Introduction to Multi-Component

Seismic

1

Bablu Prasad NoniaGeophysicist (S)

Introduction

2

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?

3

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

4

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

5

Source

• Scalar source

• Vector source

Acquisition Mode Conversion

6

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

8

Sensor

Raw Shot Gather

9

LAYOUT

10

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

12

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

13

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

14

Sin φ1/α =Sin ψ1/βRay path of incident P wave & reflected wave is asymmetrical

CCP Binning

15

Conversion Point does not fall at mid point

CCP Binning

16

P-S ray path geometry for multi layer case

CCP Binning

17

CCP binning is implemented over multiple layer, user defined windows

ACP Binning

∞

18

In the limit z

Polarity Issues

19

Polarity of back spread and advance spread will not be same

Polarity Issues

advance spread back spread Z component20

Component Rotation

Y

XSource

Receiver

21

Component Rotation

Source

Y

XReceiver

T R

θ

Acquisition co-ordinates R-T co-ordinates

22

Component Rotation

23

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

24

Base of low velocity layer is not same for P and S waves

Statics for Converted Wave Data

25

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

26

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

27

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

28

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

29

Gas Seepages

30Lomond gas field

Lithology Discrimination

31

Alba Field - North Sea

Summary

32

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

33