🪨Intro to Geophysics Unit 8 – Seismic Reflection & Refraction Methods

What's This Unit All About?

• Focuses on using seismic waves to study the Earth's subsurface structure and composition
• Covers two main methods: seismic reflection and seismic refraction
  • Reflection uses waves that bounce off subsurface layers and return to the surface
  • Refraction uses waves that bend as they pass through different layers
• Explores how these methods can reveal important geological features (faults, folds, rock layers)
• Discusses the practical applications in fields like oil and gas exploration, engineering, and environmental studies
• Emphasizes the importance of understanding wave propagation and the properties of Earth materials
• Introduces key concepts and terminology related to seismic surveying and data interpretation
• Highlights the role of advanced technology and computational methods in modern seismic studies

Key Concepts You Need to Know

• Seismic waves: elastic energy that propagates through the Earth
  • Two main types: body waves (P-waves and S-waves) and surface waves (Rayleigh and Love waves)
• Velocity: speed at which seismic waves travel through different materials, depends on properties like density and elasticity
• Acoustic impedance: product of a material's density and seismic velocity, determines how much energy is reflected at an interface
• Reflection coefficient: ratio of the amplitude of the reflected wave to the incident wave, depends on the contrast in acoustic impedance
• Snell's law: describes how seismic waves refract when they encounter a boundary between two materials with different velocities
• Travel time: time it takes for a seismic wave to travel from the source to a receiver
• Seismogram: record of ground motion detected by a seismometer, used to analyze seismic data
• Stacking: process of combining multiple seismic traces to improve signal-to-noise ratio and enhance reflections

How Seismic Waves Work

• Seismic waves are generated by a controlled source (explosives, vibrators, or air guns) or natural events (earthquakes)
• P-waves (primary or compressional waves) are the fastest and can travel through solids, liquids, and gases
  • Particle motion is parallel to the direction of wave propagation
• S-waves (secondary or shear waves) are slower than P-waves and can only travel through solids
  • Particle motion is perpendicular to the direction of wave propagation
• Surface waves travel along the Earth's surface and are slower than body waves
  • Rayleigh waves have elliptical particle motion in the vertical plane
  • Love waves have horizontal particle motion perpendicular to the direction of propagation
• Seismic wave velocity depends on the elastic moduli and density of the material they pass through
• Waves reflect and refract at boundaries between materials with different properties (velocity, density)
• Attenuation: loss of energy as waves travel through the Earth due to absorption and scattering

Reflection vs. Refraction: What's the Difference?

• Seismic reflection occurs when waves encounter a boundary and some of the energy is reflected back towards the surface
  • Reflection strength depends on the contrast in acoustic impedance between the layers
  • Used to map layered structures and detect changes in rock properties
• Seismic refraction occurs when waves cross a boundary and change direction due to a change in velocity
  • Waves bend towards the normal when entering a higher velocity layer and away from the normal when entering a lower velocity layer
  • Used to determine the velocity structure of the subsurface and map the depth to different layers
• Reflection is more sensitive to vertical changes in rock properties, while refraction is more sensitive to lateral variations
• Reflection surveys typically use a shorter source-receiver distance and higher frequencies compared to refraction surveys
• Reflection data is displayed as a time section, while refraction data is often presented as a velocity model or depth section
• Combining reflection and refraction data can provide a more comprehensive understanding of the subsurface structure

Equipment and Field Techniques

• Seismic sources: devices that generate controlled seismic waves
  • Explosives: buried charges that provide high energy but can be destructive and require special permits
  • Vibrators: truck-mounted devices that generate a sweep of frequencies and are more environmentally friendly
  • Air guns: used in marine surveys, release compressed air to create acoustic pulses in the water
• Receivers: instruments that detect and record seismic waves
  • Geophones: measure ground velocity on land, typically arranged in arrays or lines
  • Hydrophones: measure pressure changes in water, used in marine surveys
• Recording systems: devices that digitize and store the seismic data collected by the receivers
• Survey design: planning the layout of sources and receivers to optimize data quality and coverage
  • Factors to consider include target depth, resolution, logistics, and environmental constraints
• Noise reduction techniques: methods to minimize unwanted signals (wind, traffic, electrical interference) during data acquisition
  • Includes proper equipment setup, using arrays, and applying filters during processing
• Quality control: monitoring data quality in the field to ensure the survey objectives are met and to identify any issues that need to be addressed

Data Processing and Interpretation

• Pre-processing: preparing the raw seismic data for analysis
  • Includes editing, filtering, and applying static corrections to account for elevation and weathering effects
• Deconvolution: removing the effect of the source wavelet and improving temporal resolution
• Velocity analysis: estimating the seismic velocity structure of the subsurface
  • Done by analyzing the move-out of reflections on common midpoint (CMP) gathers
• Stacking: summing traces from the same CMP to enhance signal-to-noise ratio and create a stacked section
• Migration: repositioning reflections to their true subsurface locations and collapsing diffractions
  • Improves spatial resolution and creates a more accurate image of the subsurface
• Depth conversion: converting the time section to a depth section using the velocity model
• Interpretation: extracting geological information from the processed seismic data
  • Identifying and mapping key horizons, faults, and other features
  • Integrating with other data (well logs, geologic maps) to build a comprehensive subsurface model

Real-World Applications

• Oil and gas exploration: using seismic data to identify potential hydrocarbon traps and guide drilling decisions
• Geothermal energy: mapping subsurface heat sources and fluid pathways to optimize geothermal well placement
• Carbon sequestration: characterizing suitable rock formations for long-term storage of captured CO2
• Groundwater studies: imaging aquifers and monitoring changes in groundwater levels and quality
• Engineering site investigations: assessing soil and rock properties for construction projects (buildings, bridges, tunnels)
• Geohazard assessment: identifying potential risks (landslides, sinkholes, active faults) and informing mitigation strategies
• Mineral exploration: detecting ore bodies and understanding the geological setting of mineral deposits
• Archaeological surveys: mapping buried features and sites without excavation, preserving cultural heritage

Common Pitfalls and How to Avoid Them

• Poor survey design: inadequate coverage or resolution can lead to missed targets or ambiguous interpretations
  • Carefully plan the survey parameters based on the project objectives and constraints
• Noise contamination: unwanted signals can obscure the desired reflections and refraction events
  • Use proper equipment setup, deploy noise reduction techniques, and monitor data quality in the field
• Velocity errors: inaccurate velocity models can distort the seismic image and lead to incorrect depth estimates
  • Perform thorough velocity analysis and incorporate available well data or geologic information
• Spatial aliasing: insufficient spatial sampling can cause artifacts and limit the ability to resolve steep dips or complex structures
  • Choose an appropriate receiver spacing based on the target depth and desired resolution
• Over-interpretation: reading too much into the seismic data without considering its limitations or integrating other data sources
  • Be aware of the resolution limits, use complementary data when available, and acknowledge uncertainties in the interpretation
• Neglecting anisotropy: assuming isotropic velocity can lead to errors in imaging and depth conversion, especially in layered or fractured media
  • Consider using anisotropic velocity models or migration algorithms when necessary
• Ignoring multiples: failing to account for multiple reflections can create false structures or obscure true reflections
  • Apply multiple suppression techniques during processing and be cautious when interpreting areas affected by multiples