In a seaport of the Cantabrian coast a geophysical survey has been developed. It was necessary to determine the characteristics of the back part of one of the walls (the land located behind the mentioned wall and under the pavement) in order to solve the doubts on the origin of collapses and the «uncontrolled disappearance» of the cement grout which was spilled to consolidate the terrain. Several profiles of electrical tomography have allowed to know the geology of the subsoil, the depth and irregularities of the limestone substrate as well as the characteristics of the sand formation placed over the limestone. The thickness and permeability of these sands are the cause of this disappearance of the cement. So, the geophysical data have allowed to know the origin the doubts and, therefore, to solve the initial objectives