Q-Compensating PSDM - CGG Veritas

PROCESSING & IMAGING
Q-Compensating PSDM
Prestack compensation for transmission anomalies
Spatial variations in the absorption properties of the overburden cause frequency-dependent
dissipation effects. These result in seismic amplitude attenuation and wavelet phase distortion
which mask underlying events and decrease resolution. CGGVeritas proprietary Q Tomography
and Q PSDM help address these issues. Q Tomography builds a model of the anomaly using
prestack amplitudes. Q PSDM can then use this model to compensate, recovering bandwidth,
allowing meaningful AVO studies to be performed and aiding in the accurate prediction of
reservoir properties.
FEATURES:
• Implemented for Kirchhoff, Beam and Reverse Time Migration
algorithms
• Compensates for frequency-dependent loss during propagation,
giving correct phase, amplitude and resolution
• Uses the estimated absorption model from frequencydependent
Q Tomography to compensate for Q effects relative
to the background
• Goes beyond simplistic surface-consistent assumptions
to compensate for transmission effects anywhere in the
overburden
BENEFITS:
• Enables meaningful AVO analysis under complex overburdens,
for example:
- Gas pockets or gas hydrates
- Unconformities
• More accurate prediction of reservoir properties
• Easier identification and interpretation of deeper reflectors
Amplitude spectra for a good data zone (red), compared to an
anomaly zone on a standard PSDM (blue) or the broadband result
from Q PSDM (green).
Safer, Clearer, Better
Stack data migrated using a standard PSDM (top) and with Q PSDM (bottom). Data courtesy
of Reliance.
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PROCESSING & IMAGING
CONTENDING WITH GAS HYDRATES
Gas hydrates can be found in the shallow sediments of many deep ocean
areas. These form zones which strongly attenuate seismic amplitudes
and are characterized by strong reflectivity due to the higher-velocity
hydrates overlying the lower-velocity free gas interval. Dim zones occur
beneath these gas hydrate pockets. The size and shape of the dim
zones vary over different offsets, prohibiting AVO analysis and reservoir
evaluation of the deeper reflectors.
To resolve this issue, a frequency-dependent tomographic amplitude
inversion is applied to offset-domain Common Image Gathers in order to
estimate an interval Q model.
After deriving and validating a geologically meaningful Q model, Q PSDM
uses it to compensate for amplitude and phase distortions during the
migration itself. This generates a clearer image of the subsurface and
enables more accurate interpretation and AVO analysis.
The examples on this page use data taken from the extensive CGGVeritas
data library in the Gulf of Mexico.
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Superposition of conventional Reverse Time Migration (black traces) with Q Reverse Time
Migration (colour traces), showing how Q migration corrects for phase as well as the more
visually obvious effects on amplitude.
Comparison of stack data from the Alaminos Canyon area, Gulf of Mexico, after conventional Reverse Time Migration (left) and Q Reverse Time Migration (right). After Q migration, events
below the gas hydrates appear more continuous with more consistent amplitudes (dashed circle).
10F–PI-447-V1