An introduction to geophysical measurement

1. An introduction to geophysical measurement John Townend EQC Fellow in Seismic Studies john.townend@vuw.ac.nz Cotton 520, ph. 463-5411

2. What does geophysics involve? • Seismology • Passive source (earthquakes, noise) seismology • Active source (dynamite, airguns) seismology • Engineering seismology • Potential methods • Gravimetry • Magnetism • Electrical and electromagnetic methods • Resistivity • Magnetotellurics • Geodesy + geochronology, thermal methods, remote sensing, ...

3. What do we measure? • Seismology • Passive source (earthquakes, noise) seismology • Active source (dynamite, airguns) seismology • Engineering seismology • Potential methods • Gravimetry • Magnetism • Electrical and electromagnetic methods • Resistivity • Magnetotellurics • Geodesy Seismic velocity (Vs, Vp) Density (g) Magnetic susceptibility (km) Resitivity (r) Conductivity () Position (x) + geochronology, thermal methods, remote sensing, ... Age (a) Thermal cond. (kt)

4. A brief history of seismometry (1) • Early seismoscopes • Record displacements only, not timing • Chang Heng (~AD 132) • Mercury seismoscopes (late 18th century) • Left: Seismoscope record from Carson City, Nevada, of the 1906 San Francisco earthquake • Right: Telescope image recorded at the Hawaii Telescope Observatory showing the effects of a local earthquake (mb = 4.3)

5. A brief history of seismometry (2) • Mechanical seismographs • Record the time-dependence of motion • Developed in the late 19th century • A schematic illustration of the Milne horizontal seismograph (1894), and a record obtained in 1901. Image courtesy of the USGS.

6. A brief history of seismometry (3) • Electromagnetic instruments • Achieve higher amplifications by recording the current induced in a moving coil

7. Fundamental challenges • How do we measure ground motion using an instrument that is itself attached to the ground, and moving? • How do we make reliable measurements of motion occurring over a very wide range of frequencies and amplitudes?

8. Wavelength, period, frequency • Wavelength (metres, m) • The distance between successive peaks/troughs • Period (seconds, s) • The time between successive peaks/troughs passing a point • Frequency (Hertz, Hz = s–1) • The number of oscillations in a fixed time • Equal to 1/period (“reciprocal”) Period t W’length x

9. Some practical issues • Signal frequencies • Seismic waves contain frequencies of mHz–kHz • Signal amplitudes • Displacements can be as little as 10 µm–10 cm • The ideal seismometer requires • High bandwidth • High dynamic range

10. spring constant, k (t) mass, m A mass on a spring • Force on mass due to spring • F = –k • Newton’s 2nd law • F = ma  –k = m • Natural frequency • 02  k/m • Overall equation •  = –02  .. ..

11. The mass oscillates at the natural frequency, o Free resonance

12. spring constant, k (t) mass, m A simple harmonic oscillator natural frequency

13. damping coefficient, c spring constant, k (t) mass, m A damped harmonic oscillator damping parameter

14. Damping • Underdamping • Exponential decay in signal amplitude • Critical damping • Non-oscillatory motion

15. The seismometer equation Undamped oscillation term Pendulum acceleration Viscous damping term Ground acceleration

16. Electromechanical seismometers • Instead of measuring the mass’s motion by a mechanical device, we can measure the voltage induced in a moving coil • voltage  sensor velocity • This increases damping

17. Broadband seismometers STS-2 broadband data in Pennsylvania from a July 1995 earthquake in Tonga • Original data • Low-pass filtered • High-pass filtered • Zoomed high-pass filtered

18. 東日本大震災, 11 March 2011 The earth rings like a bell

19. Newton’s law of gravitation m2 r • Force exerted by m1 on m2, and vice versa • Gravitational acceleration of a mass due to m1 m1 And the mass of something is its volume multiplied by its density: m = V

20. Why measure gravity? • Measurements of gravitational acceleration (“gravity”) provide information about density • Exploration geophysics • Reconnaissance in sedimentary basins • Characterisation of ore bodies or groundwater aquifers • Solid earth geophysics • Determination of crustal/lithospheric structure • Measurement of the shape of the Earth • Investigations of glacial rebound and mantle viscosity

21. Typical densities

22. Measuring gravity We usually measure relative differences in gravity LaCoste & Romberg Model “G” Zero-length spring L = mg/k

23. Watching the earth deform Animated version Preliminary GPS displacement data (version 0.1) provided by the ARIA team at JPL and Caltech. All Original GEONET RINEX data provided to Caltech by the Geospatial Information Authority (GSI) of Japan.

24. Sea surface deformation Tanioka et al., Hokkaido U. Tanioka and Seno, 2001

25. OBS & OBP network Hino et al., Tohoku U. ＋ : OBS ▲ : OBP Epicenters: JMA (since 2003) 2008/06/14 M7.2 2003/05/23 M7.1 2011/03/09 M7.3 2011/03/11 M9.0 2005/08/16 M7.2

26. Where to with early warnings? Yamada et al., Kyoto U. A public warning was issued 8 s after seismic waves were first detected, 31 s after rupture started

27. Where to with early warnings? • Successes • Warning broadcast very quickly (in advance of S-wave arriving) • 27 bullet trains stopped • Tsunami warnings issued in time • Failures • Magnitude underestimated due to long rupture duration • Ground motion and tsunami heights underestimated • Aftershock warnings impeded by interfering waves Yamada et al., Kyoto U.

28. Suggested reading material • The solid earth: an introduction to global geophysics (Fowler, 1990, 2004) • Field geophysics (Milsom, 2003)