Seismic Interpretation

October 13, 2015- GrilledSeismic

What is seismic interpretation?

Seismic interpretation is the process of obtaining geologic information from seismic data, especially from reflections in the case of the oil and gas industry. This is the simplest explanation in which most geophysicists would agree. Nonetheless, we could write thousands of pages trying to explain everything involved around this methodology. For this post, we will only focus on the basics.

Seismic reflection.- Is the branch of exploration seismology dedicated to the acquisition, processing and interpretation of data from the reflections of acoustic waves in the different layers of the earth. Said reflections are generated  in seismic exploration through controlled sources such as dynamite and seismic vibrators, known as vibroseis, in land and, air guns (mostly) in the sea. This method is ruled by the acoustic impedance (Z) property of the rocks in which the seismic waves travel through the earth. Acoustic impedance (Z) is the result of the velocity of the seismic waves (V) times the density (ρ) of the rock in which the waves travel (Z=V ρ). When the seismic waves travel through the subsurface and find an interface with different acoustic impedance, part of the energy of the waves is refracted and the other part is reflected. The reflected waves are the ones of interest to us and we measure their travel time from the source to the receivers. Knowing the travel time and the velocity of the seismic waves we can reconstruct the path that the waves traveled to generate an image from the subsurface through seismic processing.

Fig. 1.- (Stein and Wysession, 2003)

Seismic reflection data comprise:

- Continuity of the reflections indication a geologic structure.

 - Variability of the reflections indicating stratigraphic changes, fluid  presence and the reservoir structure.

- The seismic wavelet.

- Noises of different kinds and data deffects.

Seismic interpretation is the thoughtful process to separate this effects.

The seismic wavelet starts as a pulse, which is generated through some artificial source and then travels down through the earth, it reflects in an interface and travels back up to the surface where is recorded by the receivers, bringing geologic information with it. This wavelet is minimum phase and it has some bandwidth of some frequency and, during data processing is converted (ideally) to a zero-phase wavelet, making the interpretation easier and more accurate.

Fig. 2 Seismic line from the Gulf of Mexico

The interpreter is not interested directly in the wavelet, but rather in the geologic information it carries. Because of this, understanding the wavelet and distinguishing its characteristics to define geologic details is one of the most important tasks to the interpreter.

The interpreter needs to know enough about the acquisition and processing of the seismic data to take into account the effects of undesirable noise in the data and not to think of it as the geologic features he is looking for. Noise is always present in the data and it can be random, multiples reflections, refracted energy or energy from an unknown source.

Fig. 3.- Vertical slice through a seismic volume showing the (a) before and (b) after a structural filter, and (c) difference. (Causes and Appearance of Noise in Seismic Data Volumes By Satinder Chopra, Kurt Marfurt).

Another step of seismic interpretation is the definition of the structural frame, which is accomplished by following the continuity of the reflectors through the seismic data, be it 2D or 3D. The continuity of the reflectors is ruled by the sedimentary boundaries created at the time when they deposited, and the structure imposed by the tectonic forces that worked on them. Following these continuities in an area and create structural maps is then, one of the most basic and traditional activities in seismic interpretation.

Once the structural frame has been defined, it is time to go to the interpretation of stratigraphic features and the look for hydrocarbons. The most important part of this activity is the seismic amplitude, which can be presented or extracted in many ways. The data loaded to the workstation must then be true amplitude and zero-phase, and the interpreter must check that his data meets this standard.

Fig. 4 Shows two vertical slices, interpreted faults and an interpreted horizon inside a seismic volume.

The interpreter can continue to more advanced steps in the interpretation, such as seismic attributes extraction, AVO (amplitude vs offset) analysis and seismic inversion, but that will be the subject or other posts later.

References.

STEIN, S. & WYSESSION, M. 2003.An Introduction to Seismology, Earthquakes, and Earth Structure.

Causes and Appearance of Noise in Seismic Data Volumes By Satinder Chopra, Kurt Marfurt




By GrilledSeismic