Principles of Wave Detection and Echo Sounding

A Transducer is used to both emit the wave pulses and detect the returning echoes. The device records the exact time delay of the reflected pulse on an oscilloscope, which is then used to calculate the depth or distance of that boundary.

Changes in material density also cause changes in wave velocity and Absorption:

• Velocity changes: When a wave enters a medium with a different density, its speed changes, causing it to refract (bend). Observing where waves bend helps scientists map layers of different materials.
• Absorption differences: Denser substances (like bone or metal) absorb more wave energy as heat. If the returning pulse is much smaller, it reveals information about the high density or absorbing nature of the boundary material.

Echo Sounding in Deep Water

Echo Sounding is a specific application used by ships and submarines to detect objects like fish or shipwrecks in deep water, or to map the seabed. This technique relies on the Pulse-Echo Technique, which follows a distinct step-by-step process:

1. A transducer on the ship emits a short burst of high-frequency sound waves down into the water.
2. The waves travel through the water until they hit a boundary (such as the seabed or a submarine) and reflect back upwards.
3. The transducer detects the returning echo, and a computer records the total "round-trip" time from emission to detection.

Because the pulse travels all the way to the object and then all the way back, the total distance travelled by the wave is exactly twice the actual depth of the water. Therefore, the distance is calculated using the formula:

$$s = \frac{v \times t}{2}$$

Where:

• $s$ = distance or depth (m)
• $v$ = speed of the wave in the medium (m/s)
• $t$ = total "round-trip" time from emission to detection (s)

Worked Example: Calculating Water Depth

A ship sends an ultrasound pulse and the echo returns after $1.2\text{ s}$. The speed of sound in seawater is $1500\text{ m/s}$. Calculate the depth of the water.

Step 1: Write down the known values.

• $v = 1500\text{ m/s}$
• $t = 1.2\text{ s}$

Step 2: Substitute these into the pulse-echo equation.

• $s = \frac{1500 \times 1.2}{2}$

Step 3: Calculate the total distance travelled, then divide by 2.

• Total distance = $1800\text{ m}$
• Depth = $1800 \div 2 = \mathbf{900\text{ m}}$

Seismic Waves and Earth's Structure (HT Only)

We cannot drill to the centre of the Earth, but earthquakes produce extremely low-frequency Seismic waves (infrasound, $< 20\text{ Hz}$) that travel straight through the planet. A Seismometer detects these waves on the surface, allowing scientists to map the Earth's internal structure by analysing their paths.

There are two main types of seismic waves, which behave differently inside the Earth:

• P-waves (Primary waves) are longitudinal, travel faster, and can pass through both solids and liquids.
• S-waves (Secondary waves) are transverse, travel slower, and crucially, can travel through solids only.

As seismic waves travel deeper into the Earth's mantle, increasing density causes their speed to change. This continuous change in velocity makes them refract continuously, travelling in curved paths. However, a sudden, sharp change in a wave's direction indicates it has hit a boundary with a sudden change in state or density.

Because S-waves absolutely cannot travel through liquids, they leave a massive S-wave Shadow Zone on the opposite side of the Earth from the epicentre. The complete absence of S-waves here proves the Earth's outer core is liquid.

Meanwhile, a P-wave Shadow Zone is created because P-waves slow down and refract sharply at the mantle-core boundary, leaving "blind spots" on the surface. Faint P-waves detected within these shadow zones provide the essential evidence that the inner core is solid, as they must have reflected off a solid internal boundary.