Simply defined, seismic interpretation is the science (and art) of inferring the geology at some depth from the processed seismic record. While modern multichannel data have increased the quantity and quality of interpretable data, proper interpretation still requires that the interpreter draw upon his or her geological understanding to pick the most likely interpretation from the many “valid” interpretations that the data allow.
The seismic record contains two basic elements for the interpreter to study. The first is the time of arrival of any reflection (or refraction) from a geological surface. The actual depth to this surface is a function of the thickness and velocity of overlying rock layers. The second is the shape of the reflection, which includes how strong the signal is, what frequencies it contains, and how the frequencies are distributed over the pulse. This information can often be used to support conclusions about the lithology and fluid content of the seismic reflector being evaluated.
The interpretation process can be subdivided into three interrelated categories: structural, stratigraphic, and lithologic. Structural seismic interpretation is directed toward the creation of structural maps of the subsurface from the observed three-dimensional configuration of arrival times. Seismic sequence stratigraphic interpretation relates the pattern of reflections observed to a model of cyclic episodes of deposition. The aim is to develop a chronostratigraphic framework of cyclic, genetically related strata. Lithologie interpretation is aimed at determining changes in pore fluid, porosity, fracture intensity, lithology, and so on from seismic data. Direct hydrocarbon indicators (DHI, HCIs, bright spots, or dim-outs) are elements employed in this lithologic interpretation process.
This article discusses a basic three-step methodology, which, when followed, provides for a more complete and accurate geological interpretation from seismic data.
Step one: interpretation plan Accomplishing a successful interpretation requires that the interpreter first carefully consider the following questions.
What are my objectives? An interpreter should clearly understand what conclusions are required from the data. Because so much information is available on the seismic, it is important to focus maximum attention on extracting the data pertinent to completing the objective task. Does the objective require evaluating the entire dataset from first sample to last, one stratigraphic sequence, or just one specific amplitude anomaly? This dictates what combination of the three basic interpretation types should be used, when the interpretation should be completed, and what supporting databases are required.
What are the regional tectonic, structural, and depositional trends? It is important for the interpreter to have a basic understanding of what tectonic influences and depositional systems occur within the area of the seismic survey to be investigated. Although this preconceived earth model may be vague and incomplete, particularly in frontier basins, it provides interpreters with insight and constraints as to what types of structures, faulting, and stratigraphic geometries may exist. The interpretation of fault styles, structural geometries, and facies patterns must be consistent with regional tectonic forces and basin infilling.
What seismic patterns should I be looking for? Perhaps the most common interpretational pitfall, and certainly one of the most dangerous, is the mapping of events, amplitude, or AVO changes without qualification as to what geological analog they represent. To prevent this mistake, it is critical that all types of available geological data be gathered and merged with the seismic data. Key to this merging are well-constructed synthetic seismograms, vertical seismic profiling (VSP) data, and/or seismic models (see Synthetic seismograms, Checkshots and vertical seismic profiles, and Forward modeling of seismic data). This verifies the seismic signature of the target, the location of the mapping horizon, and the adequacy of the time-depth functions. Varying the synthetic seismogram or model parameters allows for the prediction of seismic responses for various lithologics and fluid types.