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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])

 

General Statement

Seismic Interpretation is the extraction of subsurface geologic information from seismic Previous HitdataNext Hit. 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 Previous HitdataNext Hit. The danger in seismic interpretation is in thinking that everything we see is geology!

Reflection seismic Previous HitdataNext Hit comprise:

  • Continuity of reflections indicating geologic structure.
  • Variability of reflections indicating stratigraphy, fluids and reservoir fabric.
  • The seismic wavelet.
  • Noise of various kinds and Previous HitdataNext Hit defects.

    • 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 Previous HitdataNext Hit 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.

    General statement

    Figures

    Noise

    Structure

    Stratigraphy

    Conclusion

     

     

     

     

     

     

     

     

     

    General statement

    Figures

    Noise

    Structure

    Stratigraphy

    Conclusion

     

     

     

     

     

     

     

     

    General statement

    Figures

    Noise

    Structure

    Stratigraphy

    Conclusion

     

     

     

     

     

     

     

     

    Figure Captions

    Figure 1. (Left) The horizon track on Lines 57 and 60 defining the structure, and (Right) the Horizon Slice sliced through the Previous HitdataNext Hit volume 40 ms below. (From Interpretation of Three Dimensional Seismic Previous HitDataNext Hit, AAPG Memoir 42, SEG Investigations in Geophysics No. 9, Seventh Edition, 2011.)

    Presence of Noise

    Noise is ever-present in seismic Previous HitdataNext Hit. 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 Previous HitdataNext Hit may suffer defects because of:

  • Irregular Previous HitdataNext Hit acquisition showing as footprint.
  • Obstacles to the Previous HitdataNext Hit acquisition crew.
  • Equipment difficulties in the field.
  • Processing problems.

    • 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.

    Defining Structure

    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 interpretation. To aid in this endeavor the seismic interpreter can manipulate the Previous HitdataNext Hit and the display in various ways. The time-honored approach to prepare the Previous HitdataNext Hit for Previous HitstructuralNext Hit interpretation is to apply AGC (Automatic Gain Control) in the late stages of Previous HitdataNext Hit processing. This reduces amplitude variability (where most of the stratigraphic information lies), and hence increases visible Previous HitdataNext Hit continuity.

    The interpreter also may compress the display color bar to optically saturate and thus to render invisible more of the amplitude variations. Other Previous HittechniquesNext Hit 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 Previous HitstructuralNext Hit interpretation only, and that the requirements of later, more advanced types of seismic interpretation are quite different.

    Defining Stratigraphy and Reservoir

    Once the structure has been established, the interpreter turns his attention to stratigraphic interpretation and the detection of hydrocarbon fluids. Overwhelmingly important here is seismic amplitude - and the amplitude may be presented to the interpreter or extracted from the Previous HitdataNext Hit in various ways. The Previous HitdataNext Hit loaded to the workstation must be True Amplitude and Zero Phase, and the interpreter must satisfy himself that the Previous HitdataNext Hit used are such. Understanding the wavelet is complicated and very important (part of the fundamental separation of effects) but outside the scope of this article.

    In order to increase the visibility of stratigraphic variations the interpreter will remove the structure - and the best way to do this is to make a Horizon Slice. The concept behind the Horizon Slice is the reconstitution of a depositional surface at a key point in geologic history. The structure used for the reconstitution is most commonly defined at the level of the objective. However, it is often better to define the structure at one level (conformable with the objective) and to use this to remove the structure at the objective level. This very effectively separates structure into step one and stratigraphy into step two.

    This procedure is illustrated in the accompanying Figure 1. The horizon tracked on the two vertical sections follows a reflection with good Previous HitstructuralNext Hit continuity and little, if any, stratigraphic variability. The horizon track is then displaced downwards by 40 ms (a simple horizon computation on the workstation) to intersect the prominent red blob visible below it, and the amplitude is then extracted along the displaced track. The resulting Horizon Slice, on the right of the Figure 1, shows a very clear channel (the spatial pattern of the red blob) with interesting amplitude variations along it.

    When the seismic interpreter extends his analysis even further and enters the field of reservoir Previous HitevaluationNext Hit, the Previous HitdataTop 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.

    Conclusion

    So seismic interpretation is the thoughtful separation (with workstation assistance) of the various effects that the subsurface and the seismic acquisition process have mixed together!

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