Combined electrical resistivity tomography and magnetic resonance sounding investigation of the surface-water/groundwater interaction in the Urema Graben, Mozambique

This study focusses on the hydrogeology of Urema Graben, especially possible interactions between surface water and groundwater around Lake Urema, in Gorongosa National Park (GNP). Lake Urema is the only permanent water source for wildlife inside GNP, and there are concerns that it will disappear due to interferences in surface-water/groundwater interactions as a result of changes in the hydraulic environment. As the lake is the only permanent water source, thiswould be a disaster for the ecosystem of the park. The subsurface geology in Urema Graben was investigated by 20 km of electrical resistivity tomography (ERT) and three magnetic resonance sounding (MRS) surveys. The average depth penetration was 60 and 100 m, respectively. The location of the ERT lines was decided based on general rift morphology and therefore orientated perpendicular to Urema Graben, from the transitional areas of the margins of the Barue platform in the west to the Cheringoma plateau escarpments in the east. ERT andMRS both indicate a second aquifer, where Urema Lake is a window of the first upper semi-confined aquifer, while the lower aquifer is confined by a clay layer 30–40 m thick. The location and depth of this aquifer suggest that it is probably linked to the Pungwe River which could be a main source of recharge during the dry season. If a dam or any other infrastructure is constructed in Pungwe River upstream ofGNP, the groundwater level will decrease which could lead to drying out of Urema Lake

Magnetisk resonans sondering i grundvattenprospektering, med tillämpningar i Laos och Sverige

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.Godkänd; 2011; 20110517 (nilper); DISPUTATION Ämnesområde: Tillämpad geofysik/Applied Geophysics Opponent: Doctor Jean Roy, IGP Consulting, Outremont, Québec, Canada Ordförande: Professor Sten-Åke Elming, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet, Luleå Tid: Måndag den 3 oktober 2011, kl 10.00 Plats: F341, Luleå tekniska universitet</p

Magnetisk resonans sondering i grundvattenprospektering, med tillämpningar i Laos och Sverige

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.Godkänd; 2011; 20110517 (nilper); DISPUTATION Ämnesområde: Tillämpad geofysik/Applied Geophysics Opponent: Doctor Jean Roy, IGP Consulting, Outremont, Québec, Canada Ordförande: Professor Sten-Åke Elming, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet, Luleå Tid: Måndag den 3 oktober 2011, kl 10.00 Plats: F341, Luleå tekniska universitet</p

Assessment of hydrogeological and water quality parameters, using MRS and VES in the Vientiane Basin, Laos

