PROCESSING & IMAGING
Q-Compensating PSDMPrestack compensation for transmission anomalies
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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 frequency-•dependent 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•
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.
Stack data migrated using a standard PSDM (top) and with Q PSDM (bottom). Data courtesy of Reliance.
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
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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.
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).
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