MIGRATION TO ZERO OFFSET (DMO) FOR A CONSTANT VELOCITY GRADIENT: AN ANALYTICAL FORMULATION1

Migration to zero offset (MZO) is a prestack partial migration process that transforms finite-offset seismic data into a close approximation to zero-offset data, regardless of the reflector dips that are present in the data. MZO is an important step in the standard processing sequence of seismic data, but is usually restricted to constant velocity media. Thus, most MZO algorithms are unable to correct for the reflection point dispersal caused by ray bending in inhomogeneous media. We present an analytical formulation of the MZO operator for the simple possible variation of velocity within the earth, i.e. a constant gradient in the vertical direction. The derivation of the MZO operator is carried out in two steps. We first derive the equation of the constant traveltime surface for linear V(z) velocity functions and show that the isochron can be represented by a fourth-degree polynomial in x, y and z. This surface reduces to the well-known ellipsoid in the constant-velocity case, and to the spherical wavefront obtained by Slotnick in the coincident source-receiver case. We then derive the kinematic and dynamic zero-offset corrections in parametric form by using the equation of the isochron. The weighting factors are obtained in the high-frequency limit by means of a simple geometric spreading correction. Our analytical results show that the MZO operator is a multivalued, saddle-shaped operator with marked dip moveout effects in the cross-line direction. However, the amplitude analysis and the distribution of dips along the MZO impulse response show that the most important contributions of the MZO operator are concentrated in a narrow zone along the in-line direction. In practice, MZO processing requires approximately the same trace spacing in the in-line and cross-line directions to avoid spatial aliasing effects.     

The significance of pollen-rich inorganic lake sediments in the Cratére du Nouveau-Quèbec area, Ungava, Canada

Unexpectedly high pollen concentrations characterize the basal, silty part of the postglacial sediments accumulated in two lakes from the Cratère du Nouveau-Quebéc area, Ungava. These lacustrine silts and their pollen content result from early postglacial washing of a pollen bearing till. The till must therefore ahve incorporated pollen that relates to events prior to the last glacial event. The matrix of the till deposits surrounding the lakes shows outstandingly high pollen concentrations. It is hypothesized that because of the proximity of the ice divide during the last (and earlier) ice advance(s) in Ungava, the previously depoisited till and the pollen that haad accumulated in its matrix during the interglacial interval(s) were preserved in relict till plains or recycled into the till of teh last glaciation. The crater's age has been established at 1.4 Ma and holds a minimum thickness of 95 m of sediments. It is very likely filled with successive tills or related glacigenic deposits perhaps representing the whole length of time since the crater was formed. Alike the most recent till, these deposits should pollen. there is thus the prospect ofr a 1.4 Ma old pollen record for Ungava.     

Morphology and facies architecture of a falling sea level strandplain, Umiujaq, Hudson Bay, Canada

Coastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground-probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse-sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing-upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp-based bar-and-beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion.     

Fluctuations glaciaires de la fin du Würm dans les Alpes françaises, établies par des analyses polliniques

Sediments from the peat-bog La Muzelle (Massif des Ecrins, Iserc, France, alt. 2,150 m) were analyzed. The site was reached by a Pinus forest and an Abies forest during the Allerød and the Atlantic, and during the Bølling a shrubland was noted. Forests have been degrading into lawns without intermediary heath long before the appearance of the first signs of human action. The glacier has been receding ever since the Late Würm, except during the Younger Dryas when big frontal moraines were formed close to the peat-bog. The nature of the sediments allowed a determination of several frontal positions of the glacier. No notable re-advance could be evidenced during the Older or Oldest Dryas nor during the Holocene; only a minor oscillation may have occurred at the beginning of the Allerød.