The potential of the qualitative interpretation of MRS data: hydrogeophysical study of the Lagoons of Estaña (Spain)

ABSTRACT The Lagoons of Estaña, at the Spanish pre‐Pyrenees, constitute a set of natural water ponds the origin of which is not well‐known, though a karstic formation is one of the hypotheses. A project is being developed with the objective of understanding the hydrological functioning of the lagoons related to underground waters. The lagoons are in Keuper and Muschelkalk rocks in contact with the main water‐bearing structure of the zone, formed by a syncline of Cretaceous limestone. All the area is affected by a strong tectonics. In order to know the hydrogeological limits of this contact and to map the subsurface Triassic structures several resistivity profiles and eighteen magnetic resonance soundings (MRS) are made connecting the syncline with the lagoons and around them. Because of space limitations in the field the geographical distribution of the measurements is not the most appropriate one to detect possible karstic structures. A special strategy for the interpretation, mainly qualitative, is designed based on the pattern of the e(t) and E0(q) curves, including a detailed verification of the geological situation of the MRS measurements to prevent errors that could occur in case of 1D modelling is not appropriate. Part of the area is influenced by a high geomagnetic gradient due to the presence of basic rocks but good MRS measurements are obtained and it is demonstrated that this magnetic anomaly does not affect the lagoon area. The joint use of MRS and resistivity sections allows a correct interpretation and to establish the geometry of a hydrogeological model.