SEG/EAGE Distinguished Instructor Short Course 2011
Seismic Acquisition from Yesterday to Tomorrow
There seems to be a very recent acceleration in the evolution of seismic acquisition. Offshore, wide-azimuth surveys have resulted in images of remarkable clarity. On land, increase in channel count has allowed the use of denser grids leading to significant noise reduction. Both onshore and offshore, the race for bandwidth extension is tenser than ever. This course presents these developments as a natural consequence of the conjunction of our need for clearer seismic images and the availability of recent technological advances. The core of the course is the relationship between acquisition parameters and seismic image quality.
- Historical overview
Understanding the present state of seismic acquisition and, in particular, some of its surprisingly conservative aspects requires us to look back into our short history. It will be shown how improvements in recording techniques have enabled early geophysicists to distinguish reflections in the wiggling seismograms behind the first refracted arrival, how they were led to deploy their instruments at sea, and the importance for our industry of the digital revolution which allowed us to shift from 2D to 3D and now 4D seismic acquisition.
- Elementary theoretical recap
Most of what we do in seismic acquisition can be understood with very simple theories. In fact, Snell's law can describe almost all of our experiments. Since we cannot record continuous data neither in time nor, more importantly, in space, we must be familiar with sampling theory. Definitions of terms and objects used in the course are given at the end of this chapter.
- Signal and Noise
In the data we collect, one part is useful for making seismic images, and the other part may prevent these images from being useful. Even though sorting one part from the other is more of a processing task, the acquisition geophysicist must provide the conditions for this sorting to be feasible. The geophysicist must therefore understand the available leverage on the respective importance of signal and noise. Some essential properties of seismic body waves (the signal part) are reviewed in the first section. The second section is dedicated to the noise part. The third section discusses the relationship between both parts, the so-called "signal-to-noise ratio".
- Components of seismic acquisition
This chapter is about sources and receivers. After an explosion in the number of types of seismic sources only three have really survived. The vibrator is covered in the next chapter. The explosive and air-gun sources are described, as well as some alternative sources that still solicit some interest. On the receiver side, geophones and hydrophones, as well as their coupling and disposition are described. Newly introduced MEMS sensors bring high hopes but have not yet replaced conventional geophones. Recorder capacity follows an exponential curve. One of their most debated features is whether to use cables or not.
- Vibroseis: a very special source
The vibrator deserves a specific section not only as the most commonly used onshore seismic source but also for its many possibilities that include offshore usage. The acquisition geophysicist must know when to use this source: what it can do and what it cannot do.
- Acquisition geometry - Survey design
2D survey design is a problem that was fundamentally solved more than 20 years ago. Its solution happens to be economically realistic. This is not the case for 3D seismic where economics calls for compromises. To find the right compromise, the relationship between field parameters and imaging must be understood. It is shown that noise control and signal preservation are not always contradictory constraints. 4D seismic will be used to show the importance of error management.
This is not a survey design course. Survey design is discussed but not covered in detail. After attending this course, the participant should:
- Improve knowledge of where seismic acquisition comes from and where it may be going.
- Gain the capability to distinguish between signal and noise parts - at least for most cases.
- Understand the available leverage on signal and noise as well as the leverage that is not available.
- Most importantly, understand that there is a relationship between acquisition parameters and seismic image quality. Ideally increase their knowledge of this relationship.
Who Should Attend
All those interested in seismic imagery. The acquisition geophysicist may discover an unfamiliar presentation of familiar concepts. The processing geophysicist may discover the causes of some types of perturbations in seismic images. Likewise, the interpreter may gain understanding of the limitations in seismic images. Those in charge of financing these images may understand better why they are so expensive. The mathematical level of this course is elementary, although those with no idea of what the Fourier transform is may find some chapters difficult to follow. Those with no prior knowledge of seismic reflection would profit from advance reading of the chapters 2 (especially the definitions) and 3, which will be presented relatively fast in this one-day course. Young professionals or students with no industrial experience will find a presentation very different from those given in universities.
The SEG/EAGE Distinguished Instructor Short Course is sponsored by SEG and EAGE.
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