Chapter 5 - Processing of Seismic Reflection Data 1 01

7/25/2019 Chapter 5 - Processing of Seismic Reflection Data 1 01

1/70

Overview ta3520

Introduction to seismics

Fourier Analysis Basic principles of the Seismic Method

Interpretation of Raw Seismic Records

Seismic Instrumentation

Processing of Seismic Reflection Data Vertical Seismic Profiles

Practical:

Processing practical (with MATLAB)


2/70

Signal and Noise

Signal: desired

Noise: not desired

So for reflection seismology:

- Primary reflections are signal

- Everything else is noise!


3/70

Signal and Noise (2)

Direct wave: noise

Refraction: noise

Reflection: (desired) signal


4/70

Signal and Noise (3)

Direct wave: noise

Refraction: noiseReflection: signal

Multiply reflected : noise


5/70

Signal and Noise for P-wave survey

Noise: direct wave through first layer

direct air wave

direct surface wave

S-wave Multiply reflected wave

Refraction / Head wave

Desired signal:

primary reflected P-waves


6/70

Signal and Noise for P-wave survey

Signal

Noise=

Primary P-wave Reflected Energy

All but Primary Reflection Energy

Goal of Processing:

Remove effects of All-but-Primary-Reflection Energy


7/70

Processing of Signal

(Primary-reflected energy)

Goal of processing:

Focus energy to where it comes from


8/70

Understanding signal and noise:

wave theory

Basic physics underlying signal is captured by wave equation

Ray theory: approximation of wave equation (high-frequency)

Resonances: modes expansion of wave equation

S-waves, P-waves: elastic form of wave equation


9/70

Seismic Processing Basic Reflection and Transmission

Sorting of seismic data

Normal Move-Out and Velocity Analysis

Stacking

(Zero-offset) migration

Time-depth conversion


10/70

Basic Reflection and Transmission

(pdf-file with eqs)


11/70

CMP sorting

shot data

image

NMO correction

(zero-offset) migration

stack

velocity model

velocity model

Processing flow


12/70

Processing

Input: Multi-offset shot records

Results of processing:

1. Structural map of impedance contrasts

2. Velocity model


13/70

Sorting:

Common Shot gather

Seismic recording in the field:

Common Shot data

(Each shot is recorded sequentially)

Nomenclature:

- common-shot gather- common-shot panel


14/70

Common Shot gather


15/70

Sorting:

Common Receiver gather

Gather all shots belonging to one receiver position in the field

Analysis/Processing: shot variations

(e.g., different charge depths)

(Also in common-shot gathers: receiver variations, e.g.,

geophones placed at different heights)


16/70

Common Receiver gather


17/70

Sorting


18/70

Sorting:

Common Mid-Point gather


19/70

Sorting:

Common Mid-Point gatherMid-points defined as mid-points between source and receiver

in horizontal plane

Since reflections are quasi-hyperbolic:

Seismograms not so sensitive to laterally varying structures

Good for velocity analysis in depth

Stacking successful (noise suppression)

In practice, not really a point but an interval: BIN


20/70

CMP gather over structure


21/70

Sorting:

Common Offset gather

Purpose:

Very irregular structures (in which stacking does not work)

Application of Dip Move-Out (correction for dip of reflector)

Checking on migration: small and large offsets should give the

same picture : otherwise velocities are wrong

In practice, not really a point but an interval: BIN


22/70

Zero-offset gather over structure


23/70

SortingCommon-Mid-Point (CMP) gathers:x

s+ x

r= constant

Common-Offset gathers (COG):xs xr= constant

Multiplicity = Fold:N

rec

2 xs/ x

r

Nrec

= Number of receivers

xs = Spacing between subsequent shotsx

r= Spacing between subsequent receivers


24/70

Sorting


25/70

CMP sorting

shot data

image

NMO correction


stack

velocity model

velocity model

Processing flow


26/70

Reflection 1 boundary


27/70

Normal Move-Out: 1 reflectorT =

R

c=

(4d2 + x2)1/2

c

x = source-receiver distance

R = total distance travelled by ray

d = thickness of layer

c = wave speed

We do not know distance, but we know time:

T = T0 ( 1 +x2

c2 T02

)1/2

where T0 is zero-offset (x=0) traveltime: T0 = 2d/c


28/70

T = T0 ( 1 +

x2

c2 T02 )

1/2

Extra time shift compared to T0 called:

NMO- Normal Move-Out

TNMO = T T0 = T0 ( 1 +x2

c2 T02 )1/2 T0


29/70

Normal Move-Out (NMO)


