Multicomponent Seismic Data API
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Transcript of Multicomponent Seismic Data API
Introduction to Multi-Component
Seismic
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Bablu Prasad NoniaGeophysicist (S)
Introduction
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Multi-component seismic refers to seismic data which utilise
• P-wave data
• S-wave data
• Pure S-wave data
• Mode converted S wave data
Why S-wave?
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Goal of exploration is to get
structural information
lithological information
fluid content
P-wave seismic proved only partially successful.
Two wave carry different information of subsurface. So it is advantageous to record both P and S wave dataMulti-component seismic offers a better solution:
and where μ, ρ and k are
respectively shear modulus, bulk modulus and density.
The Arriving of S-Wave Imaging
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Shear wave exploration did not gain popularity till late 90’s
Non-availability of source to generate shear wave
More number of channels required for multi-component acquisition (3 times) compared to P-wave acquisition
P-wave was considered sufficient for imaging requirements, as the objective was mainly structural
No clear economic benefit was seen initially
Inadequate Processing knowledge of S-wave data
Acquisition
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Source
• Scalar source
• Vector source
Acquisition Mode Conversion
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When Seismic wave impinges on an interface at an oblique angle, different types of waves are produced.
Advantage of Mode Conversion
Converted (P-S) Pure Shear (S-S)
SOURCE Conventional Shear
RECORD LENGTH Shorter Longer
LVL Less Shallow Problem Larger Shallow Problem
PROCESSING Special Conventional7
Shear source generates very strong noise
Explosive used for P wave is buried so generates less surface waves
Signal will travel in LVL only once, so less absorption
Larger static correction for S wave source than P wave source. So chances of larger inaccuracy
PP as well as PS data is acquired simultaneously
Acquisition
3 FDU’s & Triphone DSU
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Sensor
Raw Shot Gather
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LAYOUT
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Survey Design Issues
CMP
CCP
50 meter CMP = 33.3 meter CCP for Vp/Vs=2, Vs/Vp=0.511
Binning
This bin size formula was suggested as a way to smoothen the high frequency variations fold due to the standard p-wave reflection point binning. (Lawton1993)
Larger bins reduce lateral resolution and prevent a direct match between PP and PS data.
Survey Design Issues
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Offset
• Shorter offsets in converted-wave cover the same subsurface area as P- Wave recording
• Far offsets are constrained by P-Wave• Near offsets are constrained by converted wave
NOTE: P-wave criteria for far offsets and converted wave criteria for near offsets are used
Major Issues in Processing of P-S Data
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CCP binning (mid point binning not valid)
Polarity issues
Need for rotation of recorded components
Difficulty in receiver statics estimation
Problem associated with estimation of Vs
CCP Binning
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Sin φ1/α =Sin ψ1/βRay path of incident P wave & reflected wave is asymmetrical
CCP Binning
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Conversion Point does not fall at mid point
CCP Binning
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P-S ray path geometry for multi layer case
CCP Binning
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CCP binning is implemented over multiple layer, user defined windows
ACP Binning
∞
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In the limit z
Polarity Issues
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Polarity of back spread and advance spread will not be same
Polarity Issues
advance spread back spread Z component20
Component Rotation
Y
XSource
Receiver
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Component Rotation
Source
Y
XReceiver
T R
θ
Acquisition co-ordinates R-T co-ordinates
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Component Rotation
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Radial response, r, is the horizontal ground motion in the source-reviver plane
r= x cosθ + y sinθ
Transverse response, t, is the horizontal ground motion perpendicular to the source-reviver plane
t= - x sinθ + y cosθ
Statics for Converted Wave Data
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Base of low velocity layer is not same for P and S waves
Statics for Converted Wave Data
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Shot statics same as P-P shot statics
Receiver statics scaled from P-P receiver statics
S-wave statics is far greater than P-wave’s
Vp/Vs Estimation
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No established method of estimating Vs, directly from converted wave data
Vp/Vs has more geological significance in comparison to Vs alone
There types of approach to estimate gamma
• from SP gathers
• processing baseed
• interpretation based
Vp/Vs Estimation
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Initial Gamma (from SP gathers)
Pick prominent reflector on PP gather and identify the same on PS gather
Tpp=Tp+Tp
Tps=Tp+Ts
Vp/Vs=Ts/Tp
Importants
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Fracture density and orientation
Shear wave polarised parallel to fractures( maximum stress) is FAST
Shear wave polarises perpendicular to fracture (minimum stress) is SLOW
Birefringence
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Gas Seepages
30Lomond gas field
Lithology Discrimination
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Alba Field - North Sea
Summary
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Multicomponent technology is not limited to benefiting reservoir development; additional exploration issues that can gain from multi-component seismic surveys include multiple attenuation and structural imaging
Many advancements in field, processing, and interpretation methods
A number of success stories for example lithology discrimination, DHI and fracture imaging
However, there is still room for improvement in acquisition and processing quality, cost reduction, interpretive understanding & application
Thank You
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