Modelling and processing of 3D seismic data collected over the overlapping spreading centre on the East Pacific Rise at 9° 03′ N

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Abstract Oceanic ridges are important as the locus for the generation of new crust associated with the movement of plates on the Earth’s surface. The geometry of the axial magma chamber (AMC) under these ridges is poorly constrained. This is especially true for the AMC under an overlapping spreading centre (OSC) where the ridge undergoes a small lateral offset, typically less than 8 km. One such OSC exists on the East Pacific Rise at 9° 03′ N. Both 2D and more recently 3D seismic reflection surveys have been conducted over the area in an attempt to establish the relationship of the AMC with the two overlapping limbs of the axial rise. A major problem for imaging the AMC is caused by the seafloor, which has a large velocity contrast and has rapid changes in topography. This presents a major challenge for the processing of data. This paper aims to show that the understanding the effects of acquisition and processing can be facilitated by accurate 3D modelling of the seismic wavefield. In this paper we use a modelling method based on complex-screens, which is both robust and fast for large 3D models with deep water and diffractive interfaces. We compare synthetics with real data and explore the sensitivity of processing 3D data from shot gather to final 3D depth-migrated image. We show that the combination of severe seabed topography and 3D velocity variation within the oceanic crust causes distortion and complex focusing of the reflected energy. Also, for data with only sparse reflectivity it is not possible to construct an adequate velocity model for depth imaging from the reflection data alone. This problem may be overcome if there is an alternative method to determine velocity, e.g. refraction data, but care must be exercised during interpretation given the different resolutions of the two datasets. Lessons from the modelling exercise can be used to aid the interpretation of the real data and examples are shown for comparison; however, the geological implications of the results are outside the scope of this paper.