30/70

Normal Move-Out (NMO)


31/70

TNMO = T

T0 = T0 ( 1 +

x2

c2 T02 )

1/2

T0

Larger TNMO for larger offset

Smaller TNMO for larger T0(deeper layers have smaller move-out)

Smaller TNMO for larger wave speed c(deeper layers usually larger velocities so smaller move-out)


32/70


33/70

NMO: effect velocity

NMO with right

velocity

NMO with too small

correction: too high

velocity

NMO with too large

correction: too small

velocity


34/70

NMO: 1-layer approximation

TNMO = T T0 = T0 ( 1 +x2

c2 T02

)1/2 T0

Use Taylor expansion of square root:

TNMO = T T0 T0 ( 1 +x2

2 c2 T02 ) T0 =

x2

2 c2 T0


35/70

NMO: 1-layer approximation


36/70

NMO: 2-layer

Position depends on

velocities of layer 1 and 2

(via Snells Law)


37/70

NMO: 2-layer

(pdf-eqs)


38/70

NMO: multi-layer approximation


39/70

Velocity model: RMS model

time

velocity

Interval velocity(velocity of layer)

Root-mean-square velocity

(weighted-average velocity of layers above)


40/70

Velocity Analysis

Approaches:

T2 x2 analysis

Alignment of reflectors: visually or mathematicalexpression of coherence

With T2 X2 analysis we depend on picking travel-times,

and thus signal-to-noise ratio

V l i A l i


41/70

Velocity Analysis:

Original CMP gather

Starting CMP gather


42/70

T2

-x2

analysis

V l it A l i li i


43/70

Velocity Analysis: aligning

reflectors

Starting CMP gather


44/70

Constant-velocity NMO

Velocity too high:

correction too little

Velocity too low:

correction too much


45/70

Velocity panels

CMP gather

with velocity

1300 m/s

CMP gather

with velocity

1700 m/s

CMP gather

with velocity

2100 m/s


46/70

Velocity as function of time

V l i l l d l


47/70

Velocity panels:real data example

h f l i l i


48/70

Coherence measures for velocity analysis

Stacked amplitude (normalized or not)

Cross-correlation (normalized or not)

Semblance:

related to cross-correlation

A = amplitude

Sum m over traces m in CMP

S (t , c) = m A (xm , t , c) )

2

m A2 (xm , t , c)

M


49/70

Semblance for CMP gather

V l it l l d t l


50/70


V l it l l d t l


51/70


V l it l l d t l


52/70


Factors affecting velocity estimation


53/70

Factors affecting velocity estimation

(Yilmaz, 1988)

Spread length

Stacking fold / Signal-to-Noise ratio(fold = multiplicity in CMP)

Choice of coherence measure

Departures from hyperbolic move-out

NMO stretch

Effect spread length on velocity estimation


54/70




55/70




56/70




57/70


V l it d l RMS d l


58/70


time

velocity

Interval velocity

(velocity of layer)

Root-mean-square velocity

(weighted-average velocity of layers above)

V l it d l RMS d l


59/70


Time(m-seconds)

CMP location

h t d tProcessing flow


60/70

CMP sorting

shot data

image

NMO correction


stack

velocity model

velocity model

g


61/70

Applying NMO

Amount x

2

/(c

2

T02

) never exactly on a sample:

INTERPOLATION

NMO stretch


62/70

(via picture)

T

T

Since T0 is larger:

shift is smaller

Since T0 is smaller:

shift is larger

Hyperbolic shift

NMO stretch


63/70

(mathematically)

Due to differential working on T as function of T0:

TNMO =x2

2 c2 T0

T0

T0

x2

2 c2 T02

=

This is called NMO-stretch


64/70

NMO stretch


65/70

NMO stretch

Amplitude spectra

Mute: too much NMO-stretch


66/70

We do not want too much distortion: setting it zero.

This called muting


67/70

NMO-

stretch on

field data

shot dataProcessing flow


68/70

CMP sorting

shot data

image

NMO correction


stack

velocity model

velocity model

Stacking


69/70

g

Add traces from NMO-corrected, CMP gather into ONE trace

Number of traces = stack fold

Events that are not hyperbolic, do not add up nicely and

destructively interfere

Goal of stacking : to increase signal-to-noise ratio


70/70

Primaries and multiple

primary

multiple

primary

Date post:	27-Feb-2018
Category:	Documents
Upload:	ricky-chiu
View:	229 times
Download:	1 times

Chapter 5 - Processing of Seismic Reflection Data 1 01

Documents