Exploration Geophysics - Volume 45, Issue 4, 2014

Staggered-grid finite-difference (SFD) methods have been used widely in seismic wave numerical modelling and migration. The conventional way to calculate the high-order SFD coefficients on spatial derivatives is the Taylor-series expansion method, which generally leads to great accuracy on just a small frequency zone. In this paper, we first derive the SFD coefficients of arbitrary even-order accuracy for the first-order spatial derivatives by the dispersion relation and the least-squares method, which can satisfy the specific numerical solution accuracy of the derivative on a wide frequency zone. Then we use the SFD coefficients based on the least-squares to solve the first-order spatial derivatives and analyse the accuracy of the numerical solution. Finally, we perform elastic wave numerical modelling with the least-squares staggered-grid finite-difference (LSSFD) method. Meanwhile, the numerical dispersion, the modelling accuracy and the computing costs of the new method are compared with that of the Taylor-series expansion staggered-grid finite-difference (TESFD) method. The numerical dispersion analysis and elastic wavefield modelling results demonstrate that the LSSFD method can efficiently suppress the numerical dispersion and has greater modelling accuracy than the conventional TESFD method under the same discretisations and without extra computing costs.

In this paper, we derive the staggered-grid finite-difference coefficients of arbitrary even-order accuracy for the first-order spatial derivatives by the dispersion relation and the least squares method, which can satisfy the specific numerical solution accuracy of the derivative on a wide frequency zone for elastic wave numerical modelling.

This paper introduces exploration clues for orogenic gold prospecting in the north-west of the Sanandaj-Sirjan metamorphic zone, north-west Iran. These are derived based on processing and interpreting airborne magnetic and radiometric data in order to identify favourable host rocks, and surface and subsurface structures.

The Piranshahr-Sardasht-Saqqez Zone (PSSZ) in the north-west of the Sanandaj-Sirjan metamorphic zone (SSZ) hosts some major Iranian gold deposits. In the south-east of PSSZ, there is a north-east trending orogenic gold belt which contains three gold deposits/occurrences (Qolqoleh, Kervian and Ghabaghloujeh). In this research, studies are focused on processing and analysing airborne magnetic and radiometric data in order to find applicable indicators for prospecting gold in this area.

Former studies on the gold deposits/occurrences in the study area suggest three essential factors in local orogenic gold mineralisation: (1) intersecting deep bending structures/shear zones, (2) Fe-rich mafic meta-volcanic lithologies (primary source and host rocks) and (3) altered mylonitic granites (secondary host rock). Geological structures and lithological contacts can be mapped based on locating edges in the magnetic field at different depths.

In this study, we extracted the structure from aeromagnetic data by reduction to the pole, upward continuation and applying a tilt derivative filter to the horizontal derivative of the upward continued data. Upward continuation was to several levels from 500 to 4000 m. Afterwards, a 3D architecture was built based on extracted subsurface lineaments in different levels. This 3D model can assist in the visualisation of the underground shape of structures that may influence gold mineralisation.

Moreover, mafic meta-volcanic rocks in the study area, which contain magnetic minerals such as magnetite, titanomagnetite and ilmenite, can be mapped using aeromagnetic data. Mylonitic granites, which are the other host rock in the deposits, were mapped using airborne radiometric data.

Acquisition of magnetic gradient tensor data is likely to become routine in the near future. New methods for inverting gradient tensor surveys to obtain source parameters have been developed for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets and contacts, and is independent of magnetisation direction. In combination the NSS and its vector gradient determine source locations uniquely. NSS analysis can be extended to other useful models, such as vertical pipes, by calculating eigenvalues of the vertical derivative of the gradient tensor. Inversion based on the vector gradient of the NSS over the Tallawang magnetite deposit obtained good agreement between the inferred geometry of the tabular magnetite skarn body and drill hole intersections. Besides the geological applications, the algorithms for the dipole model are readily applicable to the detection, location and characterisation (DLC) of magnetic objects, such as naval mines, unexploded ordnance, shipwrecks, archaeological artefacts, and buried drums.

New methods for inverting gradient tensor surveys, based on eigenanalysis and the normalised source strength derived from the eigenvalues, have been developed for point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models.

Assuming without evidence that magnetic sources are magnetised parallel to the geomagnetic field can seriously mislead interpretation and can result in drillholes missing their targets. This paper reviews methods that are available for estimating the natural remanent magnetisation and total magnetisation of magnetic sources, noting the strengths and weaknesses of each approach.

