Waves for Detection and Exploration | Physics

📖 In-Depth Theory

Seismic Waves

SEISMIC WAVES are produced by earthquakes (or explosions) and travel through the Earth.

TWO MAIN TYPES:

P-WAVES (Primary/Pressure waves):

Longitudinal — compressions and rarefactions.

Travel through solids AND liquids.

Faster — arrive first at seismograph.

Spread out in all directions from the earthquake focus.

S-WAVES (Secondary/Shear waves):

Transverse — particles vibrate perpendicular to direction of travel.

Travel through SOLIDS ONLY — cannot travel through liquids.

Slower — arrive after P-waves.

SEISMIC EVIDENCE FOR EARTH'S STRUCTURE:

S-waves do not pass through the outer core → outer core must be LIQUID.

P-waves are refracted (change speed) at the core boundary → core is DENSER than mantle.

Seismographs detect arrival times of P and S waves → map Earth's interior.

SHADOW ZONES:

Regions on Earth's surface that receive neither P nor S waves after a distant earthquake.

S-wave shadow zone: region directly opposite the earthquake (S-waves blocked by liquid core).

P-wave shadow zone: P-waves refracted by core → gaps in coverage.

Echo Sounding and SONAR

ECHO SOUNDING uses reflected sound (or ultrasound) pulses to measure depth and map the seabed.

PRINCIPLE:

Emit pulse → detect echo → measure time → calculate distance:

d = v × t / 2

APPLICATIONS:

OCEAN FLOOR MAPPING: ships emit ultrasound → build up contour map of seabed.

FISH DETECTION: fishing vessels detect shoals of fish by their sonar reflections.

SUBMARINE DETECTION: military sonar detects submarines.

GLACIER THICKNESS: radar waves (EM) used for ice penetration.

SEISMIC SURVEYING FOR OIL:

Explosions on Earth's surface → seismic waves travel down → reflect off rock layers.

Reflected waves detected at surface → timing reveals depth and structure of rock layers.

Geologists use this to locate oil and gas reservoirs.

Refraction, Reflection and Absorption of Waves

Different substances may ABSORB, TRANSMIT, REFRACT or REFLECT waves — in ways that vary with wavelength.

REFRACTION:

Waves change speed when passing between media → change direction.

Seismic P-waves refract as they pass through Earth's layers → curved paths.

Light refracts when passing through a lens or prism.

Sound refracts when passing between air layers at different temperatures.

REFLECTION:

Waves reflect off boundaries between media.

Echoes: sound reflected from walls.

Sonar: sound reflected from seabed.

Radar: radio waves reflected from aircraft.

ABSORPTION:

Some wavelengths absorbed by specific materials.

Infrared absorbed by greenhouse gases.

Ultraviolet absorbed by ozone layer.

X-rays absorbed by bone more than soft tissue.

TRANSMISSION:

Waves that pass through a material.

Visible light transmitted through glass.

P-waves transmitted through liquid outer core.

⚠️ Common Mistake

S-waves cannot travel through LIQUIDS — this is the key evidence that Earth's outer core is liquid. P-waves can travel through both solids and liquids. Seismic waves travel in curved paths through the Earth because their speed changes gradually with depth (refraction).

📐 Key Equations

d = v × t / 2 (echo sounding depth calculation)

📌 Key Note

P-waves: longitudinal, solid + liquid. S-waves: transverse, solid only. S-waves blocked by outer core → outer core = liquid. Echo sounding: d = vt/2. Seismic surveying: find oil layers. Refraction at boundaries: wave changes speed → changes direction.

🎯 Matching Activity — Seismic Waves

Match each wave type to its properties and what it can travel through. — drag the symbols on the right to match the component names on the left.

P-waves (Primary)

Drop here

S-waves (Secondary)

Drop here

S-wave shadow zone

Drop here

Seismic surveying

Drop here

Explosions create seismic waves — reflections from rock layers reveal oil/gas deposits

Region where S-waves don't arrive — evidence that outer core is liquid

Transverse — travel through solids ONLY — cannot pass through liquid outer core

Longitudinal — travel through solids AND liquids — arrive first at seismograph

⭐ Higher Tier Only

HT only — use seismic wave data to infer Earth's internal structure. Explain the difference between P and S waves and why S-waves don't pass through the outer core. Calculate depths from echo sounding data.

🔬 Triple Science Only

Waves for detection and exploration (HT only, physics only) — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. After a distant earthquake, S-waves are not detected on the opposite side of Earth. What does this indicate?

2. A ship sends a sonar pulse and receives the echo 0.4 s later. Speed of sound in water = 1500 m/s. What is the water depth?

⭐ How Well Do You Understand This Topic?

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⚡ Waves for Detection and Exploration

Seismic Waves

Echo Sounding and SONAR

Refraction, Reflection and Absorption of Waves

Mr. Badmus AI