5.3 Velocity models and travel time calculations
3 min read•august 9, 2024
Seismic velocity models are crucial for understanding Earth's structure. From simple layered models to complex 3D representations, they help us interpret seismic data and map the planet's interior. These models form the foundation for analyzing how seismic waves travel through the Earth.
Travel time calculations use these velocity models to predict wave arrivals and uncover subsurface features. Through and advanced analysis methods like tau-p, seismologists can refine velocity models and gain deeper insights into Earth's composition and dynamics.
Velocity Models
Types of Velocity Models
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- represents Earth's structure as distinct layers with constant velocities within each layer
- Simplifies complex subsurface structures
- Assumes abrupt velocity changes at layer boundaries
- Useful for sedimentary basins or crustal studies
- describes continuous velocity changes with depth
- Accounts for gradual variations in rock properties
- Provides more realistic representation of Earth's interior
- Commonly used in studies
- depicts velocity changes only with depth
- Assumes lateral homogeneity
- Suitable for global or regional scale studies
- Serves as a starting point for more complex models
Advanced Velocity Models
- incorporates lateral variations along a profile
- Represents velocity changes in both depth and one horizontal direction
- Useful for imaging complex geological structures (fault zones, subduction zones)
- Requires more computational resources than 1D models
- captures velocity variations in all spatial dimensions
- Provides most comprehensive representation of Earth's structure
- Allows for detailed imaging of complex subsurface features
- Demands significant computational power and data input
- Increasingly used in oil and gas exploration, earthquake studies, and global tomography
Travel Time Calculations
Inversion and Modeling Techniques
- determines subsurface velocity structure from observed travel times
- Utilizes seismic wave arrival times at different stations
- Involves solving an inverse problem to estimate velocity distribution
- Iterative process refines initial model to minimize misfit between observed and calculated travel times
- Crucial for creating accurate velocity models of Earth's interior
- calculates theoretical travel times based on assumed velocity model
- Predicts seismic wave arrivals for given source-receiver configurations
- Helps validate and refine velocity models
- Useful for designing seismic surveys and interpreting observed data
- Involves through the assumed velocity structure
Advanced Travel Time Analysis
- transforms seismic data from time-distance domain to intercept time-ray parameter domain
- Simplifies analysis of multi-layered velocity structures
- Facilitates identification and separation of different seismic phases
- Enhances resolution of velocity discontinuities
- Commonly used in reflection seismology and global seismology
- Provides efficient means for calculating travel times in complex models
Standard Earth Models
Global Reference Models
- serves as a widely used global reference model for Earth's structure
- Developed by Kennett and Engdahl in 1991
- Provides 1D velocity and density profiles for the entire Earth
- Based on travel time observations from earthquakes worldwide
- Used as a starting point for regional and global tomographic studies
- Includes both P-wave and S-wave velocity profiles
- represents an updated global reference model
- Created by Kennett, Engdahl, and Buland in 1995
- Improves upon IASP91 by incorporating more recent seismic data
- Provides better fit to observed travel times, particularly for phases
- Includes structure and density profiles
- Widely used in earthquake location algorithms and seismic tomography
Key Terms to Review (29)
1d velocity model: A 1D velocity model is a simplified representation of seismic wave velocities as a function of depth in the Earth, where the velocity is described as a single dimension over depth. This model is crucial for understanding how seismic waves travel through different geological layers, allowing for accurate predictions of travel times and wave propagation characteristics. The 1D aspect implies that variations in lateral dimensions are not considered, making it an idealized framework for initial analyses in seismology.
2D velocity model: A 2D velocity model is a representation of the Earth's subsurface velocity structure, displaying how seismic wave speeds vary with depth and horizontal distance in two dimensions. This model is crucial for understanding wave propagation and helps in accurately predicting travel times for seismic waves as they move through different geological layers, enhancing our ability to interpret seismic data effectively.
3D Velocity Model: A 3D velocity model is a representation of the subsurface geological structure that provides the velocity of seismic waves as a function of three-dimensional space. This model is essential in understanding how seismic waves travel through different materials and depths, enabling accurate predictions of travel times and improving the interpretation of seismic data.
