DENSITY STRUCTURE OF THE NATIONAL PETROLEUM RESERVE ALASKA FROM GRAVITY PROFILE INVERSIONS
For the terrain density inversion, the free-air gravity anomaly along each profile was modeled as a constant times the topographic profile plus a polynomial term representing the contributions of deeper density contrasts. The number of polynomial coefficients was varied until either the correlation with terrain was maximized or the gravity residual was minimized. Results indicate that near-surface densities on the Coastal Plain average about 2.0 g/cm3 and range from 1.0 g/cm3 on the ice to around 2.4 g/cm3, with some localized higher values. Densities in the Northern and Southern Foothills average around 2.6 g/cm3 and range from 2.4 to over 3.0 g/cm3. Some of the highest densities are found in the southernmost part of the NPRA, on the northern margin of the Brooks Range.
A Fourier technique was used to invert free-air gravity profiles along selected seismic lines into density cross sections. Before each inversion, an initial density cross section was constructed using constant or linearly increasing densities between bounding surfaces interpreted from the seismic reflection data. The inversion results maintain the bounding surfaces, but also introduce lateral variations in density within each layer. The lateral density variations in the sedimentary layers may indicate fold structures, while those in the basement layer could indicate Franklinian rift basins.