Unexpected Measuring Devices

How scientists turn things built for something else (or nature itself) into measuring instruments. Common trick: find something that already reacts to what you want to measure, then read out that reaction.

Infrastructure as sensors

Fiber optic cables → seismometers (DAS)

• Distributed Acoustic Sensing: laser pulses sent into a (often unused, "dark") telecom fiber.
• Tiny glass impurities reflect a bit of light back (Rayleigh backscatter). Timing of the echo = position along the cable.
• Ground vibration stretches the fiber → phase of the echo changes → strain measured every few meters over tens of km.
• One cable = thousands of virtual seismometers.
• Uses: earthquakes, traffic, footsteps, volcano monitoring, glacier movement, whale calls near subsea cables.
• Note: measures strain/vibration, not density directly. Subsurface structure (stiffness, density, layers) is inferred from how fast seismic waves travel.

Submarine telecom cables (without DAS)

• Normal data traffic on transoceanic cables: light polarization shifts when cable is disturbed.
• Google + Caltech (2020/21) detected earthquakes and ocean swells this way, no extra hardware.
• "SMART cables": plan to add temperature/pressure/seismic sensors to repeaters of new cables.

Mobile network microwave links → rain gauges

• Cell towers talk via microwave links. Rain attenuates microwave signal.
• Operators log signal strength anyway → convert loss to rainfall intensity.
• Rain maps for regions without weather radar (Israel, Netherlands, parts of Africa).

Smartphones

• Accelerometers → crowd-sourced earthquake detection (Android Earthquake Alerts, MyShake). Early warning seconds before shaking arrives.
• Barometers in phones → pressure data for weather models.

GPS / satellites

GPS → earthquakes & tsunamis via ionosphere

• Big quake or tsunami pushes air up → acoustic/gravity waves travel to ionosphere (~300 km).
• Waves disturb electron density (TEC, total electron content).
• GPS signals pass through ionosphere → extra delay depends on TEC → ground receivers see the disturbance.
• Can track tsunami wave across ocean from above, even far from coast.
• Bonus: GPS also measures ground motion directly (mm precision), incl. slow "silent" earthquakes.

GPS reflections → soil moisture, snow depth

• GNSS reflectometry: signal bouncing off ground interferes with direct signal. Pattern depends on soil wetness / snow height / sea level.

GRACE satellites → groundwater

• Two satellites fly ~220 km apart; distance between them measured to micrometers.
• More mass below (mountain, water) pulls first satellite ahead → distance changes → gravity map.
• Shows groundwater depletion (India, California), ice sheet loss.

Particles from space

Cosmic ray muons → X-ray of huge objects (muography)

• Muons from cosmic rays rain down constantly, penetrate rock. Denser material absorbs more.
• Count muons behind object → density image.
• Found hidden void in Great Pyramid of Giza (ScanPyramids, 2017). Also: volcano magma chambers, Fukushima reactor cores.
• This one actually gives density directly.

Cosmic ray neutrons → soil moisture

• Neutrons from cosmic rays slowed by hydrogen (= water) in soil. Fewer fast neutrons above ground = wetter soil.
• Covers ~hectare area, much bigger than a point probe (COSMOS network).

Geoneutrinos → Earth's inner heat

• Radioactive decay (U, Th) inside Earth emits antineutrinos. Detectors (KamLAND, Borexino) count them.
• Estimate how much of Earth's heat comes from radioactivity.

Animals as sensors

• Fin whale songs → seismic source: loud calls penetrate seafloor; reflections recorded by ocean-bottom seismometers used to image oceanic crust layers (2021).
• Elephant seals with sensor caps → temperature/salinity profiles under Antarctic sea ice where ships can't go. Large share of data from that region.
• Pigeons with air-quality backpacks (London "Pigeon Air Patrol") → pollution mapping.

Accidental archives (measuring the past)

• Paintings of sunsets → red/orange ratio correlates with volcanic aerosols in atmosphere (study of old masters, e.g. after Tambora 1815).
• Ship logbooks → centuries of weather/sea ice observations (oldWeather citizen science).
• Grape harvest dates (Burgundy, since 14th c.) → summer temperatures.
• Nuclear bomb carbon-14 "pulse" (1950s–60s tests) → dates when human cells were formed, age of ivory, forensic age.
• Tree rings, ice core air bubbles, stalagmites → past climate, CO₂, solar storms.

Weird physics tricks

• Atomic clocks → height: gravity slows time (general relativity). Optical clocks detect difference from ~1 cm height change → "chronometric geodesy".
• Seismometers → human activity: COVID lockdowns 2020 caused measurable drop in seismic noise worldwide; football fans jumping register as small quakes.
• Wi-Fi signals → detect presence, movement, even breathing through walls (signal reflection changes).

Pattern

1. Signal already exists (light in fiber, GPS, cosmic rays, whale calls, phone data).
2. Target phenomenon disturbs it (strain, electron density, absorption, attenuation).
3. Model inverts disturbance → quantity of interest.