SYSOEV A.P. , SELEZNEV V.S. , SOLOVIEV V.M., BRYKSIN A.A. , LISEIKIN A.V.
JSC “Sibneftegeofizika”, Russia, Geophysical Survey SB RAS , Russia

The Geophysical Survey of the Siberian Branch of the Russian Academy of Science fulfills for the last few years a number of seismoprospecting profiles on water areas of the rivers Ob, Vakh, Yenisei, Lena, etc. by the general extent more than 2500 running km. [Seleznev and other, 2004, 2006].

Together with JSC “Sibneftegeofizika” the technique of data processing of the river seismic prospecting recordings was created, which basic specificity consists in the following. First, in connection with a difficult trajectory of a river channel and impossibility for technical reasons exact matching of lines of reception and excitation, the arrangement of median points of reflexion on a observing plane represents chaotic set with a considerable deviation from a line of observation.

Secondly, at immersing of the seisphone cable assembly on a bottom displacement of its position by a watercourse is possible. Thirdly, initial seismograms in some cases possess a low ratio of signal/noise because of the hindrances connected with work of vessels, industrial hindrances at passage of sites of large cities, wave processes in a water layer and pulse hindrances from discharges of atmospheric electricity. All listed features do not allow to apply standard procedures of processing by CDP method of river seismic prospecting recordings and in this connection, their adaptation was required.

Procedure of processing the river seismoprospecting data on a way from reception initial seismogram to construction the final time section consists of following stages: the description of observing system; specification of coordinates of points of reception; a correcting filtration; correction of static and kinematic amendments; suppression of hindrances and subtraction of multiple waves; migratory transformations before stacking; definitive stacking and time section processing.

The description of observing system is carried out with use of interactive system PreProc (working out by JSC “Sibneftegeofizika”). In the given system the description of arrangements, input of topographical data, the interactive control of observing system, setting of scatter line and the export of seismograms in SEG-Y format and SPS files are carried out. The control of observing system is carried out by automatic calculation theoretical hodograph curve of the first breakings (taking into account geometry of arrangement and aprioristic static amendments) and its superposition on real seismograms in the course of their viewing on the screen. In places of abnormal discrepancy of theoretical and real hodograph curves the additional analysis of all initial data was carried out for the purpose of detection of errors in the description of position of points of geophysical observation. The choice of the scatter line is carried out on area of dispersion of midpoints.

Along this line the forming of supergather CDP-seismograms is carried out. The traces will belong to the same CDP-ensemble if they are located in one platform (bin) with the characteristic sizes of an order 25*500 meters. The choice of so big cross-section size of a bin is caused by necessity of preservation traces with the big deviation from a profile line in break points, and preservation traces with the big offsets for effective suppression of multiple waves. CSP-sorted gathers in SEG-Y format with the filled headers and standard SPS-files with the full description of observing system are exported as a result. This material registers for storage and use in processing.

In field environment it is technologically difficult precisely estimate the displacement of coordinates of bottom seisphones. Correction of coordinates is carried out in appendix SDITR of GeovecteurPlus complex using the first breaks of the refracted waves. The technique of specification of position of points of reception is based on definition of distance from a wave transit time. It is widely applied in sea seismicity and in seismology at definition of epicentres of earthquakes. Using the direct wave which speed in a water layer is known and practically constant, this method gives very high accuracy.

However, at small depths, the direct wave goes out in the first breaks only on nearby offsets. Use of the given technique for the refracted wave is probable at following assumptions about model of environment: on a site, within the used offsets, the refracting border is horizontal and speed along the border is constant. Besides, at the analysis of results following rules are accepted: the distance between receivers in arrangement cannot exceed the set step between channels; displacement of all arrangement or its part along the course of a vessel is more probable due to inertial movement of seisphone cable assembly.

The initial stage of data processing for the subsequent work consists of a combination of following procedures: assignment of the specified geometry, binning; correction of amplitudes for attenuation; superficial-coordinated minimum-phase deconvolution; a strip filtration (6-110 Hz); superficial-coordinated normalization of amplitudes; reduction of high-amplitude hindrances. Before the further processing assignment of geometry taking into account the specified positions of points of reception is carried out and new alignment of scatter line is defined. Correction of amplitudes for attenuation as a rule is carried out under the law of a geometrical divergence with selection of the scaling factor. The basic procedure of a stage of initial processing is deconvolution, considering the non-identity of superficial conditions. In used algorithm each trace is expressed in the form of a filter combination general for all traces and filters which characterize a source and the receiver. For alignment of amplitudes taking into account superficial conditions the programmed superficial-coordinated normalization is carried out. Software routine REDSP intended for frequency-dependent editing of seismic data is applied to suppress the high-amplitude hindrances.

Correction of static and kinematic amendments is carried out by means of FAKOR3D software routine in which the method of calculation of correcting static amendments to frameworks of 3 factorial models is realised. In the course of division of factors harmonization of tgace shifts is carried out, that essentially reduces probability “transition from a phase on a phase”. The program allows to carry out the set number of cycles of correction of static and kinematic amendments for one miscalculation. For each cycle parametres of work of the program are set: correlation modes, the maximum shifts, modes of correction of kinematics (in a window or in all time interval).

The stage of suppression of hindrances on seismograms, includes following procedures: suppression of irregular hindrances in CSP gathers; suppression of medium-rate hindrances in CRP gathers; subtraction of multiple waves; correction of residual kinematic amendments.

Migratory transformations before stacking are done for the purpose taking into account a seismic pulling down and increase spatial resolution. The following sequence of procedures is for this purpose carried out: suppression of irregular hindrances in CDP gathers; correction of speed for an inclination borders (DMO-transformation); correction of residual kinematic amendments; correction of variables on time of residual phase shifts; suppression of irregular hindrances in time sections with equal offsets; migration of gathers in FX-area.

The problem solved at a stage of final stacking and processing of a time section – is to increase spatial and time resolution. The following sequence of procedures is for this purpose carried out: definitive stacking with correction of residual phase shifts; a zero-phase deconvolution; suppression of casual hindrances in frequency-spatial area; harmonization of a peak spectrum; dependent on an inclination corner median filtering; definitive by-trace normalization; strip filtration.

Literature
Seleznev V. S, Soloviev V.M., Sysoev A.P., Babushkin S.M., Emanov A.F., Bryksin A.A. Seismic researches on water areas: problems, results, prospects. Ways of realisation of oil and gas potential of Khanty-Mansiysk autonomous region: Materials of the seventh scientifically-practical conference. Volume 1, Khanty-Mansiysk, The publishing house “IzdatNaukaServis”, 2004, p.204-212.
Seleznev V. S, Soloviev V.M., Sysoev A.P., Babushkin S.M.,Bryksin A.A. Detailed seismic researches on water areas and in transit (water-land) zones. Ways of increase of efficiency of geologo-prospecting works on oil and gas in the Eastern Siberia and Republic Sakha (Yakutia): Materials of the all-Russia conference, SSRIGGMP, Novosibirsk, 2006, p.212-215.

Oil and gas of Arctic shelf 2008