Ray Theoretical Traveltime Inversion of Seismic Data in Two-Dimensional Plane Dipping Layers.

A ray theoretical traveltime inversion method in two dimensional arbitrarily dipping layers has been developed and applied to synthetic and real field data. The method is based on Durbaum\u27s formula and may be applied to single-fold as well as CMP raw data. The advantage of Durbaum\u27s formula is that we can estimate dipping angles as well as accurate velocities in dipping layers. In addition, the number of traces which can be utilized for velocity estimations is drastically increased. Also, a statistical method can be used to estimate normal incident times (t\sb{\rm o}) and the take-off angles at the surface (B\sb{\rm o}) which are measured manually from a stacked section in other methods. Basically, two different statistical ways of interval parameter estimations are discussed. One is Gauss-Newton\u27s nonlinear least squares which applies to sampled traveltime data. The other is the coherency measures technique which does not need picking. To visualize the coherency measures, three different types of plotting are proposed depending on which parameter is fixed with respect to the others. The velocity-B\sb{\rm o} analysis display which plots semblance coefficients on the axes of Vnmo and B\sb{\rm o} at a fixed t\sb{\rm o} is the most efficient method of presentation. An analytical method and an iterative method for velocity inversion from interval parameters are discussed. A computer algorithm to trace rays efficiently in arbitrarily dipping plane layers is also presented because both inversion methods need ray tracing to solve the inversion equation. The accuracy of inversion is studied through models. This study shows that velocity-B\sb{\rm o} analysis is superior to Dix\u27s and Hubral\u27s method for estimating the interval velocities, depths, and dipping angles. Even in the presence of random errors, the method gives a better prediction than the others. The inversion from curved layers and the effect of multiples and diffractions on velocity-B\sb{\rm o} analysis are examined. Application of Velocity-B\sb{\rm o} analysis to real seismic data shows that it successfully predicts both velocity and dipping angle. These accurate velocities and dipping angles are essential in subsequent processing stages

Morphological and geological features of Drake Passage, Antarctica, from a new digital bathymetric model

The Drake Passage is an oceanic gateway of about 850 km width located between South America and the Antarctic Peninsula that connects the southeastern Pacific Ocean with the southwestern Atlantic Ocean. It is an important gateway for mantle flow, oceanographic water masses, and migrations of biota. This sector developed within the framework of the geodynamic evolution of the Scotia Arc, including continental fragmentation processes and oceanic crust creation, since the oblique divergence of the South American plate to the north and the Antarctic plate to the south started in the Eocene. As a consequence of its complex tectonic evolution and subsequent submarine processes, as sedimentary infill and erosion mainly controlled by bottom currents and active tectonics, this region shows a varied physiography. We present a detailed map of the bathymetry and geological setting of the Drake Passage that is mainly founded on a new compilation of precise multibeam bathymetric data obtained on 120 cruises between 1992 and 2015, resulting in a new Digital Bathymetric Model with 200 × 200 m cell spacing. The map covers an area of 1,465,000 km2 between parallels 52°S and 63°S and meridians 70°W and 50°W at scale 1:1,600,000 allowing the identification of the main seafloor features. In addition, the map includes useful geological information related to magnetism, seismicity and tectonics. This work constitutes an international cooperative effort and is part of the International Bathymetric Chart of the Southern Ocean project, under the Scientific Committee on Antarctic Research umbrella

A crustal model and its tectonic implication on the evolution of the Pacific margin of the northern Antarctic Peninsula

The Shackleton Fracture Zone (SFZ) and the South Shetland Trench (SST) are prominent bathymetric structures in the Southeast Pacific off Antarctic Peninsula. The SFZ comprises a high ridge and a deep trough. The SFZ ridge was probably formed by the uplift of low-density material like serpentinite. Two phases of deformation observed in the trough suggest that (1) a large-scale crustal faulting due to transtensional movement along the SFZ during Drake Passage opening before 6 Ma formed the deep trough, and (2) recent contractional structures around the trough are indicative of the present convergence between the Scotia and Antarctic plates. The angle of subduction of oceanic crust in the SST increases from southwest to northeast along the SST as its age increases from southwest to northeast. Because thick accumulation of sediments is not expected in active trenches with a horst and graben structure, the presence of thick trench-fill sediments (up to 1300m) over a horst and graben structure in the South Shetland Trench (SST) indicates that they accumulated after the cessation of subduction at about 4 Ma

PRELIMINARY RESULTS OF SEISMIC SURVEY IN THE CENTRAL BRANSFIELD STRAIT, ANTARCTIC PENINSULA

Multichannel seismic profiles in the central Bransfield Strait show structural variation mainly controlled by transform faults across the strait. Near King George Island, large displacement of the spreading axis, discontinuity and different style of the faults, intense deformation of the basement, and abrupt change in the morphology of the basin are indicative of the presence of a large fault zone. On the basis of the fault map, the central Bransfield Strait can be divided into three segments. Transform faults, including the large fault zone, form the boundaries of segments. The basinward-dipping reflectors concentrated in the central segment suggest that initial rifting activity was relatively strong in this region

Altimetry Enhanced Free-Air Gravity Anomalies in the High Latitude Region

Available marine free-air gravity anomalies (FAGA) derived from multiple satellite altimetry missions have had geologically useful, short wavelength features removed during processing. An approach is described for augmenting these FAGA in the high latitude region with coherent higher frequency data. This added-value approach is demonstrated over the Barents Sea in the Arctic using existing FAGA predictions from the Danish National Cadastre (KMS98) as a reference. Short wavelength components between 4 and 111 km were added from reduced and correlation-filtered ERS1 168-day mission altimetry that had been sorted into ascending and descending datasets for separate processing. The processed data were then recombined by spectral quadrant swapping to generate a correlated, high frequency gravity field related to the local geologic sources. This added-value surface adjusted the reference FAGA to better reflect features at wavelengths related to the distances between altimetry tracks

Post-subduction margin structures along Boyd Strait, Antarctic Peninsula

The Pacific margin of the Antarctic Peninsula to the southwest of the Hero Fracture Zone (HFZ) is a former subducting margin which became inactive following the arrival of ridge crest segments of the Antarctic - Phoenix ridge at the margin during the Tertiary. In contrast, the part of the margin to the northeast of the HFZ remains active. Tertiary convergence was approximately perpendicular to the margin and ongoing motion is thought to have the same orientation. A new seismic reflection profile running along Boyd Strait, just northeast of the landward projection of the HFZ, shows major structural components similar to those typically observed along the margin to the southwest of the HFZ. In order of increasing proximity to the margin, these components are: the inner shelf, the shelf basin, the mid-shelf basement high (MSBH), and the outer shelf. The continuation of the post-subduction margin structures to the active margin suggests that the boundary between crust with passive and active margins characteristics is not sharply defined. Our postulated scenario for tectonic evolution along Boyd Strait is that: (1) before the arrival of the last ridge crest segment to the southwest of the HFZ, the inner shelf and the shelf basin were part of a Cretaceous-Tertiary arc and forearc area, (2) after the arrival, thermal effects resulting from interaction of the ridge crest with the margin just southwest of the HFZ lead to the formation of the MSBH to the northeast, but MSBH uplift in Boyd Strait did not prevent concurrent cross-shelf sediment transport contributing to development of an extensive outer shelf on the seaward flank of the MSBH, (3) Recent extension in Bransfield Strait, a marginal basin to the northeast of the landward projection of the HFZ, has caused about 10 kin of seaward deflection in the strike of the part of the MSBH to the northeast of the projection of the HFZ