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    Seismic methods & elastic waves

    Seismic is one of the most important techniques of hydrocarbon exploration in terms of money spent as well as geophysicist and geologist employed to work on it. Seismic method is capable of great subsurface penetration with sufficient accuracy and resolution for decision making.

    The importance of seismic stems out of the fact that in the absence of seismic data, well-data is the only data for geoscientist that helps to determine the productivity of reservoir. Acquiring any type of well data requires drilling an exploratory well. Rapidly changing geology moving away from previous well will result into random/blind drilling and high chances of dry holes. Seismic provides the stratigraphy of area before commencing the drilling operations hence increasing drilling success ratio.

    Seismic method produce subsurface image of earth which helps geologists and geophysicists in identifying potential hydrocarbon bearing zones. Although, primarily seismic is used for hydrocarbon exploration, but it is also found to be useful for ground water exploration, earthquake studies (seismology) and monitoring of volcanoes.

    Seismic techniques employed in hydrocarbon exploration are also called exploration seismology as this technique is borrowed from earthquake studies (seismology).

    Exploration seismology involves same type of measurements like earthquake seismology. However the only difference is that in exploration seismology, elastic waves are produced in controlled manner and they are man made.

    Seismic methods work by generating an elastic wave from a seismic source to produce subsurface image.

    There are two types of elastic waves:-

    1. Surface waves:- Waves which travel along or near the surface.
    2. Body Waves:- Waves which travel inside the body of rock and energy propagation is in all direction. Body waves are of primary interest in Seismic. They are classified as :-
    1. Primary waves:- Also called P-waves, longitudinal wave or compressional waves. In these waves particle motion is parallel to direction of propagation of waves.

    Figure:- Primary or compressional waves

    2. Secondary waves:- Also known as S-Waves, shear waves or transverse wave. In these waves particle motion is perpendicular to direction of propagation of waves.

    Figure:- Secondary or shear waves

    Secondary waves are further classified as SV-Waves and SH-Waves.

    2a. SV-Waves :- In SV-Waves particle motion is in vertical plane.

    2b.SH-Waves:- In SH-Waves particle motion is in horizontal plane.

    Figure 2.3:- SH ,SV and P waves (from left to right)