Magnetic Resonance Sounding (MRS) in groundwater exploration, with applications in Laos and Sweden
Abstract: Water is essential for all life on the planet, sustaining and ensuring the earth’s ecosystem. Groundwater from a global perspective provides about 50 % of the potable water, 40 % of the industrial water and 20 % of the irrigation water. For drinking water, deep groundwater has many advantages compared to surface water and shallow groundwater, since it demands little or no treatment and the access is secured against temporary droughts. However, drilling wells for deep groundwater is expensive and unprofitable if it is made without knowing the groundwater potential and development of techniques for exploration are therefore of high priority. The petrophysical properties of aquifers are to a large extent influenced by the water content and salinity, e.g. electrical conductivity and electric permittivity, which can be determined from Vertical Electrical Sounding (VES) and Ground Penetrating Radar (GPR) measurements, respectively. Magnetic Resonance Sounding (MRS), based on the principle of nuclear magnetic resonance, is a relatively new, non invasive technique, which in contrast to other geophysical techniques gives a direct measure of the free water content, but also the pore size distribution with depth. With MRS it is possible to determine both storage and hydraulic related parameters far less ambiguously than with classical geophysical techniques. This thesis presents four studies, where the MRS technique have been tested and developed in combination with other geophysical techniques in three different geological environments; (1) The sedimentary basin of Vientiane, Laos, with naturally occurring salt in the bedrock as shallow as 50 m in depth, which inevitably affects drinking and irrigation water from deep wells; (2) In karst limestone, on the island of Gotland, Sweden, where saltwater intrusion, both recent and relic together with pollution from pesticides and fertilizers are major threats to an already exhausted drinking water supply; (3) Test of the MRS spin-echo (SE) technique in Norrbotten, Sweden, where the presence of magnetic rocks and sediments have made it impossible to do MRS with a standard measuring procedure.In Laos (1), MRS and VES have proven to be very efficient in to locate the fresh-salt water interface and distinguish freshwater aquifers from clays. The electrical conductivity (EC) of the aquifers determined from VES correlates well with EC of water collected from shallow and deep wells, which makes the method promising for future water quality estimation in the Vientiane basin. In Gotland (2), the performance of MRS, VES, GPR and Radiomagnetotelluric (RMT) were tested. The use of multiple techniques has shown to give a more coherent interpretation, but MRS and RMT showed to be more efficient in detecting groundwater and characterizing the aquifer. MRS SE measurements have also been conducted in the Bothnian bay (3). It offers a good environment to test the influence of large scale magnetic gradients on the MRS signals but also the reliability of MRS SE result. The MRS SE technique has further been tested for different soil types. The measuring procedure has subsequently been tested and optimized to meet conditions of magnetic environments.
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