The concepts of reference wave slowness (reciprocal of velocity) and an associated free reference space Green's function slowness spectrum are introduced. A modified Kirchoff surface integral, containing only the imaginary part of this free reference space Green's function slowness spectrum, is formulated, yielding an integral equation for the unknown fields and sources in the interior of a closed surface on which the (remotely sensed) fields are known. A well-posed, analytic closed form solution of this integral equation is obtained.