Tomographic reconstructions of borehole sections using the radio imaging method at Pyhäsalmi massive sulfide deposit in Finland

Abstract

We have used the radio imaging method (RIM) to delineate attenuating zones in two borehole sections in the area of the Pyhäsalmi volcanogenic massive sulfide (VMS) copper-zinc deposit located in central Finland. The frequency band (312.5–2500 kHz) is higher and thus provides better resolution and sensitivity to conductive targets than traditional ground-level and borehole electromagnetic (EM) methods. When EM waves are assumed to be propagated along straight rays, the simultaneous iterative reconstruction technique can be used and the decayed amplitudes of the electric field are converted to the attenuation coefficient in dB/m. The straight-ray assumption was, however, not met in this study. The reconstruction results of two borehole sections were compared with time-domain EM (TEM) data and electric logging data. Electric logging reveals the nearby conducive mineralizations, and when compared with RIM data, the continuation of attenuating formations can be better predicted. The intersections interpreted from the TEM data were consistent with the RIM data. However, continuation of the attenuating domains could only be established from RIM data. Low ray densities at the upper and lower edges, violation of the straight-ray assumption, and out-of-plane targets may generate artifacts. In addition, the constructions suffer from smearing in the direction of the raypath. According to the results, we can recover the shape and orientation of attenuating targets in the borehole sections, but the physical properties are underestimated due to the straight-ray assumption. The comparison studies confirmed that RIM is well-suited to estimating subsurface conductivity properties and to predicting the continuation of attenuating domains between the boreholes at the Pyhäsalmi VMS deposit.