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Advances in time–domain induced polarisation tomography : Data acquisition, processing and modelling

Olsson, Per-Ivar LU orcid (2018)
Abstract
What would you find below your feet – how do you find out? Perhaps you could look at a geological map, drill, or dig? That could work, but sometimes the maps are not detailed enough, and digging everywhere to find out is impractical. Imagine if you could develop a method that allows you to see straight through the ground, as x–rays through the body! To some extent, such methods already exist; you can for example send electric current into the ground that tells you what lies beneath when it returns. The current does not see the subsurface as we do–no soil, boulders, water or bedrock – but it can tell us about its view of the underground. Its image is in terms of electrical resistivity and chargeability and can be difficult to understand if... (More)
What would you find below your feet – how do you find out? Perhaps you could look at a geological map, drill, or dig? That could work, but sometimes the maps are not detailed enough, and digging everywhere to find out is impractical. Imagine if you could develop a method that allows you to see straight through the ground, as x–rays through the body! To some extent, such methods already exist; you can for example send electric current into the ground that tells you what lies beneath when it returns. The current does not see the subsurface as we do–no soil, boulders, water or bedrock – but it can tell us about its view of the underground. Its image is in terms of electrical resistivity and chargeability and can be difficult to understand if it is not translated. The translation is accomplished by comparing the electric image with, for example, geological maps and information from boreholes. With this method, we obtain more reliable and more detailed models of the subsurface. Additionally, electrical surveys can help us to determine where we need more subsurface information and where it would be most interesting to drill or dig.

When we plan and build structures below or above ground, search for suitable places for wells, or remediate contaminated areas, we need good and reliable information about the ground below. Incorrect or incomplete information about the subsurface can lead to unexpected problems. These problems can in turn lead to delays and reduced sustainability in the implementations. One example of such a project is the train tunnel through Hallandsåsen in Southern Sweden, which suffered several delays and took 23 years to complete, and its final price tag was approximately ten times the initial estimate.

This thesis addresses how we can develop and improve the use of electrical current to investigate the subsurface. The method has been used and developed for more than one hundred years, but bottlenecks remain that limit its use. One example is in cities where electrical installations and a complex environment in the subsurface distort the current. We then need to filter the image to make use of the results from the measurements. The method may also be limited by lack of resources needed to conduct the investigations, or to make proper interpretations of its information. By refining and optimising the method, its usefulness can be increased. For example, by enabling its use in urban areas or for projects with limited resources.

This thesis describes how we can process signals from electrical surveys and handle interference from other electrical installations, similar to a pair of noise–cancelling headphones. The processing allows us to retrieve more information about the subsurface and increase the reliability of the results. Another improvement that is introduced is a change of the shape of the current that is sent into the ground. The change of waveform results in a reduction in the time required for a survey, while the magnitude of the signals is increased, similar to completing a podcast in half of the time with better audio quality.

Another way to improve the method is to increase our understanding of what types of responses we can expect from the measurements. This thesis describes how measurement results that were previously considered erroneous can be explained, and that these are actually physically possible. By not rejecting such results, we can obtain more information from the measurements, more reliable models of the subsurface, and post–processing of the measurements is simplified. In addition, it describes how we can compensate for the effects of varying duration of current transmissions. If the effects are not considered properly, different electrical images of the same subsurface are obtained depending on whether the current is sent just one second longer or shorter.

