VẬT lý địa CHẤN a2 01 FUNDAMENTALS f SEISMIC EXPLORATION

Fundamentals for Seismic Exploration Schedule  Introduction  Fundamentals of Seismic Wave Seismic Wave Propagation  P wave and Shear wave  Reflection Coefficient  Outline of Seismic Data Processing  Topics of Seismic Data Processing  Static Correction  Ｐｏｌａｒｉｔｙ　ｏｆ　Ｓｅｉｓｍｉｃ　 wave  DMO (Dip Moveout)  Schedule  Introduction  Fundamentals of Seismic Wave Seismic Wave Propagation  P wave and Shear wave  Reflection Coefficient  Outline of Seismic Data Processing  Topics of Seismic Data Processing  Static Correction  Ｐｏｌａｒｉｔｙ　ｏｆ　Ｓｅｉｓｍｉｃ　 wave  DMO (Dip Moveout)  Exploration Seismology Introduction-1  Goal : Imaging of detailed subsurface structure and obtaining information related to rock properties  Method :  Artificially generate seismic waves and observe the seismic waves (detection)  Analysis of observed seismic waves  Artificial generator : Seismic energy source Detector : Receiver (Sensor) Introduction-2 Classification of Exploration Seismology with regard to the geometry of source and receiver  Surface Seismic Method  Reflection Seismic Method (usual for oil and gas exploration)  Refraction Seismic Method (rare case for oil & gas Exploration)  Borehole Seismic Method   VSP (surface to borehole) Cross-well (borehole to borehole) Introduction-3 Geometry of Seismic Methods Surface R S R R R S R R R R Reflector Reflection Seismic Method Surface Refraction Seismic Method S R R R R R R S S Reflector VSP (Vertical Seismic Profiling) Crosswell Seismic method Reflection Seismic Method Introduction-4 Acquisition Observe reflected waves Recording (A/D conversion) Data Processing and Analysis Standard Data Processing Special Data Processing (Pre-Stack Migration etc.) Attribute Analysis (AVO, Inversion ) Basement Results Subsurface imaging Velocity information Information for reservoir characterization Available to Exploration and development stages Introduction-3 Reflection Seismology  Acquisition   Survey Area  On-Shore (Land)  Off-Shore (Marine)  Transition Zone Method  2D method  3D method  4D method  Component Observation  4C Observation  S-Wave Survey  Data Processing  Conventional Processing  3D Processing  Special Processing  Pre-Stack Migration  Depth Migration  Multiple Attenuation  Attribute Analysis  AVO  Inversion Seismic Filed Observation  Energy Source    Receiver     Explosive source Non-explosive source Geophone Hydrophone Accelemeter Recording   Digital telemetry sysstem 24bit A/D convesion Layout of Land Seismic Survey ss e l e Wir al n Sig Recording Track Remote Station Unit Source JGI Recorder Vibrator Receiver Geophone Seismic Wave Reflected wave 図 － 反射法地震探査の概念図 Direct wave Reflector A D Vw ｉ ｉｃ ic : Critical angle Vsw ｉｃ Refracted wave C B ｒ Travel time T2 Ｔ D T1 Ｔ１ A Ｔ２ First Break Vw B C Vsw Refraction Analysis for Static Correction Time-Term Sub-weathering Velocity Weathering Structure Example of Static Correction Without static correction With static correction Schedule Introduction Fundamentals of Seismic Wave – Seismic Wave Propagation – P wave and Shear wave – Reflection Coefficient Outline of Seismic Data Processing Topics of Seismic Data Processing – Static Correction – Polarity – DMO (Dip Moveout) Polarity of Seismic Waves Surface D Z-Axis Reflected Wave Incident Wave : U = cos(ω t + kz ) Observation Reflected Wave : U r = − R * cos(ω t − kz) Incident Wave G Observed Data G ≈ cos( ω t + kz ) + R * cos( ω t − k ( z + D)) Hydrophone : H ≈ cos( ω t + kz ) − R * cos( ω t − k ( z + D)) Geophone : 2D V Amplitude of reflected waves on free surface Geophone : Twice of incident