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Ground follow up of a radio-magnetic anomaly
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deduced from AEM and IP data show good agreement in terms of resolution and sensitivity. Problems originating from the gradient array in IP measuremets are also discussed.
GROUND FOLLOW UP OF A RADIO-MAGNETIC ANOMALY LARS GRANAR SGU, Box 670, 751 28 Uppsala (Sweden)
In 1977 the map area Hofors NV in Central Sweden was surveyed by airborne measurements of the geomagnetic field, the natural gamma radiation and the radio-magnetic field from the VLF station GBR (Great Britain Rugby) transmitting at 16 kHz. A prominent 4 km long anomaly of radio-magnetic field was detected in the hilly region 5 km SE of the village Sv&lsjB. Normally, this type of anomaly indicates a water-bearing fault zone often also indicated by a negative anomaly in the total intensity of the geomagnetic field. However, since the anomaly is located in a geological formation which bears the famous orebody at Falun, 25 km to the west, one could not exclude a horizon of sulphide mineralization as the cause of the anomaly. Detailed geophysical investigations were made over the southwestern part of the anomaly located in a valley north of the hill Sparvguldsberget. Electrical, electromagnetical and seismic measurements showed a zone of about 200 m width having reduced resistivity and seismic velocity. The moraine cover was about 20 m thick. No evidence of compact sulphide ore near the surface was obtained. Finally, a drill-hole was made in order to check the central part of the zone at a depth of 100 m. It penetrated fine-grained, layered rock of vulcanic origin (leptite formation) with faint traces of sulphide mineralization. Several sections contained brecciated and crushed rock. At the end of the hole the sum of lost core amounted to 8 m of soft mylonite or open cracks. Result from the geophysical investigations will be presented and discussed. What can be learn from this “negative” case history? CALCULATION OF RESISTIVITY ANOMALIES FOR MODELS WITH SURFACE INDUCED POLARIZATION LAURI ESKOLA’,
ESKO ELORANTA% and HEIKKI SOININEN’
‘Geological Survey of Finland, Geophysics Department, Espoo 15 (Finland)
Kivimiehentie
1, SF-021 50
a Helsinki University of Technology, Department of Mining and Metallurgy, Laboratory of Applied Geophysics, SF-021 50 Espoo 15 (Finland)
A numerical method is given for calculating the apparent resistivity anomalies produced by a surface polarization model. Surface polarization is gener-
deduced from AEM and IP data show good agreement in terms of resolution and sensitivity. Problems originating from the gradient array in IP measuremets are also discussed.
GROUND FOLLOW UP OF A RADIO-MAGNETIC ANOMALY LARS GRANAR SGU, Box 670, 751 28 Uppsala (Sweden)
In 1977 the map area Hofors NV in Central Sweden was surveyed by airborne measurements of the geomagnetic field, the natural gamma radiation and the radio-magnetic field from the VLF station GBR (Great Britain Rugby) transmitting at 16 kHz. A prominent 4 km long anomaly of radio-magnetic field was detected in the hilly region 5 km SE of the village Sv&lsjB. Normally, this type of anomaly indicates a water-bearing fault zone often also indicated by a negative anomaly in the total intensity of the geomagnetic field. However, since the anomaly is located in a geological formation which bears the famous orebody at Falun, 25 km to the west, one could not exclude a horizon of sulphide mineralization as the cause of the anomaly. Detailed geophysical investigations were made over the southwestern part of the anomaly located in a valley north of the hill Sparvguldsberget. Electrical, electromagnetical and seismic measurements showed a zone of about 200 m width having reduced resistivity and seismic velocity. The moraine cover was about 20 m thick. No evidence of compact sulphide ore near the surface was obtained. Finally, a drill-hole was made in order to check the central part of the zone at a depth of 100 m. It penetrated fine-grained, layered rock of vulcanic origin (leptite formation) with faint traces of sulphide mineralization. Several sections contained brecciated and crushed rock. At the end of the hole the sum of lost core amounted to 8 m of soft mylonite or open cracks. Result from the geophysical investigations will be presented and discussed. What can be learn from this “negative” case history? CALCULATION OF RESISTIVITY ANOMALIES FOR MODELS WITH SURFACE INDUCED POLARIZATION LAURI ESKOLA’,
ESKO ELORANTA% and HEIKKI SOININEN’
‘Geological Survey of Finland, Geophysics Department, Espoo 15 (Finland)
Kivimiehentie
1, SF-021 50
a Helsinki University of Technology, Department of Mining and Metallurgy, Laboratory of Applied Geophysics, SF-021 50 Espoo 15 (Finland)
A numerical method is given for calculating the apparent resistivity anomalies produced by a surface polarization model. Surface polarization is gener-