Oleg Yu. Ganzha

The seismic seabed observations, the current state of the issue and the ways of development of bottom seismic using robotic tools are considered in the paper. An overview of the current types of bottom seismic surveys and their division into two main types — deep-water and shallow-water works and works in land-sea transition zones (transit zones) is given. The main current technologies used in both sectors of the bottom works market are discussed. The bottlenecks and problems of each of the types of technologies used are considered. The technology of self-popup seismic nodes for use in deep and shallow water is presented in detail. The advantage of using this technology for the bottom part of hybrid marine seismic surveys in deep water is shown. Several variants of the prospects for the development of self-popup technology in deep water are presented. Among them, a new direction of seabed seismic exploration in deep water has been analyzed in detail. It consists in the design and use of a large number of Autonomous Underwater Vehicles (AUV), each of which is complemented by the ability to receive and record seismic bottom data. A new technology of robotic surveys based on the use of self-popup nodes without leaving anchor on the seabed and an unmanned autonomous surface vessel-platform has been proposed for the shallowwater sector. The model of the self-popup node without leaving the anchor on the seabed has been tested. An effective way of automatically node deployment to the seabed has been found and tested. Marine tests of the emersion of the proposed model of a self-popup seismic node without anchor using compressed air technology have been carried out. The problems of the proposed model are identified. After that a stable modified version of its emersion under all weather conditions has been found.

This paper is devoted to the features of seismological observations in the Arctic seas, which are complicated by harsh climatic conditions, the presence of ice cover, stamukhi and icebergs, and limited navigation. Despite the high risk of losing expensive equipment, the deployment of local networks of bottom seismographs or stations installed on ice is still necessary for studying the seismotectonic characteristics and geodynamic processes of the region under consideration, the deep structure of the crust and upper mantle, seismic hazards, and other marine geohazards. Various types of seismic stations used for long-term and short-term deployments in the Russian sector of the Arctic Ocean, as well as various schemes and workflows for their deployment/recovery, are described. The characteristics of seafloor seismic noise and their features are also considered. The results of deployments demonstrate that the characteristics of the stations make it possible to reliably record earthquake signals and seismic noise. Based on the experience gained, it was concluded that the preferred schemes for deploying ocean-bottom seismographs are those in which their subsequent recovery does not depend on their power resources. Usually, such schemes allow for the possibility of dismantling stations via trawling and are suitable for the shelf depths of the sea. The advantages of such schemes include the possibility of installing additional hydrophysical and hydrobiological equipment. When using pop-up ocean-bottom seismographs, special attention should be paid to the careful planning of the recovery because its success depends on the possibility of a passage to the deployment site, which is not always possible due to changing meteorological and ice conditions. Seismic records obtained on the seafloor are characterized by a high noise level, especially during periods of time when there is no ice cover. Therefore, it is recommended to install bottom stations for periods of time when ice cover is present. The frequency range of the prevailing noise significantly overlaps with the frequency range of earthquake signals that must be taken into account when processing bottom seismic records.

The Russian sector of the arctic shelf is the longest in the world. Quite a lot of places of massive discharge of bubble methane from the seabed into the water column and further into the atmosphere were found there. This natural phenomenon requires an extensive complex of geological, biological, geophysical, and chemical studies. This article is devoted to aspects of the use of a complex of marine geophysical equipment applied in the Russian sector of the arctic shelf for the detection and study of areas of the water and sedimentary strata with increased saturation with natural gases, as well as a description of some of the results obtained. This complex contains a single-beam scientific high-frequency echo sounder and multibeam system, a sub-bottom profiler, ocean-bottom seismographs, and equipment for continuous seismoacoustic profiling and electrical exploration. The experience of using the above equipment and the examples of the results obtained in the Laptev Sea have shown that these marine geophysical methods are effective and of particular importance for solving most problems related to the detection, mapping, quantification, and monitoring of underwater gas release from the bottom sediments of the shelf zone of the arctic seas, as well as the study of upper and deeper geological roots of gas emission and their relationship with tectonic processes. Geophysical surveys have a significant performance advantage compared to any contact methods. The large-scale application of a wide range of marine geophysical methods is essential for a comprehensive study of the geohazards of vast shelf zones, which have significant potential for economic use.

