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Undersökning av geofysiska metoders användbarhet vid kontroll av den omättade zonen i en infiltrationsdamm vid Vombverket

Löfgren, Anneli (2011) In Examensarbeten i geologi vid Lunds universitet
Department of Geology
Abstract
Drinking-water for municipalities in south-western Sweden is produced at the Vomb water treatment plant east of Lund by first letting water from Lake Vombsjön pass through strainers and then spread out over recharge basins built on the Vomb Plain. The water infiltrates and slowly percolates down through the glaciofluvial deposits to a natural groundwater reservoir, and is thereafter drawn from 120 water wells. It is important to maintain a sufficiently high oxygen level in the unsaturated zone for an effective decomposition of the organic material from the lake. For this reason it is important that the unsaturated zone is not too thin.

The goal of this study was to assess the usefulness of geophysical methods for estimation and... (More)
Drinking-water for municipalities in south-western Sweden is produced at the Vomb water treatment plant east of Lund by first letting water from Lake Vombsjön pass through strainers and then spread out over recharge basins built on the Vomb Plain. The water infiltrates and slowly percolates down through the glaciofluvial deposits to a natural groundwater reservoir, and is thereafter drawn from 120 water wells. It is important to maintain a sufficiently high oxygen level in the unsaturated zone for an effective decomposition of the organic material from the lake. For this reason it is important that the unsaturated zone is not too thin.

The goal of this study was to assess the usefulness of geophysical methods for estimation and monitoring of the unsaturated zone above groundwater reservoirs. The geophysical methods chosen for these investigations were broadband electromagnetic sensor and Continuous Vertical Electrical Sounding (CVES).

The electromagnetic sensor is an induction method which measures the average conductivity a few meters down in the ground by transmitting electromagnetic waves. The waves are influenced by electrical and magnetic differences in the ground. With the help of receiver equipment it is possible to record and measure how the wave is changing. CVES is a method to measure the resistivity in the ground. Resistivity is the ability of the ground to function as an insulator. The resistivity is measured by transmitting direct current with a known current into the ground via two electrodes, while measuring the potential difference between two other electrodes.

Both these methods can be used to measure the resistivity in the ground and they have different advantages and disadvantages. The electromagnetic sensor is a fast method that does not require direct contact with the ground, but it shows only average conductivity values down to the depth of investigation for each frequency. With CVES resistivity- or time-lapse models (showing relative differences between measurements) can be constructed through inversion of the data. The disadvantage is that the measurements take longer time and require more equipment. Together the two methods complement each other well.

The time-lapse models based on CVES showed a reduction in resistivity of 10-25 %, approximately from the water table and down to about 17 meters above sea level, at the base of the glaciofluvial deposits. The results of the study show that geophysical methods are useful for estimation and monitoring of the unsaturated zone above groundwater reservoirs. (Less)
Abstract (Swedish)
På Vombverket produceras dricksvatten genom att först sila vatten från Vombsjön och därefter sprida ut det över infiltrationsdammar på Vombfältet. Vattnet infiltreras sakta ner genom markens isälvsavlagringar till ett naturligt grundvattenmagasin, och pumpas sedan upp från 120 brunnar. För att det organiska materialet från sjön ska kunna brytas ner effektivt är det viktigt att syrehalten i den omättade zonen är tillräckligt hög, vilket betyder att den omättade zonen inte får vara för tunn.

Målet med undersökningen var att se hur användbara de geofysiska metoderna är vid kontroll av den omättade zonen i en infiltrationsdamm. De geofysiska metoder som valdes till dessa undersökningar var flerfrekvens-stångslingram och Continuous Vertical... (More)
På Vombverket produceras dricksvatten genom att först sila vatten från Vombsjön och därefter sprida ut det över infiltrationsdammar på Vombfältet. Vattnet infiltreras sakta ner genom markens isälvsavlagringar till ett naturligt grundvattenmagasin, och pumpas sedan upp från 120 brunnar. För att det organiska materialet från sjön ska kunna brytas ner effektivt är det viktigt att syrehalten i den omättade zonen är tillräckligt hög, vilket betyder att den omättade zonen inte får vara för tunn.

Målet med undersökningen var att se hur användbara de geofysiska metoderna är vid kontroll av den omättade zonen i en infiltrationsdamm. De geofysiska metoder som valdes till dessa undersökningar var flerfrekvens-stångslingram och Continuous Vertical Electrical Sounding (CVES).

