Sometimes in geophysics and seismic method in particular, some very thin layers may not be detected from seismic surveys leading to wrong geological interpretations.
Thin beds is a well-known problem in reflection seismic profiling. Theoretically, all the information to describe a thin bed is contained in the wavelet shape and amplitude. The classic paper by Widess (1973) goes through this in quite some detail:
http://geophysics.geoscienceworld.org/content/38/6/1176.abstract
However, from an interpreter's point-of-view, it is often not possible to distinguish between variations in layer thickness and material properties by simply looking at the wavelet shape as this is a highly non-unique problem. Generally, such a qualitative approach is only valid where layers are pinching out rather than pure thin beds. There has been some recent work to use wavelet analysis in aiding interpretation, but this relies on some a priori knowledge of subsurface to get an accurate result:
http://archimer.ifremer.fr/doc/00175/28602/27019.pdf
The most reliable way to characterize a thin bed is through seismic inversion. Although all inversion techniques (pre- and post-stack) will provide some information on the nature of thin beds, full waveform inversion is the most effective the amplitude and phase variations with incidence angle can be used to constrain problem. Below is an example that uses both synthetic and field data to illustrate what can be achieved:
http://www.earthdoc.org/publication/publicationdetails/?publication=6615
Thank you very much for comments. They comments are deeply appreciated. I aloo appreciate the web links
Mark has answered about the blind zone in seismic reflection. Just to integrate the answer I will deal with blind zone in seismic refraction which are a big problem and the real limit of refraction method. In refraction seismic the blind zones are due to two model reason:
1a)Low-velocity layers
2a)Relatively thin layer overlying a layer with strong
velocity contrast;
and one acquisition reason
1b)Short travel-time branch may be missed due to spatial under sampling
(high geophone spacing).
For the a) condition it is quite impossible to resolve the problem from the data, but we can control the errors in the model appraisal if we have information about the blind zone presence (borehole). In the follow papers, for example, you can find some solutions (or blind zone control)
R. Schmöller. Some aspects of handling velocity inversion and hidden layer problems in seismic refraction work,Geophysical prospecting, Vol 30, No 6, December 1982 pp. 735 - 751. DOI: 10.1111/j.1365-2478.1982.tb01336.x
M. S. Vijaya Raghava and G. Nanda Kumar, The blind-zone problem in multiple refraction-layer overburden situations,Geophysical prospecting, Vol 27, No 2, June 1979 pp. 474 - 479. DOI: 10.1111/j.1365-2478.1979.tb00980.x
Some people, for wide angle seismic acquisition, proposed to use post-critical reflection as indicator of the blind zones presence
D. Sarkar, H. C. Tewari, M. M. Dixit and K. L. Kaila, A solution to the blind‐zone problem by modeling of postcritical reflections. GEOPHYSICS,Vol. 56,NO.2(FEBRUARY 1991)
DOI: 10.1190/1.1443044
In the b) case the blind zone problem is solved reducing the geophone spacing, in order to eliminate the spatial aliasing in slowness estimate. It should be underlined that, in general, this condition could be produced also in the interpretation phase when we interpolate inappropriately the travel-times with the linear branches, also when data are acquired with suitable (non aliased) geophone spacing.
Besides the intrinsic un-detectability, the same problems, evidenced by Mark in seismic reflection from the point of view of the signal, exist in refraction if we use a processing approach to refraction like that I proposed in the paper
de Franco R., Multi-refractor imaging with stacked refraction convolution section.
Geophysical Prospecting, v. 53, 335-348, 2005.
Thanks Roberto.Your answers and comments are deeply appreciated
The term "blind zone" in seismic reflection imaging has a different origin from that discussed above for Refraction data. In reflection prospecting, blind zone refers to that part of the subsurface which is not imaged for a given geometry of the survey. That is, if you draw ray paths for the given position of shots and receivers, the rays are not reflected from the parts in the blind zone. This can typically happen, for example, when trying to image below a thrust of high velocity formation , or under a salt dome.
One way to minimize the problem of blind zone is to do ray tracing - but not from the surface but from subsurface points - for a likely model of the subsurface and see what sort of surface geometry is required to get adequate sampling of the subsurface reflection points.
However, I don't know if there is any published work following this approach. Most commercial software do the ray tracing only from surface positions and that is not good enough to come up with proper geometry to image complex subsurface.
Chaitanya is spot on regarding bond zones in reflection surveying. I should have been clearer on this in my previous post.
In answer to the question about ray tracing from subsurface locations, the only software I know for certain that can do this is NORSAR 2D and 3D:
http://www.norsar.no/
hi ...i want to ask a question... is there any method to detecting blind zone by semblance?
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