Ak135 model: The ak135 model is a widely used seismic velocity model that describes the Earth's interior structure, specifically the seismic wave speeds of different layers of the Earth. This model is crucial for understanding how seismic waves travel through the Earth and is essential for calculating travel times of these waves during seismic events, thereby enhancing our knowledge of earthquake processes and Earth's geological structure.
Attenuation: Attenuation refers to the reduction in amplitude and intensity of seismic waves as they propagate through different materials in the Earth. This concept is crucial for understanding how energy is lost due to scattering, absorption, and geometrical spreading of seismic waves, influencing the identification of seismic phases, velocity models, and the study of Earth's structure.
Compressional Wave Velocity: Compressional wave velocity refers to the speed at which compressional waves, also known as primary or P-waves, travel through a medium. This velocity is crucial for understanding how seismic waves propagate through the Earth and plays a significant role in constructing velocity models and calculating travel times for seismic events.
Core: The core is the innermost layer of the Earth, composed mainly of iron and nickel, and is crucial for understanding Earth's internal structure and its geodynamic processes. It plays a significant role in generating the planet's magnetic field and influences seismic wave propagation and behavior, which is essential for analyzing velocity models and travel time calculations.
Crust: The crust is the outermost layer of the Earth, comprising both continental and oceanic regions. It plays a critical role in seismic activities and is where seismic waves are generated and propagated. The composition and thickness of the crust vary significantly, influencing how seismic waves travel through it and how they are recorded on the surface.
Forward modeling: Forward modeling is a computational technique used in seismology to simulate how seismic waves propagate through different geological structures. This method helps in predicting the travel times and amplitudes of these waves based on a specific velocity model, allowing researchers to understand the subsurface features of the Earth better. By inputting various parameters and geological conditions, forward modeling creates synthetic seismograms that can be compared with actual data to refine models of the Earth's interior.
Gradient velocity model: A gradient velocity model is a method used in seismology to describe how seismic wave velocity changes with depth in the Earth. This model provides a linear approximation of velocity variations, allowing for more accurate predictions of seismic wave travel times as they pass through different geological layers. Understanding this model is essential for interpreting seismic data and enhancing our knowledge of subsurface structures.
Huygens' Principle: Huygens' Principle states that every point on a wavefront can be considered as a source of secondary wavelets, which spread out in all directions at the same speed as the original wave. This principle helps to explain how waves propagate through different media and relates directly to concepts like velocity models, ray paths, and the behavior of waves during reflection and refraction.
Iasp91 model: The iasp91 model is a seismic velocity model that describes the Earth's interior structure and is particularly useful for understanding seismic wave propagation. Developed in 1991 by the International Association of Seismology and Physics of the Earth's Interior, this model provides a standardized framework for calculating travel times of seismic waves through different layers of the Earth, including the crust, mantle, and core.
Inversion techniques: Inversion techniques are methods used to interpret seismic data by estimating the properties of the Earth's subsurface from observed waveforms. These techniques are crucial in building models that represent how seismic waves travel through different materials, allowing scientists to infer physical characteristics such as density and elastic properties. The effectiveness of these techniques depends on accurate data and computational models to link observed travel times and waveforms back to subsurface structures.
Isotropic vs. Anisotropic: Isotropic materials have uniform properties in all directions, meaning their physical characteristics are the same regardless of the direction in which they are measured. In contrast, anisotropic materials exhibit different properties when measured along different axes. This distinction is crucial in understanding how seismic waves travel through various geological formations, which significantly impacts velocity models and travel time calculations.
Layered velocity model: A layered velocity model is a representation used in seismology that describes how seismic wave velocities change with depth in the Earth's subsurface. This model typically consists of several distinct layers, each with its own seismic velocity, which helps in understanding the geological structure and predicting the travel times of seismic waves as they pass through different materials.
Lithology: Lithology is the study of the physical and chemical characteristics of rocks, particularly their mineral composition and texture. Understanding lithology is crucial as it influences the behavior of seismic waves, which is important for constructing velocity models and travel time calculations. The type of rock material impacts how seismic reflection and refraction methods are applied in exploration, and detailed lithological analysis is essential for interpreting results in vertical seismic profiling and well logging.
