Seismic Reflection Method Insights

Sep 2, 2024

Seismic Reflection Method

Overview

  • Primary method to image the outer part of the Earth.
  • Reveals detailed subsurface structures.
  • Involves sending acoustic energy signals into Earth and recording reflections at the Earth's surface.

Challenges

  • Distinguishing geologically relevant reflections from noise.
  • Seismic images can be distorted.
    • Vertical scale often presented in time, not depth.
    • Time reflects signal's journey from source, to reflector, and back.

Process

  1. Shot Point (SP) and Receiver

    • Use a seismic source (SP) recorded at a receiver.
    • Initial energy arrival (direct arrival) is a straight path from source to receiver.
    • Subsequent reflections indicate subsurface interfaces.
  2. Seismic Record

    • Includes signals and noise.
    • Single records: signal-to-noise ratio is low.
    • Stacking: Combines multiple records to enhance signal-to-noise ratio.
  3. Stacking Process

    • Multiple records interfere constructively for reflections, destructively for noise.
    • Enhanced kicks represent reflections. Displayed by coloring parts of waveform.

Gathering Types

  • Common Shot Gather (CSG)
    • All records use the same shot, corrected using Normal Move Out Correction.
    • Stacking velocity often corresponds to seismic velocity of the water.
  • Common Midpoint Gather (CMP)
    • Combines traces reflecting off the same small patch from different shot-receiver pairs.
    • Used to resolve small reflector patches.

Seismic Profiles

  • Displayed in two-way time (signal journey time).
  • Real depth interpretation requires seismic velocity knowledge.
    • Velocity affects perceived geology image.

Depth and Velocity

  • Vertical Exaggeration: Higher seismic velocity layers appear thinner on two-way time plots.
  • Planar Fault Distortion: Faults appear distorted due to velocity differences.

Lateral Velocity Variations

  • Pull-Up Effect: High velocity layers (e.g., salt) create artifacts like perceived folds.
    • Misleading geological interpretation if not corrected for real velocity.

Conclusion

  • Seismic images provide detailed subsurface views but can be distorted.
  • Understanding of velocity effects is crucial for accurate geological interpretation.