DENSITY STRUCTURE OF THE NATIONAL PETROLEUM RESERVE ALASKA FROM GRAVITY PROFILE INVERSIONS

The National Petroleum Reserve Alaska (NPRA) was explored extensively using seismic reflection profiling and limited drilling during the 1970’s. Gravity stations were established at many of the seismic shot point locations. Both the seismic data and the gravity data are in the public domain, and are available from the USGS. We have reorganized the 53,000 gravity station values back into their original profile form, resulting in long profiles of closely-spaced gravity anomaly values along nearly all of the seismic lines. Some of the longer gravity profiles have been split up into shorter sections corresponding to available seismic images. The gravity profiles have been analyzed using geophysical inversion techniques (1) to map the densities of the rocks forming the topographic surface in the NPRA and (2) to estimate the density variations in the subsurface along the seismic profiles.

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.