Waveform inversion of deep seismic reflection data: the polarity of mantle reflections

Abstract

The Flannan and W-reflectors are two prominent mantle features observed on seismic reflection data off the northwest coast of Scotland. They are the brightest, most laterally extensive, intra-mantle reflectors identified on a deep seismic dataset anywhere in the world. Despite extensive study, their physical origin is still the subject of speculation. We present a scheme to determine the polarity of these mantle reflectors, and constrain their upper structure using near-normal-incidence seismic reflection data. The technique exploits the convolutional model of the earth; we use a deterministic source-signature deconvolution to invert the data. We have explored the parameterization of the inversion by testing real and synthetic data. We find that it is critical to the legitimacy of the reflectivity model that many traces are stacked prior to the inversion and that the data have a good signal-to-noise ratio. Furthermore, an accurate estimate of the effective source wavelet is a fundamental requirement for obtaining a valid reflectivity model; in particular we find the deconvolution results are most sensitive to the precise value of the water depth and reflection coefficient used in estimating the sea-bed multiple train. In the case of the Flannan-reflector, the inversion shows unequivocally that it has a positive polarity. Modelling the W-reflector is less straightforward as a result of reduced signal-to-noise ratio. None-the-less, the inversion suggests a positive polarity for the W-reflector, in agreement with observations of post-critical reflections seen on wide-angle seismic data. The near-normal-incidence polarity measurements support the suggestion that both the Flannan and W-reflectors represent an eclogitic slab, presumably a relict oceanic subduction zone, preserved within the continental lithospheric mantle.