EXPLORATION GEOPHYSICS

EARTH MODELEARTH MODEL

NORMAL-INCIDENCE REFLECTION AND TRANSMISSION COEFFICIENTS

WHERE: 1 = DENSITY OF LAYER 1

V1 = VELOCITY OF LAYER 1

2 = DENSITY OF LAYER 2

V2 = VELOCITY OF LAYER 2

DENSITY 1

VELOCITY V 1

DENSITY 2

VELOCITY V2

REFLECTION

COEFFICIENT

112

112

2

2

VV

VVR

TRANSMISSION COEFFICIENT

1122

112

VV

VT

SEISMIC ACQUISITION

GEOPHONE STRUCTURE

GEOPHONE TYPES & FREQUENCY

SOURCE TRUCKS

3D-SEISMIC COVERAGE

CDP & FOLD STACK

MARINE SEISMIC CABLE LAYOUT

MULTIPLE RAYPATH

FACTORS AFFECTING AMPLITUDE

ATTENUATION TECHNIQES

SEISMIC DATA

PROCESSING

PROCESSING FLOWCHART

OBJECTIVE OF PROCESSING

REAL EARTH

EARTH PROPERTIES PULSE

SEISMIC TRACE

PROCESSING

MODEL EARTH

EFFECT OF STACKING ON NOISE

SIGNAL PLUS RANDOM AT SIX-DETECTOR ARRAY

SIGNAL PLUS RANDOM AT SIX-DETECTOR ARRAY

SIGNALSIGNAL

SIGNAL + NOISESIGNAL + NOISE

NOISENOISE

SIGNAL + NOISE

EACH RECEIVERSUMMED OUTPUT

SEISMIC-WELL TIES

PURPOSE: TO COMBINE TWO DATA SETS FOR A MORE COMPLETE INTERPERATION

SEISMIC DATA

• TWO WAY TIME

• SEISMIC VELOCITY

• SEISMIC IMPEDENCE

• SEISMIC SEQUENCES

• SEISMIC FACIES

• STRUCTURE

WELL DATA

• LINEAR DEPTH

• VELOCITY

• DENSITY

• GEOLOGIC AGE

• LITHOLOGY

• FAULT ORIENTATION

ACOUSTIC STRUCTURE OF EARTH

LAYERED EARTH SONIC DENSITY RC

CONVOLUTION

DETAILED SYNTHETIC SEISMOGRAM

DETAILED SYNTHETIC SEISMOGRAM

POST-STACK PROCESSING

• RESIDUAL STATICS

•DISPLAY ENHANCEMENT

• MIGRATION

•ATTRIBUTE ANALYSIS

•INVERSION

•TIME-DEPTH CONVERSION

SUMMARY OF SEISMIC PROCESSING

• DATA ARE GATHERED AND CMP-ORDERED GATHERS ARE FORMED

•VELOCITY ANALYSIS IS PERFORMED ON SELECTED GATHERS

•THE GATHERS ARE MOVEOUT CORRECTED, STACKED, AND PLOTTED

•STACKING MAY BE FOLLOWED BY ADDITIONAL PROCESSING

VELOCITY ANALYSISNORMAL MOVEOUT (NMO)

X

TX

= TX - T0

IMAGE POINTWHERE

T0 = ZERO OFFSET TIME

T0T0

NMO-ISOTROPIC LAYER

AX

C

Z

IMAGE POINT

B

D

V

ZT

2

V

DC

V

BCABTX :

222 )27( XDC

2

2

2

2

2

2 27

V

X

VV

DC

2

222

V

XTTX

WHERE

THEREFORE

MULTI-LAYER CASE

FOR LAYER 1:

FOR LAYER 2:

2

222

V

XTTX

22,

22

2,2

2,NMO

X V

XTT

2/122

2

][TT

XV

XNMO

VNMO = NORMAL MOVEMEOUT VELOCITY

ASSUMED RAYPATH FOR VNMO CALCULATION

X

TX,1

Z

V1

T0,1

T0,2

TX,2

ACTUAL RAYPATHACTUAL RAYPATH

HOW IS VNMO DETERMINED?