The optimisations of the thesis are exemplified with, among others, results from a major survey that mapped a geologic site in terms of resistance and chargeability down to 200 metres below ground. Such information is important and can help us to take better decisions, for example in connection with infrastructure projects for a more sustainable future. Hopefully, the work in this thesis can increase the use of electrical surveys, ensuring we can make more informed decisions in the future. (Less)
Abstract (Swedish)
Vad står vi på – hur tar man reda på det? Kanske man kan titta på en geologisk karta, borra ett hål eller gräva en grop? Det kan gå bra, men ibland finns det inte tillräckligt detaljerade kartor och att gräva upp allt man är intresserad av är ingen lysande idé. Tänk om man kunde forska fram en metod som gör att man kan se rakt igenom marken, som röntgenundersökningar på sjukhus! På sätt och vis kan man redan det, man kan skicka ner ström i marken som berättar om hur där ser ut när den kommer tillbaka. Ström ser inte marken som vi gör – ingen jord, sten, vatten eller berg men den kan berätta för oss om dess bild av marken. Bilden är i termer av motstånd och uppladdning och kan vara svår att förstå om den inte översätts. Översättningen görs... (More)
Vad står vi på – hur tar man reda på det? Kanske man kan titta på en geologisk karta, borra ett hål eller gräva en grop? Det kan gå bra, men ibland finns det inte tillräckligt detaljerade kartor och att gräva upp allt man är intresserad av är ingen lysande idé. Tänk om man kunde forska fram en metod som gör att man kan se rakt igenom marken, som röntgenundersökningar på sjukhus! På sätt och vis kan man redan det, man kan skicka ner ström i marken som berättar om hur där ser ut när den kommer tillbaka. Ström ser inte marken som vi gör – ingen jord, sten, vatten eller berg men den kan berätta för oss om dess bild av marken. Bilden är i termer av motstånd och uppladdning och kan vara svår att förstå om den inte översätts. Översättningen görs genom att jämföra den elektriska bilden med till exempel geologiska kartor och information från borrhål. Med denna metod får vi totalt sett en säkrare och mer detaljerad modell av marken. De elektriska undersökningarna kan också hjälpa oss att avgöra var vi behöver mer information och var det kan vara som mest intressant att borra eller gräva.

När vi ska planera och bygga konstruktioner ovan eller under mark, hitta lämpliga platser för brunnar eller sanera förorenade områden behöver vi ha bra och säker information om vad marken består av. Felaktig eller ofullständig information om marken kan skapa oväntade problem. Dessa kan i sin tur leda till förseningar och minskad hållbarhet i genomförandet. Ett exempel är tågtunneln genom Hallandsåsen i Skåne som tog 23 år att färdigställa och blev cirka tio gånger dyrare att bygga än vad som först beräknats med en slutnota på över tio miljarder kronor.

Denna avhandling handlar om hur vi kan utveckla och förbättra användningen av ström för att undersöka marken. Metoden har använts och utvecklats sedan förra sekelskiftet men det finns fortfarande flaskhalsar som begränsar dess användning. Ett exempel är i städer där elinstallationer och en rörig miljö i marken förvirrar strömmen. Vi behöver då filtrera dess bild av marken för att kunna utnyttja mätresultaten. Användningen av metoden kan också begränsas av brist på resurser för att utföra undersökningarna eller för göra ordentliga tolkningar av informationen. Genom att förfina och effektivisera metoden kan den alltså vara till större nytta på fler platser och användas för att få information om marken med mer begränsade resurser.

I avhandlingen beskrivs hur man kan filtrera signaler från elektriska markundersökningar och hantera störningar från andra elinstallationer, ungefär som att sätta på den ett par brusreducerande hörlurar. Filtreringen gör att vi kan få ut mer information om marken och ett mer pålitligt mätresultat. En annan förbättring som har utvecklats är att förändra formen på strömmen som skickas ner i marken. Formändringen gör att tiden som krävs för en undersökning halveras samtidigt som man får dubbelt så stark signal, som att lyssna igenom en podcast på halva tiden med bättre ljudkvalitet.

Ett annat sätt att förbättra undersökningarna är att öka vår förståelse för vad strömmen berättar om marken. Avhandlingen beskriver hur mätresultat som tidigare betraktats som felaktiga kan uppkomma och att dessa faktiskt är fysiskt möjliga. Genom att inte förkasta dessa resultat kan vi få ut mer information från mätningarna och säkrare modeller av marken samtidigt som översättningen av mätresultat kan bli enklare. Dessutom beskrivs hur vi kan kompensera för effekter av hur länge strömmen sänds ner i marken. Om effekterna inte hanteras får man olika elektriska bilder av samma mark beroende på om man skickar ström någon sekund längre eller kortare.

