Seismic dispersion: What does it really mean and what causes it?

Seismic dispersion: What does it really mean and what causes it?

Seismic dispersion is a phenomenon that plays a crucial role in understanding the behavior of seismic waves and their propagation through the Earth’s subsurface. It refers to the variation of seismic wave velocity with frequency, where different frequencies of seismic waves travel at different speeds. This dispersion phenomenon is of great importance in seismology and has important implications for earthquake studies, exploration geophysics, and characterization of subsurface structures. In this article, we will introduce the concept of seismic dispersion, explore its underlying causes, and discuss its implications for understanding the Earth’s interior.

The basics of seismic scattering

Seismic waves are elastic waves that propagate through the Earth’s subsurface, carrying energy generated by earthquakes or artificially induced sources. These waves can be broadly classified into two types: body waves and surface waves. Body waves include primary (P) waves and secondary (S) waves, while surface waves consist of Love and Rayleigh waves. Seismic dispersion primarily affects the behavior of surface waves.
When seismic waves propagate through a homogeneous medium, their velocities remain constant regardless of their frequency. This is known as non-dispersive propagation. In real geological scenarios, however, the subsurface is rarely homogeneous, and the presence of variations in rock properties such as density, lithology, and fluid content leads to dispersive propagation.

Dispersion occurs because different frequencies of seismic waves interact with subsurface materials in different ways. High-frequency waves interact more strongly with geological structures than low-frequency waves. As a result, high-frequency waves travel slower than low-frequency waves, resulting in a dispersion of seismic velocities.

Causes of Seismic Dispersion

Several factors contribute to seismic dispersion, and understanding these causes is critical to interpreting the observed dispersion phenomena in seismic data. The main causes of seismic dispersion can be attributed to the following factors:

1. Geological Heterogeneity: Geological formations are characterized by variations in lithology, rock properties, and fluid content. These heterogeneities introduce frequency-dependent effects on seismic wave propagation. For example, in a layered medium, the presence of interfaces between different rock types can cause scattering and mode conversion, resulting in dispersive wave behavior.

2. Attenuation: Attenuation refers to the loss of energy as seismic waves propagate through the subsurface. Different frequencies of seismic waves can experience different degrees of attenuation due to interactions with subsurface materials. In general, high-frequency waves experience greater attenuation than low-frequency waves. This frequency-dependent attenuation results in a dispersion of seismic velocities.

3. Fluid saturation: The presence of fluids, such as water or hydrocarbons, in subsurface formations can significantly affect the propagation of seismic waves. Fluid-filled pores and fractures can have different mechanical properties than the surrounding rock matrix, resulting in frequency-dependent effects on seismic velocities.

4. Anisotropy: Anisotropy refers to the directional dependence of material properties. In subsurface formations with anisotropic properties, seismic waves traveling in different directions experience different velocities depending on their polarization. This directional dependence results in dispersion of seismic velocities.

Implications and Applications of Seismic Dispersion

The study of seismic dispersion has wide-ranging implications and applications in the Earth sciences. Some notable areas where an understanding of seismic dispersion is critical include

1. Earthquake studies: Seismic dispersion plays an important role in earthquake seismology. By analyzing the dispersion characteristics of seismic waves recorded during earthquakes, scientists can infer information about the Earth’s subsurface structures, including the crust, mantle, and even the presence of deep-seated faults and fractures.

2. Exploration Geophysics: Exploration geophysics uses seismic propagation to map subsurface reservoirs and identify potential hydrocarbon-bearing formations. By analyzing the dispersion characteristics of seismic waves, geophysicists can estimate reservoir properties such as porosity and fluid content, aiding in the exploration and production of oil and gas.

3. Seismic imaging: Seismic dispersion is a challenge in seismic imaging, where accurate imaging of the subsurface is critical for geological and engineering purposes. Proper understanding and correction of dispersion effects is essential to obtain high resolution images and accurate subsurface velocity models.

4. Subsurface Characterization: The study of seismic dispersion contributes to the characterization of subsurface structures and properties. By analyzing the propagation behavior of seismic waves, scientists can infer information about lithology, fracture networks, and fluid content, aiding in geotechnical investigations, groundwater studies, and geothermal energy exploration.

In summary, seismic dispersion is a fundamental phenomenon in seismology that refers to the variation of seismic wave velocities with frequency. Geological heterogeneity, attenuation, fluid saturation, and anisotropy are the main causes of seismic dispersion. Understanding and analyzing seismic dispersion has significant implications for earthquake studies, exploration geophysics, seismic imaging, and subsurface characterization. By unraveling the complexities of seismic dispersion, scientists can gain valuable insights into the Earth’s interior and make informed decisions in various fields related to Earth science. It is an ongoing area of research that continues to advance our understanding of the dynamic nature of our planet.

FAQs

Seismic dispersion: What does it really mean and what causes it?

Seismic dispersion refers to the phenomenon where seismic waves of different frequencies travel at different velocities through a medium. It is caused by the elastic properties of the subsurface materials and their interaction with the seismic waves.

How do seismic waves propagate through the Earth?

Seismic waves propagate through the Earth by traveling through different layers of the Earth’s interior. These waves can be divided into two main types: body waves and surface waves. Body waves include P-waves (primary waves) and S-waves (secondary waves), while surface waves include Love waves and Rayleigh waves.

What is the relationship between seismic dispersion and frequency?

The relationship between seismic dispersion and frequency is that higher-frequency seismic waves tend to travel faster than lower-frequency waves through a medium. This phenomenon leads to the dispersion of seismic waves, where they spread out or disperse over time and distance due to the varying velocities of different frequencies.

What are the main factors that cause seismic dispersion?

Several factors contribute to seismic dispersion. The primary factors include the elastic properties of the subsurface materials, such as their density, stiffness, and anisotropy. The presence of fractures, faults, and other geological heterogeneities can also cause seismic dispersion. Additionally, the pore fluids within the rocks can affect the dispersion characteristics of seismic waves.

Why is seismic dispersion important in geophysics?

Seismic dispersion plays a crucial role in geophysics because it carries valuable information about the subsurface properties and structures. By analyzing the dispersion characteristics of seismic waves, geophysicists can infer the composition, density, and fluid content of the subsurface materials. This information is essential for various applications, including oil and gas exploration, earthquake studies, and geological mapping.