passive acoustic monitoring of marine mammals and time series of environmental variation in the water column. With the EGIM available to all the EMSO Regional Facilities, EMSO will be reaching a milestone in standardization and interoperability, marking a key capability advancement in addressing issues of sustainability in resource and habitat management of the oceans. This work was funded by the project EMSODEV (Grant agreement No 676555) supported by DG Research and Innovation of the European Commission under the Research Infrastructures Programme of the H2020. EMSO-link EC project (Grant agreement No 731036) provided additional funding. Other projects which supported the work include Plan Estatal de Investigación Científica y Técnica y de Innovación 2017–2020, project BITER-LANDER PID2020- 114732RB-C32, iFADO (Innovation in the Framework of the Atlantic Deep Ocean, 2017–2021) EAPA_165/2016. The Spanish Government contributed through the “Severo Ochoa Centre Excellence” accreditation to ICM-CSIC (CEX2019-000928-S) and the Research Unit Tecnoterra (ICM-CSIC/UPC). UK colleagues were supported by Climate Linked Atlantic Sector Science (CLASS) project supported by NERC National Capability funding (NE/R015953/1). Peer Reviewed Article signat per 33 autors/es: Nadine Lantéri; Henry A. Ruh; Andrew Gates; Enoc Martínez; Joaquin del Rio Fernandez; Jacopo Aguzzi; Mathilde Cannat; Eric Delory; Davide Embriaco; Robert Huber; Marjolaine Matabos;George Petihakis; Kieran Reilly; Jean-François Rolin; Mike van der Schaar; Michel André; Jérôme Blandin; Andrés Cianca; Marco Francescangeli; Oscar Garcia; Susan Hartman; Jean-Romain Lagadec; Julien Legrand; Paris Pagonis; Jaume Piera; Xabier Remirez; Daniel M. Toma; Giuditta Marinaro; Bertrand Moreau; Raul Santana; Hannah Wright; Juan José Dañobeitia; Paolo Favali

17 pages, 1 figure, 1 table, Deep-sea ecosystems are reservoirs of biodiversity that are largely unexplored, but their exploration and biodiscovery are becoming a reality thanks to biotechnological advances (e.g., omics technologies) and their integration in an expanding network of marine infrastructures for the exploration of the seas, such as cabled observatories. While still in its infancy, the application of environmental DNA (eDNA) metabarcoding approaches is revolutionizing marine biodiversity monitoring capability. Indeed, the analysis of eDNA in conjunction with the collection of multidisciplinary optoacoustic and environmental data, can provide a more comprehensive monitoring of deep-sea biodiversity. Here, we describe the potential for acquiring eDNA as a core component for the expanding ecological monitoring capabilities through cabled observatories and their docked Internet Operated Vehicles (IOVs), such as crawlers. Furthermore, we provide a critical overview of four areas of development: (i) Integrating eDNA with optoacoustic imaging; (ii) Development of eDNA repositories and cross-linking with other biodiversity databases; (iii) Artificial Intelligence for eDNA analyses and integration with imaging data; and (iv) Benefits of eDNA augmented observatories for the conservation and sustainable management of deep-sea biodiversity. Finally, we discuss the technical limitations and recommendations for future eDNA monitoring of the deep-sea. It is hoped that this review will frame the future direction of an exciting journey of biodiscovery in remote and yet vulnerable areas of our planet, with the overall aim to understand deep-sea biodiversity and hence manage and protect vital marine resources, This research has been funded within the framework of the following project activities: ARIM (Autonomous Robotic Sea-Floor Infrastructure for Benthopelagic Monitoring; MarTERA ERA-Net Cofound); RESBIO (TEC2017-87861-R; Ministerio de Ciencia, Innovación y Universidades); JERICO-S3: (Horizon 2020; Grant Agreement no. 871153); ENDURUNS (Research Grant Agreement H2020-MG-2018-2019-2020 n.824348); Slovenian Research Agency (Research Core Funding Nos. P1-0237 and P1-0255 and project ARRS-RPROJ-JR-J1-3015). We also profited of the funding from the Spanish Government through the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S) and Italian Ministry of Education (MIUR) under the “Bando premiale FOE 2015” (nota prot. N. 850, dd. 27 ottobre 2017) with the project EarthCruisers “EARTH’s CRUst Imagery for Investigating Seismicity, Volcanism, and Marine Natural Resources in the Sicilian Offshore”. Ocean Networks Canada was funded through Canada Foundation for Innovation-Major Science Initiative (CFI-MSI) fund 30199

