THE FRENCH PERMANENT ACCELEROMETRIC NETWORK:

TECHNOLOGICAL AND PARAMETRICAL CHOICES

SCIENTIFIC GOALS AND RESULTS

Catherine Berge-Thierry*, J.F. Fels, P. Dervin, P. Guéguen, S. Nechtschein, D. Hatzfeld, P. Volant, P.Y. Bard,

F. Courboulex, P. Dominique, B. Feignier

 * Institut de Radioprotection et de Sûreté Nucléaire/berssin

catherine.berge@irsn.fr

http://www-rap.obs.ujf-grenoble.fr

Technicians and scientists that ensure the maintenance and improvment of the network:

Institut de Physique du Globe de Paris (West Indies), Laboratoire de Géophysique Interne et de Tectonophysique, Laboratoire des Ponts et Chaussées, Institut de Radioprotection et de Sûreté Nucléaire, Géosciences-Azur, le Bureau de Recherches Géologiques et Minières, Ecole et Observatoire des Sciences de la Terre de Strasbourg, Commissariat à l’Energie Atomique, Observatoire Midi Pyrénées, Centre d’Etudes Techniques de l’Equipement, Conseil Général de la Martinique, Ministère de l’Environnement et du Développement Durable, Direction de la Recherche et des Affaires Scientifiques et Techniques, Institut National des Sciences de l’Univers Université de Bretagne Occidentale Université de Clermont Ferrand

Strong motion stations in the 80’s: ~ the NPP map

• 0.01g threshold

• Analog instruments (SMA1, SMA2 essentially)

• Very few events detected

• not sensitive enough

• inadequacy for a SM database construction

Birth of the French Accelerometric Network

Aims of the Network: Provide accurate data for scientists and engineers Sensitive threshold, without saturation for strong motion Instruments developed by a French company

Testing phase in most of the seismic areas in France (metropolitan):

Nice, Grenoble, Alps instrumented, Emphasized significant site effects in sedimentary valleys (Grenoble) Demonstrated the feasability and accuracy of instruments

The Ministry decided sponsoring the RAP, with an objective of 100 stations

0.

4.

8.

12.

16.

20.

4.

8.

5.

10.8.

11.

15.

18.

12.

19951996

1997

19981999

20002001

2002

Stations Réseau RAP

Stations installées par année

91 « RAP » stations installed in 2003

Number of stations from 1995 up to 2002

RAP Network = RAP stations + Associated SM stations

IRSN, BRGM, LDG and Martinique General Council (metropolitan and French West indies)

Instrumentation and recordprocessing ≠ RAP stations

0.

100.

200.

300.

400.

500.

600.

18.120.

301.352.

285.339.

558. 584.

19951996

1997

19981999

20002001

2002

Réseau Accélérométrique Permanent

Nombre de séismes

0.

200.

400.

600.

800.

1000.

1200.

1400.

22. 169.

466. 487. 520.667.

974.

1205.

19951996

1997

19981999

20002001

2002

Réseau Accélérométrique Permanent

Nombre d'enregistrements

RAP strong motion Database, end of 2003

2557 seismic events4510 3 component records

Evolution of recorded events

Evolution of records

RAP station: instrumentation and data processing

Weight/dimension: 4 Kg, 260 x 160 x 120

Power requirements: 11.5 – 18 Volt, 2.5 Watt

Input voltage full scale +/- 5 Volt +/- 1g

Input impedance: 25 Kohm

Digitizers: Crystal CS5321+CS5322, sigma-delta, 24 bits.

Overall digital noise: 8 counts p-to-p.

System clock stability: 5 10-7

External time reference: DCF radio receiver.

Data time stamp: internal time + external time.

