Patent Application: US-15640108-A

Abstract:
the invention relates to a method of seismic data processing , wherein the data includes a set of seismic traces , with each trace including a signal that has been recorded by a sensor after having been propagated in a subsurface area , with the signal being defined by an amplitude as a function of time , including the steps of : migration of data according to an initial time - velocity model , picking in the time - migrated data one or more event corresponding to one or more subsurface reflector so as to obtain facets locally approximating the event , kinematic demigration of the facets plotted so as to obtain simplified seismic data in the form of a set of facets and a set of attributes associated with the facets .

Description:
the seismic data processing process shown in fig3 and 4 includes two main processing phases : a first phase 100 of constituting kinematic invariants ( fig3 ) and a second phase 200 of tomographic inversion of these kinematic invariants , i . e . an estimation of the time - or depth - velocity model ( fig4 ), on the basis of the kinematic invariants . the first phase of the processing process shown in fig3 is applied to time traces before migration . these traces correspond to the recording , as a function of time , by a sensor , of the amplitude of the signal propagated underground . according to a first step 101 , a migration of these seismic traces is performed according to an initial time - velocity model ( prestm ). according to a second step 102 , on each gather of traces obtained in the previous step , one or more events reflected in line with the surface point considered are picked . fig5 diagrammatically shows a cross - section of the pre - stack time - migrated seismic data ( prestm ) stacked on the cip gathers ( at the left ) and common image point ( cip ) gathers ( at the right ) from this data . the position of the cip gathers is indicated in the cross - section by a dotted line . rmo curves , which characterise the alignment of the data picked in the cip gathers have been superimposed with the picked data . according to a third step 103 , for each picked event , time and the associated time - migrated facet are determined . this facet is characterised by a dip ( in directions x , y ) and possibly a slope in the direction of the cip gathers . according to a fourth step 104 , the kinematic demigration of the migrated facet is performed . this facet is characterised by source and receiver positions , by a central time and by slopes , taking into consideration the acquisition geometry and the rules of focusing on the times and the time migration gradients described in chauris et al . ( 2001a , b and c ), and guillaume et al . ( 2001 and 2004 ) ( see fig8 and 9 for the rules of focusing of the kinematic time migration ). the calculation of the acoustic source - receiver pair is a well - known step that can be performed according to a plurality of established techniques ( press et al ., 1986 ). on the basis of the acoustic source - receiver pair , various attributes associated with the plotted event are also determined , such as the angles of reflection , the angles of phase , the instantaneous velocity , the stretch factor , the depth dip , and optionally the crs attributes . for all of these operations , the basic data are the components of the gradient of the time migration time . the demigrated attributes together make it possible to characterise local events that can be observed in the seismic traces before migration . as their characteristic does not depend upon the velocity model used for the migration , these data are referred to as kinematic invariants ( guillaume et al ., 2001 , 2004 ). these attributes associated with the pre - stack time - migrated image of seismic data make it possible to : 1 ) provide a depth tomography as described in the patent cited above ; 2 ) provide a new type of time tomography making it possible to determine a time - velocity model with a single update ; 3 ) calculate an instantaneous velocity ( isotropic or anisotropic ), a stretch factor , a depth dip and aperture angles associated with the pre - stack time - migrated trace gathers . this information can be used , for example , in stratigraphic inversions making it possible to characterise the nature of the subsurface ( and in particular hydrocarbon reservoirs ), or in various trace processing operations such as wavelet stretch compensation , cip angle constitution , and amplitude - versus - angle studies ; 4 ) estimate the curves of the local events in the pre - stack seismic data making it possible to reconstruct the crs attributes ( müller , 1999 ; jäger et al ., 2001 ), capable of being used in various seismic processing processes ( duveneck , 2004 ). with the determination of the acoustic source - receiver pair , it is possible to determine valuable information for the interpretation of time - migrated images . the components of the time migration gradients ( fig8 ) thus make it possible to calculate a stretch factor , a specular reflection angle , phase angles , an instantaneous velocity , a depth dip and crs attributes . the time migration stretch factor is obtained by considering the “ vertical ” component of the double time migration gradient ( fig1 ). where t sr is the double time of the time migration operator , t 0 is the time of the time - migrated image . this factor is directly derived from t sr , time of the invariant , which may itself be used to compensate for the stretch of the time - migrated trace . the calculation of the specular reflection angle θ associated with the offset - ordered common image point collections is obtained by considering the local dip of the event ( assumed to be an offset invariant ), and by comparing the values of the “ vertical ” component of the gradient of the double time of time - migration at various offsets ( fig1 ). indeed , the ratio of this component of the gradient at a given offset with that with a zero offset gives the cosine of the specular reflection angle . it should be noted that the calculation can also be performed for cip gathers arranged in shot position or in source or receiver position . cos ⁢ ⁢ θ = ∂ t sr ∂ t 0 ⁢ ⁢ current ⁢ ⁢ offset ∂ t sr ∂ t 0 ⁢ ⁢ zero ⁢ ⁢ offset where t sr is the double time of the time - migration operator , to is the time of the time - migrated