Summary
In June, 1997 and July, 2000, two separate helicopter geophysical surveys were carried out - one over an area in the vicinity of Gran, and the second over an area immediately west of Hurdalsjøen. The purpose of the surveys was to provide geophysical information for improved geological mapping. These data have been used to help evaluate geological conditions related to propose tunnel construction near Langvatnet, a lake that lies on the eastern edge of the Gran survey area and the western edge of the Hurdal survey area. Faults and fracture zones may appear as linear or curvilinear anomalies in any of the four data types collected. i.e. magnetic, radiometric, electromagnetic, and VLF data. Linear and curvilinear anomalies in the vicinity of Langvatnet were identified in the different data sets, and when plottet together and show three anomaly clusters. One appears on the west side of Langvatnet. A likely source for some of these anomalies in the contact between the metasediments that outcrop 200- 300m west of the lake and the syenitic intrusive of the Oslo igneous province. However, fracture zones mapped in a borehole also occur on this side of the lake and may be a source of some of the anomalies. Two other anomaly custers near the east end of the proposed tunnel, one cluster about 800 m from the end, the second about 200 m from the end. All three clusters show anomalies in each of the four data types. Most anomalies cut across the proposed tunnel route at a steep angle. A single exception is a magnetic anomaly that approaches the east end of the proposed tunnel at a very shallow angle. Ground geological or geophysical follow up can help clarify the nature of these anomaly clusters.
