Estimation of hydraulic transmissivity from MRS by varying the porosity exponent in detrital aquifers of the Iberian Peninsula

Abstract

ABSTRACTTo this date, estimation of hydraulic transmissivity with magnetic resonance sounding (MRS) requires transmissivity values from pumping tests in nearby wells in order to determine a calibration constant from the relationship between both types of data. The use of this technique is limited due to the fact that reported values of this calibration constant are highly variable, even within the same type of aquifer. In order to minimize this shortcoming, a new methodology, which has been developed and presented here, is used to estimate the transmissivity of detrital aquifers with heterogeneous behaviour based only on the results obtained by MRS. The exponents applied to decay time and free porosity used in the expression to compute MRS transmissivity have conventionally been fixed values. In contrast, the work presented here aims to improve the applicability of this technique using a variable exponent for free porosity in each type of aquifer, obtained only from MRS results. In order to achieve this goal, previously published MRS data from four detrital aquifers in the Iberian Peninsula have been used and a relationship between the porosity exponent and a function of decay times and free porosities has been developed. This approach eliminates the need to drill and perform pumping tests in additional boreholes, which has cost and time implications, once the correct expression is determined for a given type of aquifer. The range of values thus obtained for the porosity exponent is similar to that reported in the literature. The deviation of the results obtained with this methodology regarding the results of pumping test is similar to the deviation with conventional process. This suggests that the proposed methodology is appropriate for the aquifers considered, and the approach may be expanded to study other types of aquifers. The drawbacks of the proposal related to the conventional equivalence are explored.