February 2008
AGM & Technical Meeting
Thursday 28th February at 5:30pm
at 175 Elizabeth Street,
Brisbane
COST: ASEG Members $5 - Students Free
Soft drinks, beer, wine, and finger food provided for attendees
Schedule:
5.30 AGM and Pre-Presentation Food and Drinks.
6.00 Technical Presentations -
- Alan Meulenbroek presents: Analysis of converted refractions for shear statics and near-surface characterisation
- Shaun Strong presents: Multi-component seismic-resolution analysis using finite-difference acquisition modelling
About the Presentation:
Analysis of converted refractions for shear statics and near-surface characterisation:
In this paper we demonstrate resolution issues for representative petroleum and coal scale models,
including the classic wedge model and a coal barren-zones model. These examples illustrate
conventional P-wave resolution capabilities, and also demonstrates the comparative results
for converted-wave imagery.
In certain situations, a richer geological interpretation can be achieved through integrated
compressional (P) and shear (S) wave seismic imaging. Converted-wave reflection is an
economical methodology for such integrated analysis. However, one of the major impediments
to viable onshore converted-wave imagery relates to the difficulty in defining S-wave receiver statics.
This is because near-surface S velocities are much lower, and often more variable, than P velocities.
Refraction statics is a standard deterministic approach to conventional (P-wave) statics analysis.
This technique examines an extension whereby S-wave statics are estimated via analysis of PPS refraction
arrivals. These are refracted waves which convert from P to S for the up-going, head-wave section.
Since these PPS refractions are not first arrivals, their identification and analysis is more challenging
than for standard P-wave refraction. Based on standard P-wave practice, we believe that an optimal
production approach will include converted-refraction analysis, followed by converted-wave residual statics.
Although the thrust of this work is towards derivation of S-wave statics, an interesting auxiliary output
is also available. Based on theoretical modelling, the S-to-P time-depth ratios can be tuned to provide
P-to-S velocity ratios (and hence dynamic Poisson's ratios) for the near-surface. This has interesting implications for lithological and rock-strength analysis
in the mining, engineering, and environmental contexts.
Multi-component seismic-resolution analysis using finite-difference acquisition modelling:
A number of simple rules-of-thumb have been widely used to predict vertical and horizontal resolution limits
(e.g. Rayleigh and Widess limits; Fresnel zone). These measures provide a basic feel for the relationship
between final image-wavelength and resolution. However, seismic resolution ultimately depends on more
fundamental factors. These include survey design (fold, receiver spacing, aperture, etc), source bandwidth,
geology, and the design and sequence of algorithms used in the CMP stacking process. As targets become more
subtle, resolution analysis needs to be more controllable in terms of these individual factors.
For this investigation we use viscoelastic finite-difference modelling to simulate the acquisition of a sequence
of multi-component shot records over 2D geological models of arbitrary complexity. These shot records are then
processed and interpreted using standard real-data methods. This allows us to examine the influence different
processing algorithms have on resolution.
Attenuation (Q) is specified throughout each model for P-waves and S-waves independently.
This facilitates an instructive comparison of the resolution capabilities of conventional and converted-wave images.
Realistic numerical modelling, simulating the full acquisition sequence, leads to a more pragmatic understanding
of seismic resolution issues. It is a valuable tool, both for survey planning and image interpretation.
About the Speakers:
Alan Meulenbroek graduated from University of Queensland with a B.Sc.App. (Geoph) (Hons 1) in 2006.
His role at Velseis combines field operations and R&D, with current research focusing on multicomponent
seismology. He is enrolled in the M.Phil. Degree at the University of Queensland.
Shaun Strong graduated from University of Queensland with a first-class honours degree in geophysics
(B.Sc.(Hons)) in 2003. After a short period doing gravity acquisition, he joined Velseis, working in production
data processing and more recently R&D. Currently, his primary focus is on multi-component research, including
acquisition, data processing, algorithm development, and interpretation.
The Irish Club:
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