Mantle: The mantle is a thick layer of rock located between the Earth's crust and the outer core, making up about 84% of Earth's total volume. It plays a critical role in seismic wave propagation and the dynamics of plate tectonics, influencing everything from travel time calculations to the generation of seismic waves.
Modeling: Modeling in seismology refers to the process of creating mathematical and physical representations of the Earth's subsurface structures and seismic wave propagation. This technique is essential for understanding how seismic waves travel through different geological materials, helping scientists interpret seismic data and make predictions about earthquake behavior and impacts.
P-waves: P-waves, or primary waves, are the fastest type of seismic waves that travel through the Earth, moving in a compressional manner. They can propagate through both solid and liquid materials, making them essential for understanding the Earth's internal structure and behavior during seismic events.
Ray tracing: Ray tracing is a computational technique used in seismology to model the propagation of seismic waves through the Earth. It helps visualize and understand how seismic waves travel through different materials and structures, which is crucial for interpreting seismic data. By calculating the paths taken by waves, this method provides insights into the Earth's subsurface features and their velocity structure.
Refraction Analysis: Refraction analysis is a technique used in geophysics and seismology to determine the velocity structure of the subsurface by examining the refraction of seismic waves. This method relies on the principle that seismic waves travel at different speeds through different geological materials, allowing researchers to create models of the subsurface based on the travel times of these waves as they refract at boundaries between materials with varying densities and elastic properties.
S-waves: S-waves, or secondary waves, are a type of seismic wave that move through the Earth during an earthquake. They are characterized by their transverse motion, which means they move the ground perpendicular to the direction of wave propagation, and are only able to travel through solid materials, making them crucial for understanding Earth's internal structure.
Shear Wave Velocity: Shear wave velocity refers to the speed at which shear waves, or secondary waves, travel through a medium. These waves are crucial in seismology as they provide insights into the elastic properties of materials beneath the Earth's surface and are essential for constructing velocity models used in travel time calculations. Understanding shear wave velocity helps in determining material composition and the behavior of seismic waves during earthquakes.
Simulation: A simulation is a method used to model real-world processes or systems through computational techniques, often involving algorithms and mathematical representations. In the context of velocity models and travel time calculations, simulations are critical for predicting how seismic waves propagate through different geological structures, which helps in understanding earthquake behavior and assessing potential impacts.
Snell's Law: Snell's Law describes how seismic waves change direction when they pass through different layers of material with varying properties, specifically concerning their velocities. This fundamental principle is crucial for understanding how waves refract and reflect as they encounter boundaries within the Earth's subsurface, influencing methods of data interpretation in seismology.
Tau-p method: The tau-p method is a seismic data processing technique used for analyzing and imaging wavefields by transforming the data into a domain defined by the arrival times (tau) and the slowness (p) of seismic waves. This method allows for improved resolution and clarity in seismic imaging by effectively handling complex wave propagation, especially in heterogeneous media, which is crucial for accurate velocity models and travel time calculations.
Travel time inversion: Travel time inversion is a technique used in seismology to determine the structure of the Earth's subsurface by analyzing the differences in travel times of seismic waves as they propagate through various materials. This method allows scientists to infer the properties of underground formations and create velocity models that are essential for accurate travel time calculations.
Travel Time Tomography: Travel time tomography is a geophysical imaging technique that uses the travel times of seismic waves to create detailed models of subsurface structures. By analyzing how long it takes for seismic waves to travel through different materials, this method helps identify variations in velocity, which can reveal important information about geological features such as faults, layers, and material composition.
Velocity gradient: The velocity gradient is a measure of how the velocity of seismic waves changes with depth or position in the Earth's interior. It is essential for understanding how seismic waves propagate through different layers of the Earth and plays a crucial role in the development of velocity models that help calculate travel times. A clear grasp of velocity gradients allows scientists to infer the physical properties of geological materials, thereby aiding in the interpretation of seismic data.