Using helicopter-borne frequency domain electromagnetic (HEM) data to map magnetite has advantage... more Using helicopter-borne frequency domain electromagnetic (HEM) data to map magnetite has advantages over the magnetic method. It is independent of the earth's locally asymmetric inducing magnetic field, remanent magnetism, and magnetic anisotropy. It is also restricted to the notional "depth of exploration" of the HEM system, limiting detection to that which may be economically mined. A significant complication arises when interference between the magnetite polarisation and conductivity responses reduces the apparent magnetic content. This interference is significant over the conductive, magnetically polarisable ground common to the Hamersley Basin. The current procedures for magnetite mapping with HEM are reviewed. Both problems and successes of the method are highlighted using a case study as an example. Replacing the data transformation by an inversion increases the sensitivity of the method, but at the cost of enhancing noise.
The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Fo... more The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Formed by a cyclic weathering and eroding sequence of nominally barren banded iron formation, they may form rich proximal accumulations of iron cemented gravels distal from any obvious hard-rock iron mineralisation. While generally small, they nonetheless represent excellent economic targets as they are near surface and easy to mine. The relatively high density of iron detritals, when compared with their sedimentary hosts, makes the gravity method the primary method of exploring for these blind deposits. Identification of possible trap sites is used as a precursor to gravity surveying, with magnetics used as an ancillary method prior to drilling. Application of the gravity method in this terrain is not straightforward. Detrital trap sites are best developed next to the spectacular cliffs that form the range fronts to the Hamersley Ranges. Precise terrain corrections and use of first vertic...
Magnetic inversion programs such as mag3d from the University of British Columbia's Geophysic... more Magnetic inversion programs such as mag3d from the University of British Columbia's Geophysical Inversion Facility (UBC-GIF) have proven to be very useful for generating realistic 3D susceptibility models from surface Total Magnetic Intensity (TMI) data. These programs do not perform well when the observed data includes the response of bodies which are strongly remanently magnetised. This failure occurs because the forward model algorithm used in the inversion only generates the induced response, so the remanent component in the TMI has to be modeled using the induced response for an unrealistic distribution of susceptibility. In this paper we introduce two transforms: the analytic signal of the vertical integral (ASVI) and the vertical integral of the analytic signal (VIAS). When applied to TMI data, these transforms produce data which is qualitatively similar to a purely induced TMI response for a vertical magnetic field. We investigate the effectiveness of using mag3d to inve...
Using helicopter-borne frequency domain electromagnetic (HEM) data to map magnetite has advantage... more Using helicopter-borne frequency domain electromagnetic (HEM) data to map magnetite has advantages over the magnetic method. It is independent of the earth's locally asymmetric inducing magnetic field, remanent magnetism, and magnetic anisotropy. It is also restricted to the notional "depth of exploration" of the HEM system, limiting detection to that which may be economically mined. A significant complication arises when interference between the magnetite polarisation and conductivity responses reduces the apparent magnetic content. This interference is significant over the conductive, magnetically polarisable ground common to the Hamersley Basin. The current procedures for magnetite mapping with HEM are reviewed. Both problems and successes of the method are highlighted using a case study as an example. Replacing the data transformation by an inversion increases the sensitivity of the method, but at the cost of enhancing noise.
The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Fo... more The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Formed by a cyclic weathering and eroding sequence of nominally barren banded iron formation, they may form rich proximal accumulations of iron cemented gravels distal from any obvious hard-rock iron mineralisation. While generally small, they nonetheless represent excellent economic targets as they are near surface and easy to mine. The relatively high density of iron detritals, when compared with their sedimentary hosts, makes the gravity method the primary method of exploring for these blind deposits. Identification of possible trap sites is used as a precursor to gravity surveying, with magnetics used as an ancillary method prior to drilling. Application of the gravity method in this terrain is not straightforward. Detrital trap sites are best developed next to the spectacular cliffs that form the range fronts to the Hamersley Ranges. Precise terrain corrections and use of first vertic...
Magnetic inversion programs such as mag3d from the University of British Columbia's Geophysic... more Magnetic inversion programs such as mag3d from the University of British Columbia's Geophysical Inversion Facility (UBC-GIF) have proven to be very useful for generating realistic 3D susceptibility models from surface Total Magnetic Intensity (TMI) data. These programs do not perform well when the observed data includes the response of bodies which are strongly remanently magnetised. This failure occurs because the forward model algorithm used in the inversion only generates the induced response, so the remanent component in the TMI has to be modeled using the induced response for an unrealistic distribution of susceptibility. In this paper we introduce two transforms: the analytic signal of the vertical integral (ASVI) and the vertical integral of the analytic signal (VIAS). When applied to TMI data, these transforms produce data which is qualitatively similar to a purely induced TMI response for a vertical magnetic field. We investigate the effectiveness of using mag3d to inve...
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