FOR EACH TRACE T AND X VALUES ARE DETERMINED

VELOCITY ANALYSIS

HYPERPOLA PLOTS AS A STRAIGHT LINE ON T2 - X2 GRAPH

T2 - X2 PLOT

2,NOT

2XT

X2

OFFSET2

t2

2,NXTY

2,NOTb

2

2

2

1

X

t

Vm

NMO

2/12

22/1 )()/1(

tMVNMO

Y=b+mXWHERE

VMNO CORRECTION

KNOWING THE VNMO, T AND OFFSET, THE TIME CORRECTION NECESSARY TO SHIFT

EACH TRACE CAN BE CALCULATED

INTERVAL VELOCITY DETERMINATION FROM SEISMIC

FROM VELOCITY ANALYSIS, WE KNOW

VNMO1,T0,1

VNMO2, T0,2

SUBTITUTING THESE VALUES INTO THE DIX EQUATION

2/1

1,02,0

1,02

12,022 ]

**[

TT

TVNMOTVNMOVID

2/1

1,02,0

1,02

12,022 ]

**[

TT

TVNMOTVNMOVID

MIGRATION EFFECTS

• FLAT HORIZONS REMAIN UNCHANGED IF THERE ARE NO

VELOCITY ANOMALIES ABOVE

• DIPPING HORIZONS BECOME STEEPER, SHALOWER, AND

MOVE LATERALLY UPDIP

• SYNCLINES BECOME BROADER, WITH ANY BOW TIES

ELIMINATED

• ANTICLINES BECOME NARRWER

• DIFRACTIONS COLLPSE TO POINTS

RAY PATH MIGRATION

X

t Φ TAN

tV2

1SINΘ X

SIN V

2 TAN

WHERE

= TRUE DIP OF REFLECTOR

= APPERENT DIP OF REFLECTOR

V= VELOCITY

REFLECTIONREFLECTOR

x

t

WHY MIGRATE?IF BEDS ARE DIPPING, APPERENT POSITION OF

EVENTS ON A STACKED SECTION DIFFERS FROM THEIR TRUE POSITION

DEPTH MODEL

DEPTHTIME

ARRIVAL TIMES

7 KFT/SEC

11 KFT/SEC

9 KFT/SEC

ASSUMPTION UNDERLYING MIGRATION

• ALL EVENTS ARE PRIMARIES-

NO NOISE, NO MULTIPLES, NO SHEAR WAVES

• ALL EVENTS COME FROM THE PLANE VERTICALLY

BENEATH THE SEISMIC LINE-

NO SIDEWIPE

• VELOCITY ARE KNOWN EVERY WHERE

MIGRATION MOVES DIPPING HORIZONS

FOCUSING EFFECT OF A SHARP SYNCLINE

DEFOCUSING EFFECT OF AN ANTICLINE

FOCUSING EFFECT OF A SYNCLINE

3-D MIGRATION

• ELIMINATES ASSUMPTION OF 2--D MIGRATION THAT ALL

DATA COMES FROM WITHIN PLANE OF SECTION

• REQUIRES 3-D DATA COLLECTION TO INSURE CLOSELY

SPACED TRACES IN BOTH X, Y DIRECTIONS

• MOVES ENERGY IN BOTH INLINE AND CROSSLINE

DIRECTIONS

• INLINES AND CROSSLINES TIE AFTER 3-D MIGRATION

• MAP MIGRATION IS AN ALTERNAIVE FOR COARSE GRID OF

2-D LINES

TIME VS. DEPTH SECTIONS

CDP GATHER AFTER DEMULTIPLEXING

CDP GATHER AFTER AMPLITUDE CORRECTION

CDP GATHER AFTER DECONVOLUTION

VELOCITY ANALYSIS

CDP GATHER AFTER NMO CORRECTION

CDP GATHER AFTER MUTING STRETCHED ZONES

CDP STACKED SECTION

CDP STACKED SECTION WITH TVF FILTER