Seismic instruments and seismic networks Jens Havskov

1. Seismic instruments and seismic networks Jens Havskov

2. Generation and measurement of seismic waves

3. Measure of mass displacement Spring Mass Damping A very simple mechanical seismograph

4. Recording

5. Electromagnetic seismometer Voltage is proportional with the velocity of the coil in the magnetic field

6. Geophone

7. Accelerometer, the hart of the broad band seismometer and the accelerometer Simplified principle behind Force Balanced Accelerometer. The displacement transducer normally uses a capacitor C, whose capacitance varies with the displacement of the mass. A current, proportional to the displacement transducer output, will force the mass to remain stationary relative to the frame.

8. Displacement, velocity and acceleration A fault is displaced a given distance D A standard seismometer measures the velocity of the ground V A force is proportinal to acceleration measure by an accelerometer A The relation between these measures are: V = 2 f D A = 2 f A f : frequency in Hz Measuring one, we can therefore calculate the others Seismologists like to use nm displacement

9. Sensor output All sensore give an output in volts The output is linearly proportianl to velocity for seismometers The output is linearly proportional to acceleration for accelerometers

10. Typical frequencies generated by different seismic sources.

11. Sensor frequency response All seismometers have a natural resonance frequency f0 below which the output is no longer linearly proportinal with the ground velocity f0 =1Hz Short period seismometer

12. Filter response Left: An RC high cut filter consisting of a capacitor C and a resistor Rc. The resistance of the capacitor decreases with increasing frequency so the effect of the RC combination is to filter out higher frequencies. The input signal is x(t) and the output signal y(t). Right: The amplitude response (output y(f) divided by input x(f)) of the RC filter.

13. Inreasing price Seismic sensors for different frequencies Different sensors are distinguised by the lowest frequecy they can record linearly Geophone: 4.5 Hz → Short period: 1 Hz → Broadband: 30 s (0.03 Hz) → Very broadband: 120 s (0.008 Hz) →

14. Correction for frequency response Seismologists like to work with displacment or velocity Within certain frequency limits it is possible to correct for the instrument response and generate displacment or velocity The top trace shows the original digitally recorded signal. The bottom trace shows the signal converted to true ground displacement in nm. The seismometer is a 1 Hz sensor with an output proportional to ground velocity.

15. Raw traces of seismic noise for different sensors: Broad band, short period and accelerometer A small window of the common traces for Z-channels. The numbers above the traces to the right are max amplitude in counts and the numbers to the left, the DC offset in counts.

16. Displacement signal of seismic noise 1-20 Hz A small window of the common traces for the Z-channels. The traces have been corrected for instrument response and show displacement in the frequency band 1-20 Hz. The numbers above the traces to the right are max amplitude in nm and the numbers to the left, the DC offset in nm. Note that traces look identical.

17. Sensor to use • All sensors can, within a given frequecy range, produce the same result • Depending on task to be solved, a low price sensor, like the geophone might be suitable, e.g. for location and magnitude • Senstive accelerometers may ofen be sufficient in noisy environments • Broad band sensors are needed for doing advanced analysis for larger events • Geophone, one componnent: 100 $ • Accelerometer, three component; 3000 $ • Short period, one component: 500-1000 $ • Broadband, three component : 10000-150000 $

18. Geophone and short period seismometers, commenly used in local seismic networks

19. Examples of broad band sensors used in local and global networks Nanometrics and Guralp

20. Kinemetrics accelerometer 13 cm

21. MEMS accelerometer Principal elements of a MEMS (micro electro mechanical systems) accelerometer with capacitive transducer. The mass is the upper mobile capacitor plate which can rotate around the torsion bars. The displacement, proportional to acceleration, is sensed with the variance in the capacitance. The size of the sensor above is about 2 mm. Found in mobile phones

22. We record in 3 directions to get the 3D earth movement

23. Seismic recorder • The seismometer gives out an electric signal proportinal to ground movement • The signal is saved in a seismic recorder • The recorder migh also retransmit the signal to a center

24. Main units of a seismic recorder. The GPS can be connected to the digitizer or the recorder. The power supply may be common for all elements or each may have its own regulator, but usually the power source is unique (e.g. a battery).

25. Digitizer The analog to digital conversion process. The arrows show the location and values (amplitudes) of the samples and the signal is thus approximated with a sequence of numbers available at time intervals Δt. Normally a signal is sampled 100 times a second.

26. Example of recorders Nanometrics, SARA and GeoSig

27. Seismic recorders are everywhere A smart phone has a built in accelerometer and a digitizer. So it can work as a complete seismic station with the appropriate software. Some smartphones are actually connected to global seismic networks

28. Quake-Catcher Network Sensores can be a mobile phone or better, a very inexpensive accelerometer with built in digitizers ($100). Blue dots are stations, red dots are events.

29. Seismic station The seismic station has the sensor, the digitzer and may also have a recorder. The most crititcal part of the installation is the broad band sensor which must be well shielded from ambient noise and temperature changes High ambient noise can ruin the signals from a good sensor !

30. Microseismic noise Seismic noise in different filter bands. The short period station (1 Hz) is situated about 40 km from the North Sea and the unfiltered trace clearly shows the high level of low frequency noise (~0.3 Hz) generated by the sea.

31. Noise displacement at different sites Noise curves in a rural environment. The 3 dotted lines correspond to the maximum, mean and minimum level, the dashed lines give two extreme examples observed in the US and the full line curves give the limits of fluctuation of seismic noise at a European station on bedrock in a populated area 15 km away from heavy traffic A good site has 1 nn of ground displacement amplitude at 1 Hz

32. Noise spectra The Peterson noise curves and noise spectral level for the IRIS station BOCO. The noise level is in dB relative to 1 (ms-2)2/Hz. The Peterson high and low noise models are shown with dashed lines. The noise spectra are shown for all 3 components. Note the lower noise level for the vertical (Z) component.

33. Thermal isolation Thermal isolation of a broad band sensor and surrounding seismic pier and mechanical separation of the pier from the vault walls for the most demanding applications.

34. Typical short period station, not so critical, but it must be shielded from the wind

35. Broad band stations in Geofon network Example of BB vaults from the GEOFON network. a) Underground bunker vault for remote recording, b) Wide and shallow borehole type, c) and d) Simple bunker construction. Note that b – d allow onsite recording since there is a separate recording room. Copied from Fig. 7.55 by W. Hanka in NMSOP, Vol.1, Bormann (Ed.), 2002;

36. Broadband station insulation

37. Broad band on rock

38. Broad band cover

39. Geofon station

40. Not so good broad band

41. Seismic network A seismic network is a set of interconnected seismic stations that work together to detect the seismic waves in space and time with the main purpose of locating the earthquakes.

42. Location

43. Networked seismic stations

44. SeisComP The SeisComP data collection software. To the right is seen the traces for a trigger and to the left the location is shown together with the arrival times.

45. Old and new SeisComP and recording drums

46. GSN network, all have public access A large number of seismic stations are open to the public so any user can, using e.g. the free SeisComP, build his own seismic network.

47. Some stations in the Dominican network

48. Seismic array p=1/v Left: A plane wavefront arrives with apparent slowness p to a rectangular shaped array . The angle  with respect to North defines the ray arrival azimuth. Right: The waveforms recorded are ideally identical, except for the relative delays.

50. Potential use of a small seismic array Improved detection of weak signals Automatic detection of P and S-wave arrivals Determination of azimuth Automatic location Location of weak emergent arrivals like volcanic tremor Building a regional location capability in a small area