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

Pilser, Hannes LU (2020) In Dissertations in Geology at Lund University GEOR02 20192
Department of Geology
Abstract
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... (More)
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. (Less)
Abstract (Swedish)
Chocaya Basin ligger i den nordvästra delen av dalen Cochabamba i Bolivia, där ett halvtorrt klimat råder. Den ökande befolkningen är beroende av lokala akviferer för dricksvatten och jordbruk, där brunnar används för att utvinna vattnet. Låg nederbörd, främst under torrperioden, och uppförandet av nya hus eller fabriksbyggnader, påverkar infiltrationen av vattnet i jorden och därmed akviferernas påfyllning. Detta har resulterat i djupare brunnar, lägre grundvattennivåer och utökat risken för att tömma akvifererna. Detta arbete undersöker den nuvarande situationen av hydrologin och hydrogeologin i dalen, särskilt Chocaya-bassängen, där en potentiell akvifer förekommer. Området som utforskats ligger främst på jordbruksmark, men inkluderar... (More)
Chocaya Basin ligger i den nordvästra delen av dalen Cochabamba i Bolivia, där ett halvtorrt klimat råder. Den ökande befolkningen är beroende av lokala akviferer för dricksvatten och jordbruk, där brunnar används för att utvinna vattnet. Låg nederbörd, främst under torrperioden, och uppförandet av nya hus eller fabriksbyggnader, påverkar infiltrationen av vattnet i jorden och därmed akviferernas påfyllning. Detta har resulterat i djupare brunnar, lägre grundvattennivåer och utökat risken för att tömma akvifererna. Detta arbete undersöker den nuvarande situationen av hydrologin och hydrogeologin i dalen, särskilt Chocaya-bassängen, där en potentiell akvifer förekommer. Området som utforskats ligger främst på jordbruksmark, men inkluderar även delar av Chocaya-floden. Målet är att kartlägga geometrin för denna potentiella akvifer. Två geofysiska metoder har använts: ERT (Electrical Resistivity Tomography) och TEM (Transient Electromagnetic Method). ERT-metoden bygger på att leda en ström genom jorden vilket möjliggör mätning av jordens resistivitet, medan TEM använder transienta vågor som skickas genom i jordlager. Geologiska material/sediment kan fungera som en ledare och kan inducera en elektrisk ström till omgivande material. Detta genererar i sin tur en sekundär våg. Utrustningen vid ytan registrerar dessa sekundära vågor och beräknar därmed materialets resistivitet, det vill säga markens förmåga att motstå en elektrisk ström. Olika geologiska material ger upphov till olika resistiviteter. Med hjälp av de uppmätta resistiviteterna är det möjligt att fastställa litologin och därmed kartlägga dimensionen av den potentiella akviferen. De båda metoderna användes som komplement med syfte att uppnå detaljerade och mer exakta resultat. Dessutom användes två litologirapporter från närliggande brunnar för att identifiera hur litologin varierar i de översta 100 metrarna. Datahanteringen visade att TEM-resultaten inom detta område var mer tillförlitliga. Sålunda har främst TEM-data använts gällande tolkningar och slutsatser. Resultaten visar att höga resistiviteter (≈1000– 2000 Ωm) dominerar i de översta 50–100 metrarna av marken i studieområdet. Därefter sjunker resistiviteten till cirka 100 Ωm. Höga resistiviteter är kopplade till grova sediment, såsom grus och sand. Enligt litologirapporterna består sedimenten huvudsakligen av grova sediment ner till 75 m djup. Andelen finare sediment blir därefter högre. Resultaten av mätningarna stämmer väl överens med dalens geologiska historia: alluvialkoner deponerade över lakustrina sediment. Sammanfattningsvis kan sägas att geometrin hos den potentiella akviferen kopplas till de höga resistiviteterna och därmed de översta 50–100 metrarna i marken. I den delen av marken kan vatten infiltrera och lagras i den potentiella akviferen. En öppen yta behövs för en god infiltration av vatten genom det lokala geologiska materialet. Befolkningsökningen genererar dock stor efterfrågan på bygglov, vilket skulle resultera i en minskad infiltration av vatten i området och påverka nettouppladdningen av den potentiella akviferen. Kompletterande TEM- och ERT-mätningar genomfördes utanför studieområdet. Dessa bidrar till att klarlägga berggrundens resistivitet och finna en förklaring till förekomsten av ett termiskt källa (Liriuni) beläget i närheten. (Less)
Please use this url to cite or link to this publication:
author
Pilser, Hannes LU
supervisor
organization
course
GEOR02 20192
year
type
H2 - Master's Degree (Two Years)
subject
keywords
ERT, TEM, geophysics, Cochabamba, aquifer
publication/series
Dissertations in Geology at Lund University
report number
581
language
English
id
9007203
date added to LUP
2020-03-27 11:13:30
date last changed
2022-03-27 03:42:52
@misc{9007203,
  abstract     = {{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.}},
  author       = {{Pilser, Hannes}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Dissertations in Geology at Lund University}},
  title        = {{A geophysical survey in the Chocaya Basin in the central Valley of Cochabamba, Bolivia, using ERT and TEM}},
  year         = {{2020}},
}