articles

 

The inverse seismic kinematical problem is considered. Its correctness is discussed. Structural stability of  inversion is studied. Examples are given. 

A general problem of accuracy evaluation is very important for the CDP velocity analysis made by the  vertical or horizontal velocity spectra interpretation. The main idea of the precision estimation is based on  dependence between the velocity spectra structure and two parameters: wavelet frequency and dispersion of  the arrival time curve approximation. For the aim of the velocity spectrum sensitivity threshold evaluation  Marechal criteria was introduced. Based on the introduced strategy the modeled data have been analyzed.  Offset dependence of the velocity analysis were investigated. This example is describing the uncertainty of the  effective velocity estimation.

Ill-posedness of inverse travel time problem the CDP method in seismic exploration is an important  problem. Non-uniqueness is one of fundamental feature of travel time inversion, i. e. existence of many solutions – velocity-depth models with travel time attributes (t₀, VCDP) fitting the data. In the same time, many  algorithms of velocity-depth modeling (including tomography) have in their base minimization criterion of  difference between travel time attributes of the model and initial data – for some assumptions about the type  of inhomogeneities. This paper demonstrates results based on forward travel time modeling of experiments  allowing to evaluate consequences of using simplified layer models of the near surface section, in case of using  a misfit minimization criterion for travel time attributes (t₀, VCDP). We provide some recommendations and  emphasize the importance of the problem of information deficit about the near surface properties.

We investigate the new method that combines the migration of VSP data and updating of the velocity  model. The method is based on the comparison of subsurface images obtained by using different types of waves:  the primary reflections and surface-related multiples. As a measure of similarity of the images we use the  functional based on cross-correlation. Estimation of the velocity model parameters is carried out by maximization  of this functional. Therefore the resulting velocity model provides the maximal similarity between images  obtained by using the primary reflections and surface-related multiples. We develop an iterative procedure to  maximize the functional and demonstrate the efficiency of the method using the synthetic walk-away data. We  consider the recovery of the velocity parameters in the layer with linear velocity dependence in horizontal and  vertical directions in the case of the inclined base of the layer. We demonstrate an efficiency of the method  for curvilinear reflecting interfaces. The results of the numerical simulation on determination of the Thomsen’s  parameters  ε  and  δ  for transversely isotropic model with the vertical symmetry axis are considered.

Multiple reflections recorded during marine seismic acquisition mask the primary waves. Attenuation of  multiples still remains one of the most challenging problems of real data processing. The SRME algorithm is  among the most efficient demultiple schemes. Its appluication for 3D real data processing requires a huge  amuont of computations. The paper presents a computationally efficient 3D SRME method that provides high  scaling for computers with thousands of nodes. Practical application of the developed code allowed for com- putationally efficient multiple attenuation for data from various regions. 

A characteristic gridding method is used to model seismic wave propagation in macrofractured subsurface  where macrofracture length is comparable with seismic wave length. Thorough examination is made of diffractions  that form seismic response from subvertical macrofractures – possible fluid conduits in oil and gas reservoirs.  Evidence is provided that macrofractures, if fluid-filled, are best identifiable on the horizontal component of  recorded converted diffractions, while gas-filled macrofractures are well seen on the vertical component of  recorded P-diffractions.

The paper concerns with a method of evaluation effective seismic properties of vertical fractured rocks  based on the theory of linear slip surfaces. Basic theoretical and practical issues of the method are stated.  Extensive joints and penny-shaped cracks are examined, relations between expressions of effective seismoacoustic  parameters of such fractured media are determined. Results of physical modeling of seismic waves propagation  in porous-fractured media are compared with theoretical estimations. It is shown that Hudson’s theory predictions  diverge  from  experimental  results.  Kinematic  parameters  of  recorded  signals  are  in  good  agreement  with  theoretical estimations for energy transfer velocity in porous-fractured media obtained in frame of the theory  of linear slip surfaces. Examined physical models possessed considerable spatial dispersion which required to  take  into  account  difference  between  group  and  phase  velocities.  Utilized  approach  is  valid  for  effective  description of wide range of fractured rocks.

I analyze the unphysical caustics for qP-wave in acoustic transversely isotropic media with vertical symmetry  axis. Caustics appear at a large negative anelliptic anisotropy parameter as it shown for the geometrical spreading  factor and the phase velocity.

A unified approach has been proposed for simultaneous determination of the elastic and transport properties of reservoir rocks. The transport properties include the electrical and heat conductivity, hydraulic  permeability, and permittivity. The approach is based on the effective medium theory that makes it possible  to relate the macroscopic (effective) properties of reservoir rocks with the respective properties of solid skeleton,  fluid filling the pore space, pore/crack geometry, and pore connectivity. This approach allows one to solve the  following actual problems of prospecting geophysics: to determine the geometrical characteristics of reservoir  porous space; to explain correlations observed between different physical properties; to estimate “nonmeasurable” properties from the measured ones; to detect cracked zones in carbonate reservoir rocks and determine  their characteristics; to predict the physical properties of reservoir rocks at different scales; to reconstruct  complete stiffness tensor (or tensor of transport properties) of anisotropic rock from limited number of measurements insufficient for using standard procedures; to build scale-dependent velocity models of shales with  allowance made for their anisotropy needed to monitor the hydrofracture development.

The measurements of cross electric and seismic effects were performed in the natural conditions using  VSP tools (SEISZOND). We observed significant changes of seismic first break time versus time of DC electric passing through the media. It is concluded that electrolysis of ground water is a probable mechanism of  these phenomena.

During deep seismic studies offshore, thick water layer hinders estimation of velocities in the sedimentary cover and the upper consolidated crust. Primarily, this is due to the fact that in first arrivals there are no  turning and refracted waves propagating in these layers. But in multi-component measurements with bottom  stations, it is possible to use for interpreting converted and multiple waves related to these intervals of the  section. Using the multiple and converted waves, one can evaluate the P- and S-waves velocities in the sedimentary strata and the upper consolidated crust. The case study of the multi-wave interpretation of seismic  data along the profile in the deep water South Okhotsk basin is described in the paper. Velocity parameters of  the section resulting from the interpretation allow the identification of the Earth’s crust in this region as thinned  and extended continental crust.

The methodology of large-scale seismic land acquisition is described on example of Gazprom Libya  case-study operations in desert environment of Libya in Area 64. The details of the acquisition preparing are  shown, as well as general estimations of technical and economical efficiency of possible approaches. Detailed  description of the selected final interactive approach was given in the article, which started with 3D SPARSE  survey to cover huge area, and ended, based on interactive seismic data processing and interpretation, by putting and selection of optimal location and parameters of the dense 3D VIVID cube. All works were done  using the Q-Land technology of the WesternGeco company. In the article, results of using of the Q-technology are shown (examples of coherent noise and aliasing attenuation are given) and data with characteristic  low-frequency components (as results of using of MD-sweep and GACs receivers) are demonstrated.

The article is devoted to the comparative analysis of possibilities of VSP equipment, equipped with  various variants of digital transmission systems and time modulation system, depending on temperature and a  lengths of geophysical cable. We make conclusions concerning dependence of the basic characteristics of digital borehole probes on a ration of the cable length and a speed of the sending device.