Classic hydrogeological investigation methods consider aquifers as single units. In real life however, aquifers are heterogeneous with strong variations in their physical properties. Heterogeneities could have a strong effect on the aquifer behavior especially when dealing with transport related problems. In porous media, heterogeneities could modify flow patterns, and strongly control the transport of solutes and heat. In fractured media, the effect of heterogeneities is even more significant, as the heterogeneous features provide the main pathways for flow and transport.
The identification of these heterogeneities requires modern high-resolution investigation methods, that can observe and reconstruct the spatial variability in aquifer parameters. Tomographic methods were designed to fulfill these requirements. These methods use penetrating signals into closed domains that are affected by the heterogeneities. With enough signals transecting the domain, the heterogeneities could be reconstructed. The tomography concept originates from beyond geosciences, as the same phenomenon is used in medical imaging devices. The method is also widely used in geophysics, where seismic and electric resistivity tomography are standard tools of exploration.
In hydrogeology, hydraulic tomography uses repeated pumping tests or tracer tests to provide hydraulic signals, and pressure sensors to detect their arrivals. Tracer tomography employs repeated tracer tests for the same purpose, but it requires water sampling for detection. Thermal tracer tomography uses heat injections in combination with distributed temperature measurements.
Tomographic methods are still novelties with a lot of open questions, such as optimal experiment design, data requirements and the uncertainty of the interpretations. Further research of these problems could lead to practical tools, that could be used with the same ease in the future as pumping tests today.