An unmigrated seismic section shows a reflector with an apparent dip of 15°. After applying migration, the same reflector appears to dip at 28° and is shifted laterally. What does this indicate?
AMigration introduced an error — it should have flattened the reflector, not steepened it
BThe true reflector is steeper than it appeared; migration moved it updip to its correct subsurface position
CThe stacking velocity model was too slow, causing over-migration of the reflector
DThe reflector was a multiple reflection that migration correctly removed from the section
Unmigrated dipping reflectors always appear with shallower dip and displaced in the downdip direction compared to their true position. Migration moves them updip toward the true location and restores the correct (steeper) dip. Seeing a steeper reflector after migration is exactly the expected result for a genuinely dipping interface — the migrated section is more accurate, not erroneous.
Question 2 Multiple Choice
Why is pre-stack depth migration (PSDM) required for reliable imaging beneath a salt body, whereas post-stack time migration would fail?
ASalt bodies absorb seismic energy completely, so only PSDM's higher energy input can generate reflections beneath them
BSalt has very different seismic velocity from surrounding sediments, creating strong lateral velocity variation that violates the assumptions of time migration
CPost-stack time migration cannot handle more than one reflection per trace, and salt creates multiple reflections
DPSDM uses a denser acquisition grid that is only economically justified beneath high-value salt plays
Time migration assumes velocities vary only vertically — a 1D velocity model. Salt bodies have P-wave velocities (~4480 m/s) nearly twice those of surrounding sediments (~2000–2500 m/s), causing rays to refract strongly at the salt flanks and base. This lateral velocity variation bends ray paths in ways that a 1D velocity function cannot predict. Depth migration uses a full 2D or 3D velocity model to trace rays accurately through the salt geometry, producing correctly positioned subsalt images.
Question 3 True / False
Diffraction hyperbolas on an unmigrated seismic section indicate that the subsurface contains curved or dome-shaped reflective interfaces.
TTrue
FFalse
Answer: False
False. Diffraction hyperbolas arise from point scatterers — fault tips, pinch-outs, fractures, or any abrupt lateral discontinuity — not from curved surfaces. When a wave hits a point scatterer, it diffracts energy in all directions; receivers at varying offsets record this energy at different travel times, producing the characteristic hyperbolic pattern. Migration collapses these hyperbolas back to the point where the scatterer is actually located. A curved reflector would produce a more complex pattern, not a simple hyperbola.
Question 4 True / False
Time migration is less accurate than depth migration when subsurface geology involves significant lateral velocity variation.
TTrue
FFalse
Answer: True
True. Time migration uses stacking velocities that vary with depth but not laterally — a 1D velocity function. This works well when layers are flat and velocity contrasts are mild. But in structurally complex areas (salt bodies, overthrust belts, steep dips), seismic rays bend laterally through the velocity field in ways a 1D model cannot capture. Depth migration uses a full 3D velocity model, tracing rays accurately through lateral velocity contrasts to position reflectors correctly in depth.
Question 5 Short Answer
Why does an unmigrated seismic section misrepresent the true positions of subsurface reflectors, and what information does migration use to correct this?
Think about your answer, then reveal below.
Model answer: An unmigrated section plots each reflection at the midpoint between source and receiver at the recorded two-way travel time. For dipping reflectors, the actual reflection point is laterally offset from that midpoint (shifted updip), so the reflector appears at the wrong position and with the wrong (shallower) dip. Migration corrects this by tracing each recorded reflection backward through a velocity model — using the known wave speed — to determine where the reflecting surface must actually be, repositioning events to their true subsurface locations.
The velocity model is the critical input: an accurate model produces a correctly migrated image; a wrong model produces a migrated image that is still incorrect, just differently so. This is why velocity model building (often through tomographic analysis) is the most labor-intensive part of modern seismic processing — the migration output is only as good as the velocity model fed into it.