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Application of amplitude ratio profiles extracted with prestack Kirchhoff integral migration

Application of amplitude ratio profiles extracted with prestack Kirchhoff integral migration 2007 VolA No.1 Petroleum Science Application of Amplitude Ratio Profiles Extracted with Prestack Kirchhoff Integral Migration Wu Zhiqin', Wang Shangxu', Zhang Shaohong', Wang Yurr' and Wang Pengcheng' (1. School a/Resources and Information, China University a/Petroleum, Beijing 102249, China) (2. Institute a/Geology and Geophysics, Chinese Academy a/Sciences, Beijing 100029, China) Abstract: The Z component and X component profiles of seismic waves extracted with the prestack Kirchhoff integral migration could approximate to the primary wave (P wave) and converted shear wave (PS wave) profiles under certain conditions. The relative change of their reflection amplitude reflects the formation stress anomaly and subsurface media anisotropy. The principle and method for extracting amplitude ratios were studied and the application of amplitude ratio profiles was also examined when processing and interpreting actual seismic data. The amplitude ratio profile is an effective supplementary means of identifying the stratigraphic boundary and lithology. Key words: Prestack Kirchhoff integral migration, amplitude ratio profile, converted shear wave, primary wave, lithology wave and S wave velocity data simultaneously or 1. Introduction optionally (Li and Luo, 1998). It also can be used to pick up PS wave profiles from conventional seismic profiles The amplitude ratio profile of the P and PS waves and calculate the amplitude ratio profile of the P and PS reflects directly the phase relation and the relationship waves (Huang, et al., 1997). of amplitude between the P and PS waves, and is In this paper, actual 2-D multi-component seismic data closely related to the formation stress anomaly, were processed with the Kirchhoff prestack elastic wave subsurface media anisotropy and the presence of qil/gas depth migration to obtain depth profiles of vertical and in the formation. This reflects the physical parameters horizontal components of displacements. In order to study of subsurface media (Thornesen, 1999). Amplitude ratio the horizontal change of lithology of the target formation profiles play an important role in revealing the pattern or to study the pattern of faults, the amplitude ratio profile of faults, providing lithologic information and of the P and converted PS waves were calculated, and the interpreting geologic structures and phenomena in variation of amplitude ratio profiles and the corresponding seismic exploration. geologic implications were analyzed. The interpretation At present, prestack migration processing includes the results of actual seismic data of the central block in the Kirchhoff integral method, (single pass) wave equation Jungar Basin indicated that amplitude ratio profile could finite difference method, phase-shift method and reverse­ be an effective supplementary means for the structure and time method. All of these methods use downward lithology interpretation. continuation to realize the backward- tracing of seismic waves (Zhang, et al., 2000). Actual seismic data involve 2. Basic principles for calculating amplitude both reflected waves and diffracted waves. Indeed the data ratios with Kirchhoff integral migration recorded might primarily be diffracted waves when the structures are complex. For complex structures, especially Suppose that the ground is free and the stress is zero, with steep slopes, the diffraction stack Kirchhoff migration the time-domain displacement of a point in semi-infinite has a big advantage in obtaining seismic profiles (Bevc, space underground can be formulated with the 1997). The Kirchhoff integral solution of vector wave Kirchhoff integral equation (Qin, et al., 1988): equations, which is deduced from the Kirchhoff integral, is a simplified method for realizing diffraction stack 1f+OOf lox migration (Sheriff and Geldart, 1999). The method can be Um(x,t)=- [(-(1-2v-)oI3- + 4Jr -00 rV p r used to deal with the prestack or poststack migration of two- or three- dimensional seismic data (Chen, et al., x x x r I x ' 2v --!!!