This may mislead the interpreter to believe that there are continuous vertical fractures from the top to the maximum DOI. In fact, multiple sets of fractures could occur in a single well due to sets of horizontal to sub-horizontal and vertical to sub-vertical fractures. Thus a deep saturated fracture zone will produce a deeper DOI with an increase in the transmitter moment, indicating the presence of water at still deeper levels.
The results obtained by the SCI can be used in principle to create 3D images of subsurface electrical structures. However, we have used constrained inversions that assume the earth structures to be spatially coherent and the regularization concept results in an overall improvement of the resolution of geological structures that may not be well resolved by individual soundings A rigorous 3D inversion simultaneously considering the total survey area 71 , 72 may improve the results. However, it would require extremely large computational resources in terms of memory, time and parallelization, particularly because fracture zones need very fine meshing.
There is no software package currently available commercially to accomplish this objective. The present work emphasizes the importance of regional surveys in finding sustainable groundwater sources in hardrock terrains where most of the shallow wells located in the weathered zone have gone dry due to over exploitation and the challenge lies in locating sustainable groundwater sources in bedrock where limited secondary porosity is provided by sporadically distributed fracture zones.
The results demonstrate that the AEM surveys in combination with borehole data and geological information provide a potential tool to achieve this goal as they are capable of characterizing crystalline hardrocks and lineaments, mapping the spatial extent of fracture networks and associated hydrogeological pathways, termed as Hydrolins, that are derived from the AEM data.
The DOI information efficiently demarcates the deep groundwater bearing fracture zones. Their yield potential is determined by the connectivity provided by the Hydrolins. The study also brings out a threshold depth, TGWH, below which the well-connected fracture zones are likely to provide high yielding wells.
The AEM data also helps in studying the reduction in the groundwater bearing fracture zones with depth. The groundwater potential of a given watershed thus can be assessed by determining the availability of fracture zones below the TGWH. Lineaments are important in locating groundwater and the study reveals that Hydrolins provide the most significant clues followed by Maglins and Satlins. Additionally, a precise knowledge of Hydrolins can also be helpful in identifying suitable recharge zones.
Knowledge of the fracture network in hardrock terrains is useful to understand regional hydrogeology and provide crucial inputs to simulate the groundwater flow system, and develop sustainable groundwater management plans. Since the study deals with the important problem of locating sustainable sources of groundwater in hardrocks, it is important that the present results supported by a limited number of groundwater data are further confirmed by expanding the analysis to larger groundwater data sets Famiglietti, J.
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