SEISMIC

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SEISMIC DATA GATHERING
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GEOPHONE STRUCTURE
3
GEOPHONE TYPES FREQUENCY
4
SOURCE TRUCKS
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3D-SEISMIC COVERAGE
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CDP FOLD STACK
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MARINE SEISMIC CABLE LAYOUT
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MULTIPLE RAYPATH
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ATTENUATION TECHNIQES
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SEISMIC DATA PROCESSING
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PROCESSING FLOWCHART
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OBJECTIVE OF PROCESSING
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EFFECT OF STACKING ON NOISE
SIGNAL PLUS RANDOM AT SIX-DETECTOR ARRAY
NOISE
SIGNAL
SIGNAL NOISE
SIGNAL NOISE EACH RECEIVER
SUMMED OUTPUT
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POST-STACK PROCESSING

• RESIDUAL STATICS
• DISPLAY ENHANCEMENT
• MIGRATION
• ATTRIBUTE ANALYSIS
• INVERSION
• TIME-DEPTH CONVERSION

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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

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FACTORS AFFECTING AMPLITUDE
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VELOCITY ANALYSIS
NORMAL MOVEOUT (NMO)
X
TX - T0
TX
T0
IMAGE POINT
WHERE T0 ZERO OFFSET TIME
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NMO-ISOTROPIC LAYER
X
A
C
Z
B
D
IMAGE POINT
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MULTI-LAYER CASE
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HOW IS VNMO DETERMINED?
FOR EACH TRACE T AND X VALUES ARE DETERMINED
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VELOCITY ANALYSIS
HYPERPOLA PLOTS AS A STRAIGHT LINE ON T2 - X2
GRAPH
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VMNO CORRECTION
KNOWING THE VNMO, T? AND OFFSET, THE TIME
CORRECTION NECESSARY TO SHIFT EACH TRACE CAN BE
CALCULATED
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INTERVAL VELOCITY DETERMINATION FROM SEISMIC
FROM VELOCITY ANALYSIS, WE KNOW
VNMO1,T0,1 VNMO2, T0,2 SUBTITUTING THESE
VALUES INTO THE DIX EQUATION
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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

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RAY PATH MIGRATION
WHERE ? TRUE DIP OF REFLECTOR ? APPERENT
DIP OF REFLECTOR V VELOCITY
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WHY MIGRATE?
IF BEDS ARE DIPPING, APPERENT POSITION OF EVENTS
ON A STACKED SECTION DIFFERS FROM THEIR TRUE
POSITION
DEPTH MODEL
ARRIVAL TIMES
7 KFT/SEC
TIME
DEPTH
9 KFT/SEC
11 KFT/SEC
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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

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MIGRATION MOVES DIPPING HORIZONS
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FOCUSING EFFECT OF A SHARP SYNCLINE
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DEFOCUSING EFFECT OF AN ANTICLINE
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FOCUSING EFFECT OF A SYNCLINE
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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

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TIME VS. DEPTH SECTIONS