The use of MRS in the determination of hydraulic transmissivity: The case of alluvial aquifers

Abstract

Magnetic Resonance Sounding (MRS) is nowadays accepted as a new geophysical method that can be used for a reliable determination of the ground water content distribution in the top 150 m. A great effort has also been made in MRS development to deduce the hydraulic transmissivity, based on empiric relationships of the permeability with a factor F which is calculated with NMR parameters measured at laboratory scale. To use this relationship under field conditions a calibration coefficient C T = T pt / F has to be previously established, which demands the knowledge of the transmissivity T pt evaluated in the pumping test. The transmissivity can then be calculated at any other site of the same aquifer using the relation T mrs = C TF . The C T values reported suggest a certain relationship with the lithology, but with a great dispersion and contradictory results. MRS surveys carried out in alluvial aquifers in Spain have shown that the value of C T evaluated at one site may not be valid at another place of the same aquifer, because of the great heterogeneity of this kind of geological environment. The demand of a pumping test at each site where a MRS is measured invalidates the method actually used for MRS transmissivity evaluation. More than 50 MRS have been used to propose a new methodology. The aquifers visited cover a great range of transmissivities (from 2 × 10 − 6 to 9 × 10 − 3 m 2/s). The MRS signal amplitude varies between 20 and 1400 nV, the signal/noise ratio is in the range from 0.6 to 42, and the value of the decay time constant varies from 200 to 800 ms. It has been demonstrated that when the transmissivity increases, the value of F decreases, and C T increases, except for certain groups of MRS taken at the same aquifer or part of one aquifer, for which F increases with T pt, keeping C T constant. A function C T ( F) of the type C T = m F − n has been obtained that allows the transmissivity evaluation without the need of T pt. Considering that both values of transmissivity, T pt and T mrs, are subjected to deviations due to the experimental errors as well as due to evaluation errors, the prediction achieved by the proposed equation is rather good. To perform a better evaluation of the values of the coefficients m and n it is necessary to have a greater number of MR soundings of good quality and with a trustworthy inversion at locations where a really comparable and good performed pumping test is available, covering a sufficient range of transmissivities. Though the data we have used do not always fulfil these conditions, the result is promising. Once a trustable function is available, the forecast of the transmissivity using MRS will not need the existence of any pumping test in the area. The general extension of this methodology demands the availability of MRS taken at all kinds of geological and hydrogeological environments, which is impossible without the existence of a universal MRS data base.