wave Hydrophone : Zero (Cancelled) H Schedule Introduction Fundamentals of Seismic Wave – Seismic Wave Propagation – P wave and Shear wave – Reflection Coefficient Outline of Seismic Data Processing Topics of Seismic Data Processing – Static Correction – Polarity – DMO (Dip Moveout) Principle of DMO (Dip Moveout) L T= L S M G P V (H ) + L2 cos θ NMO ； T → Tn HP Tn = H R V DMO ； Tn → T p D M →P θ H cos θ − L2 sin θ (T p = 2H p V ) (CMP to CRP) H cos θ − L2 sin θ Tp = V H cos θ L2 tan θ MP = ∆X = H DMO operation creates CRP gather from post-NMO CMP gather S M D G R S M P ' G R DMO in Common Offset Domain Tn = Tp = V H cos θ − L2 sin θ DMO Operator -L H cos θ − L sin θ V H cos θ 2 L L2 tan θ ∆X = H Relationship between Tp and ∆X DMO Operator  Tp   ∆X  Tn Tp Reflection point exists somewhere on the ellipse of DMO operator DMO in Common Offset Domain (L= constant)   +   = L    Tn  Parameter (known) L : Offset/2 Tn : Traveltime after NMO ∆X X(CRP ) c to r e l f e R T Imaging of reflector is given by the envelop of DMO operator NMO Conventional NMO & Stack Stack V NMO = V cos θ CMP Gather CMP NMO DMO Common CRP Gather V NMO = V L NMO + DMO & Stack X L CMP DMO on Common Offset Panel L=Constant Stack Comparison of Velocity Analysis Method Stacking Velocity D MO Analysis Influence of Dipping Reflector Velocity Reflection Point V= V ( x, z ) cos θ M S1 Velocity Analysis Pre- Stack Time Migration No Inflence of Dipping Reflector No Influence of Dipping Reflector V = V ( x, z) Deviated from the Analysis Point Normal to the Reflector S2 Velocity Analysis by Ｒ1 Ｒ２ X V = V ( z) Deviated from the Analysis Point Normal to the Reflector S2 M Ｒ1 Ｒ２ S1 X J ust below the Analysis Point S2 S1 Ｒ1 M Ｒ２ Concept θ D D' D D' θ Z- Axis Z- Axis θ D Z- Axis X DMO Velocit y Soni c Conventional Stacking Velocity P-wave Reflection Coefficient vs Incident Angle Linear Approximation Incident Reflected R(θ ) θ θ R(θ ) ≈ A + B ⋅ sin θ A=Intercept (R0) ρ 1, V ρ , V2 B=Gradient (R0-2S0) AVO Analysis Cross-Plot B Transmitted R0: Reflection coefficient of vertical P-wave S0 : Reflection coefficient of vertical S-wave A Attenuation of Seismic Wave Elements of Signal Decay Reflection • • • • Spherical Divergence Anelastic Absorption Scattering Loss Signal Loss at Reflectors Scattering Loss Spherical Divergence Anelastic Absorption Amplitude Decay Factor • Spherical Divergence ∝ r • Anelastic Absorption • Loss at Reflectors ∝ e − αr ∝ (1 − Ri ) • Scattering Loss Decay of seismic wave is caused by the combination of these factors [...]... velocity information Travel time of Direct wave Disadvantage – Restricted survey area Around borehole (VSP) Interwell zone (Crosswell method) – Well distance < about 1000m for Crosswell Limitation of energy source (After Harris, 1995) (After Harris et al., 1995) Schedule  Introduction  Fundamentals of Seismic Wave Seismic Wave Propagation  P wave and Shear wave  Reflection Coefficient  Outline of Seismic. .. condition of 4D seismic acquisition (practically difficult) – – – Same specification as the baseline survey Same weather condition as the baseline survey Same noise level as the baseline survey Baseline 3D survey : the first survey Monitor 3D survey : on and after the second survey Example of 4D Seismic Survey Baseline seismic cross-section 4D Seismic Difference Section (Monitor – Baseline) after co2.. .Seismic Energy Source  Explosive Source   Dynamite Non-Explosive Source    Airgun Vibroseis (Vibrator) Weight drop type Seismic Energy Source - Vibrator Generate seismic energy by continuous vibration , starting with a low frequency and gradually