Forward and inverse modelling of self-potential data can be based on different procedures. Seepages through a fracture or a pipe-like structure can be easily detected and evaluated employing the analytical formulae obtained for simple, geometrical bodies. Some of them contain a term denominated the polarization moment. The concept of the polarization moment arises as a consequence of the boundary conditions in the solution of the Laplace equation for sphere, a cylinder section or cylinder. In spite of the fact that the polarization moment was used for several authors in the past, its existence has not been experimentally proved up to date and the best of our knowledge. The results of several experiments carried out on cylinder-like bodies show that the existence of polarization moment can be originated by a confined fluid flow in such structures. The polarization moment has two components: one linked with rotatory movement of the confined water flow and the second one with drag of the ions along the cylinder axis. The polarization moment increases significantly in nonlinear flow regime and the obtained self-potential effects allow the reliable detection of even relatively short piping phenomena in the field recorded data.