1. A surveying method for determining the subsurface density distribution at a selected resolution for a solution volume below a survey area using an airborne or other type of vehicle containing gravitational sensors for measuring the gravitational acceleration and the gravity gradients, said gravitational sensors having a predetermined resolution, the method comprising: (a) determining the size of the volume elements at various depths within the solution volume based on the selected sensor resolution, the height of the gravity survey and the predetermined sensor resolution such that the volume of the volume elements times the density resolution equals mass anomalies that are all within the predetermine detector resolution; (b) determining an observation survey grid above the solution volume comprising observation points, the number of said observation points being sufficient to perform the determination of the subsurface density distribution; (c) compiling a matrix having elements that relate the gravitational acceleration and gravity gradient values to the mass values of the volume elements; the matrix entries being dependent on the geometries between the location of the volume elements within the solution volume and the location of the observation points within the observation survey grid; (d) moving the vehicle along a path of travel over the survey area such that the gravity sensors make gravitational measurements at said observation points within the observation survey grid over the survey area at selected altitudes; and (e) based on the gravitational measurements and the inverse of the matrix, calculating values which represent the average density of the volume elements.