A comparison of 2D and 3D resistivity surveys for 3D inversion imaging

Sammanfattning: Following study focuses on electrical resistivity surveying and how it can be used as an exploration method to increase knowledge of subsurface structures. Future scenarios with increasing population and rapid urbani-zation, put high demands on reliable infrastructure; safe foundations, un-derground structures, environmental safety and access to resources. By creating a transparent underground, subsurface structures and founda-tions can be built safer, faster and cheaper, and from an enrviornmental point of view it will be possible to survey environmental factors such as contamination flows and landfills. Subsurface investigations are today largely based on drilling techniques, where probing and soil samples give detailed information at discrete points in the survey area. An alternative is to use a geophysical technique as a preliminary study method. Geophysical exploration is a discipline that uses physical field variations, and through non-intrusive measuring and data interpretation gaining a continuous image of the subsurface. Electrical re-sistivity tomography is a technique based on the soil elements varying ca-pability to conduct electricity. At first the studies were conducted in one-dimension but was soon developed to a two-dimensional technique. Three-dimensional surveys are a highly researched subject today. It is be-ing used in smaller scale, but not nearly to the same extent as a regular two-dimensional survey. The focus in this thesis is to compare three-di-mensional and two-dimensional studies. The comparison is done by con-ducting a two-dimensional and a three-dimensional field study as well as doing a synthetic study comparing different survey techniques. The field study as well as the inversion process clearly shows the difficul-ties with three-dimensional studies. They are much more time consuming, there is a need for more equipment and it also demands more user-knowledge for planning and processing. Another issue is the larger elec-tronmic spread, increasing the risk for noise-pollution. The result from both the fireld study as well as the synthetic study, shows that a three-dimensional study is far superior in detecting subsurface struc-tures. However, using the synthetic models, this study showed that a smaller three-dimensional study, i.e. arrays using 45 degrees but not a full degree cover, has almost as clear subsurface images as a full three-dimen-sional array.