Seismic Interpretation

This page is the first step of a seismic stratigraphy interpretation. Its objective is to define the genetic reflection packages by the surfaces that envelope seismic sequences and systems tracts. These bounding discontinuities are identified on the basis of reflection termination patterns and their continuity.
Boundaries are defined on a seismic line by identifying the termination of seismic reflectors at the discontinuity surfaces. These terminations occur:
  • Below a discontinuity and the definition of the upper sequence boundary. Examples of this include:
    • Toplap: termination of strata against an overlying surface, representing the result of non-deposition and/or minor erosion.
    • Truncation: this implies the deposition of strata and their subsequent tilting and removal along an unconformity surface. This termination is the most reliable top-discordant criterion of a sequence boundary. Such truncation can also be caused by termination against erosional surface, as for instance a channel.
  • Above a discontinuity and the definition of the lower sequence boundary:
    • Onlap: A base-discordant relationship in which initially horizontal strata progressively terminate against an initially inclined surface, or in which initially inclined strata terminate progressively updip against a surface of greater initial inclination.
    • Downlap: a relationship in which seismic reflections of inclined strata terminate downdip against an inclined or horizontal surface. Examples of downlap surfaces include a top basin floor fan surface, a top slope fan surface, and a maximum flooding surface.
Note: If onlap cannot be distinguished from downlap because of subsequence deformation, the term baselap is used.

Recommended procedures for performing seismic sequence analysis include:
  • Identifying the unconformities in the area of interest. Unconformities are recognized as surfaces onto which reflectors converge.
  • Mark these terminations with arrows.
  • Draw the unconformity surface between the onlapping and downlapping reflections above; and the truncating and toplapping reflections below.
  • Extend the unconformity surface over the complete section. If the boundary becomes conformable, trace its position across the section by visually correlating the reflections.
  • Continue identifying the unconformities on all the remaining seismic sections for the basin.
  • Make sure the interpretation ties correctly among all the lines.
  • Identify the type of unconformity:

Recommended color codes:
If using only black and white:
This hierarchy of surfaces now provides a framework to the reflectors. Be as objective as possible when identifying the discontinuity surfaces of the section. Where possible base your observations more on the geometric relationships of the reflectors than on an interpretation of their origin.
Your next step in the workflow is to interpret the origin of this framework. A suggested approach is to assume that the framework is result of the generation of repeated successions of accommodation and sediment fill (the accommodation succession of Neal and Abreu, 2009). These seismic units of the accommodation successions vary in magnitude and duration and can be interpreted to be the products of sedimentary packages that accumulated on a depositional profile. Successions can be seen to consist of component partial succession sets that sequentially prograde to aggrade, retrograde, and aggrade to prograde to degrade.  
Neal and Abreu, 2009 propose that when interpreting seismic it is assumed that deposition has responded to accommodation successions that range across time scales of 10–105 ka and have stacking patterns that are products of vectors of accommodation rate (δA)/sedimentation rate (δS). Thus:
  1. δA/δS < 1 and increasing = Progradation to aggradation stacking (PA or lowstand).
  2. δA/δS < 1 and decreasing = Aggradation to progradation to degradation stacking (APD or highstand).
  3. δA/δS > 1 = Retrogradation stacking (R or transgressive).
The interpretation of depositional sequence from a relatively conformable unit will be dependent on data resolution and the lateral extent of the coverage. Stacking patterns will be seen to be repeated across a range of durations and magnitudes accommodation succession. The depositional settings of these accommodation successions can be related to systems tracts, depositional sequences, sequence sets, composite sequences, composite sequence sets, and megasequences and used to describe and interpret a basin’s depositional fill. Thus the framework of the accommodation succession provides a simple, objective observation based, predictive, independent of time or sea-level terminology, flexible for all scales of data and subsequent data-resolution improvement, and provides a guide to incorporate the new observations and make predictions of previously unrecognized complexity elsewhere.
From the perspective of interpretation each full succession should be considered to consist of component partial succession sets that are sequentially, lowstand—progradation to aggradational; transgressiveretrogradation; and highstand—aggradation to progradation to degradation. To paraphrase Neal and Abreu, (2009) the “terms highstand and lowstand as originally defined to label systems tracts relative to a shelf edge, and with an implied relationship between sea level and systems tracts, have been the root of confusion. Like propose that these terms be used in the strict sense of the original definition, because their meaning has been lost when applied to the many depositional settings and high-resolution data sets to which the concepts of sequence stratigraphy are now applied". As Neal and Abreu, (2009) propose this concept of accommodation succession stacking should be used in the interpretation of stratigraphic data within a hierarchal framework of depositional sequences, sequence sets, and composite sequences. As they point out this approach allows an interpreter to accurately categorize observations, provide a basis for predictions away from control points, and develop a framework that allows revisions as higher-resolution data become available.

Boggs, S. Jr., 2001, Principles of Sedimentology and stratigraphy, 3rd Ed., Prentice-Hall, Inc., New Jersey, 726 p.
Neal, J., and Abreu, V., 2009, Sequence stratigraphy hierarchy and the accommodation succession method, Geology, v. 37, p. 779-782
Vail, P. R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A.W. (ed.), Atlas of seismic stratigraphy: AAPG Studies in Geology No. 27, Vol. 1, p. 1-10.
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