Figure 8: Back-projection, fast imaging, and inversions on data simulated starting from a model with four prisms of different shapes and resistivities (resp., 1000  m the resistive ones and 10  m the conductive ones), buried in a homogeneous medium (100  m). A comparison is made between the back-projection obtained using the transposed of matrix (a), the image obtained by a damped least-squares solution of the filtered sensitivity matrix (b), the standard least-squares constrain inversion (c), and robust data constrain inversion (d). The color scales are the same.