Seismic Wave & Imaging (September 2016)

P-waves, also known as primary waves, are a type of seismic wave that can travel through both solid and liquid media, including gases. These waves are characterized by their ability to compress and expand the material they pass through. Therefore, the statement that P-waves cannot pass through gaseous media is incorrect. P-waves can indeed travel through gases, making the given answer "False" correct.

The given statement, "Fresnel zone is a function of depth only," is incorrect. The Fresnel zone is a concept used in wireless communication to determine the quality of a signal between a transmitter and receiver. It is not solely dependent on depth but also influenced by other factors such as distance, frequency, and obstacles in the signal path. Therefore, the correct answer is false.

The Zoeppritz equations are a set of mathematical equations that describe the reflection and transmission of seismic waves at an interface between two different rock layers. These equations are commonly used in geophysics to model the behavior of seismic waves and determine the reflection coefficient, which is a measure of the ratio of the amplitude of the reflected wave to the incident wave. Therefore, it is correct to say that the reflection coefficient as a function of angle for a plane wave impinging on a plane reflector obeys Zoeppritz equations.

Rayleigh waves, also known as ground roll, are a type of surface wave that travels along the surface of the Earth. They are slower than P-waves and have a velocity close to that of S-waves. Therefore, the given statement that Rayleigh waves have a velocity close to P-waves is incorrect.

A wave is a disturbance that transfers energy from one place to another without transferring matter. It can travel in different directions and can be reflected, refracted, diffracted, and transmitted. This means that waves can bounce off surfaces, change direction when passing through different mediums, spread out when passing through an opening, and continue propagating through a medium. Waves can be attenuated by factors such as absorption, scattering, or interference, so the statement that waves can't be attenuated by any factor is incorrect. The movement of waves is not directly related to heat transfer, as waves can transfer different forms of energy, not just heat.

A seismic trace is a graphical representation of seismic waves recorded during seismic exploration. It provides information about various properties of the subsurface, such as velocity, frequency, and wavenumber. However, the bulk modulus is not directly obtained from a seismic trace. The bulk modulus is a measure of a material's resistance to compression and is typically determined through laboratory tests or other geophysical methods. Therefore, the correct answer is Bulk Modulus.

Energy absorption does not occur when the wave propagates through a fast velocity zone. In fact, energy absorption typically occurs when waves encounter zones of low velocity or high attenuation, where the energy of the wave is dissipated or absorbed. Therefore, the statement is false.

Summing the energy along the diffraction hyperbola refers to a technique called Kirchhoff migration. This migration method is used in seismic data processing to enhance the quality and resolution of subsurface images. It involves summing the energy contributions from different points on the diffraction hyperbola, which helps in accurately locating and imaging subsurface reflectors. Kirchhoff migration is a widely used technique in the oil and gas industry for interpreting seismic data and improving the understanding of subsurface structures.

The correct answer is Pwave --> Swave --> Rayleigh wave --> Love wave. This sequence indicates the correct velocity sequence from fastest to slowest. Pwaves are primary waves that travel the fastest through the Earth's interior. Swaves are secondary waves that travel slower than Pwaves. Rayleigh waves are surface waves that move slower than both Pwaves and Swaves. Love waves are also surface waves but they have the slowest velocity among the four waves listed.

Seismic migration/imaging is necessary to correct the apparent reflector position in preprocessing seismic data. During seismic data acquisition, the recorded signals can be distorted due to various factors such as the velocity variations in the subsurface, the presence of noise, and the occurrence of multiple reflections. Seismic migration helps in accurately determining the position of the reflectors by accounting for these distortions and providing a clearer image of the subsurface. This is crucial for geological interpretation and exploration purposes.

As depth increases, the diffraction curves become wider. This is because diffraction is the bending of waves around obstacles or through openings, and the wider the opening or obstacle, the more the waves spread out. Therefore, as the depth increases, the waves have more space to spread out, resulting in wider diffraction curves.

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Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets, which propagate in all directions. These secondary wavelets have the same amplitude and phase as the primary wavefront. The superposition of these secondary wavelets creates a new wavefront. Therefore, the correct answer is that the amplitude and phase in the secondary wavefront are the same as the primary wavefront.

The diffraction hyperbola refers to the pattern of diffraction caused by a constant velocity wavefront encountering an obstacle. As the depth increases, the diffraction hyperbola becomes broader. This is because as the wavefront encounters the obstacle at greater depths, the diffracted waves spread out more, resulting in a broader pattern. Therefore, the correct answer is "Broader as depth increases."

When a P-wave strikes an interface, it can be either reflected or transmitted as different types of waves. The energy of the P-wave can be partitioned into reflected shear waves, transmitted shear waves, and reflected pressure waves. However, it does not result in the generation of a reflected surface wave. Surface waves are typically generated by the interaction of S-waves with the Earth's surface, not P-waves. Therefore, the correct answer is "Reflected Surface wave."

The statement "There is discrepancies of time and depth migration" is true. Time migration and depth migration are two different methods used in geology for imaging subsurface structures. Time migration involves correcting seismic data for the effects of the velocity model, while depth migration directly images the subsurface based on the true depth. These two methods can produce different results and there can be discrepancies between them.

The statement is false because acoustic waves cannot be directly derived from Newton's First Law and Hooke's Law. Newton's First Law states that an object at rest will stay at rest unless acted upon by an external force, while Hooke's Law states that the force needed to extend or compress a spring is directly proportional to the distance it is stretched or compressed. These laws do not specifically describe the generation or propagation of sound waves, which require the presence of a medium such as air or water.

The given statement states that the diffraction response for a homogeneous single velocity media is that the amplitude decays exponentially from the apex. This means that as the diffraction wave propagates through the medium, the amplitude of the wave decreases exponentially as it moves away from the source or apex. This is a characteristic behavior of diffraction in such a medium.

Shear waves can actually pass through saturated rock. In fact, shear waves travel slower in saturated rock compared to dry rock due to the presence of water. This is because the water molecules in the rock impede the movement of shear waves, causing them to slow down. Therefore, the correct answer is False.

Lowering the fold data refers to reducing the number of times a particular point in the subsurface is sampled during seismic data acquisition. This can be done to save costs or improve efficiency. However, it is important to note that lowering the fold data can indeed affect the resolution of the seismic data. Higher fold data provides more redundant information, which helps to reduce noise and improve the signal-to-noise ratio, leading to better resolution. Therefore, reducing the fold data can result in a decrease in the quality and resolution of the seismic data.