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Lund University (2019)

A geophysical survey in the Chocaya Basin in the central Valley of Cochabamba, Bolivia, using ERT and TEM

Pilser, Hannes

Titre : A geophysical survey in the Chocaya Basin in the central Valley of Cochabamba, Bolivia, using ERT and TEM

Auteur : Pilser, Hannes

Université de soutenance : Lund University

Grade : H2 - Master’s Degree (Two Years) 2020

Résumé
The Chocaya Basin is situated in the north-western part of the Valley of Cochabamba, Bolivia, where a semi-arid climate prevails. The increasing population is dependent on local aquifers for drinking water and irrigation. Wells are used to extract water. Scarce amount of rainfall, mainly during the dry season, and constructions of buildings, are reducing the infiltration of water in the soil and consequently the net recharge of the aquifers. This has resulted in deeper wells, lower groundwater levels and the risk of depleting the aquifers. This thesis investigates the current situation of the hydrology and hydrogeology of the valley, in particular the Chocaya Basin, where a potential aquifer is located. The study area is mainly on farmland and at parts of the Chocaya River. The aim is to map the geometry of this potential aquifer. Two geophysical methods were applied, ERT (Electrical Resistivity Tomography) and TEM (Transient Electromagnetic Method). The ERT method is based on passing a current through the subsurface and measuring the resistivity of the ground, while the TEM uses transient waves that are sent through the ground. The ground acts as a conductor that induces electrical current into the surrounding material, which generates a secondary wave. Equipment at the surface registers these secondary waves and makes it possible to calculate the resistivity of the subsurface, i.e. the ability of materials to resist the flow of electrical current. Different geological materials have different resistivities, making it possible to determine the lithology and thereby the dimension of the potential aquifer. The use of both TEM and ERT helps to obtain more accurate results. In addition, lithology reports from nearby wells were used to characterise the top 100 m of the subsurface. The data processing showed that the TEM results in this area were more reliable. For this reason, data from the TEM were mainly used for interpretations and conclusions. The results show that the top 50–100 m part of the subsurface in the study area has a high resistivity (≈1000–2000 Ωm). High resistivities are linked to coarse sediments such as gravel and sand. Further down, the resistivity drops to approximately 100 Ωm. The lithology report indicates that the sediment consists of mainly gravel and sand down to 75 m and below finer sediments become more abundant. These results of the measurements are in line with the geological history of the valley : alluvial fan deposits on top lacustrine and fluviolacustrine sediments. Overall, the dimensions of the potential aquifer can be linked to the high resistivities, i.e the top 50–100 m of the ground. There, water can infiltrate and be stored in the potential aquifer. An open surface is needed for a good infiltration of water through the local geological material. However, there is a high demand of construction buildings due to the increase in the population, which would reduce the infiltration of water in this area for recharging the potential aquifer. Some additional TEM and ERT measurements were performed outside of the study area. They should contribute to assess the resistivity of the bedrock and to find an explanation for the existence of a thermal spring (Liriuni) located near the study area

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Page publiée le 3 juin 2020