Maria Klepikova 'IMVUL' Research project
Title: Flow tomography : inverse modelling of flow measurements for imaging hydrogeological systems
Supervisor: Tanguy Le Borgne
Description of the project:
In fractured aquifers flow generally takes place in a few fractured zones. The identification of these main flow paths is critical as it controls the transfer of fluids as well as transport of solutes in the subsurface. In this work we will focus on developing new methods for imaging the hydraulic properties of transmissive fractures at the scale of several meters. Traditional inversion methods have been developed for porous aquifers whose hydraulic properties vary smoothly in space (Day-Lewis et al., 2000, Yeh and Liu, 2000, Illman et al., 2009). Our objective is to develop new inversion methods that are adapted for fractured media, and that take advantage of different types of data including cross-borehole hydraulic experiments, flowmeter tests [e.g., Le Borgne et al., 2007, Hess, 1986, Paillet, 1998], temperature measurements, tracer tests. Key questions are therefore to know if these data taken together offer complementary information on the distribution of spatial hydraulic properties and how to incorporate the different types of data in an inverse problem. There is an increasing interest for using temperature data as it can be seen as a natural tracer of ground water flow (Anderson, 2005). Fluid flow carries a net amount of heat and creates anomalies in the temperature field; away from fractures there is a natural geothermal gradient. Temperature anomalies at the Ploemeur fractured rock field site attain 2.5 degrees under and upper the natural geothermal gradient for the borehole depth 100-150 m. Yet, relatively few methods exist for using temperature to image flow in fractured media. To investigate this topic, we started with studying the temperature profile at the borehole scale. Using a flow and heat transfer numerical model, we find that the slope of the temperature profile is related directly with vertical borehole flow velocity. Thus, we propose a method to invert temperature measurements to derive borehole flow velocities. During the first year of PhD project this method was applied and validated on the Ploemeur site (Klepikova et al., under review).
The results of the above study provide new insights on how to include temperature profiles in inverse problem for imaging heterogeneous fracture properties. In May 2011 we have performed a field experiment where we measured the borehole temperature profile under all possible pumping configurations. Examples of temperature profiles collected in one borehole under different pumping conditions are given in Fig.1. We thus obtained a complete data set for “transient temperature tomography”. This is a new type of experiment for which a definition of an inverse problem strategy is needed.
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