Data storage: trigger records saved on internal 8 Mb Flash memory

Communication: serial RS232, to modem or PC

Sensors: Kinemetrics (Episensor T), Guralp, Acquisition system: Titan developed by Agecodagis

• Sampling: 125 sps• FIR filters (time delays removed during processing)• « Titan » format (serial type)• Data stored on a 8 Mb Flash Memory

• Trigger mode (STA/LTA)

Time constant for LTA 30 s

Time constant for STA 1 s

Triggering threshold 4 or 6 depending on site

Pre-event 10 s

Trigger record minimum duration 30 s

Trigger record maximum duration 100 s

memory is full within 4 ou 5 days (~ 50 records)

Operations performed « on site » (regional RAP center)

Regional Center

Maintain stations

Seismic catalogues

Interogating stations& retrieving records

RAP Central Site

FTP servers Regional Centers

RAP Central Site

Offset Removal (SAC)Storage, checking- validation

Distribution through the RAP WEB site

The IRSN – Associated Network

Durance

Grenoble

Tricastin

The Moyenne Durance Fault Network:Velocimetry + Accelerometry

IRSN stations acquisition parameters

Sampling 200 sps

Frequency range usable for data  40% of the sampling using digital anti-aliasing filters, ie [0 to 80Hz] for 200 sps.

Final anti aliasing filter no causal, linear response for the phase, pure delay

Triggering with a threshold 0.5 to 1mg depending on the stations

Filter linked to the trigger IIR Bandpass filter : classic strong [0.1Hz-12.5Hz] with 200sps

Internal FBA23 Characteristic frequency: 50Hz, damping 70%, dynamic >135dB from 0.01 to 50hz

Internal Episensor  Frequency range 0 to 200Hz, dynamic >145dBK2/

Etn

as (

kine

met

rics

)

IRSN strong motion data processing

Standard Kinemetrics method : Strong Motion Analysis for computing corrected accelerograms V2 format

• Opening of the event file recorded by a K2 or Etna acquisition system (.EVT),

• Writing IRSN raw data in CDMG format (V1 File)

• Removing the continuous component (offset evaluated on the entire event file),

• Instrumental correction

• Integrating (including a high pass filter) velocity and displacement time series

• Writing IRSN corrected data in CDMG format (V2 File).

1st decimation factor  (instrumental correction) [1] keeping the maximum number of samples for corrected data

Half length of filter operator: low pass filter for offset correction, [10]

Acceleration decimation factor: high pass filter of acceleration during the offset correction,

[10]

Velocity decimation factor: high pass filter of velocity for the high-pass filter step,

[10]

Displacement decimation factor: high pass filter of displacement for the high-pass filter step,

[10]

Terminal frequency: final frequency of the Ormsby low pass filter (instrumental correction)

[50]

Roll-off width: width of the Ormsby low pass filter (instrumental correction)

[3]

Corner frequency : corner frequency for Ormsby high pass filter (offset correction and high pass filtering of velocity and displacement)

[0.25]

Roll-off width - Ormsby high pass filter - (offset correction and high pass filtering of velocity and displacement)

[0.1]

 

IRSN strong motion data processing

IRSN processing: TRIFUNAC Branch

Corrected IRSN data (under RAP format)

RAP Central Site

• Storage in the database

• Distribution via RAP WEB site

3 other associated networks maintained by:- BRGM (16 in French West Indies)- LDG (2)- Martinique General Council (28)

Geosig instruments (processing not described in this contribution)

RAP « weak motion » records

556 m

Clay and sand

sand

447 m

24 m

40 m

15 m alluvions

limestone

accelerometer

accelerometer

Significant amplifications (duration, amplitude) in sedimentary valleys

The Mw=5,1 « Besançon » event (23/02/2004) recorded by RAP Network

• compute Moment magnitude,• compare recorded acceleration with predicted one,

Conclusions

• Until the 90’s strong motion recorded associated to detection (NPP site safety)

• French Accelerometric Network: scientific and engineering aims- beginning of structural instrumentation,

- improvement of geological site characterization

• Moderate Metropolitan Seismicity conducted the choice of low threshold for RAP stations (trade-off):

- this strategy allows the recording of weak motion (site effects)- necessity to increase the Flash memory to avoid any loss of data in case of communication failures (aftershocks)

• Federation of the scientific and user communities to improve the network and the data distribution