The reflection experiment

The basic principle of seismic reflection is to send elastic waves (using an energy source such as a dynamite explosion or Vibroseis) toward the Earth, where each interface between the layers of materials within the Earth will reflect a portion of the wave energy back to the surface and allow the rest of the energy to be refracted and continue traveling to the next layer. These reflected energy waves are recorded over a certain period of time (called the recording length) by receivers that detect the displacement of the land in the place where they have been placed. On land, the receiver generally used is a small portable instrument called a geophone, which converts the displacement of the ground into an analog electrical signal. In water, hydrophones are used, which convert the pressure changes into electrical signals. The response of each receiver or sensor to an energy discharge (or explosion) is called a “trace” and is recorded in a data storage device, after which the point at which the energy is injected is moved and repeated. The process. Generally, the recorded signals go through a complex processing process before being ready for interpretation, this being an area of considerable research both in industry and in academia. In general, the more complex the geology of the terrain under study, the more sophisticated the techniques are required to eliminate the influence of measurement noise and increase the resolution. In modern seismic surveys by reflection, such large amounts of information and data are recorded that powerful computers must be used to process them. supercomputers or computer clusters "Cluster (computing)").

Reflection and transmission for the case of incidence perpendicular to the interface

When a seismic wave encounters an interface between two materials with different acoustic impedances, a portion of the wave energy will be reflected at the boundary, while the other portion of the energy will be transmitted passing through the boundary. The amplitude of the reflected wave is obtained by multiplying the amplitude of the incident wave by the seismic reflection coefficient, determined by the contrast between the impedances of the two materials.

For the case of a wave incident on an interface perpendicularly, the reflection coefficient is calculated as.

where and are the impedance of the first and second material, respectively.

Similarly, the amplitude of the incident wave is multiplied by the transmission coefficient to calculate the amplitude of the wave transmitted across the interface. The formula to calculate the transmission coefficient for normal incidence is.

Since the sum of the squares of the amplitudes of the reflected and transmitted wave must be equal to the square of the amplitude of the incident wave, it can be shown that.

By studying changes in the intensity of reflected waves, seismologists can infer changes in seismic impedances. In turn, they can use this information to infer changes in the properties of the rocks at the interface, such as their density and elastic modulus.

Reflection and transmission for the case of generic incidence

The situation is much more complicated in the case that the wave is incident obliquely, due to the mode conversion between P waves and S waves, and is described by the Zoeppritz equations. In 1919, Karl Zoeppritz obtained 4 equations that allow determining the amplitudes of the reflected and refracted waves at a plane interface for a P wave as a function of the angle of incidence and six independent elastic parameters.[1] These equations have 4 unknowns and it is possible to solve them, but they do not allow us to intuitively understand how the reflected amplitudes vary with the properties of the rocks under study.[3]​.

The reflection and transmission coefficients, which determine the amplitude of each reflection, vary with the angle of incidence and can be used to obtain information about the amount of fluid contained in the rock. The practical application of the phenomenon of oblique incidence, called AVO, has been facilitated by theoretical studies that have allowed us to obtain useful approximations of the Zoeppritz equations and by the progress experienced in computer processing capacity. AVO studies attempt with relative success to predict the fluids (oil, gas, or water) contained in potential reservoirs, to reduce the risks of drilling unproductive wells and to identify new oil reservoirs. The most frequently used 3-term simplification of the Zoeppritz equations was developed in 1985 and is known as the "Shuey equation". A 2-term simplification called the "Shuey approximation" is valid for angles of incidence less than 30 degrees (which is generally the case in seismic surveys) and is indicated below:[4]​.

where = reflection coefficient for normal incidence; = gradient AVO, describes the reflection behavior for intermediate angles for intermediate angles of incidence and = angle of incidence. For the case of normal incidence =0, this equation reduces to the one provided in the preceding section.

Insert subtitle here.

Reflection seismic is a method used in several areas and its applications can be divided into three groups:

A similar method is ground-penetrating radar (GPR), which uses radar waves instead of mechanical waves, although with considerably less penetration capacity.

