III Seismic Refraction Methods

Question 1

What are seismic refraction methods, and how are they applied in geophysical surveys?

Answer 1

Seismic refraction methods involve generating seismic waves, such as P-waves, that travel through different subsurface layers at varying velocities. The method measures the travel times of these waves to detect the boundaries between layers and calculate their depths. These methods are applied in geophysical surveys to investigate subsurface structures, such as determining the depth of bedrock, identifying faults, and detecting underground cavities

Question 2

Explain the difference between P-waves, S-waves, and surface waves in seismic methods.

Answer 2

• P-waves (Primary waves)are compressional waves that travel through solids, liquids, and gases, causing particle motion in the same direction as wave propagation (izplatīšanās).
• S-waves (Secondary waves)are shear waves that only travel through solids, with particle motion perpendicular to the wave direction.
• Surface waves(Rayleigh and Love waves) travel along the Earth’s surface, often causing noise in seismic surveys. Rayleigh waves create elliptical particle motion, while Love waves cause transverse particle oscillation

Question 3

What is Snell’s Law, and how does it apply to seismic wave refraction?

Answer 3

Snell’s Lawstates that when a seismic wave passes from one material to another with a different velocity, the angle of incidence is related to the angle of refraction by the velocity ratio of the two materials. This law helps to determine how waves bend (refract lūzt) when they hit boundaries between layers of different seismic velocities

seismic velocity → the rate at which a seismic wave travels through a medium, that is, distance divided by travel-time

Question 4

Describe the intercept time method used in seismic refraction data processing.

Answer 4

Theintercept time methodcalculates the depth and velocity of subsurface layers by analyzing travel time curves of seismic waves. The method identifies the time it takes for the waves to travel through different layers and uses the crossover distance and delay times to estimate layer thickness and velocity

Question 5

What factors affect the velocity of seismic waves in different media?

Answer 5

Seismic velocity is influenced by several factors:

• Density (blīvums): Higher density materials generally increase wave velocity.
• Elastic properties: The bulk modulus (compressibility) and shear modulus (rigidity) also affect velocity.
• Porosity (porainība): Higher porosity decreases wave velocity.
• Saturation (piesātinājums): The presence of fluids, especially water, can significantly affect P-wave velocity but has less impact on S-wave velocity

Question 6

How do hidden layers and the “sandwich effect” impact seismic refraction surveys?

Answer 6

Hidden layersare thin subsurface layers that may not produce a detectable first arrival in seismic refraction surveys, leading to errors in depth estimation. Thesandwich effectoccurs when a thin layer is “sandwiched” between two higher velocity layers, making it invisible to the refraction method, which can cause misinterpretation of deeper layers

Question 7

What are the typical applications of seismic refraction methods in environmental and geotechnical surveys?

Answer 7

Seismic refraction is used to:

• Determine the depth of bedrock.
• Map the thickness of weathered layers.
• Identify faults and fractures.
• Locate underground cavities or voids.
• Measure aquifer depthsfor water resource management

Question 8

What role do Huygens’ Principle and Fermat’s Principle play in understanding seismic wave behaviour?

Answer 8

• Huygens’ Principleexplains that each point on a wavefront acts as a secondary source of spherical waves, which helps understand diffraction patterns in seismic waves.
• Fermat’s Principlestates that seismic waves follow the path that takes the least time to travel between two points. This principle helps in understanding why seismic waves bend and how they travel through layers with varying velocities

Question 9

Why is seismic refraction useful for detecting bedrock and aquifer depths?

Answer 9

Seismic refraction is particularly useful for detectingbedrockandaquifer depthsbecause it accurately measures the travel time of seismic waves that refract at boundaries between layers with different velocities. The velocity contrast between softer sedimentary layers and harder bedrock or water-filled aquifers makes it easier to identify these transitions