The 50 Hz EM method at the Pyhäsalmi massive sulphide deposit

ABSTRACT

The main goal of this research was to develop a simple method to observe the effects of electromagnetic induction in conductors proximal to drillholes at the Pyhäsalmi volcanogenic massive sulphide (VMS) copper − zinc deposit located in central Finland, using electromagnetic fields generated by electrical power lines (50 Hz). The idea is that the mine geologist could interpret the 50 Hz measurements from several holes and infer the boundaries between rock types based on rock samples and the 50 Hz measurements. Three-component dB/dt coils were used to measure the time derivatives of the total electromagnetic field in deep drillholes. Thus, the primary field was present while taking measurements, or the induced field was not separated from the inducing field. Time-domain data were transformed to the frequency domain and three components (x, y, z) of the total 50 Hz magnetic field were extracted. The electromagnetic field down drillholes indicates anomalies, although the difference between the primary and secondary fields is difficult to distinguish. We compared the results of the power line method with those measured using the time- and frequency-domain electromagnetic methods. According to spatial variations in the total field profiles, a strong correlation with electromagnetic data from controlled source methods was observed. The results demonstrate that the electromagnetic field generated by the primary field of power lines in an active mining area can be used to identify conductive targets in the area.

KEYWORDS:

• Electromagnetic methods
• borehole geophysics
• low frequency
• mineral
• exploration

Acknowledgements

The authors express their appreciation to geologist Timo Mäki (First Quantum Minerals) for permission to present the measured drillhole and other appropriate geophysical data. I would especially like to thank geophysicists Antti Kivinen (GRM-Services Oy) and Jarkko Jokinen (Loop and Line Oy) for insightful comments and advice on the DigiAtlantis and Boliden’s BHEM-03 measurements.

Disclosure statement

No potential conflict of interest was reported by the authors.