Imaging cargo containers using gravity gradiometry

Identifying fissile materials inside cargo shipping containers is a current national security need. We propose using the geophysical technique of gravity gradiometry to image cargo shipping containers for the purpose of detecting highdensity materials. We focus on developing the necessary numerical algorithms and carrying out feasibility studies without being concerned with engineering, instrumentation, and application issues. To obtain realistic images with sharp boundaries, robust estimators are applied to the model objective function of the inversion operator and coupled with a Huber norm to provide stable numerical computation. We explicitly form the linear and nonlinear resolution matrices to quantify the resolution errors and perform a funnel analysis to quantify the variance of the recovered densities. Assuming a realistic data acquisition pattern requiring a reasonable amount of time, we show that it is possible to identify high-density materials with a minimum volume of 15 cm(3). Finally, we provide a recovered density threshold to suggest the presence of high-density materials from an arbitrary recovered density model and apply this to two examples of recovering concealed fissile material from a cargo container.