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GCWhat is Seismic Interpretation
?*
Alistair Brown1
Search and Discovery Article #41125 (2013)
Posted May 17, 2013
*Adapted from the Geophysical Corner column, prepared by the author, in AAPG Explorer, May, 2013. Editor of Geophysical Corner is Satinder Chopra ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic
1Consulting Reservoir Geophysicist, Allen, Texas ([email protected])
Seismic Interpretation
is the extraction of subsurface geologic information from seismic data. On that definition we all are agreed.
However, if we seek a more penetrating explanation, we find practitioners get tongue-tied and talk around the subject in a variety of ways. In this article I attempt to give a longer, more descriptive definition that will apply to every interpretation
project involving reflection seismic data. The danger in seismic
interpretation
is in thinking that everything we see is geology!
Reflection seismic data comprise:
Seismic interpretation
is the thoughtful procedure of separating these effects. The seismic wavelet starts as the pulse of seismic energy, which, generated by the energy source, travels down through the Earth, is reflected and travels back up to the surface receivers carrying the geological information with it. This recorded wavelet is a minimum phase of some frequency bandwidth, and during data processing it is converted (we hope) into a zero-phase wavelet, making
interpretation
easier and more accurate.
The interpreter is not directly interested in the wavelet itself but rather in the geological information that it carries. Thus, understanding the wavelet and distinguishing its characteristics from details of the geology is one of the critical tasks of today's interpreter.
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Noise is ever-present in seismic data. It may be random noise, it may be multiple reflections, it may be refracted energy, and it may be other energy of unknown source. The data may suffer defects because of: The interpreter must know enough about the acquisition and processing to recognize these undesirable features, and thus to not confuse them with the geology he/she seeks. Seismic energy is reflected from interfaces where the acoustic properties of the rocks change. These interfaces follow sedimentary boundaries created at the time of deposition of the sediments. Following the continuity of these reflections then defines for us the structure imposed on these boundaries by the tectonic forces of geologic history. Following this continuity and making structure maps is thus the most basic, and most traditional, activity of seismic The interpreter also may compress the display color bar to optically saturate and thus to render invisible more of the amplitude variations. Other techniques include the use of Instantaneous Phase (which completely destroys amplitude information) and Structurally Oriented Filtering. All these are good ideas - provided the interpreter realizes that they are directed at Defining Stratigraphy and Reservoir Once the structure has been established, the interpreter turns his attention to In order to increase the visibility of This procedure is illustrated in the accompanying
Figure 1. The horizon tracked on the two vertical sections follows a reflection with good When the seismic interpreter extends his analysis even further and enters the field of reservoir evaluation, the data requirements are even more stringent, but the Horizon Slice concept is still effective in removing the effects of structure. Some form of Inversion may be used here, and this process converts interface information (amplitude) into interval information (acoustic Impedance). The more advanced forms of inversion seek to remove the wavelet, and this is therefore part of the fundamental idea of separating effects. However, the challenge here is to exactly understand the wavelet that has to be removed. This is difficult, and many inversions suffer and projects fail because of this issue. So seismic |