5.4 Seismic refraction and reflection methods

Seismic refraction and reflection methods are key tools for peering into Earth's interior. These techniques use controlled seismic waves to map subsurface structures, providing crucial data on layer depths, velocities, and compositions.

Advanced processing techniques like CMP gathering, NMO correction, and migration transform raw seismic data into detailed subsurface images. These methods are essential for oil exploration, crustal studies, and understanding Earth's structure.

Seismic Survey Methods

Refraction and Reflection Surveys

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• Refraction survey measures seismic waves refracted along subsurface interfaces
  • Utilizes critically refracted waves traveling along layer boundaries
  • Effective for mapping horizontal and dipping layers
  • Provides information on layer velocities and depths
• Reflection survey records seismic waves reflected from subsurface interfaces
  • Detects changes in acoustic impedance between layers
  • Produces detailed images of subsurface structures
  • Widely used in oil and gas exploration (sedimentary basins)
• Both methods involve generating seismic waves using controlled sources (explosives, vibroseis trucks)
• Geophones or hydrophones detect returning waves at the surface
• Travel times of waves used to determine subsurface properties

Advanced Seismic Techniques

• Vertical seismic profiling (VSP) records seismic waves in a borehole
  • Source at surface, receivers lowered into borehole
  • Provides high-resolution image of area surrounding the well
  • Helps correlate surface seismic data with well logs
• Wide-angle reflection/refraction (WARR) uses large source-receiver offsets
  • Combines aspects of reflection and refraction methods
  • Allows imaging of deep crustal structures
  • Useful for studying continental margins and mountain belts
• Tomography creates 3D velocity models of the subsurface
  • Uses multiple source-receiver pairs to image complex structures
  • Applies inversion algorithms to reconstruct velocity distribution
  • Applications include volcano monitoring and earthquake studies

Reflection Data Processing

Common Midpoint (CMP) and Normal Moveout (NMO)

• Common midpoint (CMP) gathering groups traces with shared reflection points
  • Improves signal-to-noise ratio by stacking multiple traces
  • Assumes horizontal layering and small lateral velocity variations
  • Typical CMP fold ranges from 30 to 120 traces
• Normal moveout (NMO) correction adjusts for travel time differences
  • Accounts for increasing travel times with offset
  • Applied before stacking to align reflections
  • NMO velocity analysis determines subsurface velocities
  • Hyperbolic moveout equation: $t^2 = t_0^2 + \frac{x^2}{v^2}$
    • t: travel time, t0: zero-offset time, x: offset, v: NMO velocity

Advanced Processing Techniques

• Stacking combines multiple traces to enhance signal and reduce noise
  • Sums NMO-corrected traces within a CMP gather
  • Improves data quality by suppressing random noise
  • Stacking velocity function derived from NMO analysis
• Migration repositions reflections to their true subsurface locations
  • Corrects for dipping reflectors and diffraction effects
  • Time migration assumes vertically varying velocity
  • Depth migration handles lateral velocity variations
  • Kirchhoff and finite-difference methods commonly used
• Additional processing steps include:
  • Deconvolution to improve temporal resolution
  • Velocity analysis for accurate NMO correction and migration
  • Static corrections for near-surface velocity variations
  • Multiple suppression to remove unwanted reflections