Avhandlingens förbättringar och effektiviseringar visas bland annat med resultat från en större undersökning som kartlägger geologi i termer av motstånd och uppladdning ned till 200 meter under markytan. Sådan information är viktig och kan underlätta att ta bättre beslut, till exempel i samband med infrastrukturprojekt, för en mer hållbar framtid. Förhoppningsvis kan avhandlingen öka användandet av elektriska markundersökningar så att vi kan ta mer informerade beslut i framtiden. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Schmutz, Myriam, Université Bordeaux, France
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Tomography, Signal–to–noise ratio, Waveform, Duty–Cycle, induced polarisation, Electrical properties
pages
167 pages
publisher
Department of Biomedical Engineering, Lund university
defense location
lecture hall B, building V, John Ericssons väg 1, Lund University, Faculty of Engineering LTH, Lund
defense date
2018-11-30 10:15:00
ISBN
978-91-7753-850-9
978-91-7753-851-6
project
Geoelectrical Imaging for Site Investigation for Urban Underground Infrastructure
Geophysical and geological survey of rock quality in Dalby quarry for detailed comparison in 3D
language
English
LU publication?
yes
id
387e23a8-f1f5-4f2f-8010-288f97c0e561
date added to LUP
2018-11-05 09:41:08
date last changed
2020-04-29 15:54:58
@phdthesis{387e23a8-f1f5-4f2f-8010-288f97c0e561,
  abstract     = {What would you find below your feet – how do you find out? Perhaps you could look at a geological map, drill, or dig? That could work, but sometimes the maps are not detailed enough, and digging everywhere to find out is impractical. Imagine if you could develop a method that allows you to see straight through the ground, as x–rays through the body! To some extent, such methods already exist; you can for example send electric current into the ground that tells you what lies beneath when it returns. The current does not see the subsurface as we do–no soil, boulders, water or bedrock – but it can tell us about its view of the underground. Its image is in terms of electrical resistivity and chargeability and can be difficult to understand if it is not translated. The translation is accomplished by comparing the electric image with, for example, geological maps and information from boreholes. With this method, we obtain more reliable and more detailed models of the subsurface. Additionally, electrical surveys can help us to determine where we need more subsurface information and where it would be most interesting to drill or dig.<br/><br/>When we plan and build structures below or above ground, search for suitable places for wells, or remediate contaminated areas, we need good and reliable information about the ground below. Incorrect or incomplete information about the subsurface can lead to unexpected problems. These problems can in turn lead to delays and reduced sustainability in the implementations. One example of such a project is the train tunnel through Hallandsåsen in Southern Sweden, which suffered several delays and took 23 years to complete, and its final price tag was approximately ten times the initial estimate.<br/><br/>This thesis addresses how we can develop and improve the use of electrical current to investigate the subsurface. The method has been used and developed for more than one hundred years, but bottlenecks remain that limit its use. One example is in cities where electrical installations and a complex environment in the subsurface distort the current. We then need to filter the image to make use of the results from the measurements. The method may also be limited by lack of resources needed to conduct the investigations, or to make proper interpretations of its information. By refining and optimising the method, its usefulness can be increased. For example, by enabling its use in urban areas or for projects with limited resources.<br/><br/>This thesis describes how we can process signals from electrical surveys and handle interference from other electrical installations, similar to a pair of noise–cancelling headphones. The processing allows us to retrieve more information about the subsurface and increase the reliability of the results. Another improvement that is introduced is a change of the shape of the current that is sent into the ground. The change of waveform results in a reduction in the time required for a survey, while the magnitude of the signals is increased, similar to completing a podcast in half of the time with better audio quality.<br/><br/>Another way to improve the method is to increase our understanding of what types of responses we can expect from the measurements. This thesis describes how measurement results that were previously considered erroneous can be explained, and that these are actually physically possible. By not rejecting such results, we can obtain more information from the measurements, more reliable models of the subsurface, and post–processing of the measurements is simplified. In addition, it describes how we can compensate for the effects of varying duration of current transmissions. If the effects are not considered properly, different electrical images of the same subsurface are obtained depending on whether the current is sent just one second longer or shorter.<br/><br/>The optimisations of the thesis are exemplified with, among others, results from a major survey that mapped a geologic site in terms of resistance and chargeability down to 200 metres below ground. Such information is important and can help us to take better decisions, for example in connection with infrastructure projects for a more sustainable future. Hopefully, the work in this thesis can increase the use of electrical surveys, ensuring we can make more informed decisions in the future.},
  author       = {Olsson, Per-Ivar},
  isbn         = {978-91-7753-850-9},
  language     = {eng},
  month        = {11},
  publisher    = {Department of Biomedical Engineering, Lund university},
  school       = {Lund University},
  title        = {Advances in time–domain induced polarisation tomography : Data acquisition, processing and modelling},
  url          = {https://lup.lub.lu.se/search/files/54336389/Olsson_P._I._Advances_in_time_domain_induced_polarisation_tomography_modified_for_erratum.pdf},
  year         = {2018},
}