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A geophysical survey (TEM; ERT) of the Punata alluvial fan, Bolivia

Mårdh, Joakim LU (2017) In Dissertations in Geology at Lund University GEOR02 20162
Department of Geology
Abstract
The Punata alluvial fan, situated in central Bolivia (Valle Alto) between the Altiplano and the lowlands, is an important aquifer for the local rural population. Due to rapid development and population growth in the area, the demand for fresh water has increased in recent year. In combination with decreasing annual rainfall, in the already semi-arid climate, the groundwater level is steadily sinking and thus depleting shallow wells. The current solution is to drill new, deeper, wells and continue to overexploit the reservoir in an unsustainable manner. In order to map the aquifer geometry this paper presents a TEM (Transient Electromagnetic Method) survey with the aim to find the sediment – bedrock boundary, which is thought be at >300 m... (More)
The Punata alluvial fan, situated in central Bolivia (Valle Alto) between the Altiplano and the lowlands, is an important aquifer for the local rural population. Due to rapid development and population growth in the area, the demand for fresh water has increased in recent year. In combination with decreasing annual rainfall, in the already semi-arid climate, the groundwater level is steadily sinking and thus depleting shallow wells. The current solution is to drill new, deeper, wells and continue to overexploit the reservoir in an unsustainable manner. In order to map the aquifer geometry this paper presents a TEM (Transient Electromagnetic Method) survey with the aim to find the sediment – bedrock boundary, which is thought be at >300 m depth. The survey acts as a complement to the previously conducted ERT (Electrical Resistivity Tomography) surveys in the area.
Valle Alto is a tectonic basin, in the department of Cochabamba, with predominantly Palaeozoic sedimentary bedrock (Ordovician and Silurian) and minor Mesozoic formations from the late Cretaceous. The lithology varies between shales, siltstones and sandstones, deposited in a marine environment during the Palaeozoic and in a continental rift basin during the Mesozoic. Valle Alto is the result of tectonic activity during the Pliocene, which yielded an enclosed lake in the area. An unquantified amount of lacustrine clay was deposited in the basin before the lake was drained due to renewed tectonic activity. On top of the clay there is interfingering alluvial fans and colluvial deposits of different generations, with the Punata alluvial fan being one of them.
In an attempt to achieve adequate depth of penetration, TEM was used in this survey. The method utilizes the fact that an electrical field always yields a proportional magnetic field and vice versa. In short, an electrical pulse is sent through transmitter loop and then abruptly turned off. This induces an electromotive force which propagate into the ground and in turn induces currents proportional to the resistivity. The currents yield a secondary magnetic field which can be measured by receiver coils, and then used to create resistivity models of the subsurface.
Unfortunately, the lacustrine clay restricted the depth penetration to 90 – 200 m and the models did not reach the bedrock boundary. The survey did however present a few other interesting features such as a distinct thin layer with very low resistivity, interpreted to be brine, on top of the alluvium – clay boundary. There might also be a fault line beneath the fan, but the results of the survey are inconclusive and further studies of the tectonic regime necessary in order to verify or disregard the hypothesis. (Less)
Abstract (Swedish)
Punata alluvialkon ligger i centrala Bolivia (Valle Alto) i ett område mellan Altiplanot och lågländerna. Det är en viktig grundvattenreservoar för den lokala landsortsbefolkningen och pga. utveckling samt befolkningsökning i området har efterfrågan på färskvatten ökat de senaste åren. Tillsammans med minskande årlig nederbörd, i ett redan halvtorrt (semi-arid) klimat, sjunker grundvattennivån stadigt och torrlägger grunda brunnar. Den nuvarande lösningen är att borra nya, djupare brunnar och sedan fortsätta att överexploatera grundvattentillgången på ett ohållbart vis. För att få en bild av akvifärens geometri presenterar denna uppsats en TEM-undersökning (Transient Elektromagnetisk Metod), med målet att kartlägga gränsen mellan lösa... (More)
Punata alluvialkon ligger i centrala Bolivia (Valle Alto) i ett område mellan Altiplanot och lågländerna. Det är en viktig grundvattenreservoar för den lokala landsortsbefolkningen och pga. utveckling samt befolkningsökning i området har efterfrågan på färskvatten ökat de senaste åren. Tillsammans med minskande årlig nederbörd, i ett redan halvtorrt (semi-arid) klimat, sjunker grundvattennivån stadigt och torrlägger grunda brunnar. Den nuvarande lösningen är att borra nya, djupare brunnar och sedan fortsätta att överexploatera grundvattentillgången på ett ohållbart vis. För att få en bild av akvifärens geometri presenterar denna uppsats en TEM-undersökning (Transient Elektromagnetisk Metod), med målet att kartlägga gränsen mellan lösa sediment och berggrund som tros ligga på >300 m djup. Undersökningen är ämnad att komplettera tidigare ERT-undersökningar (Elektrisk Resistivitetstomografi) i området.
