Interpretation work flows

First Steps in Seismic Interpretation

Series	Geophysical Monograph Series
Title	First Steps in Seismic Interpretation
Author	Donald A. Herron
DOI	http://dx.doi.org/10.1190/1.9781560802938
ISBN	ISBN 978-1-56080-280-8
Store	SEG Online Store

All interpretation projects include a number of tasks which must be completed in order to meet project objectives. The sequence in which these tasks are performed is commonly referred to as an interpretation work flow; in general terms, a work flow is an outline of the steps in a procedure for doing something. The level of detail in an interpretation work flow can vary depending on how an interpreter subdivides tasks into separate components – some interpreters are “splitters” who finely divide tasks into precisely defined incremental steps, while others are “lumpers” who group closely related and interdependent steps into larger tasks. In either case, an interpretation work flow must include in logical sequence all of the essential steps needed to complete the interpretation. Such a work flow is useful for planning and tracking the progress of an interpretation and ultimately for enabling successful and timely achievement of project objectives.

The following interpretation work flow is generic in the sense that it should be customized to meet the specific objectives and requirements of the project for which it is used. Given the wide variety of interpretation projects in the oil and gas business, ranging from exploration through appraisal and development to production, it follows that there is no single “one size fits all” work flow that can or should be routinely applied to any or every project. Rather, each interpreter should build his own library of work flows based on experience gained from working on different projects and sharing knowledge with other interpreters.

Generic Interpretation Work Flow

Establish objectives of interpretation

Objectives usually are set according to business requirements, availability of resources, and project timeline.

Make sure you have all data (and verify positioning of data)

This step is all too frequently taken for granted. Too many interpreters have learned the painful lesson about the well or seismic line they didn’t know they had or was plotted in the wrong place on the base map.

Review acquisition and processing histories of data if available

These histories certainly exist but finding them can be difficult, especially in the workstation environment. Of greater concern is that some interpreters may not realize how knowing the acquisition and processing histories of their data can improve their interpretive insight.

Review data (use animation/visualization applications)

This step is often shortened or skipped entirely because an interpreter is too eager to get going with picking horizons and faults.

• Make initial assessment of data quality - Remember the three elements of data quality: detection (signal-to-noise), resolution, and image fidelity (focusing and positioning of reflections). In making this assessment an interpreter should be mindful of the purpose for which a data set was acquired and judge its quality accordingly; for example, a high-resolution shallow hazard survey is not judged to be poor quality because it can’t be used for deep exploration.

• Determine structural style - This enables an interpreter to form initial concepts for analogues or models that may be helpful when correlating horizons and faults. These concepts should be regarded as guidelines and not as rules.

• Estimate strike/dip directions (in 3D with respect to inline/crossline directions) - This is more important for 2D data to assist handling of mis-ties. For both 2D and 3D data these directions should always be referenced to compass points; for 3D data they should not be referenced solely to inline or crossline directions. Bear in mind that in some structural settings there may not consistent strike/dip directions.

• Estimate phase and polarity of data if possible - Wavelet phase can be visually estimated using reflections from boundaries with known acoustic impedance contrasts (for example, the sea floor when working with marine seismic data), but the more reliable and preferred method is via wavelet extraction. An interpreter should be certain of the display convention for his data before estimating phase and polarity.

• If multiple data sets, establish reference data set - This is easier said than done. The reference data set might be the one with highest overall quality, or is zero-phase, or consistently ties to wells, or best meets criteria set by the interpreter in conjunction with project objectives.

Establish starting points for correlation

Regardless of how this is done, an interpreter should document these starting points and record his reason(s) for selecting them.

• Make well ties - This most often involves generation of synthetic seismograms (using carefully edited well logs). Where these are not available, then an interpreter should use time-depth information from a checkshot survey, VSP, or some type of trend curve (e.g., a velocity-depth function based on a sediment compaction model).

• Identify framework horizons and sequence boundaries - As before, this requires thorough review of data before beginning to correlate horizons and faults. The exact locations at which these horizons and boundaries were identified should be carefully recorded (“a picture is worth a thousand words” is probably best for this kind of documentation).

• If multiple data sets, confirm or reselect reference data set - In the course of establishing starting points an interpreter may find reason to revisit his earlier selection of a reference data set.

Correlate horizons and faults

This is the essence of the interpreter’s work, where he demonstrates his skill and expertise.

Figure 1.1  In a 3D interpretation project the limiting case for operating on a grid is correlating every line.

• Set up nomenclature system for data management - This is another step to which some interpreters fail to give sufficient attention, especially in the early stages of an interpretation project. Some companies have hard-wired data management systems that interpreters must follow. If an interpreter builds his own nomenclature system, then he should maintain it throughout his entire project and be sure to include a description of it as an essential part of project documentation.

• Determine size of correlation grid(s) - For a 2D interpretation project the correlation grid is essentially the grid of all 2D lines, that is, every available line is correlated (time and data quality permitting). For a 3D interpretation grid, the correlation grid at minimum takes into account the size of the smallest feature of interest to be mapped, keeping in mind the concept of spatial sampling according to the Nyquist theorem.

• Determine need for manual tracking versus autotracking - This is controlled largely by data quality, mostly detection (signal-to-noise), and also whether or not features of geologic importance and interest are marked by coherent reflections which actually can be autotracked.

• Determine need for seed (automatic) tracking versus interpolation of grid(s) - This step involves combining the determinations made in the previous two steps, that is, deciding how to operate on grids in order to fully map horizons and faults (autotracking of faults is not considered here). In a 3D interpretation project the limiting case for operating on a grid is correlating every line (figure 1.1).

Review quality (QC) of all results

The final and essential step – are final maps consistent with the horizons and faults as correlated on the seismic lines? Are the interpreted lines and maps geologically reasonable, that is, are they “reasonable” in the sense of “analogous to known geology,” or “consistent with known geology or sound geologic models,” or “within the context of expectation or realization of some geologic concept or model”.

See also

• Visual estimation of wavelet phase
• Correlation procedures in building a depth migration velocity model

External links

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