Настоящее исследование посвящено разработке дизайна и прототипа наледной сейсмостанции для регистрации землетрясений и сейсмических шумов в арктических морях, а также концепции ее использования. В работе отмечается, что регионы Арктики и Субарктики оказывают существенное влияние на глобальные процессы, происходящие на Земле. Необходимость долговременного сейсмического мониторинга в арктических и субарктических акваториях России со льда, а также совершенствование соответствующей аппаратуры, обусловлены большим научным интересом к сложной геодинамике и тектонике региона, глубинному строению литосферы и необходимостью оценки различного рода геологических опасностей в процессе активного освоения Арктики и возведения там критической инфраструктуры. При разработке дизайна наледной сейсмостанции особое внимание уделялось следующим параметрам, которые, по мнению авторов работы, являются ключевыми для оптимального долговременного сейсмического мониторинга со стационарного или дрейфующего льда: применение сейсмоприемников с расширенным частотным диапазоном регистрации; присутствие гидрофонного канала регистрации; обеспечение непрерывного контроля за оборудованием и оперативного сбора данных в течение всего периода наблюдений; обеспечение гибкости в применяемом типе и количестве датчиков; обеспечение работоспособности при низких температурах и сильном ветре; обеспечение водонепроницаемости основных узлов станции. Отмечается, что обширность субарктических и арктических акваторий России и большое количество решаемых с помощью сейсмического мониторинга со льда задач приводят к возможности широкого применения описанного оборудования. Сделан вывод о том, что к преимуществам представленного оборудования относятся приспособленность к суровым арктическим условиям, возможность удаленной передачи данных, а также использование отечественной аппаратурной базы.

Based on the data of deep seismic sounding, which were obtained in 2003 on the R/V Mezen in the Bay of Bengal, two velocity sections have been constructed that characterize the structure of the crust in its southern part. Seismic studies have shown that, in the Bengal sector of the East Indian Ridge (EIR), a subcontinental type of crust is developed, the thickness of which reaches 19.5 km. At the western foot of the EIR, the velocity section is represented by a reduced transitional crust 13 km in thickness. Using the NGDS digitized database of the thickness of the sedimentary cover in the Indian Ocean, which is freely available on the Internet, a map of the depths of the acoustic basement is built which makes it possible to identify the morphological features of its structure. A seismostratigraphic analysis is carried out of the structure of the sedimentary cover that has developed (a) on horst blocks grouped at the foot of the western slope of the EIR, (b) on the crest of the EIR, and (c) in the adjacent part of the Central Basin. It is established that, in the Late Cretaceous–Paleogene, the Bengal sector of the EIR and horst blocks represented a single platform structure. In the latest phase of tectonic activation, which manifested itself in the late Miocene–Pliocene, the platform was divided into horst blocks, which experienced differentiated subsidence. It has been established that horst blocks play the main role in the structure of Ridge 85°. This ridge is not structurally unrelated to the Afanasy Nikitin Rise and, by its nature, is not a “tracer ridge.”

The Arctic seas are now of particular interest due to their prospects in terms of hydrocarbon extraction, development of marine transport routes, etc. Thus, various geohazards, including those related to seismicity, require detailed studies, especially by instrumental methods. This paper is devoted to the ocean-bottom seismographs (OBS) based on broadband molecular–electronic transfer (MET) sensors and a deployment case study in the Laptev Sea. The purpose of the study is to introduce the architecture of several modifications of OBS and to demonstrate their applicability in solving different tasks in the framework of seismic hazard assessment for the Arctic seas. To do this, we used the first results of several pilot deployments of the OBS developed by Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) and IP Ilyinskiy A.D. in the Laptev Sea that took place in 2018–2020. We highlighted various seismological applications of OBS based on broadband MET sensors CME-4311 (60 s) and CME-4111 (120 s), including the analysis of ambient seismic noise, registering the signals of large remote earthquakes and weak local microearthquakes, and the instrumental approach of the site response assessment. The main characteristics of the broadband MET sensors and OBS architectures turned out to be suitable for obtaining high-quality OBS records under the Arctic conditions to solve seismological problems. In addition, the obtained case study results showed the prospects in a broader context, such as the possible influence of the seismotectonic factor on the bottom-up thawing of subsea permafrost and massive methane release, probably from decaying hydrates and deep geological sources. The described OBS will be actively used in further Arctic expeditions.

Adding newly processed old marine and land-based wide-angle seismic data to modern geological and geophysical information about the region provides the correct localization and understanding of interaction of the Sinai, Anatolian, and African plates in the Eastern Mediterranean. Wide-angle seismic profile data acquired by ocean bottom and land-based seismometers in 1989–1990 by USSR Academy of Sciences expeditions were reprocessed. Land-based shots and a marine airgun shooting were used as seismic signal sources. As a result, the northern and western boundaries of the Sinai Plate were determined; absence of subduction in the southwestern part of the Cyprus Arc was proved; the structure of the Earth’s crust and upper mantle of oceanic crust of the African and Sinai plates and the continental crust of the island of Cyprus has been refined.