Airborne Geophysics Helps Determine Route Of Proposed Railroad Tunnel

An airborne geophysical survey of about 830 line-kilometers was flown over an area of 180 sq<br>km in southern Norway to aid in determining which route a proposed railroad tunnel should<br>follow. The average flying height was 80 meters with a nominal line spacing of 200 meters.<br>Groundwater flow is fracture controlled in much of Norway, and failure to locate water-bearing<br>fractures can cause expensive delays in tunnel construction. A primary goal of the helicopter<br>survey was to locate any fractures or other structures using a combination of airborne<br>magnetometry, frequency-domain and very low frequency (VLF) electromagnetics, and<br>radiometrics, and to attempt to estimate via electromagnetics which fractures might be waterbearing.<br>Magnetic measurements proved to be the most useful geophysical tool for locating<br>faults, fractures, and dikes in the area. Several known faults appeared clearly in the data,<br>particularly on the western side of the survey area. Other lineaments also are presumed to be<br>fractures, faults, or dikes. Although magnetometry provided more detail than any other<br>individual data set, all the data sets proved valuable by adding new information and by<br>corroborating the findings from other data sets. From the geophysical data, it appears that the<br>western half of the survey area is more intensely fractured than the eastern half, although a few<br>large lineations, presumably fractures or faults, appear in both areas.