Measuring techniques of Induced Polarization regarding data quality with an application on a test-site in Aarhus, Denmark and the tunnel construction at the Hallandsås Horst, Sweden

University essay from Lunds universitet/Geologiska institutionen

Abstract: It has been shown that surveys of Induced polarization (IP) often contain data of low quality, because it is highly sensitive against different noise sources. When measuring induced polarization, where the method uses multicore cables and continuous electrode sounding (CVES, Continuous Vertical Electrical Sounding), noise phenomena such as capacitive coupling effects have to be considered. It is possible to decrease the capacitive coupling effect by using separated cables. This noise occur between transmitting current cable and potential reading cable. Also, other effects such as electromagnetic coupling effects will generate bad quality of data. This appears between the current electrodes and the electrical properties of the subsurface. It is strongly dependent on the length of the array. However, to obtain data of good quality, the electrode contact has to be taken into account as well. The purpose with this thesis is to examine the quality of data when measuring IP, with different techniques, and then apply the obtained experience in two different geological environments, Aarhus, Denmark and the tunnel construction at the Hallandsås Horst. The geophysical methods used were CVES 2D sounding of IP and resistivity. For a better interpretation and analysis, the results of the resistivity measurement are used as a complement to the results of IP. To achieve the purpose, measurements with both normal layouts, i.e. multicore cable, and separated cables has been made. Also different types of electrode configurations have been examined. Analysis of negative data and distribution of chargeabilities gives a good picture of the data quality. Once, electrode contact resistance was measured; this was done by measuring the resistance of each electrode. The survey was performed on a lawn at the Institute of Engineering, Lunds University, Lund. From earlier measurements it has been indicated that the area of investigation in Aarhus contains something that gives IP effect. This was examined with experience from the measurements in Lund. The survey area at the Hallandsås Horst has never earlier been examined with the method that is used in this work. Therefore, this site was chosen to complement the last part of the tunnel area and also use the knowledge from the measurements in Lund. This thesis shows that the capacitive coupling is manifested for example by negative data, which affect the measured IP effects negatively. Bad electrode contact is probably a strong candidate to the low quality of data, as well as the inductive coupling effects. The technique of using separated cables contributes to data of good quality IP data, but are logistically complicated and time consuming. It was shown, both in Aarhus and at the Hallandsås Horst, that if the geology in an area contributes to a good electrode contact, the results are satisfying when using normal cable layout. The survey in Aarhus gave a strong IP-effect that is interpreted as a pipeline. This is situated about 5 to 10 m under the ground surface. The results from the measurement at the Hallandsås Horst have given three major structures: One dolerite dike that crosses the tunnel, a probable fracture zone that may be partly clay weathered and partly water bearing. Finally, one more clay weathered fracture zone was interpreted which might contain amphibolite. The upper boundary is in level with the tunnel.

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