Acoustic properties of northern Alaska shelves in relation to the regional geology

Abstract

More than 100 airgun‐sonobuoy records from the northern Alaska shelves have been reduced to yield an average of six layers from each record. Seafloor sound velocities, regional velocity functions, and high‐resolution velocity‐depth inversions were computed. Low seafloor velocities in the northwest part of the otherwise Cretaceous seafloor of the Colville foredeep represent thin Tertiary sediments that overstep the Barrow arch from the north. Anticlinal cores of ‘mid‐shelf arches’ (Grantz et al., 1981) appear as 200‐m/s velocity increases in the Neogene seafloor of the western Beaufort shelf. Very recent or currently, active sedimentation in the central Hope basin is indicated by low seafloor sound velocities. The value of K in the least squares determination of V = V0 + Kt (t is one‐way vertical travel time) decreases toward the east from 1.74 km/s2 just east of Point Barrow of 1.67 km/s2 around Prudhoe Bay, to 1.53 km/s2 near the Mackenzie River. This decrease in K corresponds with an eastward increase in the thickness of the clastic rocks within the youngest part of the Brookian sequence (Early Cretaceous to Quaternary). A 2.1‐km/s2 value of K is the Brookian sequence of the north Chukchi basin suggests a change in the relative proportions of Cretaceous and Tertiary sediments between the basin and the Beaufort shelf. High‐resolution velocity‐depth inversions, using closely digitized travel time data, yield velocities at about 40‐m vertical spacings. Calculated values of near‐surface velocity gradients determined by velocity‐depth plots of these data are generally not in good agreement with the values predicted from the velocity functions, which are statistical generalizations based on conventional refraction picks that resolve layers no thinner than about 250 m. The discrepancies are minimized by use of independently determined seafloor velocities.