-_s_I )u (x, t + -V) + -V 0ml - + (1) 2001), and to regress both reflected waves and diffracted r r ' p r s r waves. The Kirchhoff integral migration can be used to I) 3X s: XI 2 -, X ( r )]d 'd ' v -- --- U x t + - x X process three-component, two-component, even single­ m3 r r r 2 I' V 1 2 component data (Wang, et al., 1999), and to deal with P VolA No.1 37 where I = 1, 2, 3 and m = 1, 2, 3 represent coordinates concentrated in the amplitude ratio profile or the program would overflow its memory. For this reason a of X, Y and Z directions respectively; v = V / V , x' threshold value was introduced. Considering that amplitude values of the P and PS waves at the same and x are coordinates of the actual measuring point interface were both relatively strong, so the absolute 'and the evaluated point of migration inversion; u(x', t) value of the product of the P wave amplitude multiplied is the ground displacement velocity vector; U(x', t) is by the PS wave amplitude was taken as a criterion, the displacement of x' point; r is the distance between namely threshold value. When the threshold value exceeded a specific lower limit, the amplitude ratio the evaluated point of migration inversion and the actual could be calculated with Eq. (4). Otherwise, let the ratio measuring point; Vp and V are the velocities of the P be zero. The reason to take the product of both 'as a and PS waves respectively. threshold value is that it not only reflects the influence Particle displacements U in the vertical direction of the P and S waves synthetically and the mutual and U in the horizontal direction can be deduced from relationship of amplitude between the P and S waves, Eq. (1) based on the measured data of the P wave and but also it is convenient for processing. In order to the PS wave at the surface. Because determine a reasonable lower limit of the threshold value, the concept of a depth window was introduced in U p = u, cosa -U x sin a (2) this study. As the depth increases, the seismic signal is Us = -U sin a +U xcosa attenuated so that it is impossible to deal with the entire seismic profile with a fixed parameter. Moreover it is When the offset distance is small, Eq. (2) can be necessary to process the profile in subsections each simplified to: divided by the appreciate depth window. The so-called depth window means a depth range in which the seismic (3) profile is observed and processed with an identical scale. Within a depth window, statistics on amplitude values of the P and converted S waves was made. Then the Then a method for determining the amplitude ratio lower limit of the threshold value was determined can be obtained from particle displacements in the according to a specific criterion. So the lower limit was vertical and horizontal directions (Huang, et al., 1997): actually a combination of the unified specific criterion (4) and the dynamic variation absolute value in various depth windows. Therefore, after obtaining the depth profile of the P The method was realized on a SUN workstation. The and PS waves with the Kirchhoff integral migration, the amplitude ratio profile was extracted trace by trace, amplitude ratio profile of the P and PS waves could be with a given threshold value and a depth window of a computed from Eq. (4) to reflect the relative changes of specific width. The sampling points in each trace were reflection amplitude of P wave and PS wave. treated within subsections that were divided according to the depth window. This was equivalent to a type of 3. Extraction of amplitude ratio profile and balance processing of the amplitude of entire profile. geologic implication No more complex processing was done in order to reflect the primary meanings of the amplitude ratio 3.1 A method for extracting amplitude ratio profile clearly. Firstly the displacements in the vertical and Eq. (4) indicates that the amplitude ratio of the P horizontal directions were calculated; secondly the wave to the PS wave approximates to the reflection clearly outlying data were eliminated according to the amplitude ratio of the Z component to the X component. threshold value, and finally the amplitude ratios of However, it is impracticable to divide the P wave depth related positions were obtained. profile by the PS wave depth profile, which were acquired from the Kirchhoff integral migration, at 3.2 Parameters influencing the amplitude ratio corresponding positions. To avoid indeterminate forms profile 0/0, calculation of a simple amplitude ratio of The parameters influencing the amplitude ratio displacements in the vertical and horizontal directions profile include the depth window and the threshold should be avoided when computing amplitude ratio value. The greater the depth window, the more obvious profiles. Otherwise random noise disturbance would be 38 2007 Petroleum Science the smoothing effect in the vertical direction and the structures with the same velocity change in the vertical better the continuity of the profile; but there is and horizontal directions. This illustrates that the suppression of local anomalies. The smaller the depth interface could be a large stratigraphic interface but not window, the poorer the continuity of the profile, but the a lithologic interface, such as a large unconformable local anomalies are displayed more clearly. The "white" surface or a surface in which the sedimentary sequence phenomenon (the amplitude ratio is zero) of profiles changed remarkably. Generally, there was no reflects a big difference in the amplitude between the P hydrocarbon contained in such abnormal zones. and PS waves, indicating that at least one amplitude The negative anomaly