b'AEGC 2023Short abstractsgrids there are signatures of near-surface and deepermore on appropriate anomaly separation from other fields than magnetisations. The field variations of shallow and deeperon inversion methodology.sources are mostly unrelated but the shallow sourced field variations generate an envelope of geological noise thatEvaluation of 3-component fluxgates as sensors for degrades depth estimates derived for the deeper sources. The degree of this disruption depends on the relative amplitudesdrone infill of regional aeromagnetic anomalies over and wavelengths of the two sets of field variations. Acrosssmall and shallow sources.some regions shallow-source magnetic field variationsClive FossKeith Leslie , Phillip Schmidtand David Clark1 1 2have consistent characteristics. These characteristics can be mapped by simple statistics as approximate indicators of the1 CSIRO degradation to be expected in depth estimation for deeper2 MagneticEarthmagnetisations. We illustrate the relationships by imperfections in recovering input models from inversion of synthetic data toMuch of Australia is covered with regional aeromagnetic which measured shallow sourced field variations have beendata measured at line spacings of 200 to 400 m and ground added. We also derive equivalent sources from measuredclearances of 50 to 80 m. In many cases anomalies detected by shallow sourced field variations to investigate the relativethese surveys over small and shallow bodies are insufficiently disruption in estimation of depth to deeper sources fordefined to well constrain the source magnetisations (particularly different survey elevations (which changes depth to both thethose of uncertain direction). Many of the anomalies are defined shallower and deeper magnetisations). We believe that thison 3 flight-lines or less in areas of side length no greater than 1 study of near-surface magnetisations will provide a basis forkm. We investigate suitability of relatively cheap and lightweight more sophisticated, site dependant evaluation of the benefit3-component fluxgates as sensors for lower level, closer line-of higher resolution surveys in the search for magnetisationsspaced drone surveys of these small regions of high amplitude at specific depths and to assist in optimising design of suchmagnetic field variation. Individual fluxgates have uncertain surveys. zero values and sensitivity scales and their mounting as a 3-component set is not perfectly orthogonal. These problems give rise to heading errors which can be the limiting factor in Sweet-spots for estimation of source magnetisationtheir deployment as TMI sensors. We investigate the practicality direction. of overcoming these problems with an initial laboratory-based calibration of the 3-component fluxgate, followed by on-site fine Clive Foss 1 David Pratt2 and Blair McKenzie2 tuning in low field variation areas adjacent to the anomaly of interest (similar to the compensation box pre-survey calibration 1 CSIROused in aeromagnetic surveys) and finally post-processing using 2 Tensor Research redundancy of repeat lines flown in different directions. The The direction of a magnetisation is expressed in the patternobjective is to develop a lightweight system and procedure of the magnetic field anomaly it generates. Helbigs proofwhich can be applied to upgrade regional aeromagnetic survey establishes that the direction of a dipole magnetisation (acoverage in the specific areas in which that data is insufficient, compact source or homogeneous sphere) can be recoveredand the magnetisations are of particular interest.from magnetic field analysis provided the anomalous field is isolated from other overlapping fields and its horizontal centreNon-destructive bulk remanent magnetisation of magnetisation is correctly determined. We term these andmeasurement from outcrop magnetic field mapping.other samples of the field suitable for magnetisation analysis sweet-spots. Clive Foss, Keith Leslie and Andreas BjorkVoxel inversions with free source magnetisation directions areCSIRO commonly accepted as mappings of magnetisation direction in three dimensions across the complete subsurface butWe present a case study in determination of remanent there is no analytic justification for this, even if the model- magnetisation from inversion of magnetic field data measured computed field everywhere matches the input measureddirectly over blocks of rock with side lengths of several metres. field. For subcircular sweet-spot anomalies the vector sumThe profile measurements were made across edges of the blocks of magnetisations due to contributing elements in a free- using a 3-component fluxgate magnetometer travelling along a magnetisation-direction voxel model should approximately1.4 m track. The measured tracks were centred over the vertical match the true direction of total magnetisation. However,edges (half over and half off the blocks). Both multi-channel the apparent mapping of local voxel variations in direction isinversions of the independent components and TMI inversions unlikely to have any reliability. using the combined components were successful in recovering magnetisation estimates. The field variations are due to resultant To selectively recover estimates of magnetisation direction from(induced plus remanent) magnetisation but we were able to sweet-spots we apply gradient analysis of the normalised sourcespecify the contribution from induced magnetisation using strength (NSS) to identify peak locations. Several grid-baseddirect measurements of magnetic susceptibility on the surfaces analyses can be applied to estimate magnetisation directionof the blocks. Single profile inversions have low sensitivity to from the TMI data at and immediately surrounding thosemagnetisation direction because of the weak contribution of locations. For improved resolution the data can be submittedmagnetisation components perpendicular to the vertical plane to either parametric or voxel inversion to estimate the totalof the profiles, but this is overcome by measuring multiple magnetisation vector and centre of magnetisation (but not toprofiles in different orientations. The volume of investigation recover intricate details of any internal variability in direction).determined by the geometry of the blocks, the track length The success in recovering magnetisation direction dependsand elevation of the sensor above the block is significantly FEBRUARY 2023 PREVIEW 98'