Three-parameter Radon transform in layered transversely isotropic media

Radon transform is a powerful tool with many applications in different stages of seismic data processing, because of its capability to focus seismic events in the transform domain. Three-parameter Radon transform can optimally focus and separate different seismic events, if its basis functions accurately match the events. In anisotropic media, the conventional hyperbolic or shifted hyperbolic basis functions lose their accuracy and cannot preserve data fidelity, especially at large offsets. To address this issue, we propose an accurate traveltime approximation for transversely isotropic media with vertical symmetry axis, and derive two versions of Radon basis functions, time-variant and time-invariant. A time-variant basis function can be used in time domain Radon transform algorithms while a time-invariant version can be used in, generally more efficient, frequency domain algorithms. Comparing the time-variant and time-invariant Radon transform by the proposed basis functions, the time-invariant version can better focus different seismic events; it is also more accurate, especially in presence of vertical heterogeneity. However, the proposed time-invariant basis functions are suitable for a specific type of layered anisotropic media, known as factorized media. We test the proposed methods and illustrate successful applications of them for trace interpolation and coherent noise attenuation.     

Distribution of Sessile Invertebrates in a Rocky Sublittoral Community off Rimsky‐Korsakov Islands (Sea of Japan)

Abstract. The dependence of the vertical distribution of sessile macrobenthos upon environmental physical factors was studied in a community inhabiting rocky substrata at 2 to 15 m depth near the Rimsky‐Korsakov Islands (Peter the Great Bay, Sea of Japan). Three separate groups of species were distinguished within this community. The main dominant species are the anthozoan Metridium senile and the mussel Crenomytilus grayanus. Water movement, bottom inclination and topographic position of the studied sites in respect to prevailing water flows influence the spatial structure of sessile invertebrates. No vertical zonation in the scale of the studied sublittoral area below the 2 m depth level was observed. The studied factors are believed to condition the observed mosaic of patches characterized by specific composition, with a single species being dominant.     

Woolly rhino discovery in the lower Kolyma River

A nearly complete frozen mummy of a woolly rhinoceros (Coelodonta antiquitatis Blum., 1799) was discovered in a gold mine on the lower reaches of the Kolyma River, north–eastern Siberia. This is the first find of the whole body of woolly rhino in permafrost. A large part of the mummified body was preserved, including the left part of the body, covered by skin, including skin of the head and ear, fore and hind legs. The skull with 2 horns and the lower jaw were also preserved. Most of the internal organs were lost, except the intestines, stomach, and their contents. A rib fragment from this individual was dated by AMS-radiocarbon method to 39,140 ± 390 years BP (OxA-18755). Spore and pollen analyses of the stomach contents indicate that grasses and sagebrushes formed the main part of the diet of C. antiquitatis in this region of Arctic Siberia.     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.     

Subgrade-reinforcement techniques for the dangerously deforming sections of railway lines in the north of the Russian Far East

This paper lists difficult railway construction conditions that are typical for the northern areas of the Russian Far East. Permafrost-bound issues of railway and highway construction and operation are described. Some of the authors'' developments for subgrade stabilization in the foregoing conditions are given as examples.     

Low-frequency anisotropy in fractured and layered media

Computing effective medium properties is very important when upscaling data measured at small scale. In the presence of stratigraphic layering, seismic velocities and anisotropy parameters are scale and frequency dependent. For a porous layer permeated by aligned fractures, wave-induced fluid flow between pores and fractures can also cause significant dispersion in velocities and anisotropy parameters. In this study, we compare the dispersion of anisotropy parameters due to fracturing and layering at low frequencies. We consider a two-layer model consisting of an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce anisotropy parameters dispersion due to fractures (meso-scale) in the sand layer. This intrinsic dispersion is added to anisotropy parameters dispersion induced by layering (macro-scale) at low frequencies. We derive the series coefficients that control the behaviour of anisotropy parameters at low frequencies. We investigate the influences of fracture length and fracture density on fracturing effect, layering effect and combined effect versus frequency and volume fraction of sand layer. Numerical modelling results indicate that the frequency dependence due to layering is not always the dominant effect of the effective properties of the medium. The intrinsic dispersion is not negligible compared with the layering effect while evaluating the frequency-dependent properties of the layered medium.     