in the amplitude ratio profile value of the P or PS waves approximates zero. indicated that the reflections of the P and PS waves were Generally it is the PS wave because the reflection in opposite phases or phase shift. The phenomenon amplitude of the P wave is always greater than that of appeared generally in the abnormal zone of horizontal the PS wave. The greater the threshold value, the more shear stress, because the shear stress had no influence on the propagation "white" phenomena occur, so the better the optical of the P wave, but it could speed up or effect of the profile becomes because the local noise can slow down the velocity of the PS wave, thus resulting in be suppressed. However it is unfavorable to the display a phase shift of the PS wave to the P wave. Generally, of local anomalies. If the threshold value becomes abnormal zones of shear stress corresponded to fault­ smaller, the "white" phenomena are reduced, and the developed zones or phase change zones of lithology. overall quality of the profile is improved. But if the 4. Case study threshold value is too small, the local noise increases as The block studied in this paper is located in the a result, which is unfavorable to the improvement of the interior of the Jungar Basin. The result of drilling profile quality. The depth window and threshold value verified that the major target formations in this block must be determined on the basis of integrated are the Jurassic system and Cretaceous system, so we consideration of practical situations, and must be studied mainly the formation, structure and adjusted according to the processing results and lithologicfeatures of Badaowan, Sangonghe, Xishanyao practical needs. and Toutunhe groups in the Jurassic and the bottom of 3.3 Geologic implication of the amplitude ratio the Cretaceous. The Prestack Kirchhoff elastic wave depth migration profile was used to process the P and PS wave velocities Possible geologic explanations for "white" simultaneously. The initial velocity model of the P wave phenomena in the amplitude ratio profile are that: 1) The was developed as a regularized grid velocity with lithology of sedimentary rocks exhibited obvious rhythm interpolation of the stacking velocity spectrum in a or grain-size changes in the transverse direction, i.e. the conventional processing. And the initial velocity model difference in the horizontal velocity was relatively great. of the S wave was built by the theoretical conversion However, the vertical deposition was stable, and then the from a conventional stacking velocity spectrum of the P difference in the vertical velocity was relatively small. 2) wave. After having the depth migration profiles of the Because of high rock porosity and fluids in rock pores, vertical and horizontal displacements, the amplitude the velocity of the P wave decreased; on the other hand, ratio profiles were obtained. What we used was seismic the velocity of the PS wave changed slightly, and the data of full waves, including all kinds of seismic waves. reflection amplitude value of the PS wave approximated The results of migration and amplitude ratio profiles zero. This "white" phenomenon was caused by fluids in indicated that the images of interfaces were displayed rocks, not by the change of lithology. The second case and restored very well. could be interpreted as the oil-bearing formation. In the process of extracting amplitude ratios, the depth However, the presence of noise in the profile can also window and the threshold value were determined according generate "white" phenomena. It is essential to to experiments and demands of geologic interpretations. synthetically analyze seismic and geologic data and Experiments were conducted with depth window widths of compare the data with the surrounding media when the 1,600 m (80 dots) and 800 m (40 dots), and threshold values amplitude ratio profile was used to interpret the presence of 1110, 1120, 1150, 11100, respectively. of oil/gas in the formation. Fig. 1 shows a great number of local anomalies in The stratum in which the P and PS waves behaved the left image, but it was not as clear as the right image conformably displayed a positive anomaly in the in reflecting the main bed boundaries. amplitude ratio profile, indicating the geologic VolA No.1 Application of Amplitude Ratio Profiles Extracted with Prestack Kirchhoff Integral Migration 39 COP COP 22644 22944 23244 23544 23844 22644 22944 23244 23544 23844 ! ! 4.50 ...~ '< 4 -. 'c-, '7'r·.;·.;.--:-:'· . ':" .. ,:;', :', ;.; .',' ~::,'.,~':.;.i:~:;'ii "':''''~ 5,50 ;..... ' , ' " ,.~; , :' ,;." :,:::;;''::~', :-;.~.;:;.:~..~~, :.,";. t'. ,AI'';~-;''7. 6,50 '",::;' ,,' ,- ,-;..;,~:;:;;..:"fl~::'·'· c) i:= 850 ',' ••~-'l'".,.,., .~".,"" ",,: ..;.;.' r:.)':~.>". ;~~\~':~"',,,~...~~,,:~,,~.~;<:.~.:,', /.: j 9.50 ! ...;.:.. ~ ~ ~ . -: '~,;'.'~::~.'."".....;~.:,,:.:.~ ~:, • v··;. ".~:,: '.