introduction to seismic migration

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13/05/2010 1 Introduction to Seismic Migration One-way traveltime V=1 m/s Homogeneous dipping planar reflector

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13/05/20101Introduction to Seismic MigrationOne-way traveltimeV=1 m/sHomogeneous dipping planar reflector13/05/20102One-way traveltimeV=1 m/sHomogeneous dipping planar reflectorOne-way traveltimeV=1 m/sHomogeneous dipping planar reflector13/05/20103Homogeneous dipping planar reflectorOne-way traveltimeV=1 m/sStacked position= reflection positionMigrated position=true their subsurface locationDipping reflections13/05/20104More complex structureDefinitionProcess which moves dipping reflections to their true subsurface position and collapesdiffractionsProcess which reconstructs seismic image from stack section so that reflections and difractions are plotted at their true locationStacked section MigratedsectionMigrationOperationVelocity13/05/20105Objectives Moves dipping reflections to their true dip (up dip) and subsurface location Collapes diffraction Un-tie bow-tieSeismic Velocity13/05/20106Seismic Velocity Instantaneous Represents actual velocity Similar to the well log velocity Interval Instantaneous velocity over a defined interval Root mean square (RMS) Used during NMO and diffraction modeling Average Total distance with a total traveltimedtdzVins =( ) ( )( ) ( )==212122 , 12 , 122 , 12 , 111TTins insTTins insdt t VTT Vdt t VTT V( ) ( )=== T ttins rms dt t VTt V02 2 1=== T ttins ave dt t VTT V0) (1) (RMS and Average Velocity==AA= niiniin rmstt VV112int2,==AA= niiniin avett VV11int,RMS velocity Average velocity13/05/20107How to derive velocityPre-stack seismic gather stacking velocityVelocity analysisRMS velocity) cos(dip V V stack rms =Interval velocityDix equationDix Equation(Dix,1955)Assumption Horizontal planar reflectors Small offset2 / 1112 2int) 1 ( ) () (||.|

\| =n nn rms n rmst tt n V t n Vn VVintVrms(n-1)Vrms(n)TWTtn-1tnCDP13/05/20108Exercise-1Compute RMS and average velocities at reflector B,C and D!Z=1000 mZ=2000 mBVab=2000 m/sVcd=6000 m/sVbc=4000 m/sCDAZ=3000 mSolution-1Depth Vint DTi V_ave V_rms1000 2000 0.5 2000.0 2000.02000 4000 0.25 2666.7 2828.43000 6000 0.167 3272.7 3618.1V_aveV_rmsV_intVelocity [m/s]TWT [s]13/05/20109Exercise-2Semicircle superpositionImpulse response migration13/05/201010Diffraction summationKirchhoff Migration13/05/201011Huygenss secondary sourceHuygens traveltime curve13/05/201012Kirchhoff Summation

A=xinRMSout P tr VxP * ) (cos2 put Obliquity Spherical spreading Wavelet shaping factor) / , 0 , ( v r t z x Pin =) 0 , 2 / , ( 0 = = t v z x Pout t( ) 220 z x x r + =Kirchhoff time and depth13/05/201013Kirchhoff migration parameters Velocity Aperture Maximum dipMigration velocitiesOvermigrated UndermigratedZODesired migration2500 m/s5 %10 %20 %13/05/201014Test for velocityTest for velocity13/05/201015Migration velocities13/05/20101613/05/201017Tests for maximum dip to migratea. ZO sectionb. Desired migrationc. 4 ms/traced. 24 ms/tracecdTests for maximum dip13/05/201018UndermigrationMigration strategy (Yilmaz)2D versus 3D migrationPost- versus post- migrationTime versus depth migrationCase Migration Case Migrationdipping event time migration strong lateral velocity variations associated with complex overburden structuredepth migrationconflicting dips with different stacking velocitiesprestack migration3D behavior of fault planes and salt flanks3D migrationcomplex nonhyperbolic moveoutprestackmigration3D structure 3D migration13/05/201019ZO versus stack /CMP stack section1. Complex structure nonhyperbolicmoveout2. Conflicting dipsPre-stack migrationMigration algorithm Integral solution to the scalar wave equation Finite-difference solution Frequency-wavenumber implementation: Stolt, phase-shift/Gazdag1. Handle steep dips with sufficient accuracy2. Handle lateral and vertical velocity variations3. Be implemented, efficiently13/05/201020Kirchhoff depth migration