T23D-06:
Evidence of Fluid/Gas Along the Backthrust Offshore Central Sumatra from Seismic Full Waveform Inversion
Abstract:
The Sumatra subduction zone is one of the most seismically active zones on the earth and has experienced great earthquakes in the last decade, including one of the largest earthquakes of the 21st century on December 26, 2004 (M 9.3) producing a devastating tsunami. Geodetic and seismological studies suggest that a part of the subduction is still fully locked and may produce a great earthquake up to Mw=9 in the coming future.In order to understand the earthquake and subduction zone processes, we applied elastic full waveform inversion (FWI) on 50km deep seismic reflection data from Benkulu forearc basin. Data were acquired in partnership with CGG in 2009 using a 15 km ultra-long offset streamer, with shots every 50m and receivers every 12.5m. Unlike usual exploration data, the streamer was dived at 22.5m and the source was triggered at 15m. These characteristics provided wide aperture and low frequencies to the data which are required for FWI.
To match our modelling with the observed data before applying FWI, we first estimated the source wavelet from simulated far-offset signature. Then, the same pass-band filter was applied to both the source and the observed data from 1 to 15 Hz frequency bandwidth. Additionally, an f-k filter was applied to the real data to remove the remaining swell noise up to 4 Hz, without losing any useful signal. Our starting 2D Vp model was found by applying traveltime tomography to a smooth 1D model. Our Vs model was linked by empirical relationship to our Vp model.
By inverting first for far-offset refraction to recover the background velocity model and using a multi-scale strategy by going from 1Hz to 15Hz, we image the forearc basin down to 10km depth. We highlight the presence of a 10km wide low velocity structure at 3km depth. The sharp velocity change from 4km/s to 2km/s indicates the presence of gaz or fluid which could come from the mantle through the backthrust fault. The complexity of the sea floor makes the reflection challenging to invert but could give higher resolution. We are expecting improvement in the next months.