Research On Technology Of Processing And Interpretation Of 2-D Digital Three-component Seismic Data

IntroductionPetroleum exploration requires more and more imaging quality and resolution ofseismic data and relative lithology information with the development of oil-gasprospecting difficulty. However, the complexity of underground media compels usto invent new technology to identify, so the excellences of three-component seismicexploration (TCSE) such as researching cracks, anisotropy, prediction of reservoirsand liquids and so on, are unveiled gradually. TCSE involved in this paper uses P-wave source to vibrate and three-componentgeophone to receive seismic data. It is well known that incident P-wave is createdwhen P-wave source vibrates, and P-wave is reflected and converted S-wave iscreated at the reflected interfaces. And reflected P-SV and P-SH wave are createdwhen the underground media is asymmetric or anisotropic. So not only theunderground geologic structures but also strata lithology, cracks and bearing oil-gasare analyzed using TCSE.Three-component seismic data acquisition To know the performance of the digital three-component sensors and itsapplication in the interbeded geology condition of the northern Songliao Basin, the2D trial of the three-component seismic acquisition was carried out in Daqing OilField using SERCEL's DSU3 digital sensors in 2004. And the bestthree-component seismic data of the civil terrene sediment was collected at present.There were 6 2D lines in this trial whose total length of full coverage was 89.62km.There are many villages and roads in the trial project. Whose surface arefarmland and grassland and the northern project is the oil-extraction area.Exploding lithology are sand and mud. The advantage conditions in the trial projectare that the surface is flat and the lithology of the surface layer is even layout, andstrata obliquity is low and makeup is simple. And there are many bearing-oil stratasuch as Putaohua, Fuyang, Denglouku, Yingcheng and so on, which can verify theTCSE's application effects. The disadvantage conditions are that the volcanicreservoirs are deep and uneven evidently, and the trial project lies in theoil-extraction area and the background noise is strong.Before the field operation design, the scheme must be reasoned in the theoryfirstly. The geological modeling should be designed based of P-wave data andlogging data in the trial zone, and the multi-component seismic modeling should becarried out. And the joint observation system of P and PS wave should be designedon the basis of the difference of P-P and converted wave mechanism. Referred tothe depth of target horizons, the relationship of P-and P-S reflective coefficientsand array length should be calculated in the theory to obtain the most offset.Several observation systems and operation parameters were designed to carry outthe wild trial in order to define the best observation systems based of above.Converted wave processingThe difference between converted wave and P-P wave is the asymmetry ofconverted-wave transmission trace. So the processing modules of P-P wave is notfit for converted-wave processing, including P-P CMP gathers, velocity analysis,NMO, migration, etc. The flow of converted-wave processing includes data input,preprocessing, common converted-point (CCP) gathers, pre-stacked de-convolution,stacked velocity analysis, NMO, CCP stacking, post-stacked de-convolution,migration, etc.That the converted wave processing differs primarily from P-P wave consists ofthe negative-offset reversion of horizontal component, CCP stacking,converted-wave velocity analysis and NMO.1. The negative-offset reversion of horizontal component. It is known that thereare positive and negative offsets in the horizontal components when the source liesin the middle of geophones array. Whose data should be processed respectivelyaccording to positive and negative direction, or the records of the certain directionare reversed.2. CCP stacking. It is known that the converted-wave is P-wave incidence andS-wave reflection, so CMP stacking is not adopted. The converted points are nearreceived points from CMP points if the reflective interface is horizontal. Owing tothe offset (x) coming up to the sum of the horizontal distance (xp) between theconverted points and the source and that (xs) between the converted points and thereceived point, and that is 2 h =x=xp +xs. The overlap equation is derived( )++???? ( ?)????=+?????????22221111gzxggzxxppp(1)Where g = vs vpfor PS wave, z is the depth which can be estimated by thereflective time t.3. Converted-wave velocity. Non-hyperbolic velocity analysis is often used inthe converted-wave velocity analysis. That is to say, converted-wave time-distancecurve of CCP gathers in the symmetrical media can be written as:2202210 11tps = ???? +t????+????vxpp????+???? +t????