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Results
Summary The combination of high fidelity and late time channels make SQUID magnetometers an attractive sensor for deep and highly conducting targets. Here we focus upon a data set acquired over the Lalor Lake VMS deposit. The deposit consists of a zinc zone between 700m and 1000m depth and a deeper gold/copper zone. The SQUID data set had previously been interpreted using the plate modeling to yield two conductors respectively corresponding to the zinc zone and the gold-copper zone. In this paper we invert these data with our 3D TEM inversion algorithm to produce a 3D voxel conductivity volume. Our 3D model recovers a shallow dipping conductor, which coincides well with the plate model of the zinc zone, as well as a large conductor below 1000m depth. Our inversion is done without incorporating prior knowledge and there is always the potential that large features at depth can be an artifact of the inversion algorithm. To investigate this, and to have confidence, or not, in the existence of the deep body, we carry out a hypothesis analysis where we attempt to find a model that does not have a high conductivity at depth. Forward modeling and subsequent inversion confirms that there must be another conductor below the zinc zone. Much of the concentration of conductivity lies near the region indicated by the initial blind inversion, but the amplitudes and distribution are different. Nevertheless, despite lack of knowledge about geometric details, there is some highly conductive material at depth and it would warrant a drill hole. In a final analysis we look at the relative merits of using B or dB/dt for the particular geometry of this survey. We generate synthetic B and dB/dt data based on our inversion model of Lalor Lake deposit. While the B-field data inversion recovers the correct locations and geometries of the two compact conductors, the dB/dt inversion shows the shallow conductor in a distorted geometry and the deep conductor as a blurred conductive region with conductivity much smaller than the true model. This demonstrates that B-field data can be are superior to dB/dt data for this survey in which we have surface transmitters and receivers.
SUMMARY We propose a new methodology for processing frequency domain EM data to identify the presence of IP effects in observations of the magnetic fields arising from an inductive source. The method makes used of the asymptotic behaviour of the imaginary part of secondary magnetic fields at low frequency. A new quantity, referred to as the ISIP datum, is defined so that it equals zero at low frequencies for any frequency-independent (non-chargeable) conductivity distribution. Thus any non-zero response in the ISIP data indicates the presence of chargeable material. Once the data are defined, the change in the real component of the resistivity, which is indicative of chargeability, is obtained by solving a linear inverse problem.
This ambiguity has motivated a number of modeling studies which seek to determine features of TEM data that can be used We develop a new technique for forward modeling the threedimensional to identify polarization effects. The majority of these compute electromagnetic (EM) response of chargeable material the EM response in the frequency domain and then transform in the time domain. The method can be applied to any those results back to the time domain (Flis et al. (1989), Beran type of transmitter (loop or grounded electrodes) or receiver and Oldenburg (2008)). In order to achieve an accurate solution (H or E field measurements) and operates directly in the time in the time domain over large time intervals, these techniques domain, requiring no transformations. A synthetic example of must solve the frequency domain forward problem at a an airborne time domain survey passing above a vertical, polarizable large number of frequencies, spanning many decades. This is dike is presented.
SUMMARY Electromagnetic (EM) and electrical geophysical datasets were acquired over the Antonio high sulfidation epithermal gold deposit in Northern Peru, and each dataset produced a three-dimensional resistivity inversion model which helped map geologic alteration. Time-domain airborne-EM and pole-dipole Induced-Polarization (IP) data were independently inverted, and compared to a Controlled-Source Audio- Frequency Magnetotellurics (CSAMT) 3D inversion. Absolute resistivity values were consistent across all three surveys and collectively they effectively outlined silicic alteration mapped from previous geologic work and drilling. For airborne-EM modeling, all 270 transmitter sources over the deposit region were included in the final inversion. This ability to model airborne-EM data in 3D with multiple transmitter sources demonstrates that large advancements have been made in recent years with inversion codes, data acquisition and processing. Using multiple datasets to accurately delineate silicic alteration is valuable for exploration purposes, as this highly resistive zone hosts much of the gold mineralization in the area.