The reflection experiment

The basic principle of seismic reflection is to send elastic waves (using an energy source such as a dynamite explosion or Vibroseis) toward the Earth, where each interface between the layers of materials within the Earth will reflect a portion of the wave energy back to the surface and allow the rest of the energy to be refracted and continue traveling to the next layer. These reflected energy waves are recorded over a certain period of time (called the recording length) by receivers that detect the displacement of the land in the place where they have been placed. On land, the receiver generally used is a small portable instrument called a geophone, which converts the displacement of the ground into an analog electrical signal. In water, hydrophones are used, which convert the pressure changes into electrical signals. The response of each receiver or sensor to an energy discharge (or explosion) is called a “trace” and is recorded in a data storage device, after which the point at which the energy is injected is moved and repeated. The process. Generally, the recorded signals go through a complex processing process before being ready for interpretation, this being an area of considerable research both in industry and in academia. In general, the more complex the geology of the terrain under study, the more sophisticated the techniques are required to eliminate the influence of measurement noise and increase the resolution. In modern seismic surveys by reflection, such large amounts of information and data are recorded that powerful computers must be used to process them. supercomputers or computer clusters "Cluster (computing)").

Reflection and transmission for the case of incidence perpendicular to the interface

When a seismic wave encounters an interface between two materials with different acoustic impedances, a portion of the wave energy will be reflected at the boundary, while the other portion of the energy will be transmitted passing through the boundary. The amplitude of the reflected wave is obtained by multiplying the amplitude of the incident wave by the seismic reflection coefficient, determined by the contrast between the impedances of the two materials.

For the case of a wave incident on an interface perpendicularly, the reflection coefficient is calculated as.

where and are the impedance of the first and second material, respectively.

Similarly, the amplitude of the incident wave is multiplied by the transmission coefficient to calculate the amplitude of the wave transmitted across the interface. The formula to calculate the transmission coefficient for normal incidence is.

Since the sum of the squares of the amplitudes of the reflected and transmitted wave must be equal to the square of the amplitude of the incident wave, it can be shown that.

By studying changes in the intensity of reflected waves, seismologists can infer changes in seismic impedances. In turn, they can use this information to infer changes in the properties of the rocks at the interface, such as their density and elastic modulus.

Reflection and transmission for the case of generic incidence

The situation is much more complicated in the case that the wave is incident obliquely, due to the mode conversion between P waves and S waves, and is described by the Zoeppritz equations. In 1919, Karl Zoeppritz obtained 4 equations that allow determining the amplitudes of the reflected and refracted waves at a plane interface for a P wave as a function of the angle of incidence and six independent elastic parameters.[1] These equations have 4 unknowns and it is possible to solve them, but they do not allow us to intuitively understand how the reflected amplitudes vary with the properties of the rocks under study.[3]​.

The reflection and transmission coefficients, which determine the amplitude of each reflection, vary with the angle of incidence and can be used to obtain information about the amount of fluid contained in the rock. The practical application of the phenomenon of oblique incidence, called AVO, has been facilitated by theoretical studies that have allowed us to obtain useful approximations of the Zoeppritz equations and by the progress experienced in computer processing capacity. AVO studies attempt with relative success to predict the fluids (oil, gas, or water) contained in potential reservoirs, to reduce the risks of drilling unproductive wells and to identify new oil reservoirs. The most frequently used 3-term simplification of the Zoeppritz equations was developed in 1985 and is known as the "Shuey equation". A 2-term simplification called the "Shuey approximation" is valid for angles of incidence less than 30 degrees (which is generally the case in seismic surveys) and is indicated below:[4]​.

where = reflection coefficient for normal incidence; = gradient AVO, describes the reflection behavior for intermediate angles for intermediate angles of incidence and = angle of incidence. For the case of normal incidence =0, this equation reduces to the one provided in the preceding section.

Insert subtitle here.

Reflection seismic is a method used in several areas and its applications can be divided into three groups:

A similar method is ground-penetrating radar (GPR), which uses radar waves instead of mechanical waves, although with considerably less penetration capacity.