Recent Advances in Multichannel Seismic Imaging for Academic Research in Deep Oceanic Environments

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Pacific Rise, with prominent crustal reflections and their interpretation indicated (Canales et al., 2010).Color scale is bathymetry, in meters.
applied in a few deep-ocean settings to investigate problems such as seismic Layer 2A structure beneath an MAR high-temperature hydrothermal field (Arnulf et al., 2011), the heterogeneity of slow-spread lithosphere exposed on oceanic core complexes (Canales et al., 2008;Henig et al., 2009;Xu et al., 2009; Figure 2), or the evolution of seismic Layer 2B across the flanks of the Juan de Fuca Ridge in the Northeast Pacific Ocean (Newman et al., 2011).
The densely sampled wavefield also allows applying more advanced techniques such as 2D refraction waveform tomography for obtaining seismic velocity information at scales close to the seismic wavelength (Canales, 2010;Delescluse et al., 2011).
The effectiveness of both traveltime and waveform tomography can be enhanced by downward continuation of data originally recorded at the sea surface to a datum at, or just above, the seafloor, creating a SOBE (Berryhill, 1979;Harding et al., 2007).In surface the EPR can be recognized despite considerable acquisition differences (Mutter et al., 2010).
In summary, steady access to and employment of advanced MCS methods by the academic community in the near future will be a key factor in expanding our understanding of oceanic crustal u A n PA B l O C A n A l E S , H é l è n E C A R t O n , J O H n C .M u t t E R , A l I S tA I R H A R D I n g , S u z A n n E M .C A R B O t t E , A n D M l A D E n R .n E D I M O v I ć O C E A n I C S P R E A D I n g C E n t E R P R O C E S S E S | Ridge 2000 P R O g R A M R E S E A R C H Academic research using marine multichannel seismic (MCS) methods to investigate processes related to Earth's oceanic crust has made substantial advances in the last decade.These advances were made possible by access to state-ofthe-art MCS acquisition systems, and by development of data processing and modeling techniques that specifically deal with the particularities of oceanic crustal structure and the challenges of subseafloor imaging in the deep ocean.Among these methods, we highlight multistreamer three-dimensional (3D) imaging, streamer refraction tomography, synthetic ocean bottom experiments (SOBE), and time-lapse (4D) studies.A strength of 3D MCS reflection imaging lies in the ability to migrate the seismic wavefield in three dimensions, resulting in improved accuracy and resolution of structure, particularly in rugged terrains such as at mid-ocean ridges (MORs) where energy scattered by rough seafloor contaminates and degrades the recorded signal.Magmatic systems along MORs drive and link fundamental physical and chemical processes such as volcanic construction, hydrothermal circulation, seafloor mineralization, and biological colonization.The fine-scale structure of these systems was explored earlier by single-streamer 3D MCS imaging across a ridge discontinuity at the East Pacific Rise (EPR; Kent et al., 2000) and beneath a Mid-Atlantic Ridge (MAR) volcano (Singh et al., 2006), and more recently by multistreamer 3D MCS imaging at the Ridge 2000 Integrated Study Site at the EPR(Mutter et al., 2010;Carbotte et al., 2012, in this issue; Figure1).Advanced 3D MCS methods can also contribute to imaging the complex geometry of oceanic detachment faults, whose dips presumably vary in depth and with azimuth, and which have been recently recognized as a fundamental component of a distinct mode of seafloor spreading(Escartín and Canales, 2011).Long-offset streamers make it possible to record wide-angle MCS data that can be used to infer subseafloor velocity structure via streamer refraction tomography(Zelt et al., 2004) with high resolution thanks to a densely sampled (in space) wavefield.Streamer refraction traveltime tomography was recently Figure 1.Threedimensional visualization of a subset of the Mgl0812 3D MCS data set across the East Pacific Rise, with prominent crustal reflections and their interpretation indicated(Canales et al., 2010).Color scale is bathymetry, in meters.

Figure 2 .
Figure2.Example of subseafloor variations in shallow seismic velocity (red-blue image) derived from streamer refraction traveltime tomography across the Kane oceanic core complex.velocity perturbation, contoured every 0.2 km s -1 , is shown relative to a one-dimensional structure with constant vertical gradient of 1.75 s -1 and V P = 3.7 km s -1 at the seafloor.gray shaded relief is bathymetry, and dashed yellow lines delineate the smooth, corrugated detachment surface.Horizontal axes are latitude and longitude, and vertical axes show depth in meters.FromCanales et al. (2008) modeling, including 3D pre-stack migration, 3D waveform inversion (e.g., Ben-Hadj-Ali et al., 2008), and 3D time-lapse monitoring offer tremendous opportunities to improve understanding of the dynamics and fine-scale structure of complex deep-sea geological systems such as axial magma chambers and oceanic detachment faults.

Figure 3 .
Figure 3. Example illustrating the synthetic ocean bottom experiments, or SOBE, procedure.(a) Common midpoint gather recorded on the sea surface.(b) Same gather downward continued to 1,365 m below the sea surface.Corresponding seismic arrivals are indicated in both panels.note that in (a), layer 2A prograde and most of layer 2B prograde arrivals are secondary (i.e., arrive after the seafloor reflection), while in (b), collapsing of the seafloor reflection makes layer 2A and 2B prograde branches become first arrivals and thus easier to interpret and to use in standard traveltime tomographic inversions.FromArnulf et al. (2011)