A 3D tomography algorithm of self-potential (SP) signals is applied for the first time to the localization of subsurface cavities. A specific application is made to a marl-pit in Normandy (North-West of France). A SP map with a total of 221 (5 m-spaced) measurements shows a negative anomaly with an amplitude of -8 mV associated with the position of the marl pit. To explain these data, we solved the boundary-value problem for the coupled hydro-electric problem associated with the presence of the cavity using a finite-element code. The numerical simulations point out the role of open conduits in electrical charge accumulation near the roof of the cavity and the resistivity contrast between the cavity and the surrounding formation. We applied successfully a SP tomography algorithm showing that the roof of the cavity was associated with a monopole charge accumulation due to the entrance of the ground water flow in a network of open cracks.