An innovative booster is proposed with the aim of increasing the final penetration depth of the OMNI-Max anchor in the clayey seabed with high strength gradient. The booster is attached to the tail of the OMNI-Max anchor, which is beneficial in improving both gravitational and kinetic energies of the hybrid anchor (i.e., booster + OMNI-Max anchor) during installation and can be retrieved after dynamic installation. The present study carried out two categories of large deformation numerical analyses to simulate the dynamic penetration processes of OMNI-Max anchors and hybrid anchors in normally consolidated and lightly overconsolidated clay. The coupled Eulerian–Lagrangian (CEL) approach was used to investigate the effects of impact velocity, booster weight, and soil strength characteristics (including the strain-rate behavior, the strain-softening behavior, and the undrained shear strength) on the final penetration depth of the anchor. Due to the limitations of the CEL approach in simulating the adhesion friction at the anchor–soil interface, a thin layer region method coupled in the computational fluid dynamics (CFD) approach was used to investigate the effect of the friction coefficient at the anchor–soil interface on the final penetration depth of the anchor. Based on numerical simulation results, a comprehensive prediction model based on the anchor total energy was established to rapidly predict the final penetration depth of the OMNI-Max anchor and the hybrid anchor by considering the strain-rate effect, strain-softening effect, and friction coefficient at the anchor–soil interface.

National Research Institute for Earth Science and Disaster Resilience (NIED) integrated the land observation networks established since the 1995 Kobe earthquake with the seafloor observation networks established since the 2011 Tohoku earthquake and tsunami as MOWLAS (Monitoring of Waves on Land and Seafloor) in November 2017. The purpose of MOWLAS is to provide comprehensive, accurate, and rapid observation and monitoring of earthquake, tsunami, and volcano events throughout Japan and its offshore areas. MOWLAS data are widely utilized for long-term earthquake forecasting, the monitoring of current seismic activity, seismic and tsunami hazard assessments, earthquake early warning, tsunami warning, and earthquake engineering, as well as earthquake science. Ocean bottom observations provide an extension of observations to areas where no people are living and have the advantage of increasing lead time of earthquake early warning and tsunami warning. The application of recent technology advancements to real-time observations as well as the processing of MOWLAS data has contributed to the direct disaster mitigation of ongoing earthquakes. These observations are fundamental for both science and disaster resilience, and thus it is necessary to continue ceaseless operation and maintenance.

A remotely operated vehicle (ROV) can install submarine cables along a planned route accurately, thereby deploying cabled seafloor observatories at the intended locations. In this study, we developed an ROV-based cable-laying system. Its manual and automated versions have been successfully used for the installation of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). The automated cable-laying system shows many advantages; for example, the cable-laying speed increased by approximately 0.1 knot from 0.5 knot, and the number of operators was reduced by half. Thus, the physical and mental burdens on the shipboard members including ROV operators were relieved. In this paper, we share our experiences when laying cables using an ROV during DONET installation. First, the issues that should be addressed to complete a successful cable-laying operation are discussed; subsequently, our manual and automated cable-laying systems are introduced. Then, the observations made during field operations for DONET installation are presented, which provide useful tips for developing an ROV-based cable-laying system.

