Reflection seismology is a type of geophysical imaging technique used to image the subsurface part of the earth and understand its geology. The idea of using waves to infer the geology was first introduced by Canadian scientist Reginald Fessenden in its primitive form in 1917. [1] One of the greatest proponents of reflection seismology is the oil and gas industry where it is widely used in the exploration stage of the hydrocarbons. Recent advances in signal processing, computer vision and parallel computing enabled increase in the size of the problem considered in reflection seismology.
Reflection seismology can be best understood by dividing it into the following steps:
A controlled source of seismic energy such as dynamite explosion is used to send elastic waves into the earth. Due to different properties of the layers of the earth, these waves are partially reflected and partially refracted at each layer. Receivers located at specific locations record the reflected waves over a specified period of time. This recorded data is then processed to reduce noise and enhance resolution. [2,3]
Processing of the 2D or 3D dataset acquired using the above technique is carried out using three main techniques: [4]
Deconvolution: The principle of this process is that seismic trace is assumed to be made up of convolution of the reflectivity series of the earth with distorting filters. By collapsing the seismic wavelet, the deconvolution process improves the resolution in time. However, it is non-unique, and further information or assumptions are required to ensure uniqueness.
Common-Midpoint Stacking (CMP): A subsurface location is samples multiple times using a range of values of the offset, allowing construction of a group of traces (gather) known as Common Midpoint Gather.
Migration: In simple words, migration refers to the technique of shifting the seismic event in either time or space to its occurrence in the subsurface instead of its recorded occurrence at the surface. This helps create a more accurate subsurface image.
Seismic interpretation refers to creating maps of spatial variation in geological layers by using tracing and correlation along continuous reflectors in the dataset. For hydrocarbon exploration, seismic interpretation focuses on the source rock, reservoir rock, seal and trap for the petroleum reservoir. Seismic attribute analysis refers to the process of analyzing the effects of various quantities or attributes in the seismic dataset. [5,6]
Near-Surface Applications: These applications focus on exploring and imaging the geological structure at a distance of up to 1km under the earth's surface. These typically include exploration of coal and minerals.
Hydrocarbon Exploration: These involve imaging the earth's subsurface to depth of up to 10km, and can be combined with other techniques to build a geological model useful for hydrocarbon exploration. This exploration is carried out in land and marine environments, as well as in the transition zone between these.
Crustal Study: These involve exploring the earth's crust up to depths as much as 100 km to understand tectonic processes.
Reflection seismology is a widely used technique to understand the Earth's subsurface at various depths for a variety of applications, and is the subject of active research including on better data acquisition, noise removal and data interpretation techniques to produce high-resolution and high-accuracy geological images of the earth's subsurface.
© Sumeet Trehan. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. A. Fessenden, "Method and Apparatus For Locating Ore-Bodies," U.S. Patent 1240328, September 18, 1917.
[2] J. F. Claerbout, Imaging the Earth's Interior (Blackwell Science, 1985).
[3] K. H. Waters, Reflection Seismology: A Tool for Energy Resource Exploration (Krieger Publishing, 1992).
[4] L. Thomsen et al., "Reflection Seismology Over Azimuthally Anisotropic Media", Geophysics 53, 304 (1988).
[5] R. H. Burger, Exploration Geophysics of the Shallow Subsurface, (Prentice Hall, 1992).
[6] K. P. Bube and R. Burridge, "The One-Dimensional Inverse Problem of Reflection Seismology", SIAM Review 25, 497 (1983).