Valle Alto är en tektonisk bassäng i ”department of Cochabamba”, med främst paleozoisk sedimentär berggrund (Ordovicium and Silur) och sporadiska mesozoiska formationer från sen Krita. Litologin varierar mellan lerskiffer, siltsten och sandsten, som är avsatta i en marin miljö under Paleozoikum och i en kontinental riftbassäng under Mesozoikum. Valle Alto uppkom under Pliocen pga. tektonisk aktivitet under Pliocen och en sluten sjö bildades i bassängen. En okänd mängd lakustrin lera avsattes i området innan sjön tappades som resultat av ny tektonisk aktivitet. Ovanpå leran ligger olika generationer alluvialkoner och kolluviala sediment om vartannat, där Punata alluvialkon är en av dem.
I ett försök att uppnå tillräckligt penetrationsdjup har TEM används i denna undersökning. Metoden använder sig av det faktum att elektriska fält alltid förekommer tillsammans med ett proportionellt magnetiskt fält och vice versa. Metoden bygger i korthet på att en kort elektrisk puls skickas i en sändarslinga och sedan abrupts stängs av. Det resulterar i en elektromotorisk kraft som sprider sig i marken och inducerar en ström som är proportionell mot resistiviteten. Strömmen ger i sin tur ett sekundärt magnetfält som kan mätas med hjälp av mottagarslingor, och sedan användas för att skapa resistivitetsmodeller av marken.
Den lakustrina leran begränsade tyvärr djupnedträngningen till 90 – 200 m och modellerna nådde inte berggrundsytan. Undersökningen påvisade däremot ett par andra intressanta saker, såsom ett tydligt tunt lager med väldigt låg resistivitet, vilket är tolkat som saltlake ovanpå alluvium – lergränsen. Där finns även indikationer på en förkastning under konen, men resultaten från undersökningen är inte tillräckliga för en säker slutsats och fortsatta studier av den tektoniska historien i området är nödvändig för att verifiera eller förkasta hypotesen. (Less)
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author
Mårdh, Joakim LU
supervisor
organization
course
GEOR02 20162
year
type
H2 - Master's Degree (Two Years)
subject
keywords
electromagnetic, geophysics, TEM, Punata, alluvial fan
publication/series
Dissertations in Geology at Lund University
report number
496
funder
SIDA, Minor Field Study programme (MFS)
language
English
id
8901723
date added to LUP
2017-01-31 10:05:18
date last changed
2017-08-01 04:10:04
@misc{8901723,
  abstract     = {The Punata alluvial fan, situated in central Bolivia (Valle Alto) between the Altiplano and the lowlands, is an important aquifer for the local rural population. Due to rapid development and population growth in the area, the demand for fresh water has increased in recent year. In combination with decreasing annual rainfall, in the already semi-arid climate, the groundwater level is steadily sinking and thus depleting shallow wells. The current solution is to drill new, deeper, wells and continue to overexploit the reservoir in an unsustainable manner. In order to map the aquifer geometry this paper presents a TEM (Transient Electromagnetic Method) survey with the aim to find the sediment – bedrock boundary, which is thought be at >300 m depth. The survey acts as a complement to the previously conducted ERT (Electrical Resistivity Tomography) surveys in the area.
Valle Alto is a tectonic basin, in the department of Cochabamba, with predominantly Palaeozoic sedimentary bedrock (Ordovician and Silurian) and minor Mesozoic formations from the late Cretaceous. The lithology varies between shales, siltstones and sandstones, deposited in a marine environment during the Palaeozoic and in a continental rift basin during the Mesozoic. Valle Alto is the result of tectonic activity during the Pliocene, which yielded an enclosed lake in the area. An unquantified amount of lacustrine clay was deposited in the basin before the lake was drained due to renewed tectonic activity. On top of the clay there is interfingering alluvial fans and colluvial deposits of different generations, with the Punata alluvial fan being one of them.
In an attempt to achieve adequate depth of penetration, TEM was used in this survey. The method utilizes the fact that an electrical field always yields a proportional magnetic field and vice versa. In short, an electrical pulse is sent through transmitter loop and then abruptly turned off. This induces an electromotive force which propagate into the ground and in turn induces currents proportional to the resistivity. The currents yield a secondary magnetic field which can be measured by receiver coils, and then used to create resistivity models of the subsurface.
Unfortunately, the lacustrine clay restricted the depth penetration to 90 – 200 m and the models did not reach the bedrock boundary. The survey did however present a few other interesting features such as a distinct thin layer with very low resistivity, interpreted to be brine, on top of the alluvium – clay boundary. There might also be a fault line beneath the fan, but the results of the survey are inconclusive and further studies of the tectonic regime necessary in order to verify or disregard the hypothesis.},
  author       = {Mårdh, Joakim},
  keyword      = {electromagnetic,geophysics,TEM,Punata,alluvial fan},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {A geophysical survey (TEM; ERT) of the Punata alluvial fan, Bolivia},
  year         = {2017},
}