Aquifer characterization

Our limited insight in the ground makes it very difficult to reliably describe flow and transport processes. This is in conflict with the great data hunger of numerical models, which depend on techniques that resolve spatial heterogeneity of aquifers as much as possible. My research is directed at developing integrated approaches, combining geological, hydrogeological, geophysical, and thermal field methods to reconstruct a detailed image of the properties of the subsurface. I am particularly interested in using innovative geostatistical zonation and multipoint approaches with coupled hydrogeophysical inversion schemes to accomplish the fusion of data from disparate sources. Appropriate choice of field methods and technologies are of equal importance to the numerical inversion and integration approaches used to combine the acquired field data. Especially pressure and tracer tomography shows a great potential to provide hydraulic properties with a resolution and accuracy superior to that possible with standard methods.

Probably the only way to obtain an exact insight into natural heterogeneity at various scales is the use of aquifer analogues. An outcrop composed of similar structures and lithologies as the aquifer may be viewed as an analogue of this aquifer. Such analogues provide benchmarks to test the applicability of simulation techniques, to tune field experiments and inversion procedures. They allow detailed observation at the surface of the geological structures that are present in inaccessible deep reservoirs. They can also be utilized to inform geostatistical techniques, such as multiple point geostatistics, to capture and reproduce realistic geological structures. We developed two- and three- dimensional analogs of alluvial and aeolian deposits based on detailed outcrop studies in Germany and Brazil. Currently, we work on a free data base of such three-dimensional heterogeneous portrayals of aquifers, with sets of multiple inter- and extrapolated realizations of lithological, hydraulic, chemical and thermal parameters.

High-resolution hydraulic and tracer tomography shows a great potential to provide hydraulic properties with a resolution and accuracy superior to that possible with conventional techniques. Hydraulic tomography consists of a series of short-term hydraulic tests. Varying the location of the source (pumping interval) and the receivers (observation intervals) generates streamline patterns that are comparable to the crossed ray paths of a seismic tomography experiment. With such a vast amount of information, an appropriate inverse model can thus capture the detailed two- or three-dimensional hydraulic heterogeneity of the subsurface. The focus of our current work is on the design of field experiments with tomographic measurement set-ups in unconsolidated sediments and hard rocks, as well as the development of innovative inversion methods. This involves coupling of hydraulic tomography and tracer testing, as well as combined approaches coupling with near-surface geophysics.

Active heating or cooling of the ground is subject to a family of field testing techniques. It is applied to derive thermal ground or aquifer properties, to examine the stability of geothermal installations, or to inspect the prevailing groundwater flow regime. For instance, in the active thermal tracer test, we inject warm water. By thermal monitoring in nearby observation wells, we deduce the flow and transport properties. We have done such testing at the Lauswiesen site close to Tubingen, and at the Widen site in the Thur valley in Switzerland. As an alternative to injection of water, in-situ heating is performed in thermal response tests. Here, we utilize borehole heat exchangers, which are heated up for some days and the response of the ground is recorded. This is based on the same principles as standard hydraulic pumping tests. We studied in detail parameter sensitivities for TRT interpretation, and developed a methodology to directly quantify the effect of groundwater. This means, TRTs are adopted as aquifer tests, and the groundwater flow velocity is derived. The inversion procedure is validated by attuned lab testing and in field experiments.