1. Seismic instruments and seismic networks
Jens Havskov

2. Generation and measurement of seismic waves

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

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.
Frequency (Hz)
Type of measurements
Earth tides
Earth free oscillations, earthquakes
Surface waves, earthquakes
Surface waves, P and S waves, earthquakes with M > 6
0.1-10
P and S waves, earthquakes with M> 2
P and S waves, earthquakes, M< 2

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. Very broadband: 120 s (0.008 Hz) →
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) →
Inreasing price

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: $
Broadband, three component : $

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
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. Nanometrics, SARA and GeoSig
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. High ambient noise can ruin the signals from a good sensor !
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 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. SeisComP and recording drums
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

51. Future New communcations will make local recording redundant
Some networks will consist of only low cost acceelrometers connected in a global or local network
Broad band seismometers become better and smaller but not cheaper in the short run
MEMS technology might take over and even broad band sensors with MEMS might become cheaper
There will be a large standardization in recording and processing systems, already happened to a large extent