Quantitative Depth to Bedrock Extraction from AEM Data

A new road segment is being planned northeast of Norway’s capital city, Oslo. In this context, knowledge of sediment thickness is vital, as is information about occurrence and extent of highly sensitive marine clay (so-called quick clay).

Airborne EM measurements were conducted to provide information of depth to bedrock/sediment thickness between drilling sites and guide the further drilling program. AEM data indicate a variable bedrock depth with a general trend towards shallower bedrock in the northeastern part of the investigation area. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings.

Various methods for estimating depth to bedrock were compared: (1) Using a constant resistivity threshold either survey-wide or for each profile line; or (2) by using an appropriate resistivity value as a function of position. A contouring algorithm was developed to for the second method to incorporate both borehole and AEM data and hence account for apparent variations in resistivity at the sediment-bedrock boundary. Though both exhibit the same general trends, predictions for some locations differed significantly. Based on the AEM results recommendations for further drillings were given, thus reducing the overall costs of the project