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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
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.
Seismic Record
Includes signals and noise.
Single records: signal-to-noise ratio is low.
Stacking
: Combines multiple records to enhance signal-to-noise ratio.
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.
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