MRS survey in a detrital coastal aquifer in Castellon‐Spain
Authors: Rubio F.M. , Plata J.L. 
Published: August 2009

A magnetic resonance sounding (MRS) survey was carried out by IGME in an aquifer on the Mediterranean coast of Spain, in the province of Castellon. The hydrogeological target was to use this geophysical method to discriminate between the existing different saturated levels within the aquifer, and to identify its lateral continuity and productivity, as an alternative to drilling new boreholes. Seven MRSs were taken along a profile five kilometres long. The sites were situated between boreholes with known lithology, and the information from various vertical electrical soundings was also available. Measurements were taken with NUMIS Plus equipment. In spite of the high electromagnetic noise in the area, good field data were obtained with a square antenna of side 150 m, due to the strong MRS signal found and the relative stability of the noise during the measuring time.

Using a conductive geoelectrical model for the ground, the continuity and variable thickness of the aquifer were defined by correlation of the MRS inversion results, which are in good agreement with the information from the wells except for the expected depth for the bottom of the aquifer. A shallower aquifer appears to the south.

To confirm this situation, another interpretation was carried out, using a more resistive model, and taking into account the geoelectrical and geological variations along the profile. The use of this model modifies the bottom of the aquifer in accordance with expectation, reducing the water content of each layer, so that the total amount of water remains approximately constant. The values of are larger and can give rise to the presence of a ‘ghost’ deeper layer of high permeability.

The presence of the shallower aquifer was confirmed using different values of the regularization parameter. Very high values of the regularization parameter produce a smoother distribution of water content with depth, and gives lower values for the water content, but both aquifers can still be differentiated. With low values for the regularization parameter, the water layers are better defined and contain a larger amount of water. In general, a high value of the regularization parameter gives higher values of , and produces apparent deeper information. The results from both extreme determinations can be considered as possible equivalent aquifer models.

The qualitative interpretation of the MRS is not greatly affected by variation of the geoelectrical model and of the regularization parameter, thus making it possible to determine the existence of water layers and their location to a good approximation, using a wide range of models and regularization parameter values. However, for a quantitative interpretation, it is necessary to adjust the cited parameters in order to obtain representative values for the aquifer; extreme variations of these parameters give a range of variations of the water content and of the decay‐time values, providing equivalent solutions. The decay‐time value distributions are more affected by the variations of the inversion parameters.