Discrimination of P-Wave Interval in Time-Quefrency Analysis for Proximity Microseismic Doublets Koji Nagano Muroran Institute of Technology Muroran, Hokkaido, , Japan

2 What we want to know Fine structures Flow paths in geothermal reservoir Cloud Soultz in France Geothermal fracture reservoir

3 Proximity microseismic doublets Similar waveforms Interval between events is short, <1s.

4 Accurate location in proximity MS doublets microseismic doublets with long interval long interval different velocity model proximity microseismic doublets More precise than doublets with long interval 1st event2nd event

5 Relative location P-wave interval Relative location Fine structure of fracture reservoir The relative location estimated based on the interval is much precise than that calculated from the each sources.

6 Cepstrum analysis for proximity MS doublets Cepstrum analysis is used to detect interval

7 Cepstrum Interval

8 Two cepstrum peaks Which is the P-wave interval in the two cepstrum peaks ? P S P ? S ?

9 Time-quefrency analysis - Window function suppresses data after the onset of the 2nd S-wave. - I shorten the window function. - When the window function suppresses the 2nd S-wave, S-waves lose similarity and the S-wave peak decreases. cepstrum quefrency Terminal of window - Cepstrum-quefrency graph is used to detect the interval. cepstrum

10 Contour plot and cepstrum graph Contour plot: To discriminate the P-wave peak from the S-wave peak identification of the cepstrum peaks - continuity of the peak, - comparison of the peak width - exclusion of artifacts - low resolution in quefrency Cepstrum graph: - precise detection of quefrency - difficult discrimination from artifacts quefrency Terminal of window cepstrum quefrency

11 4 check points in contour plot P-wave interval peak (1) starts more early. The window function suppresses the 2nd S-wave. The 1st and 2nd P-waves are isolated. (2) is more stable. The S-wave peak decreases because the S-wave loses similarity (3) is lower. The P-wave is smaller than the S-wave (4) is narrower. P S quefrency Terminal of window cepstrum

12 Soultz geothermal field MS events were located during hydraulic injection in The data were monitored at 4 stations Data length was 1.6 s. Sampling frequency was 5000 Hz. Proximity MS doublets were 230

13 Typical result of the time-quefrency analysis Early start Stable, low, and narrow peak Some artifacts are visible quefrency Terminal of window cepstrum quefrency

14 9 events show P-wave interval peak The P-wave interval should be detected at 4 stations to determine relative location. quefrency Terminal of window quefrency Terminal of window quefrency Terminal of window quefrency Terminal of window

15 Relative location The P-wave intervals in the Proximity MS doublets are compared among the 4 stations. Moriya 2003 showed 4752 relative locations of MS doublets with long interval. We will examine my relative locations and his data.

16 Conclusions Proximity MS doublets are a good tool to estimate fine structures of a geothermal reservoir. Time-quefrency analysis can provide the P-wave interval between the proximity MS doublets. 4 check points are indicated to discriminate the P-wave interval in a contour plot of the time-quefrency analysis. I have estimated relative locations of 9 events recorded in Soultz geothermal field.

17 difficult result of the time-quefrency analysis Which peak starts more early ???? The upper peak is more stable, lower, and narrower than the lower peak. SP quefrency Terminal of window cepstrum quefrency

18 Cepstrum and auto-correlation

19 Rotation of coordinates 3 component seismic detector was used. 3D particle motion associated with seismic waves was measured. Data in the direction of the P-wave polarization is analyzed in the time-quefrency analysis. The data is larger than that in the other directions We have clear P-wave peak in the contour plot when the data in the direction of the P-wave polarization is analyzed in the time-quefrency analysis.

20 A result of the rotation X-Y-Z P-S1-S2 S1: the maximum variance on S-wave plane S2: P and S1 are perpendicular P-wave polarization is estimated in the data at the onset of the P-wave