~~. . • ~ .......... ,,~ Fig. I A comparison of processing effect (Left: Depth window width 800 m, and threshold value 1/100; Right: Depth window width 1,600 m, and threshold value 1/20) Intersections existed among velocities of the P the time section, it could be interpreted as sandstones. wave propagated in different rocks; as a result multiple However, subsequently deposition was sandstone and solutions would occur if the P wave data alone were mudstone cycle, accompanied with a large-scale used to predict lithology. Synthetically analyzing the P lenticular sand body. wave and PS wave data to interpret lithology could COP reduce the uncertainty of lithology forecast. The amplitude ratio profile provided a supplementary tool for the determination of lithology, thus enhancing the accuracy in lithology interpretations. Moreover, the geometric contact relations among strata could be :~ 6.5 more clearly interpreted by using the amplitude ratio i:= profile. The sedimentary morphology could be seen clearly in Fig. 2 which showed the J2x and J1b groups, with fine stratified sediments, mainly mudstones and shales (including coal seam). This was consistent with the two sets of coal measures in this region. In the J2t and Fig. 2 Stratum geometric contact relation and lithology of J1s groups clastic rock deposits were mainly 531 line amplitude ratio profile developed on the intermediate section of the profile, (Depth window width 800 m, threshold value 1/100) with a rather large grain size. There was a conversion of positive and negative anomalies in the horizontal 5. Conclusion amplitude ratios, which indicated phase shifts. On the The elastic wave Kirchhoff integral migration could intermediate section of the profile, the bottom of the make full use of the information of the P and PS waves Kl group exhibited obvious downward denudation, so as to overcome shortages resulting from the and the amplitude ratio profile behaved as negative migration just using the P wave (acoustic migration). anomaly that corresponded to the phase change zone The amplitude ratio profile was obtained by using the P of lithology. This indicated that a set of large clastic and PS wave profiles. Using the absolute value of the facies fans was originally deposited. Combined with 40 2007 Petroleum Science Qin F. H., Guo Y. X. and Wang M. Y. (1988) Kirchhoff elastic product of the P wave amplitude multiplied by the PS wave migration method. Chinese Journal of Geophysics, wave amplitude as a threshold value avoided the 31(9),577-578 (in Chinese) problem caused by the indeterminate 0/0. The amplitude Sheriff R. E. and Geldart L. P. (1999) Exploration Seismology ratio profile could eliminate random disturbance and (2nd edition). Cambridge Univ, Press highlight primary aspects. It not only reconstructed Thomesen L. (1999) Converted-wave reflection seismology over underground structures clearly and improved spatial inhomogeneous, anisotropic media. Geophysics, 64, 678-690 resolution, but also reflected the properties of Wang J. L., Li Y. M. Leng C. B., et al. (1999) Methodological underground rocks, such as lithology and physical study of two-dimensional prestack depth migration and properties. The amplitude ratio profile could provide a auxiliary treatment scheme. Oil Geophysical Prospecting, supplementary means for seismic interpretation. 34(supplement), 1-12 (in Chinese) The horizontal shifts of imaging points will be Zhang P., Li Y. M. and Liu H. (2000) The situation of several caused by big horizontal velocity variety with the Eq. prestack depth migration methods. Progress in Geophysics, 15(2),30-39 (in Chinese) (l). Consequently, fault position will be changed somehow. Solving this problem will be our object of About the first author further studying. Wu Zhiqin was born in 1973 and References received his Ph.D degree from China Bevc D. (1997) Imaging complex structures with semi-recursive Universityof Mining and Technology Kirchhoffmigration. Geophysics, 62, 577-588 (Beijing) in 2005. He is undertaking Chen B. S., Li S. K. and Zhang L. H. (2001) Application of post-doctoral research in the School seismic prestack migration techniques. China Offshore Oil of Resources and Information, China and Gas (Geology), 15(5),361-364 (in Chinese) University of Petroleum (Beijing), Huang K., Yang X. M., Xu Q. Z., et al. (1997) The extraction of with his interests in earth detecting P-wave and S-wave for hydrocarbon detection using prestack and information technology, E-mail: Kirchhoff integral migration. Geophysical Prospecting for underskyblue@gmail.com Petroleum, 36(: ', 1-6 (in Chinese) Li L. M. and Luo S. X. (1998) A method for establishing P-P and (Received March 27,2006) P-SV wave prestack depth migration velocity model. Chinese (Edited by Sun Yanhua) Journal of Geophysics, 41(6), 305-318 (in Chinese) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Application of amplitude ratio profiles extracted with prestack Kirchhoff integral migration

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Publisher
Springer Journals
Copyright
Copyright © 2007 by China University of Petroleum
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
ISSN
1672-5107
eISSN
1995-8226
DOI
10.1007/BF03186571
Publisher site
See Article on Publisher Site

Abstract

2007 VolA No.1 Petroleum Science Application of Amplitude Ratio Profiles Extracted with Prestack Kirchhoff Integral Migration Wu Zhiqin', Wang Shangxu', Zhang Shaohong', Wang Yurr' and Wang Pengcheng' (1. School a/Resources and Information, China University a/Petroleum, Beijing 102249, China) (2. Institute a/Geology and Geophysics, Chinese Academy a/Sciences, Beijing 100029, China) Abstract: The Z component and X component profiles of seismic waves extracted with the prestack Kirchhoff integral migration could approximate to the primary wave (P wave) and converted shear wave (PS wave) profiles under certain conditions. The relative change of their reflection amplitude reflects the formation stress anomaly and subsurface media anisotropy. The principle and method for extracting amplitude ratios were studied and the application of amplitude ratio profiles was also examined when processing and interpreting actual seismic data. The amplitude ratio profile is an effective supplementary means of identifying the stratigraphic boundary and lithology. Key words: Prestack Kirchhoff integral migration, amplitude ratio profile, converted shear wave, primary wave, lithology wave and S wave velocity data simultaneously or 1. Introduction optionally (Li and Luo, 1998). It also can be used to pick up PS wave profiles from conventional seismic profiles The amplitude ratio profile of the P and PS waves and calculate the amplitude ratio profile of the P and PS reflects directly the phase relation and the relationship waves (Huang, et al., 1997). of amplitude between the P and PS waves, and is In this paper, actual 2-D multi-component seismic data closely related to the formation stress anomaly, were processed with the Kirchhoff prestack elastic wave subsurface media anisotropy and the presence of qil/gas depth migration to obtain depth profiles of vertical and in the formation. This reflects the physical parameters horizontal components of displacements. In order to study of subsurface media (Thornesen, 1999). Amplitude ratio the horizontal change of lithology of the target formation profiles play an important role in revealing the pattern or to study the pattern of faults, the amplitude ratio profile of faults, providing lithologic information and of the P and converted PS waves were calculated, and the interpreting geologic structures and phenomena in variation of amplitude ratio profiles and the corresponding seismic exploration. geologic implications were analyzed. The interpretation At present, prestack migration processing includes the results of actual seismic data of the central block in the Kirchhoff integral method, (single pass) wave equation Jungar Basin indicated that amplitude ratio profile could finite difference method, phase-shift method and reverse­ be an effective supplementary means for the structure and time method. All of these methods use downward lithology interpretation. continuation to realize the backward- tracing of seismic waves (Zhang, et al., 2000). Actual seismic data involve 2. Basic principles for calculating amplitude both reflected waves and diffracted waves. Indeed the data ratios with Kirchhoff integral migration recorded might primarily be diffracted waves when the structures are complex. For complex structures, especially Suppose that the ground is free and the stress is zero, with steep slopes, the diffraction stack Kirchhoff migration the time-domain displacement of a point in semi-infinite has a big advantage in obtaining seismic profiles (Bevc, space underground can be formulated with the 1997). The Kirchhoff integral solution of vector wave Kirchhoff integral equation (Qin, et al., 1988): equations, which is deduced from the Kirchhoff integral, is a simplified method for realizing diffraction stack 1f+OOf lox migration (Sheriff and Geldart, 1999). The method can be Um(x,t)=- [(-(1-2v-)oI3- + 4Jr -00 rV p r used to deal with the prestack or poststack migration of two- or three- dimensional seismic data (Chen, et al., x x x r I x ' 2v --!!!