Assuming without evidence that magnetic sources are magnetised parallel to the geomagnetic field can seriously mislead interpretation and can result in drill holes missing their targets. This article reviews methods that are available for estimating, directly or indirectly, the natural remanent magnetisation (NRM) and total magnetisation of magnetic sources, noting the strengths and weaknesses of each approach. These methods are: (i) magnetic property measurements of samples; (ii) borehole magnetic measurements; (iii) inference of properties from petrographic/petrological information, supplemented by palaeomagnetic databases; (iv) constrained modelling/inversion of magnetic sources; (v) direct inversions of measured or calculated vector and gradient tensor data for simple sources; (vi) retrospective inference of magnetisation of a mined deposit by comparing magnetic data acquired pre- and post-mining; (vii) combined analysis of magnetic and gravity anomalies using Poisson’s theorem; (viii) using a controlled magnetic source to probe the susceptibility distribution of the subsurface; (ix) Helbig-type analysis of gridded vector components, gradient tensor elements, and tensor invariants; (x) methods based on reduction to the pole and related transforms; and (xi) remote in situ determination of NRM direction, total magnetisation direction and Koenigsberger ratio by deploying dual vector magnetometers or a single combined gradiometer/magnetometer to monitor local perturbation of natural geomagnetic variations, operating in base station mode within a magnetic anomaly of interest.

Characterising the total and remanent magnetisations of sources is important for several reasons. Knowledge of total magnetisation is often critical for accurate determination of source geometry and position. Knowledge of magnetic properties such as magnetisation intensity and Koenigsberger ratio constrains the likely magnetic mineralogy (composition and grain size) of a source, which gives an indication of its geological nature. Determining the direction of a stable ancient remanence gives an indication of the age of magnetisation, which provides useful information about the geological history of the source and its environs.

Here we consider how measurements of magnetic susceptibility, magnetic remanence and Königsberger ratios (Q) can be made in the field. A basic refresher is given on how induced magnetisation differs from remanent magnetisation and what distinguishes multidomain from single domain behaviour of magnetite particles. The approximation of an infinite half-space, which is the usual assumption for using most handheld susceptibility meters, is experimentally investigated and it is found that a block 100 × 100 × 60 mm is the minimum requirement for the meters tested here. The susceptibilities of chips of a dacite, an andesite and a spilite (altered basalt) are also experimentally investigated for a range of chip sizes from a few mm down to 200 μm. The relationship is quite flat until very small grain sizes are reached where the susceptibility either decreases or increases, which is interpreted as an indication of the grain-size fraction where the magnetite resides. Making susceptibility measurements on bags of rock chips is investigated and guidelines given. The temperature of susceptibility meters is also found to be a factor and five meters have been tested for temperatures from 0°C to 50°C, the stated operating range of most meters. Finally Breiner’s method to separate induced magnetisation from remanent magnetisation using a field magnetometer is discussed. A new fluxgate based pendulum instrument to allow a more controlled implementation of Breiner’s method is also described.

The measurement of magnetic susceptibility, magnetic remanence and Königsberger ratios can be made in the field, either using a total field magnetometer or a new portable fluxgate device that is described. Various problems of using a magnetic susceptibility meter on non-ideal rock, core and chip samples can be avoided.

The resultant magnetisation vector of a compact magnetic source can be obtained through simultaneous inversion for geometry and magnetisation. The resultant magnetisation vector and induced field vector lie on a plane that intersects the transformed apparent polar wander path to provide estimates for magnetic remanence, susceptibility and age.

The remote determination of magnetic remanence in rocks is a method that has largely been ignored because of the ambiguity associated with the estimation of both the Koenigsberger ratio and remanent magnetisation direction. Our research shows that the resultant magnetisation direction can be derived directly through inversion of magnetic data for an isolated magnetic anomaly from a compact magnetic source. The resultant magnetisation direction is a property of the target magnetic rocks and a robust inversion parameter. The departure angle of the resultant magnetisation vector from that of the inducing magnetic field is an important indicator of the existence of remanent magnetisation and the inversion process can detect departures that are not easily detected by visual inspection. This departure angle is called the apparent resultant rotation angle or ARRA.

The induced field vector, remanent magnetisation vector and resultant magnetisation vector lie on the plane of a great circle. We find the intersection of the transformed polar wander vector trace with the great circle plane to obtain one or more possible solutions for the remanent magnetisation vector. Geological deduction will normally allow us to reduce the ambiguity for multiple solutions to obtain the most likely remanent magnetisation direction. Once the remanent magnetisation direction is established, it is then possible to determine the Koenigsberger ratio and magnetic susceptibility for the target.

We illustrate the methodology using survey data over the Black Hill Norite which also has extensive palaeomagnetic data available for comparison with the inversion results. We then apply the remote remanence estimation (RRE) method to a systematic study of a large number of intrusive pipes in the Thomson Orogen, New South Wales. The corrected magnetic susceptibility and remanence properties, spatial distribution and underlying uncertainties are evaluated for their potential use by diamond explorers. The additional information assists with differentiating kimberlites from other intrusive pipes based on age and remanence properties.