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<br />1 <br />:. ~'.y ~~ ..-.:' . <br />Acquisition steps <br />Steps taken at the acquisition stage <br />can help ensure the accuracy so essen- <br />tial in this building block process. <br />Diamond Geophysical uses two <br />boats to produce a multilane seismic <br />profile. Each boat tows three, 3,000 m <br />streamers, ++'ith the trailing boat tow- <br />ing hvo energy sources that fire alter- <br />nately. In effect, the boats acquire six, <br />6,000 m seismic lines of data with each <br />pass, with 40 m separation between <br />lines. The long offsets are crucial to <br />later processing steps. <br /> <br />Cr055 <br />ae <br />r~~~ ~~i~l~l'~ ; ~i <br />Fig 11 <br />?I:' <br />{r., - - <br />;.; - <br />. - t ...'f <br />~: <br />The multilane profile produced by <br />this h-pe of sun'ey helps in eelocity <br />analysis. In conventional velocity <br />analysis, the interpreter must use <br />what is effectively 2D data. <br />Data from a multilane profile can be <br />processed like a ntiniature .D sun'ey, <br />six lines at a time, with an effective <br />width of 200 m. The data are pro- <br />cessed with 3D dip moceout (DMO) to <br />preserve steeply dipping events. Then <br />the six lines are stacked and migrated. <br />in addition, all data in the six lines <br />can be stacked in one direction to <br />produce one line out of six. IE the <br /> <br /> <br />y, of <br />relat <br />rfora <br />Ire, t <br />of a <br />ng n <br />wse <br />ngin <br />reset <br />Hers I <br />lual r <br />the F <br />xec <br />cap: <br />t any <br />whit <br />ns A4. <br />letioi <br />ne & <br />sele <br />echo <br />ition <br />-~• r` <br />it r th~ <br />Ice or <br />large <br />ull Iii <br />and <br />ahle i <br />;~,..nr . <br />r world'de~ <br />_ :•=5chlumb~ <br />-asystems it <br />cnGulf of <br />i;~been pert <br />i`+deep-pen <br />duce. Scl <br />~'.;; , , <br />x'Y,and perfc <br />'!"~~4/rln u <br />~,., penetrate <br />ti~:making 3 <br />lfJ r¢;ialn.anr <br />Circle 445 on Reader Service Card -~ <br />seismic line is fairly perpendicular tp <br />the structure, the procedure improves' <br />imaging above and below salt, im= <br />proves the signal to noise ratio below <br />salt, and tan provide information very <br />useful in velocity analysis. Among <br />other things, the information helps <br />stack in-plane events. <br />Fig. 5 compares a 2D com'entionally <br />time-migrated seition with a migrated <br />multilane seismic profile. The latter <br />sho+as better top of salt, faulting <br />above salt, and improved imaging be- <br />low salt. <br />Fig. 6 shows cross-line gathers, <br />which help in the determination of in- <br />plane and out-of-plane events and in <br />the interpretation of velocities. <br />Existing technology <br />Most of the steps described <o far <br />involve technology that has ~^een <br />available for some time but the ~ has <br />not, until now, been integrated into a <br />carefulh• controlled sequence fir con- <br />struction of a velocit}~ model accurate <br />enough to use in 3D PSDM. <br />Fig. 7 sho+as the sequence at work <br />over the 1`lahogam• salt sill. It starts <br />with a brute stack, essentially a first <br />guess of velocities based on a coarse <br />sampling of celocitt• analysis-every? <br />km. Data stacked with the resulting <br />velocity field produce the image in <br />Fig. 7a. <br />A tight +elocity analysis foJaws. <br />The interpreter uses information from <br />migrated sections, cross-line gathers, <br />stack panels, and interval velocities <br />from data spaced every 500 m in the <br />in-line and cross-line directions to up- <br />date the velocity field. <br />An image based on these steps <br />shows amuch-improved base of salt <br />and better evidence of subsalt reflec- <br />Pg. 12 <br />sections 1.500 f1 apart. ~ ~ ... •' <br />