The determination of the height in the vertical reference frame in force in Poland is based both on national regulations (Council of Ministers Regulation, 2012) and on the resolution adopted by the EUREF subcommittee in Tromsø (Resolution No. 5, EUREF Symposium, 2000). Currently, the PL-KRON86-NH vertical reference frame is in use in Poland – a normal height system referred to a quasi-geoid of the average level of the Baltic Sea determined by the zero level of the mareograph in Kronstadt. According to the Regulation, by the end of 2019, Poland will adopt the PL-EVRF2007-NH vertical reference frame, i.e. a normal height system referred to the zero level of the mareograph in Amsterdam. The authors present a method of determining normal heights of seabed referred to the zero level of the mareograph in Amsterdam for coastal areas of the Baltic Sea. This method uses GNSS measurements, the EGM 2008 model and depth measuring methods typical for underwater mining.

Joint marine electromagnetic (EM) and seismic interpretations are widely used for offshore gas hydrate and petroleum exploration to produce improved estimates of lithology and fluids and to decrease the risk of low gas saturation. However, joint data acquisition is not commonly employed. Current marine EM data acquisition depends on an ocean bottom electromagnetic receiver (OBEM), and current seismic exploration methods use seismometers. Joint simultaneous data acquisition can decrease costs and improve efficiency, but conventional independent data receivers have several drawbacks, including a large size, high costs, position errors, and low operational efficiencies. To address these limitations, we developed a compact ocean bottom electromagnetic receiver and seismometer (OBEMS). Based on existing ocean bottom E-field receiver (OBE) specifications, including low noise levels, low power consumption, and low time drift errors, we integrated two induction coils for the magnetic sensor and a three-axis omnidirectional geophone for the seismic sensor to assemble an ultra-short baseline (USBL) transponder as the position sensor, which improved position accuracy and operational efficiency while reducing field data acquisition costs. The resulting OBEMS has a noise level of 0.1 nV m−1 rt−1 (Hz) at 1 Hz in the E-field, 0.1 pT rt−1 (Hz) at 1 Hz in the B-field, and a 30 d battery lifetime. This device also supports a Wi-Fi interface for the configuration of data acquisition parameters and data download. Offshore acquisition was performed to evaluate the system's field performance during offshore gas hydrate exploration. The OBEMS operated effectively throughout the operation and field testing. Therefore, the OBEMS can function as a low-cost, compact, and highly efficient joint data acquisition method.

In marine seismic acquisition, seismic reflections at the sea surface, such as sea-surface ghosts and multiples, affect the accuracy of the retrieved subsurface reflections and reduce the usable frequency bandwidth. These sea-surface effects tend to increase with the increasing roughness of the weather conditions. Consequently, processing techniques that neglect the roughness and time variation of the sea surface induce errors in the data that could compromise the validity of the final images and interpretations. We study the impact of time-varying rough sea surfaces using a modeling method derived from the Rayleigh reciprocity theorem for time-varying surfaces, and we analyze errors in the source-deghosting operation. We show that the source-deghosting limitations are weather dependent for data including sea-surface multiples: For calm sea states (wave heights below 1.25 m), the error made by the source-deghosting process is negligible; however, for rough seas (wave heights above 1.5 m), it becomes significant and blurs the deghosted image at the sea-surface multiple signals. To accurately remove all sea-surface effects from the seismic data, we simultaneously apply source-deghosting and multiple-removal operations to the same up-going wavefield. This procedure is shown to be weather independent based on our theoretical derivation and the synthetic results. Finally, this is tested on a 2D OBC data set. Comparing the proposed inversion to up-down deconvolution, we observe similar features in both wavefields: Source ghosts and sea-surface multiples seem to have been correctly removed from the data, and the inverted result indicates a slightly better resolution for deeper reflections.

