Abstract
Fractures serve as highly conductive preferential flow paths for fluids in rocks, which are difficult to exactly reconstruct in numerical models. Especially, in low-conductive rocks, fractures are often the only pathways for advection of solutes and heat. The presented study compares the results from hydraulic and tracer tomography applied to invert a theoretical discrete fracture network (DFN) that is based on data from synthetic cross-well testing. For hydraulic tomography, pressure pulses in various injection intervals are induced and the pressure responses in the monitoring intervals of a nearby observation well are recorded. For tracer tomography, a conservative tracer is injected in different well levels and the depth-dependent breakthrough of the tracer is monitored. A recently introduced transdimensional Bayesian inversion procedure is applied for both tomographical methods, which adjusts the fracture positions, orientations, and numbers based on given geometrical fracture statistics. The used Metropolis-Hastings-Green algorithm is refined by the simultaneous estimation of the measurement error´s variance, that is, the measurement noise. Based on the presented application to invert the two-dimensional cross-section between source and the receiver well, the hydraulic tomography reveals itself to be more suitable for reconstructing the original DFN. This is based on a probabilistic representation of the inverted results by means of fracture probabilities.
Highlights
Tomographic methods are suited for characterizing the spatial heterogeneity of aquifers
The presented study deals with an exemplary hypothetical case, which makes it difficult to the two tomographic concepts allows for the comparison of characteristic differences
The application of the tomography provides better results than tracer tomography, since the variability of the result is smaller two tomographic concepts allows for the comparison of characteristic differences
Summary
Tomographic methods are suited for characterizing the spatial heterogeneity of aquifers They are based on the inversion of signals that are recorded among multiple sources and receivers, which are employed to infer two-dimensional (2D) or three-dimensional (3D) images of the spatial hydraulic parameter distribution. For identifying those structural characteristics that are relevant for groundwater flow, solute, and heat transport, invasive concepts are available that apply hydraulic [1,2,3,4,5,6] or pneumatic pressure [7,8,9,10], or that use tracers, such as dye [11,12,13], salt [14,15,16,17], or heat tracers [18,19,20,21]. Recordings from pressure sensors in adjacent observation wells obtained tomographic data sets for two- or three-dimensional inversion of Geosciences 2019, 9, 274; doi:10.3390/geosciences9060274 www.mdpi.com/journal/geosciences
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