Geophysical mapping of groundwater properties for transport infrastructure construction planning - Final report

The success and costs of infrastructure projects largely depend on reliable characterization of the subsoil, where information on groundwater is essential to protect groundwater resources and to avoid stability problems. To determine the hydrogeological characteristics, drilling is carried out followed by hydraulic tests which are reliable but expensive and provide limited information which, in some cases, may not be representative of the entire area that may be affected. The use of geophysical methods can overcome this problem and by providing continuous information that can be used to optimize well placement and execution. The esults of the drilling and hydraulic tests can then in turn be fed back to improve the interpretation of the geophysical results. It is thereby possible to get more comprehensive and relevant results that reduce the risk of problems in the construction phase, thus saving resources, time and costs. The geoelectrical method DCIP (Direct Current resistivity and time-domain Induced Polarization) can provide information on the intrinsic permeability. In addition, MRS (Magnetic Resonance Sounding) can provide information on the water content and properties of the pore spaces, and thus also information related to the hydraulic conductivity. By combining both methods and using them in a two- or three-dimensional layout, a more comprehensive description of the subsoil is possible.
The purpose of the project is to find out how both methods can contribute to a reliable characterisation of the subsoil's hydrogeological properties. The methods were tested alongside conventional tests of the hydraulic conductivity using boreholes, slug tests and HPT (hydraulic profiling tool) to investigate three different test sites. The test sites were chosen to reflect different hydrogeological conditions and to provide access to reference data. Furthermore, their electromagnetic noise level was a crucial factor as it can affect the geophysical results. Measured data were processed, interpreted and compared, to evaluate the geophysical results with regard to hydrogeological information value, as well as robustness in measurement environments with different signal interference conditions. The results show that DCIP tomography provided inverted depth sections with hydraulic conductivity along the survey lines that mostly agree with the reference data from conventional methods at all three test locations. They also show that DCIP is robust enough to give good results along all test lines performed. However, it is not a guarantee that the method works everywhere due to the presence of noise/disturbances, for example in urban environments. It should also be mentioned that the algorithms that have been used for the interpretation of the hydraulic properties are part of research software, and that there is great potential for further development but also a need to adapt the user interface for a wider use. The results also show that MRS can provide information on water content and hydraulic properties that are mainly consistent with the reference data from other methods, thereby providing valuable complementary information. However, MRS measured from the ground surface, as tested here, is significantly more sensitive to electromagnetic interference, which was manifested in the fact that the method only worked fully at one of the test sites while giving limited or no useful results at the other test sites, due to the presence of noise generated by adjacent
infrastructure.