VII Annual International Conference and Exhibition - Galperin Readings 2007

The work offers integrated approach in depth section mapping based on continuation of wavefields recorded on land surface. Field is continued by integral operator, first proposed by Goldin. Geometric seismology laws are applied in the continued field. Besides, depending upon kernel of continuation operator, amplitudes of wave fronts change according to null approximation of ray-tracing method. Thus, the obtained continuation can be used for estimate of dynamic medium properties. Algorithm of wavefield continuation can be applied both for migrated sections imaging, and for measurement of medium velocity using focusing technique. Velocity analysis algorithm and depth migration results were studied on the example of one seismic profile.

The problem on tube waves excitation in a well, intersected by thin fluidfilled fracture under action of external seismic wave is considered in the report. This situation arises in VSP practice at hydro-fracturing. Based on averaging procedure the non-uniform pseudo-differential wave equation for slow eigen mode in fluid-filled fracture is derived in space-time representation for the long wave approximation by fracture opening. Numerical calculations show that high dispersive slow eigen mode is excited by fracture edges. This mode generates additional tube waves in a well. Registration of these tube waves can be used for fracture size characterization.

The method of complex velocity model parameters estimation using travel times from many sources is represented. The velocity model is constructed with smooth curvilinear reflective boundaries. Parameters approximation is guided with optimizing inversion of hodographs of direct, reflected and converted waves from different 3D VSP sources. Model hodographs are calculated with the using of ray method.

The simple and economic way of wave field selection in a direction of wave propagation on two adjacent seismic traces is offered.

With reference to the problem of look-ahead prediction using offset VSP data the singular value decomposition analysis of linearized operator of dynamic elasticity is presented. Such analysis allows to justify strictly main features of the problem that should be taken into account when the numerical algorithm for solving the inverse problem is developed. Using this analysis the optimal parametrization for inversion was founded out. It was shown that density can't be inverted. Also it was shown the principal impossibility of determining the smooth components of solution. These feature don't depend on the method of optimization because they defined by the linearized operator.

Paper describes the method of automatic building of velocity model. The method based on approximation of vertical travel times by truncated Fourier series. Computation algorithm is discussed and approximation errors are estimated. Paper also analyzes the possible sources of VSP and surface seismic data stratigrahy tie mismatches. Authors suggest the method off corrections computation to be applied to VSP vertical travel times to increase the accuracy of depth transformations.

In this paper we propose a modified Reverse-Time Migration algorithm applied to OVSP data which gives a possibility to reconstruct depth structure image having no knowledge about overburden by using original registered seismograms without their redatuming. Velocity model estimated in some depth interval in vicinity of borehole can be used for migration. Its main feature is approximation of source wavefield in the target area using downgoing wavefield registered in a borehole. To do it one needs to perform back-in-time extrapolation of downgoing wavefield using finite-difference (FD) scheme for solving the scalar wave-equation (or using FD for elasticity). It is possible to migrate PP-reflections or transmitted PS-converted waves for formation boundaries or salt flank imaging. Our numerous experiments with synthetic and field OVSP data demonstrate that modified RTM provides a good quality image in complex geological conditions.

The paper generalizes processed VSP data from 10 wells located in the Republic of Sakha (Yakutia). Specificities about VSP wavefields characteristic of the survey area are established. Velocity characteristics on the East Siberian section are analyzed.

Geophysical, seismic survey allows the examination of the geological subsurface with different precision both in vertical and horizontal scale. The precision of the above survey depends on seismic resolution that is the detailed imaging of medium built, both concerning structure (geometric) and physical, reserve parameters built. The survey of elasticity properties used in geophysics can be methodically grouped in four ranges – Fig. 1a).