Bedrock mapping

Practically all of the metals we use today are extracted from a number of minerals that different rocks are made up of. A lot of other minerals create the basis for production of important materials and products, such as calcite for cement, quartz for glass, feldspar for porcelain and apatite for fertilizers. White minerals are crushed and used as filler material in plastics and paper products. Many important raw materials for energy extractions, such as coal, oil and uranium, can also be found in the bedrock. Stone is used as a construction material, for example as paving stones, countertops and gravestones.

Information about the properties in the bedrock

Underground construction requires the bedrock to have solid values in properties like crack occurrences and strength of material. When constructing roads and railroads, as well as in cement and concrete production, a large volume of filler materials is used. This filler material, or ballast, is often composed of crushed rock. The preferred properties of the ballast often vary, leading to many surveys over bedrock quality.

Occurrences of cracks in areas with sedimentary bedrock have a large importance for groundwater. These are used for both drinking water and energy extraction (geothermal). There may even be possibilities for storing hydrocarbons in sedimentary rocks, if the properties are right.

Moraine, which is the dominate soil in Sweden, is for the most part composed of mechanically eroded material from the bedrock below it. With knowledge about the bedrock we are able to extract more information about the soil and properties like nutritional values and groundwater quality. Soils that lay on top of calcareous rocks can withstand acidification better, and soils above mafic rocks usually have more nutrients.

How is the mapping performed?

A mapping survey covers the rock type propagation, their relationships, mineralogical composition, genesis and age (both relative and absolute) as well as structural data.

Some projects take into account technical properties, for example strength of material for construction use. These surveys are composed in a “rock quality map” that is used as a support for evaluating the best use of different rocks.

Surveying the bedrock is done in four steps: preparations, field work, processing/compilation and reporting results. A geophysical survey is done parallel to the mapping.

• Preparations consist of putting together existing geological information about the areas that are being mapped. Both digital and analogue maps and databases are procured. Information from SGUs soil mapping projects, or other sources, about rock faces are transferred to field maps.
• Rock faces are examined in the field to establish rock type and specific minerals. Structures of any kind, such as primary layering, tectonic foliation, folds, faults, cracks and the rock’s magnetic susceptibility is recorded. Samples are taken for representative, as well as hard-to-determine, rocks.
• The samples are prepared for thin section and chemical analysis in the processing- and compilation-phase. Sometimes the samples are candidates for radiometric dating. The chemical results and evaluated while the thin sections provide data over mineral composition and content. Field observations are compiled to a map covering the surface propagation of the rocks, even areas where soil is present. Geophysical data taken from airplane surveys are used as support. The complied data is uploaded to SGUs digital databases.
• Reports of the preliminary results from the field work are presented every year. A geological map of the bedrock is often printed when a mapping project is completed. A short text about the bedrock in the area is presented on the map. A longer, more comprehensive, description is provided for some maps. 

Read more about geophysical surveys