A geologist is investigating the structure of rock layers beneath the Earth's surface. She sends seismic waves downward from the surface and detects the waves that return. Explain how the geologist can use the returning seismic waves to determine both the depth at which a rock boundary lies and whether more than one boundary is present underground.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves are produced by earthquakes or by controlled explosions at the surface. When they reach a boundary between two different rock layers, they are partially reflected back to the surface, where detectors record them. The speed of seismic waves through rock is known.
Model answer (5 marks)
1. Seismic waves are partially reflected when they reach a boundary between two rock layers.
2. The geologist records the time interval between emission and detection of the reflected wave.
3. Using the known wave speed, the depth of the boundary is calculated from distance = speed × time, divided by two to account for the two‑way travel.
4. If more than one boundary exists, several reflected waves will arrive at different times, each corresponding to a different depth.
5. Unreflected waves continue through the boundary (transmitted), reach deeper layers, and can be reflected back, providing further depth information.
2. The geologist records the time interval between emission and detection of the reflected wave.
3. Using the known wave speed, the depth of the boundary is calculated from distance = speed × time, divided by two to account for the two‑way travel.
4. If more than one boundary exists, several reflected waves will arrive at different times, each corresponding to a different depth.
5. Unreflected waves continue through the boundary (transmitted), reach deeper layers, and can be reflected back, providing further depth information.
Examiner tips
- Use the command word ‘Explain’ – give a clear sequence of steps. Show the two‑way travel correction (divide by 2) when calculating depth. Mention that multiple arrival times indicate multiple boundaries. Include the role of transmitted waves for deeper layers.
Common mistakes
- Forgetting to divide the distance by two for the round‑trip. Assuming only one reflected wave when multiple boundaries are present. Not recognising that transmitted waves can reveal deeper boundaries.
Mark scheme (5 marks)
- When seismic waves reach a boundary between two media / rock layers, they are partially reflected back
- The time between emitting the wave and detecting the reflected wave is measured
- Since the speed of the waves is known (constant through a given medium), the depth of the boundary can be calculated using distance = speed × time (divided by two because the wave travels to the boundary and back)
- If more than one boundary is present, multiple reflected waves will return at different times
- The remainder of the waves that are not reflected continue through the boundary (are transmitted), allowing them to reach and reflect from deeper boundaries
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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