1. A method for nuclear magnetic resonance (NMR) sensing of a material under investigation, comprising the steps of: (a) imparting a polarizing magnetic field signal to the material for a predetermined period of time; (b) receiving NMR signals from a population of particles in the material, following the polarization period; (c) constructing a mapping of the received signals from the signal space into a parameter space associated with a predetermined set of basis functions, wherein the mapping step comprises the steps of: 1. sampling and digitizing of the received signals to obtain a measurement vector b; 2. defining a matrix A incorporating information about the set of basis functions and forming a linear system of equations for the defined matrix A and measurement vector b; 3. solving the system of linear equations Ax=b to obtain a solution vector x to provide an approximation of the received signals; wherein the solution is obtained by applying an orthogonal decomposition to matrix A; and (d) determining from the provided approximation an indication of attributes of the material under investigation; wherein only basis functions for which the corresponding component of the solution vector is non-negative are used in obtaining the approximation parameters, and steps 2-3 are iterated until a non-negative solution vector is obtained by defining at each iteration a reduced size matrix A.sub.i ' for which the i-th basis function corresponding to the component of the solution vector x having the largest negative amplitude is removed.