Lineament analysis for characterizing regional fracture system – A case study from the Oslo region, Norway

Lineament analysis from topographic maps is a well-known method to identify regional fracture systems and the potential for weakness zones, but the significance of lineaments can easily be misinterpreted. A topographic lineament (topolineament) is just an elongated depression, which may or may not contain important structural information. In which case lineaments represent faults or fractures, and do all brittle structures appear as a lineaments? This is the focus of our project.

In this study, we investigate the applicability of lineament analysis to characterize the fracture system in the Oslo region, Norway. Topolineaments, derived by automatic detection using an in house-developed algorithm (OttoDetect) on a 10x10m digital elevation model, are combined with field structural observations and measurements. Special emphasis was placed on comparing the orientation of brittle structures from field data with that of the detected topolineaments.

The fieldwork was carried out by measuring fractures at selected locations both along and away from larger topolineaments. In some areas, data collection also included measuring scanlines parallel and orthogonal to selected topolineaments. Structural measurements were split into different geographical areas of Oslo and plotted on stereonets. The topolineaments were analyzed and classified by parameters such as orientation, length, density (number of lineaments over a given distance) and width-depth ratio (within the lineament), etc. Rose diagrams of all lineaments within a given radius from a geographical center point were used to show the dominant lineament orientations in the studied area. We pay especially close attention to lineaments in regions dominated by bedrock, in order to represent only bedrock-incised topolineaments, so that the rose diagrams could be used to compare with fracture orientations.

Preliminary results show that both field data and topolineaments are dominated by two orthogonal sets: E-W and N-S. However, the relative dominance of either set in the two datasets seems less correlated. Further analysis is ongoing to also constrain the relationship between fracture and lineament densities.