The 50 Hz EM method at the Pyhäsalmi massive sulphide deposit

Abstract

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\u2009−\u2009zinc deposit located in central Finland, using electromagnetic fields generated by electrical power lines (50\u2009Hz). The idea is that the mine geologist could interpret the 50\u2009Hz measurements from several holes and infer the boundaries between rock types based on rock samples and the 50\u2009Hz 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\u2009Hz 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.