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Summary Geophysical methods have recently been increasingly used in geoecological studies, including engineering and geological surveys and state monitoring of the shelf subsoil. The most widely used are various types of seismoacoustic profiling, side-scan sonar, multi-beam echo sounding. Seismoacoustic methods give an idea of the morphology of geological bodies and indirectly indicate their genesis and composition of the enclosing sediments. They are most widely used in the assessment of gaseous sediments and in the identification of gravitational processes. Side-scan sonar makes it possible to identify zones of ice scouring, to give the dimensions of relief mesoforms, as well as to estimate the intensity, type and direction of lithodynamic flows. Multi-beam echo sounding allows one to assess the morphlology of the seabed over large areas and to distinguish various genetic types of relief with their quantitative characteristics. The report provides numerous examples of the use of geophysical methods for solving various geoecological problems. It is concluded that for tasks such as determining the thickness and distribution of frozen rocks, geophysical methods at the present stage cannot give an unambiguous answer. The need to prepare a methodological manual on the use of geophysical methods for solving geoecological problems is indicated.

Summary Shallow permafrost is associated with significant potential risks for offshore construction and drilling as well as ecological risks that may occur when the permafrost is disturbed due to the human activities. Identification and mapping of the shallow permafrost areas is one of the major tasks of the site survey investigations in the Russian Arctic. Due to the lack of engineering drilling data on the Arctic shelf, remote geological and geophysical studies, such as multichannel high, very-high and ultra-high resolution (HR, VHR and UHR) seismic surveys, are increasingly being used to map and characterize the subaquatic permafrost. In the paper, we present an approach to 3D VHR reflection seismic data interpretation that allowed us to the built a volumetric model of the shallow permafrost and assess its acoustic properties in the shelf of the Kara sea. To our knowledge, this is the first ever demonstrated detailed 3D model of the shallow permafrost in the Russian Artic.

Summary The paper is dedicated to the analysis of 2D high, very high, ultrahigh resolution seismic and multibeam echo-sounder surveys data. Near-surface section (NSS) geological features were studied based on acquired data analysis on the example of three separated regions of Vostochno-Prinovozemelsky area. The main goal was clarification of NSS and geological hazards identification inside studied area. Gas-saturated sediments, paleovalleys and faults were distinguished in each region. Faults genesis and morphological traits issues, definition of deformation nature of NSS and geological hazards identification at the bottom and in near-bottom sediments were studied. Similarities and differences in studied section structure were found. In the first two regions Touronian-Santonian seismic sequence complicated by series of faults formed due to clay dehydration was defined. In the second region deformations inside Oligocene-Miocene seismic sequence were found. They were likely formed by glacial activity. Moreover, there were paleovalleys that had different nature compared to the one that depressions in the third region had. In general, glacial sediments wide spread was defined in the first two regions, which was unnatural for the third region. That means that Barentsevo-Kara glacial boundary, reviewed in the work ( Svendsen et al., 2004 ), is located between the second and the third regions.

Summary Mitigation of drilling risks associated with geological hazards is a priority task of the site survey offshore investigations. One of the most challenging regions for offshore exploration in terms of near-surface geological hazards is the western Artic shelf. In this region, the near-surface is complicated by presence of subaqueous relict permafrost, intrapermafrost and subpermafrost gas caps, pingo-like structures and various cryogenic deformations. The paper presents our experience of mapping and characterization of the shallow offshore geohazards (including permafrost) using 3D seismic data acquired in different frequency ranges in the Russian Arctic.

Series of two papers are dedicated to study of erosional and depositional processes in the northern part of the previously distinguished contourite system on the Patagonian continental slope between 46-42° at a water depth range of 2100-5200 m. The first paper is focused on interpretation of high-resolution seiscmoacoustic data collected using the parametric echo-sounder SES 2000 deep with acoustic penetration of 60 mbsf. Analysis of seismic data revealed seismic facies of contourites and gravitites as well as erosional features formed under the influence of gravity flows (gravity-driven processes) moving downslope through the canyon system. It was demonstrated that gravitites deposited by gravity flows represent a source of sediment material for contourite drift formation. Domination of erosion over deposition in the study area might be explained by the intense dynamics of bottom waters, including contour currnets, internal waves, benthic storms and gravity flows.