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 supplies, 40% of the industrial water and 20% of the irrigated agriculture. 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, salinity in deep groundwater is common in coastal areas and in areas where rock salt is occurring naturally in the soil and bedrock. Drilling wells is expensive and unprofitable if it is made without knowing the groundwater potential and the location of salt affected groundwater. Vertical Electrical Soundings (VES) has been widely used to characterize aquifers and to identify salt affected water since there may be a relation between the conductivity of the aquifer and the salinity of the water. However, it is not possible to distinguish high conductive groundwater from e.g. increased clay content. Magnetic Resonance Sounding (MRS) gives a direct image of the water content of the ground and hence the vertical distribution of an aquifer. By combining MRS with VES, salt affected groundwater could be distinguished from high conductive sediments and freshwater aquifers. The aim of this study is to define and characterize water bearing geological formation and to test the possibility of using geophysical techniques to determine hydrogeological parameters and water quality parameters relating to salinity in the Vientiane basin, Laos. The investigation area is part of the Khorat Plateau where halite is naturally occurring as shallow as 50 m in depth in the Thangon Formation. In total, 32 sites and 28 sampled wells, located in three different areas were investigated. MRS and VES recognized the stratigraphic unit N2Q1-3, consisting of alluvial unconsolidated sediments, as the main water bearing unit. The aquifer thickness varies between 10 to 40 m and the depth to the main aquifer range from 5 to 15 m. The water content is here relatively high, up to 16 % and decay times varying between 100 and 400 ms, suggest a mean pore size equivalent to medium sand to gravel. The resistivity is highly variable, but usually around 10-100 ohm-m, which suggests fresh water. Hydraulic and storage related parameters such as transmissivity, hydraulic column, specific yield and specific storage have been estimated from MRS and transverse resistance has been estimated from VES. MRS together with VES has also shown to be a useful and important tool in identifying the salt related clay layer of the Thangon Formation. This layer is characterized by low water content and a resistivity lower than 5 ohm-m. This formation can be found in all 3 areas on depth of 20 to 50 m. Several approaches have been used to look for a relation between the water conductivity collected from wells and the aquifer conductivity determined from VES, including Archie's law. The best correlation is achieved using a linear fit between VES conductivity and water conductivity for deep wells (EC[μS/cm ]= 0.3821VES[μS/cm], R2=0.81). The conductivity of water from shallow wells does not show any relation to VES conductivity (R2=0.09). This is probably because deep wells usually have higher conductivity than shallow wells and hence contributing more to the aquifer conductivity. Interpolated maps of TDS in area 3 from deep wells together with maps of the conductivity of the bottom layer and aquifer layer, MRS transmissivity and hydrostatic column indicates that the salt originate from the underlying layer situated beneath the main aquifer. The most important water quality parameters influencing the water conductivity are TDS, hardness and chloride, which all have a high correlation to the EC of water and thus VES conductivity. This makes MRS and VES a very promising tool for guidance of future drillings and water quality estimation.Godkänd; 2008; 20081021 (ysko

Tools for groundwater prospecting and geophysical prospecting for water in Ocotal, Nicaragua : a minor field study

Detta examensarbete har utförts i staden Ocotal och dess närliggande omgivning, i nordvästra Nicaragua. Projektet utfördes som ett Minor Field Study (MFS) finansierat av Styrelsen för Internationellt Utvecklingssamarbete (SIDA), genom det Internationella Programkontoret. I norra Nicaragua har vattenförsörjning blivit ett påtagligt problem. Detta p.g.a. bristen på grundvatten och det ofta förorenade ytvattnet. Därför finns det ett stort intresse att kartlägga den geohydrologiska situationen samt lokalisera grundvatten i området. Detta projekt är en fortsättning på Sequeira och Mendozas (2003) arbete, där den västra delen av staden undersöktes m h a resistivitetsmetoder. Mätningarna har intensifierats i denna undersökning för att täcka det 7,5 km2 området bättre. I denna geofysiska undersökning uppmättes 14 pseudosektioner, även här med resistivitetsmetoder, alla mellan 400 och 2200 meter långa. Tolkningen av våra resistivitetsmodeller skapade från inversion av mätdata har tillsammans med redan existerande geologiska och geohydrologiska data resulterat i följande slutsatser. Den mest gynnsamma platsen för att borra efter en akvifer, där grundvattnet ligger nära markytan, är lokaliserad i mätområdets nord-östra del. Minst gynnsamma platsen ligger i mätområdets nord-västra del. Mittregionen på området anses vara måttligt gynnsam. Data från lokala brunnar i området, där djupet ner till grundvattenytan är mätt, stöder denna tolkning. Även vertikala strukturer var av intresse då dessa kan vara tänkbara förkastningar och/eller krosszoner och således möjliga vattenförande formationer. I syd-västra delen av mätområdet återfinns tecken på vertikala strukturer i flera av profilerna. Även i andra profiler påträffades vertikala strukturer som kan vara av värde i framtida undersökningar.Validerat; 20101217 (root

Geophysical investigations to identify groundwater pathways at a small open-pit copper mine reclaimed by backfilling and sealing

Godkänd; 2011; 20120926 (nilgra

Geophysical investigations to identify groundwater pathways at a small open-pit copper mine reclaimed by backfilling and sealing

Godkänd; 2011; 20120926 (nilgra)</p