Correlation of offshore velocity spectra with onshore sonic logs in North Carolina
Synthetic seismograms computed from sonic logs of three exploratory holes in Dare County, North Carolina, on the Atlantic Coastal Plain allow chronostratigraphic correlations of prominent seismic reflections to be made with offshore seismic velocity spectra. Four prominent reflections are defined on the synthetic seismograms and followed by use of velocity spectra under the continental shelf. These reflections originate from the top and bottom of the Eocene Claiborne high-velocity limestone interval (the reflections are denoted “Ecl” and “Ecs,” respectively), from the middle Cretaceous limestone (Kdel), Early Cretaceous shale (Kgsh), and at the sediment-crystalline basement interface (B). A generalized geologic cross section is constructed over the continental shelf off North Carolina. Two layers under the shelf thicken only slightly to seaward; the layer between water bottom and Ecs thickens from 0.7 to 0.8 km and the layer between reflections Kdel and Kgsh thickens from 0.8 to 1.2 km. The other two layers outlined on the shelf double their thicknesses at the shelf edge relative to their landward thicknesses. The interval between reflections Ecs and Kdel thickens from 0.4 to 1.0 km in a seaward direction, and the deepest interval, between reflections Kgsh and B, thickens seaward from 0.7 to 1.3 km. All of the thickening in the deepest layer occurs in the seaward half of the continental shelf.