A multidimensional horizontal-loop controlled-source electromagnetic inversion method and its use to characterize heterogeneity in aquiferous fractured crystalline rocks

Abstract

SUMMARY The controlled-source dual horizontal-loop harmonic electromagnetic (HLEM) profiling method is well suited to the problem of investigating fracture zones in crystalline rocks but there are still limitations in the way that experimental data are currently interpreted—the use of 1-D data inversion leads to inaccurate determination of geological structure. To allow accurate characterization of zones of fractured rock especially underneath heterogeneous overburden, we have developed an efficient 2.5-D regularized inversion method for reconstructing subsurface electrical resistivity distributions from multifrequency HLEM data, with the forward problem solved in 3-D using a staggered-grid finite-difference method. The inversion method is validated using a synthetic example and practical data sets from four borehole sites in a granitic terrain in northeast Brazil. An appraisal of our results for sites with boreholes sited using conventional data analysis procedures shows that we can distinguish between optimally located productive wells in fracture-zone lineaments and those with diminished yields in weathered layer with no major underlying fracture zones. We suggest that 2.5-D inversion can aid in developing better strategies for sustainable groundwater resource development in the basement terrains.