Abstract:

Throughout the Sudbury basin, physical property measurements have been taken in boreholes in order to extract information about the geology close to the borehole. Due to a large number of geophysical instruments currently being deployed for the purpose of downhole logging, it is difficult to determine the strengths and weaknesses of each probe. Since each system uses different physical principles, measurement frequencies and calibration procedures, results from different instruments can vary greatly. By qualitatively comparing newly acquired data to information previously collected, strengths and weaknesses of each system can be identified. The instruments compared in this study were the Geonics EM39, the IFG BIC-01 and the IFG BMP-06. Conductivity was also collected during surveys completed by DGI Geosciences. The DGI surveys collected conductivity measurements using both a galvanic and an inductive technique. The downhole measurements were also compared to conductivity values collected on diamond drill core hand samples. The surveys of the hand samples were not expected to produce the same results as the measurements collected downhole because the rock being measured in the hand samples is under much less pressure, is dry and is a smaller volume of rock measured than in the downhole measurements. The inductive methods provided results that showed similar trends to one another but dramatically different values. This was due to the sensors being spaced much further apart in the EM39 than the other instruments. This created an averaging effect which caused zones of patchy mineralization to appear as one large range of conductive material. The IFG inductive tools were able to detect the changes from patchy mineralization to resistive host rock.<br/>Since the EM39 was designed for groundwater measurements, the instrument appeared to have an upper limit of 3 S/m which was often reached in zones of mineralization. Above this threshold, no accurate results could be measured. The DGI galvanic survey provided values that were closest to what would be expected in resistive regions, however, the galvanic instrument was less successful when observing regions of mineralization. The DGI inductive survey was completed on one hole but appeared to be influenced by a magnetic susceptibility survey that was acquired simultaneously. Of the conductivity measurements, the instruments that provided the most similar results were the two handheld instruments, the Terraplus KT-10 and the GDD MPP-EM2S+, however, even with these instruments measuring very similar areas and operating on nearly identical principles, the correlation was still not exact and the conductivity values delivered by the GDD were approximately 2.5 times those collected by the KT-10.<br/>The IFG BMP-06 and the EM39 also provided measurements of the amount of naturally occurring radioactive isotopes (natural gamma). While little was known about the specific measurement crystals within each unit, the results obtained provided a correlation with a trend and the magnitude of collected values that were very similar between the two instruments.<br/>This study showed that these instruments are not capable of producing the same results, even when collecting measurements on the same hole. There is a definite need to properly calibrate these instruments to a set of known conductivity values in order to ensure the results can be used as factual information, rather than a guiding tool to confirm mineralization.