Geophysics - Exploration method (Elastic Impedance)

Hello friends of Steemit, I share with you a method for the detection of hydrocarbons in the subsoil through elastic impedance based on seismic techniques.

Geophysics has developed seismic methods that have allowed great advances in the exploration of the subsoil. Among the best known are: the method of reflection and refraction, gravimetric, magnetic and electrical. The largest number of oil and gas fields are discovered through these methods.

In this work we use the method of reflection, which is based on the laws of reflection and refraction of Snell.

Figures 1 y 2 shows the acquisition method applied on land or sea.

Figure 1 - Acquisition of seismic data on land Source

Figure 2 - Acquisition of seismic data in the sea Source

The method consists of measuring the speed of a seismic wave generated by air cannons or vibrating trucks (Figure 3), which travels great distances at low frequencies. These waves are picked up by receivers known as Geophones or hydrophones depending on whether the data collection is on land or sea respectively (Figure 4 y 5).

Figure 3 - Vibrating truck (seismic source) Source

Figure 4 - Geophone Source

Figure 5 - Hydrophone Source

With the data of the transmitter, receiver and travel times of the seismic waves it is possible to calculate the propagation velocities of these waves. This parameter allows us, among other things, the location in depth of the reflectors in the subsoil and infer about the physical and mechanical properties of the materials by which the wave propagates.

The velocities of the waves are known as primary wave velocity (Vp) and secondary wave velocity (Vs). Velocity maps and a large number of elements such as seismic profiles, synthetic seismograms and reflection coefficients that facilitate the interpretation of the subsoil are constructed from the velocity data obtained.

In the following figure 6 a section of wave velocities (S) is observed.

Figure 6 - Section of wave velocities (S) Source

These sections are built with specialized software that facilitate their construction. The software used for this research is the Surfer, which allows the construction of 2D or 3D maps according to the available data.

Figure 7 - Specialized software Surfer Source

The speed data obtained in the study areas with the aforementioned methods represent an important cost for the industry and once used for the construction of speed maps are discarded in most cases.

The purpose of this work is to use the velocity data available for the calculation and mapping of the elastic impedance, which is a function of the velocity data. The inclusion of this parameter facilitates the task of obtaining a more reliable seismic interpretation of the study area.

The elastic impedance represents the resistance of the medium to the propagation of the seismic wave and provides fundamental information in the interpretation of seismic records and the characterization of the deposits for oil exploitation. Depending on how the elastic impedance values vary, it is possible to make inferences regarding lithology, type of fluid within the rock, fluid saturation among others.

The elastic impedance is obtained using the expression (1) obtained by Conolly ( Connolly P., (1999) Elastic impedance, The Leading Edge, Vol 18, No. 4438-452).

Where ¨EI¨ is elastic impedance ¨θ¨ the angle of incidence, Vp and Vs are the values of the velocity of the wave P and S, ρ is the density and the constant k is given by the equation:

The following image shows a section of elastic impedance obtained from the velocity data (Figure 8).

Figure 8 - Section of elastic impedance obtained between two wells

This model allows an approximation to the real structure of the subsoil and its composition. Between the wells A-B significant anomalies of variation of elastic impedance are observed that are associated to stratigraphic and lithological variations typical of the zone.

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I hope that this publication has given you knowledge about the subject. If you have any questions, please leave your comment and then I will gladly try to clarify them.

Thanks for reading my publication.