Marine seismic exploration with ocean-bottom cable technology is able to record P- and S-wave information simultaneously. Elastic full-waveform inversion (EFWI) uses P- and S-waves to invert multiple parameters with adequate amplitude information and complete illumination of the subsurface. To calculate the wavefield within EFWI, we use different formats of wave equations in fluid and solid mediums and an appropriate boundary condition to convert waves on the interface. This partitioned simulation scheme is more stable and efficient than the traditional integrated simulation scheme. However, if the fluid-solid coupled medium has an extremely irregular interface, the conventional finite-difference method with rectangular grids cannot obtain accurate source and receiver wavefields. We use the curvilinear coordinates to overcome this limitation. In the curvilinear coordinates, the irregular interface can be transformed into a horizontal interface. To reduce the crosstalk of inverted P- ([Formula: see text]) and S-velocities ([Formula: see text]), we derive the gradient formulas of [Formula: see text] and [Formula: see text] based on P- and S-wave mode separation in the curvilinear coordinates, and, finally, we develop a 2D curvilinear-grid-based fluid-solid separated-wavefield EFWI (CFS-SEFWI) method. Numerical examples that include an anomaly model and a modified Marmousi II model demonstrate that CFS-SEFWI overcomes the influence of the irregular fluid-solid interface and efficiently reduces crosstalk effects between [Formula: see text] and [Formula: see text]. Our results also demonstrate that this method is less sensitive to noise compared to the conventional CFS FWI method without separating wave modes.

The erroneous flipping of polarity in seismic records of ocean-bottom seismometers (OBSs) could go unnoticed and undiagnosed because it is coupled with unknown horizontal orientation of OBS instruments on the seafloor. In this study, we present detailed approaches to first identify potential errors in the flipping polarity of individual OBS instruments, and then determine the correct orientation of OBS instruments on the seafloor. We first conduct a series of tests by artificially flipping the polarity of seismic records of the Global Seismographic Network stations to determine the effects on orientation estimates, utilizing polarization characteristics of teleseismic P and Rayleigh waves, respectively. The tests demonstrate that erroneous polarity reversal in seismic recording could cause false estimates and reverse radial (R) and tangential (T) components. We determine the sensor orientations through comparing the observed waveforms to the synthetic waveforms, which could solve the ambiguity of R and T directions caused by potential erroneous polarity reversal of OBS data. We then apply the approaches to an OBS data set collected in the southern Mariana subduction zone to obtain the correct orientation for 9 out of 12 OBS instruments.

Vertical records of ocean-bottom seismographs (OBSs) are usually noisy at low frequencies, and one important noise source is the varying ocean-bottom pressure that results from ocean-surface water waves. The relation between the ocean-bottom pressure and the vertical seafloor motion, called the compliance pressure transfer function (PTF), can be derived using background seismic data. During an earthquake, earthquake signals also generate ocean-bottom pressure fluctuations, and the relation between the ocean-bottom pressure and the vertical seafloor motion is named the seismic PTF in this article. Conventionally, we use the whole pressure records and the compliance PTF to remove the compliance noise; the earthquake-induced pressure and the seismic PTF are ignored, which may distort the original signals. In this article, we analyze the data from 24 OBSs with water depth ranging from 107 to 4462 m. We find that for most stations, the investigated frequency range (0.01–0.2 Hz) can be divided into four bands depending on the water depth. In band (I) of lowest frequencies (0.11 and >0.08 Hz for water depth of 1109 and 2650 m, respectively), the compliance noise is negligible, and the ocean-bottom pressure is mostly caused by the seafloor motion. Thus, the compliance can be safely ignored in frequency band (I).

