nature03944-s1.doc - Supplementary Information...

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Supplementary Information Supplementary Methods 1. Data acquisition. The data used for this study were collected in 2002 on R/V Ewing during the EW0207 cruise using a 6 km-long, 480 channel Syntron digital streamer with receiver groups spaced at 12.5 m. Streamer depth and feathering were monitored with a mix of 13 depth controlling and 11 compass-enhanced DigiCourse birds, plus a GPS receiver on the tail buoy. A 10-element, 49.2 L (3005 in 3 ) tuned airgun array was used as the source of acoustic energy with shot-by-distance at a 37.5 m spacing. Listening time was 10.24 s and sampling rate 2 ms. Data were recorded on 3490e tapes in SEGD format using the Syntron Syntrack 480 seismic data acquisition system. The recorded signal has a bandwidth ranging from a couple to over 100 Hz. The nominal common midpoint (CMP) bin spacing is 6.25 m and the data trace fold is 81. 2. Data processing. The prestack processing strategy adopted for the EW0207 multichannel seismic (MCS) data consisted of: Standard straight-line CMP bin geometry; F-K and bandpass (2-7-100-125 Hz) filtering to remove the low frequency cable noise; amplitude correction for geometrical spreading; surface consistent minimum phase predictive deconvolution to balance the spectrum and remove short period multiples; surface consistent amplitude correction to correct for anomalous shot and receiver-group amplitudes not related to wave propagation; trace editing; velocity analysis using the velocity spectrum method; normal moveout and dip moveout corrections to align signal for stacking; and CMP mute to remove overly stretched data. The prepared prestack data, with and without the automatic gain control, were then stacked (averaged). The poststack processing included seafloor mute, primary multiple mute to reduce migration noise, bandpass filtering (2-7-100-125 Hz) and time migration to collapse diffractions and position the recorded reflection events to their true
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subsurface locations. Extracting an image of the layer 2A/2B boundary, often referred to as the 2A event, requires a somewhat different processing scheme because this event is not a true reflection 1 . The prestack data preparation is identical up to the velocity analysis, which is done on bandpass filtered (2-7-40-60 Hz) constant velocity stacks. When the normal moveout velocities that best flatten the retrograde branch of the 2A refraction are chosen, the data are stacked. The stacked layer 2A event is time migrated and coherency filtered. Surgical mute is then used to extract the layer 2A event, which is afterwards merged with the reflection section to form the final, composite seismic image. 3. Modeling. The synthetic ray-amplitude curves for a variety of potential MTZ interfaces shown in Fig. 3b-f were calculated by ray tracing through a 1D layered media. Modelled amplitudes depend on both the reflection and transmission coefficients but are scaled before plotting to equal the reflection coefficient at zero-offset. The 1D model used for ray tracing was constructed for the area centered at the largest group of MTZ reflection events imaged (Figs 2b and S1). Maximum source-receiver offset was 6000 m, identical to that of the field
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  • Spring '13
  • L
  • Seismology, Reflection seismology, Data acquisition

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