6.1.5 Waves for detection and exploration (HT)

Detecting hidden structures with waves

1. A wave may travel at a different velocity through different materials.
2. Some of a wave's energy is absorbed, reducing the energy that reaches a detector.
3. Part of a wave is reflected when it meets a boundary between two materials.
4. By detecting reflected waves and timing how long they take to return, scientists can locate hidden boundaries and work out what materials the waves passed through.

Ultrasound imaging

1. Ultrasound is partially reflected at a boundary between two different media, while the rest carries on or is absorbed.
2. A detector records the reflected pulse, and if the velocity is known the time for the pulse to return gives the distance to the boundary. $d=\frac{vt}{2}$.
3. $d$ is the distance to the boundary in $m$, $v$ is the wave velocity in $m/s$, and $t$ is the total time to travel there and back in $s$.
4. The division by $2$ is needed because the measured time covers both the outward and return journeys.
5. In medical imaging, reflections from boundaries between tissues build up an image inside the body, and in industrial imaging ultrasound reveals hidden cracks or flaws in solid objects.

Seismic waves and the Earth's core

1. P-waves are longitudinal seismic waves that travel through both solids and liquids, at different speeds in each.
2. S-waves are transverse seismic waves that travel through solids but cannot travel through liquids.
3. Seismic waves are recorded at monitoring stations around the world, which compare which waves arrive, when, and where.
4. S-waves are not detected after passing through the liquid part of the Earth's core, which is evidence that this region is liquid.
5. P-waves pass through both solid and liquid regions but change velocity between them, so their paths and arrival times reveal the positions of boundaries inside the Earth.
6. Together, P-wave and S-wave observations provide evidence for the structure and size of the Earth's core.

Echo sounding

1. Echo sounding uses high-frequency sound waves to detect objects in deep water and to measure water depth.
2. A transmitter sends a sound pulse down through the water, it is reflected by the seabed or an object, and the echo returns to a detector.
3. The depth is found from the sound velocity and the total echo time, again halving because the pulse travels there and back. $depth=\frac{sound\; velocity\times echo\; time}{2}$.
4. For example, if sound travels at $1500m/s$ and the echo returns after $0.40s$, the depth is $\frac{1500\times 0.40}{2}=300m$.

Evidence from indirect observations

1. The Earth's deep interior cannot be observed directly, so seismic waves are used as indirect evidence.
2. As more earthquake data were gathered worldwide, patterns in the detection of P-waves and S-waves led scientists to discover hidden regions and improve the model of the core's structure and size.