Wave Types & Properties¶
Introduction¶
When an earthquake occurs, it emits energy that propagates as waves that travel throughout the Earth. These waves, called seismic waves, are sensitive to many physical and chemical rock properties. Seismic waves can therefore be used to study the properties of the rocks that they passed through, based on recordings made at seismometers. There are several types of seismic waves that are sensitive to different properties, each of which are important tools for mapping geophysical features in the subsurface.
Propagation types¶
As seismic waves travel through a medium, they propagate as two main types of waves: compressional waves, where material is deformed in the same direction as propagation, and shear waves, where material is deformed orthogonally to the direction of propagation. Shear waves can be further defined in terms of polarity, where one component of shear motion occurs horizontally and one component occurs vertically. Thus, all three spatial dimensions are accounted for.
In wave propagation, frequency is directly proportional to velocity. Compressional waves generally have higher frequencies and travel about 1.7 times faster than shear waves, primarily due to the natural resistance materials have to compression. Shear waves are only sustainable if a material is rigid enough to withstand shear stress, and hence cannot travel through liquids.
Body waves & Surface waves¶
In addition to shear and compressional waves, seismic waves are categorized into types based on the paths they take between the event and the station. Body waves travel through the interior of the Earth, and surface waves travel along the boundary between the crust and the atmosphere (i.e., the surface).
Body waves are generally higher frequency than surface waves, and so they arrive at the station first. As they radiate through the Earth, body waves spread in three dimensions, and their energy is distributed spherically. These waves’ amplitude decreases proportionally to the square of the distance they travel. Surface waves, on the other hand, radiate in two dimensions, and their amplitudes decrease linearly with the distance they travel. Because of these relationships, higher-frequency body waves arrive first but have lower amplitudes, while lower-frequency surface waves arrive later but have higher amplitudes.
Body waves travel as both compressional and shear waves. Since compressional waves travel faster than shear waves, body waves on a seismogram are often referred to as P-waves, where “P” stands for “Primary”, and shear waves as S-waves, where “S” stands for “Secondary”. Similarly, surface waves are categorized as Rayleigh and Love waves, each of which have a different motion of propagation. Rayleigh waves oscillate in a ‘retrograde elliptical’ fashion, meaning that both the vertical-shear and compressional components are captured in recordings. On the other hand, Love waves oscillate purely horizontally, perpendicular to the direction of propagation, and thus only the horizontal-shear component is captured on seismograms. The difference between the arrival times of Rayleigh and Love waves helps us determine the depth of earthquakes.
Why is this topic important for understanding a caldera using geophysics?¶
Seismic surveys can be used to distinguish between materials with different physical properties, like densities or fluid content. Subsurface structures can be mapped by identifying how seismic velocities change as waves travel into new materials. For example, waves will travel faster through denser plutons than through sedimentary rock. Additionally, magma bodies, which contain fluid, greatly reduce the speed of P- and S-waves. Depending on the crystalline structure of said magma, shear waves may be completely unable to pass through, since these waves cannot travel through liquids. Some rock units, such as magma chambers, also contain a mixture of both anisotropic and isotropic materials. Anisotropic rocks cause waves to travel at different speeds depending on the direction from which they arrive. This is a product of crystal alignments, microfractures, and other physical properties. On the other hand, isotropic rocks have the same properties in all directions. By identifying how waveforms and travel times from waves with different components of motion are affected, we can gain further insight into subsurface structure, deformation fabrics, and mineral makeup.
What is the physical property measured and associated units?¶
We measure the velocity and arrival times of seismic waves, which have the respective units of kilometers/second or meters/second, and seconds. Seismic velocities and travel times can be used to infer the physical properties of the sampled rocks due to their relationships to velocity. For example, the density (kilograms/meters^3, or grams/meters^3) of materials significantly affects the velocity at which waves travel.
What types of rocks in the subsurface would show a good contrast for this property?¶
Seismic waves travel more slowly when passing through porous, less dense materials, such as limestone. Fluids, or rocks with high fluid contents like groundwater reservoirs or magma bodies, also reduce wave velocities, and will inhibit S-wave propagation altogether in the rare case that the reservoir is purely liquid (i.e., there is no rock matrix or crystal mush). Solid, denser materials are a better conduit for seismic waves due to their higher incompressibility and rigidity. If there is a strong contrast in physical properties like density, porosity, or water content between layers, waves will reflect and refract along those layer interfaces.
-Sal & Claire
Reflection & Refraction¶
Refraction and Reflection surveys are two of the main geophysical surveys conducted to better understand underlying rock layers. Both surveys are conducted by generating a high-energy acoustic wave at the surface, and measuring the time it takes for sensors to record either the reflected wave or the refracted wave. As these acoustic waves travel through the different subsurface layers, they experience changes in wave velocity (m / s) depending on the density of the material they are passing though. Thus, we can determine the density (kg / m3) and thickness of each underlying layer. While both surveys use acoustic waves, they are conducted slightly differently and result in different ‘visuals’ of the subsurface.
Refraction surveys typically involve a straight line of evenly spaced geophones (sensors) that record the arrival time of acoustic waves. When a strong acoustic wave is generated at one end of the geophone line, it will travel across the surface and trigger each of the sensors. However, the wave also travels deeper through the subsurface until it reaches the next rock layer. Typically this bedrock layer should be more dense than the surface layer, which increases the velocity of the acoustic waves. However, refraction surveys depend on the acoustic wave hitting the bedrock at a ‘critical angle’ determined by Snell’s Law. The refracted wave will travel along the surface of the rock layer and propagate new smaller waves up to the surface. These new waves will trigger the geophones before the original surface wave due to the increased velocity. From the recorded arrival times, we would be able to create a velocity profile of any layer the waves traveled through, along with the depth of the layers. A refraction survey could be used when studying the fens within the Valles since it will give us an idea of the underlying bedrock and the density of the near surface layers.
Reflection surveys also use high-energy acoustic waves and a line of geophones, but they utilize the waves reflected at each change in rock boundary instead of the refracted wave. As the wave encounters a new layer, it experiences a change in density and acoustic velocity. This sudden change converts some of the wave’s energy into a reflection that travels back up towards the surface. The amount of energy reflected is determined by the acoustic impedance of the new rock layer. Acoustic impedance is calculated by multiplying the rock’s density (kg / m3) by the velocity of the traveling wave (m / s). A large change in acoustic impedance will result in a stronger reflected wave. Recording the strength and arrival times of these reflected waves allows us to create a 2D map of the subsurface. We can also easily visualize any faults since cracks in rocks will result in a strong difference of acoustic impedance. Reflection surveys should be extremely helpful in mapping out the surrounding faults in the caldera, and even help map the different layers that a refraction survey won’t reach.
-Brenden