The Limitations of Seismic Reflection Data

TUTORIAL 1

The limitations of seismic reflection data

Seismic reflection data is an essential tool for the modern geologist and most particularly for the petroleum geologist and this tutorial is about the limitations of these data – what you can’t see as much as what you can. This includes the following concepts;

 Superposition of wavelets - the convolutional model of the earth  Seismic vertical resolution  Seismic horizontal resolution - Fresnel zones  The effects of seismic migration

These concepts are partially explained in ♣ Kearey, P. and Brooks, M., An Introduction to Geophysical Exploration, 2nd edition, or the 3rd edition by Kearey, Brooks and Hill., Blackwell Scientific Publications, chapter 4 (This is on restricted loan in the geology library). The two key references are:

♣ Sheriff, R.E., 1977, Limitations on resolution of seismic reflections and geologic detail derived from them, in Seismic stratigraphy, AAPG Memoir 26. BOX FILE

♣ Farmer et al, 1993, Structural imaging: toward a sharper subsurface view, Oilfield Review, Jan 1993. ON WEBSITE, WEEK 1 REFERENCE: MIGRATION.

If necessary, begin by refreshing your memory about the basic principles of reflection seismic data processing, which we covered in the second year, with the help of Kearey, Brooks and Hill. Then read Sheriff (1977) and Farmer et al (1993).

It would be sensible to make notes of the references you read. Also read carefully through the relevant section of my course notes.

In the tutorial, I will be showing you seismic data and asking you to interpret it with particular reference to limitations of horizontal and vertical resolution, and differences between time and depth images. An aid to reading Farmer et al:

Start from an understanding of the CDP stacking process to produce a 2-D seismic section, known as a post-stack time section. In general, reflectors on this section will not be in their true spatial position. If we have an accurate velocity model, referred to as the macro model, it is comparatively easy to calculate the true spatial position of reflectors from the time section (distance = velocity× time). It is particularly easy if we can assume that the velocity is constant. The key problem is that we do not normally have a reliable velocity model. So we make do with a simple model and obtain what is somewhat confusingly known as a migrated time section. This will be better that the unmigrated section because diffraction tails will have been removed etc but lateral positioning cannot be trusted. To do better, we have two alternatives; (1) Calculate how a ray would bend if it left the surface vertically and descended into the earth obeying Snell’s law at all interfaces. This is known as the image ray. It is the computationally cheap (though not totally accurate) way of calculating the true lateral position of a reflector that has only been time migrated. (2) Commit yourself to a macro model, probably obtained from additional data, and do a proper job of migration to obtain a migrated depth section. The problem is that if the macro model is wrong, then the depth section will be seriously wrong, which is why people are cautious about creating depth sections too soon and are happy to initially look at a time section.

So what about pre-stack migration? The CDP stacking process works fine if structures are relatively horizontal. But as structures become progressively complex, the whole basis of the CDP stacking process becomes invalid. If we construct a seismic section made up of ray paths of one constant source/receiver offset only (a single fold stack), then this can be migrated in the usual way. So construct, for example, 120 single fold seismic sections for the 120 different source/receiver offsets and time or depth migrate each of them individually. Finally, we can stack the 120 migrated sections to obtain the pre-stack migrated section. Clearly, the computational effort compared to post-stack migration, has increased by a factor of 120. Until a very few years ago, computers were not powerful enough to make this a feasible option. However, this really is the correct way of proceeding.

Finally, all these various options can be performed in 2-D or 3-D. 3-D pre-stack depth migration of an average size survey still takes about a month of computer time on the fastest industry computers.

Do not worry about the final section titled which migration algorithm? Also ignore all references in the article to yet another approach called Dip Moveout (DMO). Your aim is to obtain a qualitative understanding of migration options at a level of understanding expressed in Farmer et al and this summary plus handout/lecture notes.