-_s_I )u (x, t + -V) + -V 0ml - + (1) 2001), and to regress both reflected waves and diffracted r r ' p r s r waves. The Kirchhoff integral migration can be used to I) 3X s: XI 2 -, X ( r )]d 'd ' v -- --- U x t + - x X process three-component, two-component, even single­ m3 r r r 2 I' V 1 2 component data (Wang, et al., 1999), and to deal with P VolA No.1 37 where I = 1, 2, 3 and m = 1, 2, 3 represent coordinates concentrated in the amplitude ratio profile or the program would overflow its memory. For this reason a of X, Y and Z directions respectively; v = V / V , x' threshold value was introduced. Considering that amplitude values of the P and PS waves at the same and x are coordinates of the actual measuring point interface were both relatively strong, so the absolute 'and the evaluated point of migration inversion; u(x', t) value of the product of the P wave amplitude multiplied is the ground displacement velocity vector; U(x', t) is by the PS wave amplitude was taken as a criterion, the displacement of x' point; r is the distance between namely threshold value. When the threshold value exceeded a specific lower limit, the amplitude ratio the evaluated point of migration inversion and the actual could be calculated with Eq. (4). Otherwise, let the ratio measuring point; Vp and V are the velocities of the P be zero. The reason to take the product of both 'as a and PS waves respectively. threshold value is that it not only reflects the influence Particle displacements U in the vertical direction of the P and S waves synthetically and the mutual and U in the horizontal direction can be deduced from relationship of amplitude between the P and S waves, Eq. (1) based on the measured data of the P wave and but also it is convenient for processing. In order to the PS wave at the surface. Because determine a reasonable lower limit of the threshold value, the concept of a depth window was introduced in U p = u, cosa -U x sin a (2) this study. As the depth increases, the seismic signal is Us = -U sin a +U xcosa attenuated so that it is impossible to deal with the entire seismic profile with a fixed parameter. Moreover it is When the offset distance is small, Eq. (2) can be necessary to process the profile in subsections each simplified to: divided by the appreciate depth window. The so-called depth window means a depth range in which the seismic (3) profile is observed and processed with an identical scale. Within a depth window, statistics on amplitude values of the P and converted S waves was made. Then the Then a method for determining the amplitude ratio lower limit of the threshold value was determined can be obtained from particle displacements in the according to a specific criterion. So the lower limit was vertical and horizontal directions (Huang, et al., 1997): actually a combination of the unified specific criterion (4) and the dynamic variation absolute value in various depth windows. Therefore, after obtaining the depth profile of the P The method was realized on a SUN workstation. The and PS waves with the Kirchhoff integral migration, the amplitude ratio profile was extracted trace by trace, amplitude ratio profile of the P and PS waves could be with a given threshold value and a depth window of a computed from Eq. (4) to reflect the relative changes of specific width. The sampling points in each trace were reflection amplitude of P wave and PS wave. treated within subsections that were divided according to the depth window. This was equivalent to a type of 3. Extraction of amplitude ratio profile and balance processing of the amplitude of entire profile. geologic implication No more complex processing was done in order to reflect the primary meanings of the amplitude ratio 3.1 A method for extracting amplitude ratio profile clearly. Firstly the displacements in the vertical and Eq. (4) indicates that the amplitude ratio of the P horizontal directions were calculated; secondly the wave to the PS wave approximates to the reflection clearly outlying data were eliminated according to the amplitude ratio of the Z component to the X component. threshold value, and finally the amplitude ratios of However, it is impracticable to divide the P wave depth related positions were obtained. profile by the PS wave depth profile, which were acquired from the Kirchhoff integral migration, at 3.2 Parameters influencing the amplitude ratio corresponding positions. To avoid indeterminate forms profile 0/0, calculation of a simple amplitude ratio of The parameters influencing the amplitude ratio displacements in the vertical and horizontal directions profile include the depth window and the threshold should be avoided when computing amplitude ratio value. The greater the depth window, the more obvious profiles. Otherwise random noise disturbance would be 38 2007 Petroleum Science the smoothing effect in the vertical direction and the structures with the same velocity change in the vertical better the continuity of the profile; but there is and horizontal directions. This illustrates that the suppression of local anomalies. The smaller the depth interface could be a large stratigraphic interface but not window, the poorer the continuity of the profile, but the a lithologic interface, such as a large unconformable local anomalies are displayed more clearly. The "white" surface or a surface in which the sedimentary sequence phenomenon (the amplitude ratio is zero) of profiles changed remarkably. Generally, there was no reflects a big difference in the amplitude between the P hydrocarbon contained in such abnormal zones. and PS waves, indicating that at least one amplitude