image . the calculation of an instantaneous velocity v inst on the basis of horizontal and vertical components of the gradient of the single time of time - migration is obtained by expressing this vector using the angle of incidence of the wave and the instantaneous velocity ( fig1 ). the calculation can be performed for all of the source and receiver geometries , which makes it possible to access anisotropic parameters using the information on the phase angle and the dip estimated below . v inst . = 1 - 4 ⁢ ( ∂ t ∂ t 0 ) 2  ∂ t ∂ x  where t is the single time of the time - migration operator ( for the source or receiver path ), the calculation of the depth dip of the event considered is obtained by considering the picked time dip and the estimation of the instantaneous velocity estimated above ( fig1 ). dip depth = ∂ z ∂ x dip depth = v inst . × dip the calculation of phase angles makes it possible to free from the assumption of isotropy used for the calculation of reflection angles . it relies on the use of instantaneous velocity which enables slowness vectors p s and p r to be calculated at the image point . where t s and t r denote the simple times of the migration operator ( for the source and the receiver , respectively ). the directions of these vectors are characterised by phase angles . from these vectors , it is also possible to derive the cosine of the phase angle of reflection by equations the estimation of crs attributes is obtained by adjustment on the distribution of kinematic invariants obtained by time - demigration of local curve events . these crs attributes can be used to estimate a depth - velocity model ( duveneck 2004 ), or for various other applications of the crs attributes ( jäger et al ., 2001 ). we should note that while fig8 to 13 diagrammatically show a two - dimensional case , the process can be generalised directly to a three - dimensional case . fig8 diagrammatically shows various elements involved in the kinematic migration and demigration steps . these elements include in particular the dip of the time - migrated image , and the gradients of the source , t s , and receiver t r travel times , at the basis of the estimation of various attributes . these travel times are dependent on the position in the migrated image ( x , t 0 ), the source s or receiver r position , and finally the velocity model . these elements are used in the time migration , where their sum is involved , t sr = t s + t r . fig9 diagrammatically shows a focusing condition of the kinematic time migration . for a given dip , the position of the migrated point ( x , t 0 ) and those of the source and the receiver will satisfy this condition . fig1 diagrammatically shows a step of determining the vertical stretch . fig1 diagrammatically shows a step of determining the cosine of the reflection angle . the ratio between the vertical stretch values with a given offset and that with a zero offset is determined . this estimation assumes an isotropic medium . fig1 diagrammatically shows a step of determining the instantaneous velocity . the components of the single , source or receiver transit time gradients are used . the velocity can therefore be obtained for the source and receiver trajectories . fig1 diagrammatically shows a step of determining the depth dip on the basis of the time dip and the instantaneous velocity . fig1 diagrammatically shows a step of determining phase angles . slowness vectors are calculated from instantaneous velocities and are used to derive phase angles , in particular reflection phase angles . the second phase 200 of the processing process shown in fig4 is applied to the kinematic invariants . on the basis of these kinematic invariants , a processing operation is implemented , making it possible to estimate a time - velocity or a depth - velocity model according to which the following steps are iterated . according to first step 201 , a kinematic migration ( time or depth ) of the kinematic invariants obtained above using a model of the velocity field ( time or depth ) of the subsurface . according to a second step 202 , an alignment of the migrated points obtained is characterised . it is also possible to apply a criterion for minimising the slope of the migrated events in the cip gathers ( chauris et al ., 2001a ). according to a third step 203 , the parameters of the velocity model ( time or depth ) are updated . step 203 consists of selecting a time - or depth - velocity field setting of parameters that optimises the alignment of the migrated facets . steps 201 , 202 and 203 can be repeated until an alignment deemed to be sufficient is obtained . it should be noted that the processing operation for selecting the parameter values of the velocity field as proposed does not systematically require a time or depth migration of the seismic data ( which is very costly ) after each updating of the velocity model . it is consequently understood that the process proposed by the invention can be implemented without requiring high computing powers . fig6 diagrammatically shows an example of pre - stack depth - migrated cip gathers ( presdm ) obtained by respectively applying the final velocity model obtained using a picking in the pre - stack time - migrated data ( above ) and the final velocity model obtained by using a picking in the pre - stack depth - migrated data ( below ). fig7 diagrammatically shows an initial depth - velocity model ( at left ) and a final depth - velocity model ( at right ) obtained in the second phase of the process . chauris , h ., noble , m . s ., lambaré , g ., and podvin , p ., 2001 . migration velocity analysis from locally coherent events in 2 - d laterally heterogeneous media , part i : theoretical aspects , geophysics , vol . 67 , no . 4 , pages 1202 - 1212 . chauris , h ., noble , m . s ., lambaré , g ., and podvin , p ., 2001 . migration velocity analysis from locally coherent events in 2 - d laterally heterogeneous media , part ii : applications on synthetic and real data , geophysics , vol . 67 , no . 4 , pages 1213 - 1224 . dix , c . h ., 1955 . seismic velocities from surface measurements , geophysics 20 , 68 . duquet , b ., lailly , p ., and ehinger , a ., 2001 . 3d plane wave migration of streamer data , seg expanded abstracts 20 , 1033 . duveneck , e ., 2004 . velocity model estimation with data - 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