Geophysics Flashcards
Name four main types of seismic waves, and define their polarization (i.e. how are the particles moving with respect to the direction of the propagation of the waves).
The p-wave has polarization along the propagation direction
The s-wave has polarization direction to the propagation direction
The Rayleigh wave has polarization direction in a plane normal to the surface and along the propagation direction it polarizes elliptically
The love wave has polarization direction in horizontal direction and normal to the propagating direction
What is the difference between surface waves and body waves?
Body waves like the s- and p-waves can travel through the earths inner layers, while the surface waves can only travel along the surface like ripples in water.
P- and S-wave velocity equation
Vp = sqrt(lambda+2*mu/p)
Vs = sqrt(mu/p)
(b) Can S-waves propagate through water? How can we record S-waves in a marine environment?
The S-wave only travels through solids as compared to the p-wave which goes through both solids and liquids. We can record S-waves in a marine environment by using sensors on the seafloor that catch waves.
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List the main steps of seismic processing, describe each step briefly.
Preprocessing enhances the signal by removing, correcting and modifying signal.
Deconvolution is a process designed to restore a waveshape to the form it had before convolution
Velocity analysis and normal moveout corection. Flatten reflection hyperbolasto increase signal-to-noise ratio.
Stacking reduces the amount of data and improves the signal to noise ratio.
Migration. Rearrangement of seismic information elements so that reflections and diffractions are plotted at their true locations.
List four different data domains and briefly describe how data (seismic traces) are sorted in each domain.
When data comes from a single shot and several receivers it’s called a common shot gather. If you have a single receiver with many shots it’s called a common receiver gather.
The common offset gather is normally used for basic quality control because it approximates a structural section. All the traces are at the same offset.
The common midpoint gather is a fairly normal gather. Traces are sorted by surface geometry to approximate a single reflection point from earth. Data from several shots are combined into a single gather. The traces are then sorted by offset to perform velocity analysis for data processing and hyperbolic moveout correction.
What is the P-wave velocity and S-wave velocity respectively for water?
For water: Vp = 1500 m/s and Vs = 0 m/s
Seismic waves are attenuated when traveling through the earth(loss of waves). What are the most important factors attenuating P- and S-waves.
Attenuations of seismic waves:
- Reflection and transmission
- Absorption (Anelastic attenuation)
- Spherical spread
What principle can be used to derive Snell’s law?
We can use Fermat’s principle to derive Snell’s law. Fermat’s principle says that the path taken between two points by a ray is the path that can be traversed in the least time.
What is Snells law
Snell’s law describes the relationship between the angles of incidence and refraction when a wave passes through a boundary between two media with different seismic velocities.
What is the critical angle?
The critical angle is the angle of incidents that gives us an angle of refraction of 90 degrees. Always happens when we go from a more dense to a less dense substance.
θ1 = arcsin(V_1/V_2 )