increasing frequencies Concept of cross correlation for vibroseis system Reference Sweep Reflector Reflection (1) Reflection (2) Reflection (3) Reflection... Reflection (4) Reflection (5) Observed data After cross correlation with sweep signal Seismic Energy Source - Impactor Wight-drop type seismic energy source Seismic Energy Source - Airgun Generate seismic waves by releasing compressed air into water 2D Seismic Survey 2D Seismic Line Shots and Receivers are on the same line Reflection Point Subsurface imaging along a seismic line Example of 2D seismic imaging... ine Direction of Shooting (Perpendicular to the line) Left 8 sec ic L Right H2 1.2km V Horizontal (H1) S-Wave (Direct) S-Wave (Refraction) P-Wave (Direct) Vertical m /s e 1500 0 sec c Surface Wave 340 m/ sec Horizontal (H2) P-Wave (Reflection) 2 sec PS Converted Wave (Reflection) ? 0m 40 c / se 4 sec S-Wave (Reflection) 6 sec 8 sec 1.2km Borehole Seismic Methods Advantage – Detailed subsurface imaging... Topics of Seismic Data Processing  Static Correction  Polarity  DMO (Dip Moveout)  Basic Concept of Wave Propagation A progress disturbance propagates from point to point in a medium The disturbance is generated by a pushing and pulling of material particles around the baseline X Note: Particles do not flow along the wave Displacement of particles is limited around the baseline Waveform : Configuration... receiver lines for each shot Y X t Distribution of Reflection points Geometry of Land 3D Survey Swath type (Shot lines and Receiver lines are perpendicular) JGI Recording Track JG I Receiver Line Reflection Points n Li t o Sh Bin e Survey area is divided into small rectangle cells, named “bin” and the bin center is defined as CMP in 3D seismic survey CMP (Bin Center) Note : In the case of regular geometry,... of regular geometry, all reflection points are exactly located at each bin center However, in the case of irregular geometry, reflection points distribute around bin center Reflection point Field Operation of Marine 3D Survey Sail Line Obtain several CMP lines by multiple streamer cables and alternative shooting of twin airgun strings through a single sail line Example of 3D Volume (Marine 3D) in L... line Example of 2D seismic imaging Layout of 2D seismic reflection survey Geophone JGI JGI Source Reflection Points １ ２ ３ ４ ５ ６ ７ ８ Receiver Location ９ １０ １１ １２ １３ １４ １５ １６ １７ １８ １９ ２０ ２１ ２２ ２３ ２４ ２５ ２６ ２７ Relationship between Source & Receiver Position Relationship between Source & Refection Points CMP (Common Mid Point) 3D Seismic Survey 3D Imaging by reflection points covering on the whole survey... i l eS m Ti os s - Li ne Definitions In Line : Pararell to sail lines Cross-line : Perpendicular to sail lines 4D Seismic Survey Definition : 4D = 3D + time (Time-lapse 3D) – Repeat 3D seismic survey several times – Compare the monitor 3D survey with the baseline 3D 　 survey Purpose – Reservoir Monitoring Obtain time –lapse images associated with fluid and pressure front of reservoir during hydrocarbon ...Schedule  Introduction  Fundamentals of Seismic Wave Seismic Wave Propagation  P wave and Shear wave  Reflection Coefficient  Outline of Seismic Data Processing  Topics of Seismic Data Processing... Schedule  Introduction  Fundamentals of Seismic Wave Seismic Wave Propagation  P wave and Shear wave  Reflection Coefficient  Outline of Seismic Data Processing  Topics of Seismic Data Processing... of source and receiver  Surface Seismic Method  Reflection Seismic Method (usual for oil and gas exploration)  Refraction Seismic Method (rare case for oil & gas Exploration)  Borehole Seismic