We present the methods and results of numerical experiments studying the low-frequency sound propagation in one of the areas of the Arctic shelf with a randomly inhomogeneous gas-saturated bottom. The characteristics of the upper layer of bottom sedimentary rocks (sediments) used in calculations were obtained during a 3D seismic survey and trial drilling of the seafloor. We demonstrate the possibilities of substituting in numerical simulation a real bottom with a fluid homogeneous half-space where the effective value of the sound speed is equal to the average sound speed in the bottom, with averaging along the sound propagation path to a sediment depth of 0.6 wavelength in the bottom. An original technique is proposed for estimating the sound speed propagation in an upper inhomogeneous sediment layer. The technique is based on measurements of acoustic wave attenuation in water during waveguide propagation.

Summary Offshore construction and exploration require preliminary characterisation of the near-subsurface, which includes evaluation of potential geological hazards. In this study, we present the novel data on the geohazards on the shelf of the Kara Sea. Integrated interpretation of the 2D high-resolution (HR) and 3D exploration reflection seismic data allowed us to evaluate geohazards at different scales from the sea bottom to nearly 1 km depth. Potential geohazards on the shelf of the Kara Sea include paleovalleys, gas-bearing sediments, accumulations of permafrost, intervals with severe faulting and tectonically weakened zones possibly associated with the upward migration of fluids.

Summary Subsea permafrost and gas-saturated deposits represent the main geohazards for prospective engineering construction prior offshore Artic exploration. Similar evidences of permafrost and gas saturated deposits in seismic wavefields lead to the necessitaty in the development of a specific methodology of seismic site survey investigations that will allow accurate identification and reliable separation of these two types of geological hazards. One technical solution to this problem is the use of seismic survey with multifrequency bands. In this paper basic principles and examples of permafrost and gas saturated deposits identification are presented using multifrequency seismic data acquired during site survey in the Laptev Sea in 2014–2015.

New high-resolution seismic reflection profiles collected on the western slope of the Caspian Sea during cruises of RV “Rift” in 2004, 2006 and 2007 allowed the identification of regional sedimentary processes formed its present morphologic and structural image. The upper, gentle part of the slope, from the shelf edge to 400 m water depth, was formed in the Pleistocene–Holocene. Its architecture is dominated by very prominent wavy features, sedimentary wave bedforms, which were formed mostly by density gravitational (turbidity) streams downslope during lowstands of the Caspian Sea, i.e. during the regressive intervals, and affected by bottom currents partly. This specific sedimentary process was slowed during transgressive phases. Downsection, several distinct generations of the sediment waves are distinguished within the upper sedimentary sequence. Sampling by gravity corer characterized the uppermost undulation only. Therefore, several deeper generations of the sediment waves distinguished within the upper sedimentary sequence were compared with the available drilling results from boreholes in the shallow-water northern Caspian Sea shelf, by means of extrapolation and correlation of horizons along the available seismic profiles. Three strong wavy reflectors on the western slope were identified on the shelf as unconformities, marking the Atelian, Enotaevian, and Mangyshlakan regressions. Lacking submarine boreholes, a rough estimation for general regional seismic-stratigraphy of the upper sedimentary section on the western slope of the Caspian Sea is proposed from the stratigraphic identification of the observed sediment waves.

In the present there is an active offshore oil& gas shelf fields development. The standard methods applied for investigation of near surface geological conditions for site survey goals are high resolution seismic (HRS) for top hole drilling hazard assessment and ultrahigh resolution seismic (UHRS) mutually used with hydrographic surveying for sub-bottom geohazard assessment. On the other hand, application of 3D seismic technology for site survey investigations requires improvements both in data processing and interpretation. This paper focuses on the generalization of experience in the geohazards assessment in offshore fields based on the 3D seismic data. Involving of modern interpretation procedures standard for prospecting and exploration: inversion, wavelet analysis, pore pressure prediction based on spectral time analysis, Poisson ratio evaluation can significantly improve the accuracy and efficiency of 3D seismic data interpretation for site survey investigation. But still remain unsolved problems related to resolution of sub-bottom part, which caused by insufficient sources frequency range and absence of near offsets.