The physical background for magnetic and VLF measurements and their response to fracture zones is shortly reviewed. The ability for the identification of fracture zones using a combined interpretation of magnetic and VLF measurements is then demonstrated with a number of cases and examples where both airborne and ground measurements are used. Attention is drawn to the necesstity of combining various regional geophysical methods in order to achieve a more complete coverage of information on fracture zones. It is concluded that the demonstrated methods give a powerful tool to localize and characterize fracture zones.
A programme of systematic low altitude airborne measurements including magnetic total field and VLF measurements is run by the Geological Survey of Sweden. The programme is mainly financed by the ore prospecting and mapping budgets. The measurements are made at an altitude of 30 m above ground and with a line spacing of 200 m. Measurements are made at every 40 m along the flight lines. Figure 1 shows an index map on airborne measurements in Sweden. The possibilities to prognostic mapping of bedrock qualities (regarding fracturing) using a combination of the magnetic and electrical measurements, both in a regional and a more detailed scale, have been strongly increased in the light of the results from recent methodological studies. Part of this work has been carried out in connection with the Swedish radwaste geophysical programme. The experience from a large number of bedrock investigations for tunne1projecting, powerstation- and reactorsites, waterprospecting etc, has shown that valuable information is gained from different kinds of electrical methods and from magnetics. (Eriksson, 1974; Muellern and Eriksson. 1979).
Airborne electromagnetic measurements with ground follow up in the frequency range 10 - 30 kHz have proved to be very sensitive to water bearing zones, èven when these have a comparatively low resistivity contrast to surrounding rocks. The electrical measurements could be made in the near- or induction field, i.e. very near a fixed or moving transmitter (slingram, turam, etc). An other possibility are measurements in the radiation field far away from VLF radio transmitters generally used for submarine navigation. As water bearing fracture zones act as very weak electrical conductors, the indications caused by the near or induction field could be looked upon as mainly inductive phenomena. However indications caused by the radiation field from very distant VLF transmitters could be seen as mainly conductive phenomena. Thus the extending radio wave field causes currents within the bedrock which tend to concentrate in the weak conductors thereby reducing the current strength in the surrounding areas. Figure 2 shows typical VLF total field anomalies. Depending on measuring technique, measuring configuration or frequency, we will get different sensitivity for zones with different geometrical positions (strike and dip), thickness, length and depth. Different methods also have a varying sensitivity to disturbancies from conductive overburden (such as salty clay or salt water sediments) which sometimes mask the indications from the bedrock. Powerlines. te1ephonelines etc could also disturb the measurements. The results from VLF interpretation are collected on interpretation maps which show the position and character of various conducting zones.