We observed a harmonic noise (HN) in DEutscher Geräte-Pool für Amphibische Seismologie ocean-bottom seismometers (OBSs) data recorded from the Andaman–Nicobar region. The HN is characterized by sharp spectral peaks with a fundamental frequency and several overtones occurring at integer multiples of the fundamental frequency. We used an automated algorithm to quantify the occurrence of HN for the entire four-month deployment period (1 January 2014 to 30 April 2014). The algorithm detected more than 23 days of HN for some OBS stations. The spectral analysis of the hourly count of HN shows distinct lunar and solar tidal periodicities at 4.14, 6.1, 6.22, 12, and 12.4 hr as well as 13.66 days. The observed periodicities provide evidence of tidal triggering of HN. The HN is generated by the strumming of head buoys due to seafloor currents initiated by oceanic tides in the Andaman–Nicobar region.

Multimode and multicomponent elastic Gaussian-beam migration is attractive for its efficiency, flexibility, and accuracy. However, when it is used for ocean-bottom seismic data, the incomplete boundary condition will yield some nonphysical artifacts in the final migrated images. To solve this problem, we extend the elastic Gaussian-beam migration method from 3C to 4C by introducing the pressure recording to represent the stress tensor on the ocean bottom. Based on the elastic wave equation and the complete boundary condition for the ocean-bottom model, we derive effective formulas of accurate multimode wave downward continuation. With our method, different wave modes are separated and the receiver ghost is removed simultaneously by applying a decomposition matrix to 4C data during the migration without prior data separation and deghosting, which eliminates the artifacts better and reduces the processing cost. Three synthetic experiments were provided to validate the method for 4C ocean-bottom data migration.

We consider trends of sea level steric oscillations in the North Atlantic in 2003–2015 using two independent approaches. The first method is based on the multidisciplinary application of altimetry (AVISO) and gravimetry (GRACE mission) data. The second method is based on the integral assessment of steric oscillations using variations in the fluid volume caused by variations in water density: the calculations are carried out using the SODA, EN4, and ARMOR reanalysis data. It is shown that the application of the combined altimetry and GRACE data result in overestimated values of steric oscillations and their trends. This is related to the fact that the GRACE observations show the variations in the ocean mass; hence the sea level variations are presented in a relative coordinate system which is not the geocentric system. This system does not take into account the effects of the elastic deformation of the ocean bottom and the corresponding redistribution of the water volumes. It is shown that the maximum bias of these estimates and the errors in determining the steric oscillations and their trends based on the first method is characteristic of regions located near Greenland. This is caused by the contribution of the negative trend component to the GRACE data. If the estimates are made in regions remote from Greenland coasts, the trend component in the GRACE measurements is insignificantly manifested and the trends in the steric oscillations calculated using the method that jointly uses the AVISO and GRACE data are similar to the trends in the sea level variations based on the altimetry data. The trends of the steric oscillations of the sea level calculated from the reanalysis data are similar in the spatial distribution between both calculations and also similar to the trends in the sea level variations based on the altimetry data.

Ocean bottom electromagnetometers (OBEMs) installed on the seafloor around Nishinoshima Island (Bonin Islands) were missing after a December volcanic eruption. In February 2021, one was found on a beach on Iriomote Island (Ryukyu Islands), implying that it drifted westward for 1,700 km. The reason(s) for the disappearance of the OBEMs and the path followed by the recovered OBEM while drifting are important information for future ocean bottom observations and seafloor volcanology in general. We conducted particle drifting simulations with and without horizonal eddy diffusion to estimate the possible drift path and duration of the recovered OBEM. Our simulations show that particles transported from Nishinoshima have a 7-10 % probability of arriving at Iriomote Island, which is thus not a rare occurrence. Transport durations in our simulations varied widely between 140 and 602 days depending on the drift paths. The most likely drift duration in our simulation was 150 – 180 days, with or without eddy diffusion, corresponding to the release from the seafloor of the OBEM between 22 August and 21 September 2020. These dates follow shortly after intensifying eruptions at Nishinoshima, which may have affected the seafloor around the island. A similar drift duration and path was reported for pumices that erupted from Fukutoku-Oka-no-Ba submarine volcano (northern Bonin Islands) during 18-21 January 1986 and arrived in the Ryukyu Islands in late May 1986. Such drifting simulations may prove useful for identifying the sources of drift pumices, and thus otherwise undetectable eruptions. Finally, the Fukutoku-Oka-no-Ba submarine volcano erupted on 13 August 2021, producing abundant pumice rafts that, based on our results, will likely arrive in the Ryukyu Islands in the coming months.

