Seismic Signals Lesson: Waves, Imaging, and Earth’s Layers

Lesson Overview

• Types of Seismic Waves
• Wave Behaviors and Phenomena
• Principles Underpinning Wave Propagation
• Special Wave Behaviors
• Seismic Signal Analysis
• Seismic Imaging
• Frequency Considerations in Imaging
• Ray Tracing and High-Frequency Approximation

Seismic signals are elastic waves that travel through the Earth, generated naturally by earthquakes or artificially in exploration. These waves help geologists and geophysicists to understand Earth's interior and image underground layers.

Seismic imaging refers to techniques used to construct visuals of subsurface features by analyzing how seismic waves reflect, refract, or diffract through different geological structures.

Types of Seismic Waves

Seismic waves are categorized into body waves (travel through Earth's interior) and surface waves (travel along the surface). Each wave type behaves differently and provides different types of information.

Properties of Seismic Waves

Wave Behaviors and Phenomena

Seismic waves change as they encounter different materials underground. Understanding how waves behave-reflect, refract, or attenuate-is essential for interpreting seismic signals.

Reflection, Refraction & Mode Conversion

• Reflection occurs when waves bounce off a boundary.
• Refraction is the bending of waves as they pass into a new material.
• Mode Conversion happens when a P-wave converts to an S-wave (or vice versa) at interfaces.
• Important Note: Surface waves are not produced by deep reflections; they mostly form near the surface.

Diffraction

• Occurs when waves encounter sharp edges or points.
• Creates hyperbolic patterns in seismic records.

Diffraction Patterns

Principles Underpinning Wave Propagation

Huygens' Principle

Every point on a wavefront acts as a source of secondary wavelets. These wavelets combine to form a new wavefront. It helps explain wave bending, diffraction, and forms the basis for migration techniques in seismic imaging.

Geometrical Spreading

As waves travel, their energy spreads over a larger area, reducing amplitude. This is not due to absorption but is simply energy distribution over space.

Absorption & Attenuation

Waves lose energy due to friction and heat:

• Reduces amplitude
• Delays phase
• Preferentially removes high frequencies

Special Wave Behaviors

Dispersion

In a dispersive medium, wave velocity depends on frequency.

• Surface waves are usually dispersive.
• Different frequencies travel at different speeds.

Evanescent Waves

Waves that do not propagate far and decay rapidly.

• Generated near interfaces.
• Cannot be recorded in the far field.

Seismic Signal Analysis

Seismic records (traces) contain valuable information. But not everything can be directly extracted.

Information from a Seismic Trace

Fourier Transform

• Converts time-domain signals into frequency domain.
• Breaks complex waveforms into simpler sine waves.

Seismic Imaging

Once data is acquired, it must be processed to produce usable images of the subsurface.

Stacking & Fold

• Fold: Number of traces contributing to the same subsurface point.
• Stacking: Combines traces to enhance signal-to-noise ratio.

Migration

• Repositions reflectors to their true spatial location.
• Corrects for mispositioning of dipping reflectors and diffraction hyperbolas.

Frequency Considerations in Imaging

Low Frequency for Inversion

• Low frequencies penetrate deeper and are more stable in inversion methods.
• Full Waveform Inversion (FWI) uses low-frequency content to build reliable velocity models.

Resolution Factors

• Vertical Resolution: Depends on wavelength (shorter = better).
• Horizontal Resolution: Related to Fresnel zone and migration accuracy.

Ray Tracing and High-Frequency Approximation

• By applying high-frequency approximation, we derive ray theory.
• Rays represent energy paths in the Earth, useful for quick modeling.

Understanding seismic wave behavior, how we process their signals, and how we use them to image Earth's interior is central to geophysics. This lesson provided a complete breakdown of wave types, propagation mechanics, signal analysis, and imaging processes. With this foundation, students can approach seismic questions confidently, understanding not only the "what" but the "why" behind each concept.