articles

 

Layer, average and interval velocities which are studied with the VSP method are the only plausible source of velocities which are necessary for surface seismic. High accuracy of determining of the velocities can be attainable by means of up-to-day approaches based on optimizing methods but not on out-dated approaches based on reduction of the offset time-depth curve to the zero-offset one under the assumption of medium homogeneity between vibration source and receiver. Results of kinematic inversion are correlated with the medium image, both are obtained for a complex constructed geological media conditions. It was shown that joint determining of layer velocities together with the geometry of refracting and reflecting boundaries allows avoiding absurd results which show up as un-interfacing of results from different shot points. Examples of interval velocities determination by means of VSP method are shown with detailing which approaches to one of the sonic log and of the maximum feasible detailing of the surface seismic.

“SeisWin-QC” is multifunctional system of analysis and seismic data quality control, which is developed in “Geophysical Data Systems” Ltd. company and works in “Windows” OS, including “Vista” and “Windows 7”. The software provides automated control of technical and geophysical quality of seismic data in the flow and in interactive (“real-time”) mode. Technical quality is provided with control of layout geometry, amplitude and spectrum for each channel for the purpose to find out the broken, noisy and bad placed channels. Geophysical quality is estimated on the weighed sum of basic attributes of seismic record. The set of attributes includes: spectrum width, dominant frequency, signal/noise, etc. The system “SeisWin-QC” offers convenient user interface and wide set of advanced program-algorithmic features for seismic record attributes control, including statistical histogram, selection and sorting of attributes, representation in form of maps. “SeisWin- QC” use allows to make the estimation of seismic data quality more objective and complex at level of field works contractor, supervisor and customer.

The current state of applied techniques of quality control (QC) the seismic data is considered at carrying out 2D and 3D surveys, the variants of algorithms of the separate attributes calculations of a seismic records and their efficiency, criteria of the seismogram quality estimations. Tendencies of development and optimizations of QC technology on the basis of modern representations about structure of a wave field registered and requirements to carrying out of field works, possibilities of computing means and the software are considered. Examples of use of the interactive QC technology in “GDS” Ltd. developed on the basis of the software package “SeisWin-QC”, for supervision quality control of the seismic data in the field and from the Customer side are resulted. The conclusion about necessity of transition from absolute values of attributes used, to the relative values formed on the basis of the statistical data, received in the given region and on the area where seismic survey has been carrying out, is made.

On examples of some oilfields from north of Western Siberia we show an approach to validate prospects for hydrocarbon traps using optimal combination of 3D seismic research and well-logging data.

Quality of VSP data is influenced by many reasons, which may be divided in two groups. The first group contains reasons that not quite depend on abilities of the executor. The second one contains reasons depending on the executor and quite responsive for control during the borehole running. The main attention in the article was paid to the second of the groups. During running of the borehole controlling the quality of raw records, obtained not only from vertical but also from horizontal components, should be recommended. It will allow avoiding difficulties at the processing and interpreting stages, caused by low quality of the records. Procedures for quantitative evaluating of the VSP records quality are recommended and examples of such evaluating for different components of a borehole probe are given.

Near surface on the Siberian Platform is characterized by strong azimuthal anisotropy with large lateral variations in value as well as in symmetry direction. These make in-phase stacking of reflected PS-waves im- possible and obstruct analyzing any dynamic features of PS-waves including estimation of the target interval anisotropy parameters. In this paper the new technology of anisotropy compensation is presented. The com- pensation of anisotropy influence has been carried out using the common receiver point stacks and has allowed removing the distortions caused by near surface. This operation resulted in increased coherence of the PS-wave reflections on the common conversion point sections.

The article on the basis of synthetic model studies possibilities of refining acoustic inversion results with laterally variable wavelet. An individual wavelet is defined for each seismic traces using statistical approaches. Wavelet phase spectrum is assumed constant and equal to zero. Seismic wavefield model is analyzed, where laterally-variable low-pass filter is applied. The created area with suppressed high frequencies imitates absorbing impact of unconsolidated near-surface sediments. Results of seismic acoustic inversion are presented with using different wavelets: two wavelets which are the same for the entire area, and two laterally variable wavelets.

A software suite to compute theoretical seismograms for multi-layer absorptive subsurface along with assessing contribution made by elementary boundaries and sequences was tested on real-subsurface models. The data characterizing the mechanism that forms interference wavefield are analyzed. The elementary boundary (elementary layer) concept is considered that characterizes maximum possible degree of detail of the real subsurface and the subsurface thin-layered structure within the step function class and time sampling interval of seismic record. Disregard of contribution made by elementary boundaries and sequences would make it highly difficult to sort out wavefield structure elements and informativity of acoustic heterogeneities associated with those elements. Seismic absorption may substantially alter the wavefield interference pattern as compared with a wavefield corresponding to a non-absorptive-subsurface model.

In this article we study impact of amplitude nonidentity of pulses in single-polarity linearly varying period sequences and linearly varying period and additional bi-polarity sequences used for code pulse seismic accumulation of vibrations on dynamics of corresponding functions correlation and on interference immunity of code pulse method. It is shown that dynamics of correlation functions is practically not distorted for the most efficient code pulse linearity varying period signals (Fmax ≈ 80 Hz) in the case of double decreasing the pulse amplitude in the scanning frequency range of 10–80 Hz, and for efficient signals of linearly varying period sequences (Fmax = 30–40 Hz) in the case of double decreasing the pulse amplitude in the scanning frequency range of 10–30 Hz. Along with it, it should be noted that to preserve necessary interference immunity of code pulse method in relation to noncoherent interference the pulse amplitude in efficient scanning of linearly varying period shall not alter by more than ≈ 1,4 times within the scanning frequency band of 10–80 Hz and in efficient scanning of linearly varying period sequences by more than ≈ 1,4 times within the frequency interval range of 10–30 Hz. It is also shown that there is possibility to preserve qualitative dynamics of correlation functions for additional bi-polarity linearly varying period sequences when impulses with positive and negative polarity are processed separately in the case of decreasing the pulse amplitude in the scanning fre- quency band of 10–30 Hz by not more than 1,15–1,2 times.