What Is Seismic Interpretation?

Ray Hawk
Ray Hawk
Scientist with beakers
Scientist with beakers

Seismic interpretation is a process of analyzing seismic data for underground minerals, oil, natural gas, or fresh water deposits. Technical issues 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 first must be clearly distinguished, and they are often superimposed upon one another. Enhancing the data with spectral features or color coding in seismic software, as well as trying to improve on the resolution of imagery, is one of the main components used in determining seismic attributes.

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

There is no one dominant way to view seismic data, and different approaches to seismic interpretation must be adapted to local mining, prospecting, or research needs. The fields where seismic interpretation are 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 a former focus on the accurate detection of the volume and extent of underground fossil fuels. New techniques used in the industry are focused 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 between each other and is useful in determining the porosity level of layers. In the mid 1980s, AVO technology became popular in the oil industry and, coupled with 3D imagery, has seen a revival in interest, though the process works better in some regions of the world than others. AVO has sometimes received a bad reputation as unreliable, because the geophysics of rock and fluid characteristics must first be determined to be suitable for AVO analysis. Feasibility studies beforehand are therefore an essential seismic modeling practice for AVO to be of value. A geologist's extensive understanding of local geology conditions is also necessary for AVO calculations to produce meaningful results.

Seismic services are most effective at interpretation when they are well-informed about what the details of the seismic imagery actually represents. For instance, the contrast in seismic data is due to the actual bedding of material and not lateral or facies changes in layers. The resolution of 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 a quarter of the size of the actual wavelength of the seismic imaging equipment, which, in practical terms, means that only layers 82 feet (25 meters) or greater in depth can be resolved by software.

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

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      Scientist with beakers