Seismic Reflection Profiles Across the Northern Reykjanes Ridge

SEGY data of time-migrated seismic reflection profiles across the Northern Reykjanes Ridge collected during Expedition M201. The seismic source was an array of two GI-Guns towed at approximately 2 m depth. We used a single G. I. Gun in True GI Mode (Generator Vol.: 45 in³, Injector Volume: 105 in³) with a shot interval of 4 seconds. Seismic data were recorded with a 120-channel Hydroscience Technologies SeaMUX digital streamer (500 m active length) equipped with Concord Navigator birds and towed at 4 m depth. Recording lengths ranged from 1 to 6 seconds depending on water depth, and data were recorded at 1 ms and stored in SEG-Y format.

Onboard pre-processing was performed using Schlumberger Vista, and included assigning source and receiver coordinates, crooked-line common-midpoint (CMP) binning at 10 m intervals, lowpass filtering (at X Hz) to attenuate swell noise and de-spike to remove electrical interference. A sparse velocity field was also picked using semblance analysis on unmigrated CMP gathers.

We performed further onshore seismic data processing using Shearwater Reveal. The onshore flow included trace edit (removal and interpolation of persistently noisy channels), and shot scaling based on the amplitude of the direct arrival to boost several series of persistently weak shots. This was followed by a variable-depth de-ghost (receiver-side only), de-bubble and zero-phase conversion. The source signature for de-bubble and zero-phase conversion was estimated by shift-stacking the near-trace seafloor reflection for all profiles for incidence angles >20°. A single de-bubble operator for all traces was derived using a predictive deconvolution (24 ms gap, 2% pre-whitening) to attenuate the bubble pulse. The zero-phase conversion operator was generated using a match filter between the de-bubbled source signature and its zero-phase equivalent, and similarly applied to all traces, rotating the phase of the wavelet from approximately minimum-phase to approximately zero-phase. As only a very small amount of the reflected energy was above 200 Hz, we resampled the data to 2 ms (including an anti-aliasing filter) to reduce the data size and improve computation time for further processing steps. We then performed surface-related multiple elimination (SRME) to attenuate multiples associated with the sea surface and applied a pre-stack phase-Q compensation (Q=120 below the seafloor) to correct for dispersion effects.

We then performed a semblance-based migration velocity analysis (2 km interval) on pre-stack time migrated (PSTM) CMP gathers migrated with the sparse velocity field picked offshore.

The final migration included offset regularisation using a partial moveout correction and a Kirchhoff PSTM (including a geometric spreading correction) using the picked migration velocity field. This was followed by semblance-based stacking velocity analysis (1 km interval) to further flatten the reflectors before stacking. Post-migration we performed a parabolic Radon de-multiple and de-noise with a seafloor-relative Radon mute and an automated residual moveout (RMO) correction (50 m interval). We then applied a 30° outer angle mute (preserving 200 m near-offset) to remove post-critical refractions, NMO stretch, and mode-converted energy and stacked the data to obtain the migrated image.

Post-stack we applied an amplitude-Q compensation (Q=120 below the seafloor) to correct for attenuation and improve the resolution of deeper reflectors. This was followed by time-variant bandpass filtering (relative to the seafloor) to attenuate both low- and high-frequency noise, an amplitude boost of +10 dB/s below the seafloor to further balance the amplitudes for interpretation, and a top mute above the seafloor reflection.