Stångslingramen är en elektromagnetisk induktionsmetod som mäter den genomsnittliga elektriska konduktiviteten nere i marken genom att sända ut elektromagnetiska vågor. Med olika frekvenser når man till olika djup. Dessa vågor påverkas av elektriska och magnetiska skillnader i marken. Med hjälp av en mottagarutrustning kan man registrera och mäta hur vågen förändras. Med CVES mäter man resistiviteten i marken, vilket är markens förmåga att fungera som isolator. Man mäter resistiviteten genom att skicka ner likström med känd strömstyrka i marken via två elektroder, samtidigt som man mäter potentialskillnaden mellan två andra elektroder.

Time-lapse-modellerna, som jämför resultaten från två mättillfällen, visade en minskning i resistivitet på 10-25 procent inom damm området ungefär från grundvattenytan till cirka 17 m ö.h. där sanden övergår i lera. Resultaten från undersökningen visar att geofysiska metoder är användbara för mätning och övervakning av den omättade zonens i infiltrationsdammar. (Less)
Please use this url to cite or link to this publication:
author
Löfgren, Anneli
supervisor
organization
alternative title
Investigation of the usefulness of geophysical methods when supervising the unsaturated zone of a recharge basin at the Vomb water treatment plant
year
type
M2 - Bachelor Degree
subject
keywords
unsaturated zone, groundwater, conductivity, resistivity, recharge basin, Vomb water treatment plant, electromagnetic sensor, omättade zonen, grundvatten, resistivitet, konduktivitet, CVES, infiltrationsdamm, Gem 2, terrameter, Vombverket, stångslingram
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
284
language
Swedish
additional info
Hans Jeppsson, konsult WSP Sverige (Malmö).
id
2278068
date added to LUP
2012-01-09 12:28:54
date last changed
2012-01-09 12:28:54
@misc{2278068,
  abstract     = {Drinking-water for municipalities in south-western Sweden is produced at the Vomb water treatment plant east of Lund by first letting water from Lake Vombsjön pass through strainers and then spread out over recharge basins built on the Vomb Plain. The water infiltrates and slowly percolates down through the glaciofluvial deposits to a natural groundwater reservoir, and is thereafter drawn from 120 water wells. It is important to maintain a sufficiently high oxygen level in the unsaturated zone for an effective decomposition of the organic material from the lake. For this reason it is important that the unsaturated zone is not too thin. 

The goal of this study was to assess the usefulness of geophysical methods for estimation and monitoring of the unsaturated zone above groundwater reservoirs. The geophysical methods chosen for these investigations were broadband electromagnetic sensor and Continuous Vertical Electrical Sounding (CVES).

The electromagnetic sensor is an induction method which measures the average conductivity a few meters down in the ground by transmitting electromagnetic waves. The waves are influenced by electrical and magnetic differences in the ground. With the help of receiver equipment it is possible to record and measure how the wave is changing. CVES is a method to measure the resistivity in the ground. Resistivity is the ability of the ground to function as an insulator. The resistivity is measured by transmitting direct current with a known current into the ground via two electrodes, while measuring the potential difference between two other electrodes. 

Both these methods can be used to measure the resistivity in the ground and they have different advantages and disadvantages. The electromagnetic sensor is a fast method that does not require direct contact with the ground, but it shows only average conductivity values down to the depth of investigation for each frequency. With CVES resistivity- or time-lapse models (showing relative differences between measurements) can be constructed through inversion of the data. The disadvantage is that the measurements take longer time and require more equipment. Together the two methods complement each other well.

The time-lapse models based on CVES showed a reduction in resistivity of 10-25 %, approximately from the water table and down to about 17 meters above sea level, at the base of the glaciofluvial deposits. The results of the study show that geophysical methods are useful for estimation and monitoring of the unsaturated zone above groundwater reservoirs.},
  author       = {Löfgren, Anneli},
  keyword      = {unsaturated zone,groundwater,conductivity,resistivity,recharge basin,Vomb water treatment plant,electromagnetic sensor,omättade zonen,grundvatten,resistivitet,konduktivitet,CVES,infiltrationsdamm,Gem 2,terrameter,Vombverket,stångslingram},
  language     = {swe},
  note         = {Student Paper},
  series       = {Examensarbeten i geologi vid Lunds universitet},
  title        = {Undersökning av geofysiska metoders användbarhet vid kontroll av den omättade zonen i en infiltrationsdamm vid Vombverket},
  year         = {2011},
}