+????vxss????pspsγγγ (2)Where γ =v p /vs, which is the ratio of P-P and P-S waves;and are thehorizontal distance between the converted points and the sources and receivedpoints respectively. In fact, this method views the strata above the target horizon asthe symmetrical media that can be described by P and S velocity which are thestacked velocities. The material realization flow is: at first, P velocity can beobtained by P wave velocity analysis;given P velocity, the scan of the velocityratio is carried out by the equation (2) to get the spectrum of the velocity ratio;thex pxsvelocity ratio can be obtained by interpreting whose spectrum, and at last, Svelocity can be calculated.4. Converted-wave NMO. The relationship of the travel time and offset of theconverted wave is no hyperbola thanks to the asymmetry of ray trace, so thenon-hyperbolic equation of time and distance is introduced generally to improvethe precision of NMO and stacking. That isPSpspsPStvtxvxttttssppps 0220220 101Δ =?=???? +1 γ ????+????????+???? +γγ????+????????? (3)Data interpretationTo know about the TCSE's resolving power of the lithology in Xingcheng areafrom the rock physics, which includes mudstone, conglomerate, sandstone, tuff andlava. We chose Yingcheng Group in XS1 well and Denglouku and YingchengzuGroups in SS1 well to analyze. For XS1 well, its dominating reservoirs aresandstones and volcanic rocks, and the discrimination among lava and mudstoneand conglomerate is very large from crossing graph of P and S velocities. However,there are many complex components in the tuff that changed largely in the crossinggraph of P and S velocities, sometimes interlaced with the conglomerate not todistinguish from the conglomerate and tuff. For SS1 well, the discriminationamong mudstone and sandstone and basalt is evident from crossing graph of P andS velocities in the second and third segment of Denglouku Group and Jurassic.This interpretation workflow of TCSE is as follows. At first, the syntheticrecords should be created. It is well known that the converted wave is not createdwhen the incidence angle of P wave is 900, so synthetic records of the convertedwave can be obtained after the records of the different offsets that are made usinglogging data of S wave are processed by NMO and then stacking. Then is thehorizontal calibration. The controlled horizons of P and PS sections are confirmedbased of the synthetic records calibration. At last, the attributes sections includingthe amplitude ratios and Poisson's ratio of P and S waves are calculated.The horizontal calibration is the first important problem of the jointinterpretation of P and S waves and the extract and application of lithologyinterpretation, and which is the most significant step whether the TCSE can acquirethe geological effects and the economic benefits or not. Comparing with theseismic data interpretation of P wave, the interpretation of multi-component data isfirst the waveform identification and the horizontal calibration. The waveformidentification is carried out by the polarization, velocity propagation rule,frequency spectrum, amplitude contrasts, offset and anisotropy of P and PS waves.The laws of the horizontal calibration are: the invariability of structureinterpretation and lithology characteristics, the consistency of strata thickness andembedding depth, the comparability of stratigraphic succession and seismic facies.The unconspicuous corresponding relationship of P and PS sections can beobtained by compressed calibration. Owing to the drilling data of XS1 and XS6wells, the first segment in Yingcheng Group is volcanic reservoirs and the fourth issandstone and conglomerate reservoirs, and both are dominant gas-bearing strata inXushen Gas Field.From the amplitude ratio section of P and PS wave, the fourth and first segmentsin Yingcheng Group are expressed locally as yellow or low value, and we knowthat the fourth segment is gas-bearing sandstones and conglomerates and the firstsegment is gas-bearing volcanic rocks from the drilling data of XS6 and XS1 wells.So the corresponding relationship between the result obtained by amplitude ratiosection and the actual wells is very good.Contrast between the digital sensors and simulative geophonesBased on the knowledge of the digital sensors' capability, the single shot recordsand the frequency spectrum and the stacked section (resolution and recognizinglittle faults, etc.) are analyzed detailedly for the digital sensors and the geophones.Summarizations are followed.The digital sensors resist 50Hz disturbing waves. Forresolution, the frequency of the digital sensors is more than that of geophones at themiddle and shallow strata, and they are similar at deep strata. For S/N ratio, thedigital sensors are less than the geophones. After all, there is only one digital sensorat every trace. For the width of frequency, the digital sensors are better than thegeophones.ConclusionsThe TCSE technology is mastered basically through the acquisition, processingand interpretation of multi-component seismic data. However, there is a long waybefore us. For example, the precision including S-wave static correction, CCP,velocity analysis and so on, is very low, and how to recognize and apply Ycomponent data. Still we have progressed in TCSE technology. With the gradualperfect of TCSE technology, we believe that TCSE technology will play animportant role in the re-exploitation of Oil Field.