WAVE-FORM INVERSION OF DEEP SEISMIC-REFLECTION DATA - THE POLARITY OFMANTLE REFLECTIONS

The Flannan and W-reflectors are two prominent mantle features observe d 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 ex tensive study, their physical origin is still the subject of speculati on. We present a scheme to determine the polarity of these mantle refl ectors, and constrain their upper structure using near-normal-incidenc e seismic reflection data. The technique exploits the convolutional mo del of the earth; we use a deterministic source-signature deconvolutio n to invert the data. We have explored the parameterization of the inv ersion by testing real and synthetic data. We find that it is critical to the legitimacy of the reflectivity model that many traces are stac ked prior to the inversion and that the data have a good signal-to-noi se ratio. Furthermore, an accurate estimate of the effective source wa velet is a fundamental requirement for obtaining a valid reflectivity model; in particular we find the deconvolution results are most sensit ive to the precise value of the water depth and reflection coefficient used in estimating the sea-bed multiple train. In the case of the Fla nnan-reflector, the inversion shows unequivocally that it has a positi ve polarity. Modelling the W-reflector is less straightforward as a re sult of reduced signal-to-noise ratio. None-the-less, the inversion su ggests a positive polarity for the W-reflector, in agreement with obse rvations of post-critical reflections seen on wide-angle seismic data. The near-normal-incidence polarity measurements support the suggestio n that both the Flannan and W-reflectors represent an eclogitic slab, presumably a relict oceanic subduction zone, preserved within the cont inental lithospheric mantle.