TY - BOOK

T1 - Hydrogeological characterisation using cross-borehole ground penetration radar and electrical resistivity tomography

AU - Zibar, Majken Caroline Looms

PY - 2007/2

Y1 - 2007/2

N2 - Water and solute transport through the unsaturated zone have a majorquantitative impact on the high-quality groundwater resources available forexploitation. The protection of these valuable resources requires anaccurate understanding of the unsaturated processes in order to producereliable decision tools. This Ph.D. thesis was initiated with the objective ofdetermining unsaturated hydraulic parameters at a scale more appropriatethan traditional laboratory derived parameterization. Unsaturated flow andtransport processes were therefore studied at a field site in Denmark. Thefield site was characterized by ~30 m thick unsaturated zone consistingmainly of sands of varying coarseness. Following an instrumentation of 16boreholes two geophysical methods (cross-borehole ground penetratingradar and electrical resistivity tomography) were applied during naturalprecipitation and forced infiltration. The methods provided estimates of soilmoisture content and electrical resistivity variations among 12 m deepboreholes located 5 – 7 m apart.The moisture content change following natural precipitation was observed tobe practically negligible, providing minimal information to constrain thedynamic properties of the subsurface. On the other hand, volumetricmoisture content variations of up to 5% were observed during a 20-daylong forced infiltration experiment. The cross-borehole electrical resistancetomography and ground penetrating radar data collected during thisexperiment were subsequently combined to produce estimates of tracerconcentration profiles and images suitable for moment analysis. In spite ofthe extensive use of the cross-borehole geophysical methods, there are stillsome limitations and uncertainties associated with tomographic imagesresulting from the methods. To avoid these effects, a framework to estimateunsaturated hydraulic parameters using multiple data types was developed.In this methodology, the collected geophysical data was used directlywithout producing tomographic images. Unfortunately, the data did notprovide sufficient information to constrain all the parameters of theparametric function describing the unsaturated hydraulic properties. Onlythe saturated hydraulic conductivity values of the top 7 m were partiallyconstrained.In order to improve the tomographic estimates obtained through inversion,two additional methodologies were investigated. (1) A method wasdeveloped that incorporated information regarding correlated data errors inthe inversion. Unwanted artefacts were in this way dampened considerablywithout reducing information of the subsurface. (2) A stochastic inversiontechnique was evaluated that maintained the variability of the subsurfacephysical properties. This method also provided estimates of the subsurfacecorrelation structures which may serve as input for stochastic simulationtechniques.

AB - Water and solute transport through the unsaturated zone have a majorquantitative impact on the high-quality groundwater resources available forexploitation. The protection of these valuable resources requires anaccurate understanding of the unsaturated processes in order to producereliable decision tools. This Ph.D. thesis was initiated with the objective ofdetermining unsaturated hydraulic parameters at a scale more appropriatethan traditional laboratory derived parameterization. Unsaturated flow andtransport processes were therefore studied at a field site in Denmark. Thefield site was characterized by ~30 m thick unsaturated zone consistingmainly of sands of varying coarseness. Following an instrumentation of 16boreholes two geophysical methods (cross-borehole ground penetratingradar and electrical resistivity tomography) were applied during naturalprecipitation and forced infiltration. The methods provided estimates of soilmoisture content and electrical resistivity variations among 12 m deepboreholes located 5 – 7 m apart.The moisture content change following natural precipitation was observed tobe practically negligible, providing minimal information to constrain thedynamic properties of the subsurface. On the other hand, volumetricmoisture content variations of up to 5% were observed during a 20-daylong forced infiltration experiment. The cross-borehole electrical resistancetomography and ground penetrating radar data collected during thisexperiment were subsequently combined to produce estimates of tracerconcentration profiles and images suitable for moment analysis. In spite ofthe extensive use of the cross-borehole geophysical methods, there are stillsome limitations and uncertainties associated with tomographic imagesresulting from the methods. To avoid these effects, a framework to estimateunsaturated hydraulic parameters using multiple data types was developed.In this methodology, the collected geophysical data was used directlywithout producing tomographic images. Unfortunately, the data did notprovide sufficient information to constrain all the parameters of theparametric function describing the unsaturated hydraulic properties. Onlythe saturated hydraulic conductivity values of the top 7 m were partiallyconstrained.In order to improve the tomographic estimates obtained through inversion,two additional methodologies were investigated. (1) A method wasdeveloped that incorporated information regarding correlated data errors inthe inversion. Unwanted artefacts were in this way dampened considerablywithout reducing information of the subsurface. (2) A stochastic inversiontechnique was evaluated that maintained the variability of the subsurfacephysical properties. This method also provided estimates of the subsurfacecorrelation structures which may serve as input for stochastic simulationtechniques.

M3 - Ph.D. thesis

BT - Hydrogeological characterisation using cross-borehole ground penetration radar and electrical resistivity tomography

ER -