TGS - AVO Processing

Accurate algorithms are used to correct for losses in an offset dependent manner.
Less precise prior corrections are backed off and replaced.
Corrections for angular (offset) variation of receiver array sensitivity are made if significant.

Windowed trace amplitude variations are displayed as color-coded plots of CDP or Shot location v offset. Windowed trace amplitudes are decomposed into surface consistent components and used to:
a) correct for systematic acquisition effects.
b) monitor effects of processing on amplitude change with offset
Horizon amplitudes are displayed either as color-coded horizon slices showing CDP location v offset or as line plots above gather displays

Irreversible scaling operations and offset dependent processes present in the normal processing (imaging) sequence are eliminated to produce “true amplitude post-PSTM gathers”
True amplitude gathers limited by angle or offset are three term NMO corrected, checked for flatness, re-picked if necessary and stacked to produce angle or offset limited fold normalized stacks
Incidence angle ? for each sample is estimated on the assumption of hyperbolic moveout and no reflector dip
Derived difference stacks (FAR-NEAR) or (FAR-NEAR)*NEAR are generated to highlight the location of data having unusual AVO behavior.

Sample amplitudes A(?) of 3 term NMO corrected true amplitude gathers are analyzed and fitted to “Shuey’s” approximation to the Zoeppritz equations A(? )= A+B SIN2(? ) (accurate to about 30º)
Statistically unreliable points and non-linear fits are rejected.
Attribute A, also known as the Intercept “Stack” represents vertical incidence P wave reflectivity convolved with the average seismic wavelet
Attribute B, also known as the Gradient “Stack” represents change of P reflectivity with SIN2(? ) convolved with the same wavelet. For normal decay of reflectivity with offset A and B have opposite polarity and A*B is negative

Attributes A and B can be combined to produce:

(A* B)
This can be strong and positive for top and base of gas filled low impedance sandstones (Type 3 AVO anomaly)
(A-B/)
This can be non-zero where hydrocarbon present B is scaled to B/ so as to equal A on average
(A+B)
This can be anomalous at top of reflector above gas. Also known as “Poisson ratio Reflectivity”
FLUID FACTOR
A combination of A and B based on the local Vp/Vs ratio. Can be non-zero for gas filled reservoir.

Plots of A/B can be made and anomalous regions on these plots posted back onto section of A or B to identify their origin.