Kombinerad förundersökning för tunnlar med resistivitet och refraktionsseismik i urban undervattensmiljö

Abstract

Due to expanding cities and increasing population infrastructure needs to expand in a safe and sustainable way with little effect on the environment and economy. Years of urbanization doesn’t make it easy, no free spaces are available and culture marked buildings are protected. The only alternative left is to construct in layers. Roads and other kinds of infrastructure beneath ground level is therefore attractive and promotes sustainable development.
The master thesis is a cooperation with TRUST s´ research and development project in sustainable development of urban underground infrastructure. The project is wide but this thesis will focus on pre-investigation with geophysical mapping and integration between these and geotechnical data, like probes and boreholes.
Stockholm is a good example of an expanding urban area with surroundings of water. The risk that new infrastructure will have to cross water passages is high. With the use of two different geophysical methods, resistivity (ERT) and refraction seismic, four different underwater areas has been investigated focusing on locating the hard rock surface and weakness zones. Knowing their locations are most important when constructing a tunnel. The areas of investigation are located both in fresh and saline water in Mälaren and Saltsjön. Investigations with combined resistivity and refraction seismic have been done several times before, on land. Underwater surveys, on the other hand, are not as common and has only been done a few times in Sweden.
The results from the four investigation lines show both similarities and differences when comparing potential weakness zones and the hard rock surface between the two methods. This was expected since resistivity and refraction seismic investigates different properties of the ground and therefore can observe different structures. Both methods show a good estimation of the hard rock surface. The seismic method generally show a deeper level due to the weathered condition of the hard rock and the presence of till. Noise from the urban area was a problem that easily could be reduced by measuring during night time. Some difficulties was found in surveying resistivity in saline water, since its high conductivity properties, and the presence of gas in the sediments for the seismic which complicated the analyzing process. The authors recommends using both methods when doing complicated surveys like those done in this thesis, both for verifying and supplementing each other’s result.
The fieldwork was set in September/October 2015 in Stockholm.

Popular Abstract

Geophysics can provide important information of underground structures which is beneficial in tunnel construction, but will they work where we need them the most? In the heart of our cities and under water.
Purpose
Due to expanding cities and increasing population infrastructure needs to expand in a safe and sustainable way with little effect on the environment and economy. Years of urbanization doesn’t make it easy though, no free spaces are available and culture marked buildings are everywhere. The only alternative left is to construct in layers. Roads and other kinds of infrastructure beneath ground level are therefore attractive and promotes sustainable development. Stockholm is just one of many expanding cities with surroundings of water and there is a high probability that new infrastructure will have to cross straits. With the help of geophysics, underground structures can be mapped, but will they work in these noisy and challenging, urban underwater conditions?
Method
With the use of two different geophysical methods, resistivity (ERT) and refraction seismic, four different underwater areas have been investigated focusing on locating the hard rock surface and weakness zones. The aim of the project was to find differences and similarities between the methods and determine if the they are suitable for urban underwater surveying. The areas of investigation are located both in fresh in Mälaren and saline water in Saltsjön.
The two methods differ from each other by measuring different physical properties of the ground. The resistivity method uses electricity to determine the composition of the ground resistivity while refraction seismics uses mechanical waves to determine the velocity composition.
Result and conclusion
The results from the four investigation lines show both similarities and differences of the methods when comparing potential weakness zones and the hard rock surface. This was expected since resistivity and refraction seismic investigates different properties of the ground and therefore can observe different structures. Both methods show a good estimation of the hard rock surface when compared to probes in the area.

Figure 1 - Hard rock surface and weakness zones interpretations of resistivity, seismic and probes.
The resistivity method generally shows a more elevated hard rock surface level than the seismics due to the different interpretation of geological structures, such as till or weathered hard rock.

Figure 2 - Deviation between resistivity and seismic interpretations of hard rock surface level. The seismic interpretation is represented by the value of '0' on the y-axis. A positive bar means a more shallow interpretation of the resistivity.
Noise from the urban area was a problem that easily could be reduced by measuring during nighttime. Movements creating vibrations in the ground effects the seismic data while the subway probably is the No.1 source of noise in the resistivity data, this since it uses a strong direct current to power its motors.

Figure 3 - Comparison of variance caused by noise from the area between two resistivity measurements, one during nighttime and one during the morning rush. The size of the data points represent the distance between the current electrodes. The larger the point, the greater the distance.
Some difficulties were found in surveying resistivity in saline water since it´s high conductivity properties reduced the depth of investigation. Another complication was the presence of gas in the sediments that affected the seismic data by damping the signal and complicating the analysis. Since the two methods are based on different theories and therefore show different strengths and weaknesses the authors recommend using both methods when doing complicated surveys like those done in this thesis, both for verifying and supplementing each other’s result.

• author: Lindvall, Elisabeth and Warberg Larsson, Erik
• supervisor: Roger Wisén; Torleif Dahlin
• organization: Engineering Geology; Civil Engineering (M.Sc.Eng.)
• alternative title: Combined tunneling site investigations with resistivity and refraction seismic in urban underwater environments
• course: VTG820 20151
• year: 2016
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: Geophysics, underground, structures, tunneling, construction, urban, TRUST, seismic, resistivity, underwater, survey, geofysik, resistivitet, seismik, undervatten, undersökning
• report number: ISRN LUTVDG / (TVTG-5145) / 1-85 /(2016)
• other publication id: ISRN LUTVDG / (TVTG-5145) / 1-85 /(2016)
• language: Swedish
• additional info: Examiner: Peter Ulriksen
• id: 8727718
• date added to LUP: 2016-03-07 11:36:51
• date last changed: 2020-09-10 09:41:43