Equivalent time-average and effective medium for periodic layers

The propagation of acoustic waves through a periodic layered medium is analyzed by an eigenvalue decomposition of the propagator matrix. This reveals how the velocity and attenuation of the layered medium vary as function of the periodic structure, material parameters and frequency. There are two important parameters which control the wave propagation in the periodic medium: the reflection coefficient and the ratio between one‐way traveltimes of the two parts of the cyclic layered medium. For low frequencies (large values of wavelength to layer thickness), the layered structure behaves as an effective medium, then there is a transition zone, and for higher frequencies (small values of wavelength to layer thickness) the medium is described by the time‐average velocity. In this paper we mostly concentrate on the transition zone between an effective medium and time‐average medium regimes. The width of the transition zone increases with larger values of the reflection coefficient. The transition zone corresponds to a blocking regime for which the transmission response of the layered structure is close to zero. For even higher frequencies, the time‐average medium is replaced by a new transition zone, and then again a time‐average medium. This pattern is periodically repeated with higher frequencies. For small values of the reflection coefficient, the transition between effective medium and time‐average medium occurs around a value of wavelength to layer thickness equal to 4.     

S-wave in 2D acoustic transversely isotropic media with a tilted symmetry axis

In an acoustic transversely isotropic medium, there are two waves that propagate. One is the P-wave and another one is the S-wave (also known as S-wave artefact). This paper is devoted to analyse the S-wave in two-dimensional acoustic transversely isotropic media with a tilted symmetry axis. We derive the S-wave slowness surface and traveltime function in a homogeneous acoustic transversely isotropic medium with a tilted symmetry axis. The S-wave traveltime approximations in acoustic transversely isotropic media with a tilted symmetry axis can be mapped from the counterparts for acoustic transversely isotropic media with a vertical symmetry axis. We consider a layered two-dimensional acoustic transversely isotropic medium with a tilted symmetry axis to analyse the S-wave moveout. We also illustrate the behaviour of the moveout for reflected S-wave and converted waves.     

Frequency-dependent PP and PS reflection coefficients in fractured media

When a porous layer is permeated by mesoscale fractures, wave-induced fluid flow between pores and fractures can cause significant attenuation and dispersion of velocities and anisotropy parameters in the seismic frequency band. This intrinsic dispersion due to fracturing can create frequency-dependent reflection coefficients in the layered medium. In this study, we derive the frequency-dependent PP and PS reflection coefficients versus incidence angle in the fractured medium. We consider a two-layer vertical transverse isotropy model constituted by an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce the intrinsic dispersion due to fracturing in the sand layer. Based on the series coefficients that control the behaviour of velocity and anisotropy parameters in the fractured medium at low frequencies, we extend the conventional amplitude-versus-offset equations into frequency domain and derive frequency-dependent amplitude-versus-offset equations at the elastic–anelastic surface. Increase in fracture length or fracture density can enlarge the frequency dependence of amplitude-versus-offset attributes of PP and PS waves. Also, the frequency dependence of magnitude and phase angle of PP and PS reflection coefficients increases as fracture length or fracture density increases. Amplitude-versus-offset type of PP and PS reflection varies with fracture parameters and frequency. What is more, fracture length shows little impact on the frequency-dependent critical phase angle, while the frequency dependence of the critical phase angle increases with fracture density.     

Traveltime parameters in a tilted elliptical anisotropic medium‡

In this paper, we derive analytical expressions for one‐way and two‐way kinematical parameters in elliptical tilted transverse isotropy media. We show that the homogeneous elliptical tilted transverse isotropy models result in hyperbolic moveout with a reflection point sideslip x₀, which can be considered as an additional traveltime parameter for one‐way wave propagation. For homogeneous elliptical tilted transverse isotropy models we show that the inversion of one‐way traveltime parameters suffers from the ambiguity for large tilts. It is shown that the accuracy of the inversion is sensitive to the error in x₀. We also derive and invert the traveltime parameters for a vertically heterogeneous elliptical tilted transverse isotropy model with a tilt gradually changing with depth. The a priori knowledge of parameter δ is very important for inversion. The wrong choise of this parameter results in significant errors in inverted model parameters.