Li Zhen Cheng

Using reversed polarity airborne transient electromagnetic data to map tailings around mine sites

The response measured by transient electromagnetic (TEM) systems is normally of a predictable sign. This is particularly true for coincident-loop ground TEM systems, where the response should in theory always be positive when the ground conductivity is a constant function of frequency and the quasi-static assumption is satisfied. In most cases, these are good assumptions, but, occasionally, coincident-loop negative transients are ob-served. These negatives can be explained if material in the ground has a conductivity that changes with frequency, or, equivalently, has nonzero polarizability.

Rapid detection and classification of airborne time-domain electromagnetic anomalies using weighted multi-linear regression

Abstract We propose a rapid and efficient methodology for the detection and interpretation of airborne time-domain electromagnetic anomalies generated by thin sheet-like volcanogenic massive sulphides (VMS) deposits in a resistive environment, which are representative of VMS deposits in the Canadian Shield. In the first step of the approach, we use high-order statistics for the detection and the recognition of a MEGATEM anomaly as indicating a thin sheet-like VMS deposit with respect to three criteria of detection: the minimum level of detection, the length of detection, and the coherence of detection over time. We adapt these criteria in order to optimise the detection of thin sheet-like VMS deposits against geological noise models. Once the anomaly is detected and recognised as the response to a thin sheet conductor, we interpret the model geometry and physical property using attributes calculated from the MEGATEM anomaly. We develop a system of weighted multi-linear regression to find the most significant attributes to estimate the dip, depth, conductance, and dimensions of a thin sheet-like VMS deposit. Stepwise regression suggests that shape attributes are most significant to estimate dip while depth is most strongly estimated by size attributes. The most significant attribute to estimate the conductance is the time constant. The size is best estimated by attributes related to the size of the anomaly. We test the regression system on thin sheet models with excellent performance. Most of the parameters of the thin sheet models were estimated within an interval of confidence about the initial property. We further test the system by estimating properties of three VMS deposits in the Abitibi Greenstone Belt, Québec, Canada, for which the geometries and geological properties are known. Most parameters are estimated within the interval of confidence for ISO, a thin sheet body, while the estimates for New-Insco and Gallen show more variability caused by departure from the reference thin sheet model.

Mapping tailings around mine sites with reverse polarity airborne transient EM data

Reversed polarity (negative) transients have been observed in airborne EM data over tailings at the Fox mine (in Manitoba, Canada) and the Aldermac mine (in Quebec, Canada). In the latter case, the negatives were confirmed by PROTEM readings taken on the ground. The airborne and ground EM measurements have been modeled with a thin conductive layer at surface that is chargeable. Two resistivity/IP traverses did not show coincident conductive and chargeable material.

An Analysis of Geophysical And Geological Data From the Gallen Test Site, Quebec, Canada

The MEGATEM system was flown over a test site containing the Gallen deposit. The measured response is consistent with the known geology and shows a rapid decay indicative of a poor conductor. Comparisons with older GEOTEM and INPUT data are problematic, as each dataset was acquired at different stages in the mining of the deposit. Although the GEOTEM data has the best signal to noise, the MEGATEM data were acquired after a significant portion of the deposit was removed.

An Analysis of Geophysical And Geological Data From the Iso/New Insco Test Site, Quebec, Canada

The MEGATEM system was flown over a site containing the Iso and New Insco deposits. The existing geological information for these sites was compiled and the MEGATEM response was found to be consistent with the geological information and previous geophysical information. The MEGATEM system was found to provide better information than the historical INPUT data. Height attenuation tests infer the MEGATEM system could see the Iso deposit if it were buried 230 m deep, whereas the INPUT could only see it if it were buried less than 60 m deep. A comparison of 90 and 30 Hz data shows that 30 Hz is better for discriminating the better conductors (e.g. New Insco), while 90 Hz sees the poorer conductors more clearly.