The negative anomaly in the amplitude ratio profile value of the P or PS waves approximates zero. indicated that the reflections of the P and PS waves were Generally it is the PS wave because the reflection in opposite phases or phase shift. The phenomenon amplitude of the P wave is always greater than that of appeared generally in the abnormal zone of horizontal the PS wave. The greater the threshold value, the more shear stress, because the shear stress had no influence on the propagation "white" phenomena occur, so the better the optical of the P wave, but it could speed up or effect of the profile becomes because the local noise can slow down the velocity of the PS wave, thus resulting in be suppressed. However it is unfavorable to the display a phase shift of the PS wave to the P wave. Generally, of local anomalies. If the threshold value becomes abnormal zones of shear stress corresponded to fault­ smaller, the "white" phenomena are reduced, and the developed zones or phase change zones of lithology. overall quality of the profile is improved. But if the 4. Case study threshold value is too small, the local noise increases as The block studied in this paper is located in the a result, which is unfavorable to the improvement of the interior of the Jungar Basin. The result of drilling profile quality. The depth window and threshold value verified that the major target formations in this block must be determined on the basis of integrated are the Jurassic system and Cretaceous system, so we consideration of practical situations, and must be studied mainly the formation, structure and adjusted according to the processing results and lithologicfeatures of Badaowan, Sangonghe, Xishanyao practical needs. and Toutunhe groups in the Jurassic and the bottom of 3.3 Geologic implication of the amplitude ratio the Cretaceous. The Prestack Kirchhoff elastic wave depth migration profile was used to process the P and PS wave velocities Possible geologic explanations for "white" simultaneously. The initial velocity model of the P wave phenomena in the amplitude ratio profile are that: 1) The was developed as a regularized grid velocity with lithology of sedimentary rocks exhibited obvious rhythm interpolation of the stacking velocity spectrum in a or grain-size changes in the transverse direction, i.e. the conventional processing. And the initial velocity model difference in the horizontal velocity was relatively great. of the S wave was built by the theoretical conversion However, the vertical deposition was stable, and then the from a conventional stacking velocity spectrum of the P difference in the vertical velocity was relatively small. 2) wave. After having the depth migration profiles of the Because of high rock porosity and fluids in rock pores, vertical and horizontal displacements, the amplitude the velocity of the P wave decreased; on the other hand, ratio profiles were obtained. What we used was seismic the velocity of the PS wave changed slightly, and the data of full waves, including all kinds of seismic waves. reflection amplitude value of the PS wave approximated The results of migration and amplitude ratio profiles zero. This "white" phenomenon was caused by fluids in indicated that the images of interfaces were displayed rocks, not by the change of lithology. The second case and restored very well. could be interpreted as the oil-bearing formation. In the process of extracting amplitude ratios, the depth However, the presence of noise in the profile can also window and the threshold value were determined according generate "white" phenomena. It is essential to to experiments and demands of geologic interpretations. synthetically analyze seismic and geologic data and Experiments were conducted with depth window widths of compare the data with the surrounding media when the 1,600 m (80 dots) and 800 m (40 dots), and threshold values amplitude ratio profile was used to interpret the presence of 1110, 1120, 1150, 11100, respectively. of oil/gas in the formation. Fig. 1 shows a great number of local anomalies in The stratum in which the P and PS waves behaved the left image, but it was not as clear as the right image conformably displayed a positive anomaly in the in reflecting the main bed boundaries. amplitude ratio profile, indicating the geologic VolA No.1 Application of Amplitude Ratio Profiles Extracted with Prestack Kirchhoff Integral Migration 39 COP COP 22644 22944 23244 23544 23844 22644 22944 23244 23544 23844 ! ! 4.50 ...~ '< 4 -. 'c-, '7'r·.;·.;.--:-:'· . ':" .. ,:;', :', ;.; .',' ~::,'.,~':.;.i:~:;'ii "':''''~ 5,50 ;..... ' , ' " ,.~; , :' ,;." :,:::;;''::~', :-;.~.;:;.:~..~~, :.,";. t'. ,AI'';~-;''7. 6,50 '",::;' ,,' ,- ,-;..;,~:;:;;..:"fl~::'·'· c) i:= 850 ',' ••~-'l'".,.,., .~".,"" ",,: ..;.;.' r:.)':~.>". ;~~\~':~"',,,~...~~,,:~,,~.~;<:.~.:,', /.: j 9.50 ! ...;.:.. ~ ~ ~ . -: '~,;'.'~::~.'."".....;~.:,,:.:.~ ~:, • v··;. ".~:,: '.