A geologist is investigating layers of rock beneath the ground using seismic waves produced by a small controlled explosion at the surface. The waves travel downward, partially reflect at each boundary between different rock layers, and return to detectors at the surface. Explain how this method allows the geologist to determine both the depth of a rock layer boundary and the nature of the boundary.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves travel at a known, constant speed through a given rock type. When they reach a boundary between two different rock types, some of the wave energy is reflected back to the surface while the remainder continues downward into the next layer.
Model answer (5 marks)
When a seismic wave reaches a boundary between two rock types it is partially reflected back to the surface.
The geologist records the time between the explosion and the arrival of the reflected wave.
Because the wave speed in the rock is known and constant, the depth of the boundary is calculated as depth = ½ × speed × travel time.
A larger amplitude of the reflected wave means a greater contrast in density or elastic properties, so the nature of the boundary can be inferred.
Different reflected waves arriving at different times correspond to different boundaries, allowing several layers to be mapped.
The geologist records the time between the explosion and the arrival of the reflected wave.
Because the wave speed in the rock is known and constant, the depth of the boundary is calculated as depth = ½ × speed × travel time.
A larger amplitude of the reflected wave means a greater contrast in density or elastic properties, so the nature of the boundary can be inferred.
Different reflected waves arriving at different times correspond to different boundaries, allowing several layers to be mapped.
Examiner tips
- Use the formula depth = ½ × speed × time to show the calculation
- Explain that amplitude relates to property contrast
- Mention multiple reflections for multiple layers
Common mistakes
- Forgetting the factor ½ in the depth calculation
- Confusing reflected with refracted waves
- Not linking amplitude to property difference
Mark scheme (5 marks)
- When seismic waves reach a boundary between two different media/rock types, they are partially reflected back
- The time between emission and detection of the reflected wave is measured
- Because the wave velocity/speed is known and constant, the depth of the boundary can be calculated from the travel time
- A stronger/larger amplitude reflected wave indicates a greater difference in the properties of the two rock types at the boundary
- Multiple reflections arriving at different times correspond to different boundaries, allowing the geologist to map several rock layers
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Thermal physics questions
- An engineer is checking a metal pipeline for internal cracks using ultrasound. T…
- A ship uses sonar to map the ocean floor. The sonar system emits ultrasound puls…
- A student is investigating how the human ear detects different sounds. Explain h…
- A medical team uses ultrasound waves to produce an image of a foetus inside a wo…