DATA PROCESSING
AVO
AVO describes the effect
of variation in reflection
amplitude with incident
angle (offset) at an interface.
This is as a result of differences in rock properties either side of the
interface. The relative change in reflection coefficient is particularly
significant when the Poisson’s ratio changes greatly across the
interface. It is this phenomenon which allows AVO effects to be used
in the detection of gas.
For a detailed study of AVO character, rock property information is
ideally required. If a well log is available then parameters such as
undrilled
compressional velocity and bulk density can be derived. Shear velocity
is also needed and can be calculated approximately by other methods.
Forward modelling can then be done and synthetic gathers derived.
These can then be compared to real gathers at the well location.
Spectrum’s AVO programs offer a cost-effective method of examining
large volumes of data for AVO anomalies. They use a qualitative
Class 2 anomaly (undrilled)
approach designed to highlight those areas where further in depth
studies such as modelling or inversion may be appropriate.
Intercept & Gradient Stacks
Spectrum offers two methods of straight-line fitting of amplitude
against offset. The first is a least squares technique to estimate
the intercept and gradient values for a straight-line fit of amplitude
against offset profiles.
The second follows the work of Walden (1990), and uses a robust
statistical method to calculate the intercept and gradient values for a
Fluid factor stack
straight-line fit of amplitude against angle of incidence. The straightline assumption is tested and where this breaks down, the gradient
values are edited. These are then presented as pseudo-seismic
sections, allowing correlation with conventional data.
Angle Stacks
Data can be stacked by corridors, offset ranges or incident angle
ranges – this way the amplitude anomalies are enhanced and can
be studied in greater detail. An option is also available to transform
normal X-T gathers to incident angle(q)-T gathers for further analysis.
These AVO programs can produce various AVO attribute volumes to
Intercept stack
aid the interpreter such as Pseudo Poissons ratio, Pseudo S wave,
Fluid Factor or various derivatives and combinations from intercept (I)
and gradient (G) stacks such as I X G, I – G, Standard Deviation of
I & G. A wide variety of cross plotting is also available.
DATA PROCESSING
Azimuthally dependent Amplitude
vs. Offset in 3D seismic (AVAZ)
Spectrum offers a practical
method of evaluating true
AVAZ(AVO as a function of
azimuth) effects in 3D.
Seismic data may be recorded with different offsets and azimuths at each mid
point, and there are geologic factors which cause the AVO response to vary
with respect to azimuth. Conventionally extracted AVO attributes are incorrect
if the azimuth dependencies are not considered. AVAZ behaviour is measured,
together with the provision of a statistical significance attribute which adds a
level of confidence in the analysis and interpretation of the results.
A cause for AVAZ to exist might be the presence of an orientated anisotropic
medium, either in the overburden, or at the reflecting interface. Anisotropy
may for example be due to highly organized fracturing.
Amplitude
A less obvious AVAZ effect would be anticipated, and has been observed, for
mode converted waves in structured media. There may be preferential orientation
for mode conversion, leading to strong shear wave energy in one direction, but
perhaps little or no mode converted energy in a perpendicular direction.
Without AVAZ, AVO variations describe a planar circular pattern with azimuth,
that is a circle can be fitted through the same amplitude which must exist at
all azimuths at constant offset. Where AVAZ occurs, one would expect AVO
variations to differ and exhibit an elliptical pattern with azimuth (Lefeuvre,
1994), or to be sinusoidal as azimuth varies at constant offset. The analysis
of the pattern variation is our key to measuring AVAZ.
Offset
AVO line fit
Amplitude
AVAZ must follow certain rules:
•At zero offset, there can be no azimuthal dependence.
•It is possible for AVO gradients to vary with azimuth from positive through
zero to negative. The method to analyze AVAZ has to be able to recognize
and measure any of these situations.
•While there can be no AVAZ in the absence of AVO, there can be AVO
and no AVAZ if no azimuthal variations are present.
Offset
Azimuth
AVAZ surface fit
Further details of AVAZ can be
found in the paper given at the
SEG 2000
The method of solution allows generation of a set of attributes from
adequately sampled geometry and permits examination of several 3D
cubes of attributes as a final result. Cubes of eccentricity and phase can
be produced as well as the more usual intercept and slope, which tend to
be more stable when analyzed in azimuth sectors. This technique is best
described in terms of surface fitting as opposed to line or curve fitting. The
computation employs the robust statistical methods of AVO determination
(Walden, 1991) expanded to estimate AVO in azimuth sectors. The statistical
methods permit quality control of the results in terms of error analysis
cubes that give information about the stability and reliability of the result.
Walden estimates are less susceptible to spurious noise than least-squares
solutions, and consequently provide a more robust analysis.
The above method can be performed on Spectrum’s PSTM outputs as our
software has the capability of outputing independently migrated azimuth
sectors for subsequent recombination and/or analysis.
email: [email protected]
www.spectrumasa.com