International audience; Ocean-bottom seismic acquisition is attractive in the exploration of complex deep-water environments due to its source-receiver decoupling, which makes it possible to get a wide-azimuth coverage and long source-receiver distance to significantly improve the illumination at depth. However, such acquisition systems also provide information on the elastic properties of the subsurface by recording the displacement on the seabed with 3C geophones. This information is mostly overlooked up to now, while reconstructing jointly P-wave and S-wave velocity models would significantly improve the subsurface characterization. Achieving such a high-resolution multi-parameter reconstruction requires the design of an efficient 3D fluid-solid coupled inversion engine. The purpose of this study is to present such a tool, based on an acoustic-elastic coupled wave-equation system and a spectral-element discretization in space. The method is illustrated on a bilayered 2D model and a 3D extended Marmousi model, to show how P-wave and S-wave velocity models can be inferred from the data, and the resolution improvement obtained from the reconstruction of the S-wave velocity model.

Although tsunamis are dispersive water waves, hazard maps for earthquake-generated tsunamis neglect dispersive effects because the spatial dimensions of tsunamis are much greater than the water depth, and dispersive effects are generally small. Furthermore, calculations that include non-dispersive effects tend to predict higher tsunamis than ones that include dispersive effects. Although non-dispersive models may overestimate the tsunami height, this conservative approach is acceptable in disaster management, where the goal is to save lives and protect property. However, we demonstrate that offshore frequency dispersion amplifies tsunamis caused by outer-rise earthquakes, which displace the ocean bottom downward in a narrow area, generating a dispersive short-wavelength and pulling-dominant (water withdrawn) tsunami. We compared observational evidence and calculations of tsunami for a 1933 Mw 8.3 outer-rise earthquake along the Japan Trench. Dispersive (Boussinesq) calculations predicted significant frequency dispersion in the 1933 tsunami. The dispersive tsunami deformation offshore produced tsunami inundation heights that were about 10% larger than those predicted by non-dispersive (long-wave) calculations. The dispersive tsunami calculations simulated the observed tsunami inundation heights better than did the non-dispersive tsunami calculations. Contrary to conventional practice, we conclude that dispersive calculations are essential when preparing deterministic hazard maps for outer-rise tsunamis.

International audience; Ocean-bottom seismic acquisition is attractive for the exploration of challenging marine environments. Compared with conventional streamer acquisitions, its separation of sources and receivers makes a significant improvement in terms of illumination, especially at depth. Furthermore, such acquisition system makes it possible to record fourcomponent data through a combination of hydrophones and threecomponent (3C) geophones. The information on elastic properties of the subsurface is better captured by those 3C geophones on the seabed. This information is mostly overlooked up to now, while reconstructing jointly P-wave and S-wave velocity models would significantly improve the subsurface characterization. To achieve such a highresolution multi-parameter reconstruction, we design an efficient 3D fluid-solid coupled full waveform inversion (FWI) engine. It is based on the acoustic-elastic coupled wave-equation system, in which fluid and solid domains are divided explicitly and handled with acousticand elastic-wave equations, respectively. A hybrid approach for the misfit gradient building is proposed in such fluid-solid coupled FWI, in which multi-parameter gradient kernels, including the one related to S-wave velocity, are constructed by using a similar modeling solver in both forward and adjoint simulations. This FWI engine is illustrated on a bilayered 2D model and a 3D extended Marmousi model. We show how P-wave and S-wave velocity models can be inferred from the data, and that the resolution improvement can be obtained from the reconstruction of the S-wave velocity model, which highlights the important contribution of 3C geophone dataset.