COMPARISON OF INDUCED POLARIZATION AND CONTROLLED-SOURCE AUDIO-MAGNETOTELLURICS METHODS FOR MASSIVE CHALCOPYRITE EXPLORATION IN A VOLCANIC AREA

In this paper, we compare and contrast the results of field experiment s with the dipole-dipole-induced polarization (IP) and controlled-sour ce audio-magnetotellurics (CSAMT) methods, along the same survey profi les, at a test area that was subject to extensive drilling and detaile d geological investigation. The ore bodies are interbedded between two series of dacitic tuff. The depth and thickness of the massive chalco pyrite-pyrite-sphalerite body vary between 25 and 100 m and 0.5 and 16 m, respectively. Resistivity and IP phase measurements on the core sa mples collected from the test area provide some idea of the relative d ifferences between the background rock units and the target. The resis tivity of the chalcopyrite samples varies between 0.6 to 2 ohm-m and p rovides sufficient resistivity contrast with surrounding volcanic rock units for target detection. The results of dipole-dipole IP profiling with a 50-m dipole length conducted along two profiles are presented in the form of apparent resistivity and phase pseudosections. CSAMT me asurements were made at 13 frequencies from 2 Hz to 8192 Hz along thre e profiles. The receiver dipole length was 25 m. The CSAMT data are pr esented in the form of pseudosections using conventional and new defin itions of apparent resistivity. The elliptical contours of low apparen t resistivity generated by the transition-field notch can be misleadin g with respect to the real anomaly of the ore body. These artificial a nomaly patterns are suppressed by making use of an alternative apparen t resistivity definition derived from the frequency-normalized impedan ce. The qualitative interpretation based on the IP and CSAMT pseudosec tions shows that the location and the extension of the ore body are in dicated better in the CSAMT apparent resistivity data computed from th e alternative definition. The qualitative interpretation of the IP dat a is difficult because of the 3-D effect arising from the neighboring thicker parts of the ore body and pyrite particles within the basement . At the final stage, the far-field range of the CSAMT apparent resist ivity and phase data is identified by the visual inspection of the ind ividual sounding diagrams. The models derived from the 2-D Occam inver sion carried out on the far-field data are compared with the drill-hol e information and are found to describe the actual geological situatio n.