What’s seismic interpretation?

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Seismic interpretation is a process of analyzing seismic data for underground minerals, oil, natural gas, or freshwater deposits. Technical problems can arise in correctly interpreting the data where noise is present in seismic imaging and where three-dimensional (3D) seismic interpretation of subsurface structures is attempted. Geological features such as channel faults and stratigraphic formations must first be clearly distinguished and are often superimposed on each other. Enhancing data with spectral features or color coding in seismic software, as well as seeking to improve image resolution, is one of the major components used in determining seismic attributes.

3D seismic maps have become popular with advances in imaging software that allow various features of a seismic reading to be highlighted. This has brought geophysicists into the field of seismic mapping which was once dominated by geologists in the petroleum industry. Geophysicists are often very familiar with the intricacies of 3D mapping features in seismic interpretation, such as azimuth distributions, which are variations in the horizontal deviations of subsurface structures. Geologists have less exposure to such sophisticated mapping techniques and must acquire additional training in geophysics to make sense of it.

There is no dominant way to view seismic data, and different approaches to seismic interpretation must be tailored to local mining, prospecting, or research needs. The fields in which seismic interpretation is now being applied can range from structural geology for building construction to environmental geology for determining fault lines. The process is considered both an art and a skill, with an earlier focus on accurately detecting the volume and extent of underground fossil fuels. New techniques used in the industry focus on post-stack amplitude analysis, offset-dependent amplitude analysis (AVO), acoustic impedance inversion, and more.

Amplitude analysis is used to determine the ability of subsurface layers to demonstrate elastic properties to each other and is useful in determining the level of porosity of layers. In the mid-1980s, AVO technology became popular in the oil industry and, along with 3D imaging, has seen a resurgence of interest, although the process works better in some regions of the world than others. AVO has sometimes received a bad reputation as unreliable, because the geophysical characteristics of rocks and fluids must first be determined to be suitable for AVO analysis. Advance feasibility studies are therefore an essential seismic modeling practice for AVO to be of value. A geologist’s extensive knowledge of local geological conditions is also required for AVO calculations to produce meaningful results.

Seismic services are most effective at interpreting when they are well informed about what detail seismic images actually represent. For example, the contrast in the seismic data is due to the actual settlement of the material and not to lateral or facies changes in the strata. The resolution of the data is also limited by the frequency of the seismic wave used. A stratigraphic layer can only be resolved if its thickness is at least one-quarter the size of the effective wavelength of seismic imaging equipment, which, in practical terms, means that only layers 82 feet (25 meters) or more than depth can be solved by software.

Other factors such as degradation of image resolution with increasing depth occur when using acoustic impedance. The Earth itself also filters seismic signals. The higher the noise level in the data, the more the software has to filter it out, which degrades the remaining necessary information. Seismic interpretation must involve expert geologists and geophysicists to make use of the growing levels of data being returned, especially as the environment for seismic scanning has increased to include marine and terrestrial locations of ever increasing diversity.

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