Seismic instruments and seismic networks Jens Havskov Generation
- Slides: 51
Seismic instruments and seismic networks Jens Havskov
Generation and measurement of seismic waves
A very simple mechanical seismograph Measure of mass displacement Spring Mass Damping
Recording
Electromagnetic seismometer Voltage is proportional with the velocity of the coil in the magnetic field
Geophone
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.
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
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
Typical frequencies generated by different seismic sources. Frequency (Hz) Type of measurements 0. 00001 -0. 0001 Earth tides 0. 0001 -0. 001 Earth free oscillations, earthquakes 0. 001 -0. 01 Surface waves, earthquakes 0. 01 -0. 1 -10 Surface waves, P and S waves, earthquakes with M > 6 P and S waves, earthquakes with M> 2 10 -1000 P and S waves, earthquakes, M< 2
Sensor frequency response All seismometers have a natural resonance frequency f 0 below which the output is no longer linearly proportinal with the ground velocity f 0 =1 Hz Short period seismometer
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.
Seismic sensors for different frequencies Geophone: 4. 5 Hz → Short period: 1 Hz → Broadband: 30 s (0. 03 Hz) → Very broadband: 120 s (0. 008 Hz) → Inreasing price Different sensors are distinguised by the lowest frequecy they can record linearly
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.
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.
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.
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 $
Geophone and short period seismometers, commenly used in local seismic networks
Examples of broad band sensors used in local and global networks Nanometrics and Guralp
Kinemetrics accelerometer 13 cm
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
We record in 3 directions to get the 3 D earth movement
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
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).
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.
Example of recorders Nanometrics, SARA and Geo. Sig
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
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.
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 !
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.
Noise displacement at different sites A good site has 1 nn of ground displacement amplitude at 1 Hz 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
Noise spectra The Peterson noise curves and noise spectral level for the IRIS station BOCO. The noise level is in d. B 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.
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.
Typical short period station, not so critical, but it must be shielded from the wind
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;
Broadband station insulation
Broad band on rock
Broad band cover
Geofon station
Not so good broad band
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.
Location
Networked seismic stations
Seis. Com. P The Seis. Com. P 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.
Old and new Seis. Com. P and recording drums
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 Seis. Com. P, build his own seismic network.
Some stations in the Dominican network
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.
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
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
- Geofon mic
- Comparison of virtual circuit and datagram network
- Example of passive instrument
- Indicating instruments and instruments with a signal output
- First generation antipsychotics vs second
- Lord you are good and your mercy endureth forever
- Palo alto networks next generation security platform
- Sinkhole palo alto
- Palo alto networks next generation security platform
- Basestore iptv
- Jens peter friis
- Jens martensson
- Patofisiology
- Jens lindström
- Jens eggers
- Jens martensson ppt
- Jens turowski
- Jens groth
- Jens elmelund
- Jens erik paulsen
- Zero knowledge university
- Sis 300
- Jens turowski
- Jens weingarten
- Emelding
- Jens hilke
- Jens hilke
- Htw lsd
- Jens de sme
- Jens bejer damgaard
- Vadter
- Jens grell
- Sis 18.com
- Jens erik zebis
- Jens kurschat
- Tekst til hvor du sætter din fod
- Jens kjartansson
- Jens parse
- Jens lindström
- Jens balt
- Jens eistrup
- Jens bu
- Jens bollerslev
- Jens bomberg
- Jens reddersen
- Jens heger
- Negativwiederholungen
- Jens peter bonde
- Anja steinbacher
- Jens ernst
- Jens martensson powerpoint
- Jens horbach