~~. . • ~ .......... ,,~ Fig. I A comparison of processing effect (Left: Depth window width 800 m, and threshold value 1/100; Right: Depth window width 1,600 m, and threshold value 1/20) Intersections existed among velocities of the P the time section, it could be interpreted as sandstones. wave propagated in different rocks; as a result multiple However, subsequently deposition was sandstone and solutions would occur if the P wave data alone were mudstone cycle, accompanied with a large-scale used to predict lithology. Synthetically analyzing the P lenticular sand body. wave and PS wave data to interpret lithology could COP reduce the uncertainty of lithology forecast. The amplitude ratio profile provided a supplementary tool for the determination of lithology, thus enhancing the accuracy in lithology interpretations. Moreover, the geometric contact relations among strata could be :~ 6.5 more clearly interpreted by using the amplitude ratio i:= profile. The sedimentary morphology could be seen clearly in Fig. 2 which showed the J2x and J1b groups, with fine stratified sediments, mainly mudstones and shales (including coal seam). This was consistent with the two sets of coal measures in this region. In the J2t and Fig. 2 Stratum geometric contact relation and lithology of J1s groups clastic rock deposits were mainly 531 line amplitude ratio profile developed on the intermediate section of the profile, (Depth window width 800 m, threshold value 1/100) with a rather large grain size. There was a conversion of positive and negative anomalies in the horizontal 5. Conclusion amplitude ratios, which indicated phase shifts. On the The elastic wave Kirchhoff integral migration could intermediate section of the profile, the bottom of the make full use of the information of the P and PS waves Kl group exhibited obvious downward denudation, so as to overcome shortages resulting from the and the amplitude ratio profile behaved as negative migration just using the P wave (acoustic migration). anomaly that corresponded to the phase change zone The amplitude ratio profile was obtained by using the P of lithology. This indicated that a set of large clastic and PS wave profiles. Using the absolute value of the facies fans was originally deposited. Combined with 40 2007 Petroleum Science Qin F. H., Guo Y. X. and Wang M. Y. (1988) Kirchhoff elastic product of the P wave amplitude multiplied by the PS wave migration method. Chinese Journal of Geophysics, wave amplitude as a threshold value avoided the 31(9),577-578 (in Chinese) problem caused by the indeterminate 0/0. The amplitude Sheriff R. E. and Geldart L. P. (1999) Exploration Seismology ratio profile could eliminate random disturbance and (2nd edition). Cambridge Univ, Press highlight primary aspects. It not only reconstructed Thomesen L. (1999) Converted-wave reflection seismology over underground structures clearly and improved spatial inhomogeneous, anisotropic media. Geophysics, 64, 678-690 resolution, but also reflected the properties of Wang J. L., Li Y. M. Leng C. B., et al. (1999) Methodological underground rocks, such as lithology and physical study of two-dimensional prestack depth migration and properties. The amplitude ratio profile could provide a auxiliary treatment scheme. Oil Geophysical Prospecting, supplementary means for seismic interpretation. 34(supplement), 1-12 (in Chinese) The horizontal shifts of imaging points will be Zhang P., Li Y. M. and Liu H. (2000) The situation of several caused by big horizontal velocity variety with the Eq. prestack depth migration methods. Progress in Geophysics, 15(2),30-39 (in Chinese) (l). Consequently, fault position will be changed somehow. Solving this problem will be our object of About the first author further studying. Wu Zhiqin was born in 1973 and References received his Ph.D degree from China Bevc D. (1997) Imaging complex structures with semi-recursive Universityof Mining and Technology Kirchhoffmigration. Geophysics, 62, 577-588 (Beijing) in 2005. He is undertaking Chen B. S., Li S. K. and Zhang L. H. (2001) Application of post-doctoral research in the School seismic prestack migration techniques. China Offshore Oil of Resources and Information, China and Gas (Geology), 15(5),361-364 (in Chinese) University of Petroleum (Beijing), Huang K., Yang X. M., Xu Q. Z., et al. (1997) The extraction of with his interests in earth detecting P-wave and S-wave for hydrocarbon detection using prestack and information technology, E-mail: Kirchhoff integral migration. Geophysical Prospecting for underskyblue@gmail.com Petroleum, 36(: ', 1-6 (in Chinese) Li L. M. and Luo S. X. (1998) A method for establishing P-P and (Received March 27,2006) P-SV wave prestack depth migration velocity model. Chinese (Edited by Sun Yanhua) Journal of Geophysics, 41(6), 305-318 (in Chinese)

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Petroleum ScienceSpringer Journals

Published: Apr 14, 2010

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