VI Seismic reflection methods

Question 1

What is two-way travel time (TWTT), and why is it important in seismic reflection surveying?

Answer 1

Two-Way Travel Time (TWTT) is the time it takes for a seismic wave to travel from the source to a subsurface boundary and back to the surface. It is critical because TWTT directly relates to the depth and properties of subsurface layers. Calculating TWTT for various reflection points allows geophysicists to create a time-depth model of the subsurface structure, essential for identifying layers and potential resources.

Question 2

How do seismic waves reflect at subsurface interfaces, and what determines the reflection coefficient?

Answer 2

Seismic waves reflect at subsurface interfaces where there is a contrast in acoustic impedance (a product of density and velocity). The reflection coefficientRis given by the ratio of impedance differences across layers. High contrasts (e.g., between soft soil and bedrock) result in stronger reflections, providing clear markers for layer boundaries in seismic data.

Question 3

What is Normal Move-Out (NMO), and how does it improve the accuracy of seismic reflection data?

Answer 3

NMO is a correction applied to seismic data to adjust for the increased travel time of waves recorded at farther distances from the source. As the distance (or offset) between the source and receiver increases, travel time becomes longer due to the wave’s path curvature. NMO flattens the reflection hyperbolas, allowing stacking of traces, which enhances signal-to-noise ratio and clarifies subsurface images.

Question 4

How does the Common Mid-Point (CMP) technique help in improving seismic data quality?

Answer 4

• The CMP technique collects multiple reflections from the same subsurface point by using varied source and receiver positions. This approach allows multiple reflections to be stacked after applying NMO, increasing the signal-to-noise ratio and providing a more accurate depiction of subsurface layers. It is essential in generating high-resolution images for detailed geological interpretations.

Question 5

Explain the significance of migration in seismic reflection processing.

Answer 5

Migration corrects for the fact that recorded reflections may appear displaced due to the angle of incidence or subsurface dips. By re-positioning reflections to their true spatial locations, migration reveals accurate geological structures (e.g., anticlines and synclines) and corrects distortions, essential for accurate depth mapping and resource identification. Time migration adjusts for time-domain distortions, while depth migration corrects for depth-based imaging when lateral velocity variations are significant.