In the former mining district of La Carolina (southern Spain), waste materials were accumulated until the beginning of the 1980s without first waterproofing the substrate. Additionally, these mining dumps were abandoned without any type of restoration, which are conditions that present a significant risk of contamination of the surrounding soils and waters. Prior to carrying out the necessary work for isolating these residues, it is essential to know the geometry, thickness and distribution of wastes accumulated on the base substrate. For that purpose, in recent years, different indirect study techniques have been used, among which electrical tomography stands out. In this work, a complementary technique, magnetometry, both in its magnetic and gradiometric modalities, is used as an additional method to analyse the geometry and structure of a tailings dam. The results obtained with this methodology are compared with the results provided by electrical techniques, as well as with direct data from mechanical surveys and aerial photographs prior to waste deposition. Four total field magnetic anomaly profiles were selected for forward modelling using Mag2dc software. A correlation between total field magnetic anomaly values and changes in the accumulated thickness of mining waste was established. In this example, the lowest magnetic values coincide with the smallest thicknesses of the tailings dam. This comparative study shows the usefulness of magnetic prospecting in characterizing the base geometry of these mining structures and therefore the application to environmental characterization studies.