This issue of the RECORDER is the first of two 'Special Issues' (June and September 2001) being published, focussing on the theme 'Significant developments in Seismic Exploration in the last decade and future directions'. The overwhelming response that we received to our request for contributions to the 'Special Issue' has prompted us to crank out two special issues, instead of one as originally planned. We are all aware that advances in seismic technology have resulted in greatly improved performance in exploration and development in the last decade. So, applauding these past developments, it is possible to identify the trends that are afoot for the future. Of the different geophysical area s identified (see May 2001 issue, p6), we have selected eleven papers from the following areas for this issue as shown below.
- 3D Seismic data acquisition (2)
- Wavelet analysis (signature deconvolution) (1)
- AVO/ LMR analysis (6)
- Seismic Inversion (2)
In the first paper, '3D seismic survey designs - past, present and future', Mike Galbraith gives an overview of onshore 3D seismic survey designs. He discusses the early days of 3D, in particular covering the many 3D geometries which have been developed and used. He addresses the various issues that have driven these developments, such as sampling requirements, aperture considerations, and especially the different types of noise plaguing 3D's. He comments on the various 3D acquisition schemes in use today, and muses about areas in which better or different 3D geometries may be required in the future, such as 3D time lapse, 3D AVO, and integrated 3D / VSP.
Gijs Vermeer in his paper 'Seismic data acquisition developments' lists the significant developments in the last decade, and discusses recent developments and future directions. On the marine side, MSMS acquisition, feathering, airgun arrays, 4C cables, etc., are some interesting issues discussed. On the onshore side of things, Vermeer comments on 3D spatial sampling, single-sensor receivers, converted versus shear source acquisition, and finishes with some interesting remarks on 3D survey design.
Anton Ziolkowski in his paper 'Seismic wavelet estimation without the invalid whiteness assumption' questions the validity of processes based on the whiteness assumption, arguing that this assumption has no evidence to support it. He discusses the deterministic approach of source signature measurement and deconvolution the seismic data which would allow the wave let to be short and constant throughout the data.
John Castagna in his paper 'AVO Analysis' describes the current state of the art in AVO analysis and then discusses future directions. Castagna believes that poor utilization of AVO, lack of robustness, interpretation complexity, and uncertainty analysis are some issues that dog the AVO method. However, he concludes that theoretically AVO analysis is sound, it's just that it requires time and effort to understand the technology and see through the complexity.
Mike Graul in his paper 'AVO: Yesterday, Today and Tomorrow' begins by taking a retrolook at what has happened in AVO in the last decade and how it points to the future. He discusses various techniques being used in AVO analysis and what effects/advantages/influences they have. Included in this are forward modeling, crossplotting, pressure predictions, pore fluid and litho discriminators, processing considerations.
Bill Goodway in his ambitious paper entitled 'AVO and Lame constants for rock parameterization and fluid detection' demonstrates that understanding velocity and impedance measurements in terms of Lame parameters, incompressibility and rigidity and their attributes offers improved identification of reservoir zones and a better understanding of lithologic variations.
Brian Russell and Ken Hedlin, in their paper 'Fluid property discrimination with AVO', examine the LMR, pore modulus, and fluid discriminant techniques in terms of Biot-Gassmann theory. They show how all three approaches are related through the constant value used in a weighted difference stack between the square of P- and S- impedances. The discussion is illustrated through the use of some interesting real data examples.
Jon Downton and Larry Lines in their paper ‘AVO feasibility and reliability analysis in the presence of random noise’ show that in the presence of noise, constraints are needed to make the reflectivity attribute estimates for linearized AVO inversion more certain. These could potentially introduce theoretical error into the problems. However, the correctness of different approximations could be viewed in terms of how geologically plausible the used constraints are.
In ‘AVO and the general inverse theory’, Guillaume Cambois raises some intriguing questions about how reliable conventional AVO extraction is, and offers an alternative which perhaps avoids some of the potential pitfalls. He does this using general inverse theory. Cambois presents unsettling evidence showing a statistical correlation between intercept and gradient, and the possibility of systematic non-random extractions of intercept and gradient from pure noise.
Subhashis Mallick in his paper ‘Prestack waveform inversion using a genetic algorithm - the present and the future’ discusses the simple assumption that is made in AVO analysis - that there is no contamination of P wave amplitudes by other modes. This assumption is usually not valid. Mallick proposes that full waveform prestack inversion needs to be used, which gives an estimate of the optimum earth model at a given CMP location.
Mrinal Sen in his paper ‘Prestack waveform inversion on plane wave seismograms’ focuses on the challenging issues of waveform inversion. In his methodology Sen shows that τ−ρ appears to be the most appropriate domain for carrying out prestack waveform inversion, in that the inherent forward modeling is very fast and it generalizes easily to anisotropy and multicomponent analysis.
These 11 articles represent some of the outstanding technical work being done in our field. We trust this issue will provide you, our readers, stimulating reading, inspire you in your own technical work, and perhaps challenge you to submit an article for a future issue of the RECORDER. Keep in mind that you still have the September RECORDER to look forward to, the second part of the “Special Issue”, and it will contain 12 more excellent articles on a variety of topics.
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