HR: 1340h
AN: S23B-0328    [Abstracts]
TI: New Waveform Analysis Method: the Chirplet Transform. Characterisation of the Induced-Seismicity From the Lacq gas Field (Western Pyrenees, France).
AU: * Bardainne, T
EM: Thomas.bardainne@univ-pau.fr
AF: Imagerie G‚ophysique FRE 2639 UPPA, Avenue de l'Universite BP1155, PAU, 64013 France
AU: Gaillot, P
EM: philippe.gaillot@univ-pau.fr
AF: Imagerie G‚ophysique FRE 2639 UPPA, Avenue de l'Universite BP1155, PAU, 64013 France
AU: Senechal, G
EM: guy.senechal@univ-pau.fr
AF: Imagerie G‚ophysique FRE 2639 UPPA, Avenue de l'Universite BP1155, PAU, 64013 France
AB: Whole waveform records of waves radiated by induced-seismic events contain considerable information on event location, propagation medium, failure type, source size, and state of stress. Modern seismological studies of induced-seismicity related with hydrocarbons exploitation or storage benefit from time-frequency (TF) analysis techniques such as the windowed Fourier transform or the wavelet transform. These TF representations with fixed windows or kernels perform well only for limited classes of seismograms. Representations with signal-dependent kernels can overcome this limitation. A new five-parameter atomic decomposition of chirplets is developed for compact and precise representation of seismograms with chirp components. The five-parameter chirplet atom is obtained from the unit Gaussian function by successive applications of scaling, fractional Fourier transform, time-shift and frequency-shift operators of various duration. So, the chirplet transform unifies many of the disparate signal representation methods. In particular, the wide-range of TF methods such as the Fourier transform, spectrogram, Wigner distribution, and wavelet transform may each be shown to be a special case of the chirplet transform. We developed a methodology that recognizes the attributes of the waveforms of each seismic events. We first present and illustrate on synthetic examples this new formalism. Next, we illustrate its potentialities on seismograms recorded by 17 seismometers (Mark-Product L-4C - 12 bits) during the monitoring period (1974-1997) of the Lacq gas field (Western Pyrenees, France). Using the chirplet transform, each wave packet of the seismogram (mostly single component, sampling frequency = 150 Hz, duration = 15 s) is decomposed in terms of energy, arrival time, frequency, duration, shape (time-shear) and frequency modulation (frequency-shear). Thus, selected seismograms (N $\sim$ 5500 for 592 well-localized events) are described by a set of sextuplets which provide compact and precise representation of its inner structure, and form the basis for a classification of the seismic traces in the TF plane. Results obtained in terms of trace characterization and event parameter (date, location, magnitude, focal mechanism) are compared with the documented history of the reservoir (exploitation, fluid-injection, pressure, permeabilities and porosities maps, ...). Together, they allow to investigate earthquake induction and geomechanics processes at this site.
DE: 7260 Theory and modeling
DE: 7299 General or miscellaneous
DE: 7209 Earthquake dynamics and mechanics
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting