Exploration Geophysics - Volume 56, Issue 6, 2025

The study examines a small, unconventional Fe-skarn deposit in the Archean Nucleus of São José do Campestre in NE Brazil. By using terrestrial geophysics and magnetic processing modelling techniques, the researchers identified mineralised iron ore bodies, determined their shapes and geometries, classified the deposits, and studied the patterns and processes of magnetite enrichment. The study used magnetic susceptibility, SATMAGAN data, geochemical data, petrographic analysis of drill core samples, and field observations to validate and interpret the geophysical model. The information was integrated and analysed statistically to improve the model, leading to more realistic values. Parametric probabilistic tests were conducted to evaluate statistical procedures. The results identified two mineralised bodies with magnetic susceptibility values ranging from 95.00 to 1,591.00 x 10^-3 SI, similar to those observed during fieldwork (91.25 to 2,867.25 x 10^-3 SI) and in other Fe-skarn deposits. Geometry and depth of the two main causative sources showed one cone- or pipe-shaped body extending to~ 300 m depth; and another body in the form of a dike extending to ~700 m maximum depth. The integration of geophysical, physical, and geological information helped classify the Fe-skarn deposit, assigning its enrichment of magnetite to the participation of Proterozoic granitic dike swarms. The results of this study demonstrate the value of ground geophysics in mineral prospecting, as it is an efficient and environmentally friendly method of investigating mineral deposits.

The finite difference scheme has been widely applied in the Laplace-Fourier (L-F) domain forward modeling. The average derivative method (ADM) of the finite-difference scheme is widely adopted, but it represents a special case of the general optimal scheme, which offers better accuracy and efficiency. However, the classical general optimal scheme suffers from inaccuracy at low wavenumbers and is not rigorously convergent. In this study, we propose an accuracy-constrained general optimal 25-point scheme for the scalar wave equation in L-F domain. Based on the optimized scheme, we calculate the accuracy-constrained general optimized 25-point difference coefficients for various spatial sampling intervals. Dispersion analysis shows that, to maintain relative error within 1%, the average derivative 25-point scheme requires 5 grid points per smallest wavelength and pseudo-wavelength, while the accuracy-constrained general optimal 25-point scheme only requires 4 grid points. To suppress the boundary reflection, the perfectly matched layer boundary is employed. Numerical results demonstrate that the accuracy-constrained general optimal 25-point scheme shows better accuracy than the average derivative 25-point scheme.

The result of the standard penetration test (SPT) is expressed as an N-value and is commonly used for site characterisation in geotechnical engineering. Another indicator of integrity of the ground is the S-wave velocity, typically estimated by borehole measurement or seismic survey.

Many researchers have attempted to find precise relationships between these parameters. However, N-values estimated from S-wave velocity (Vs) using any formulae are subject to substantial errors, and the errors are inevitable due to the different nature of the parameters.

Pseudo-N value (Ñ) was first proposed in 2011 as Ñ= (Vs/60)^2.5. This is a simplified formula derived from those by previous authors. Using this simple formula as a common practice, with understanding of existence of error, the results can be compared from site to site.

This paper first compares the N-value and S-wave velocity in their natures, methods, practice and cost. Then it examines the previously published correlation formulae and proposed formula of pseudo-N value. Some examples of use of the pseudo-N values are presented.

During land seismic acquisition, the sources and receivers are commonly referenced to what is effectively a local grid. Assignment of nodes to this grid can be done by programming the node directly during deployment, or by recording its position and serial number. Although a seemingly insignificant part of the process, this can still represent 20–30% of the time spent deploying the nodes. A better alternative is to use the internal GPS position recorded by the node to assign the line and point number. This study has derived both theoretical and practical methods for determining the accuracy of the self-assignment of nodes to their planned positions. I have also developed an assignment methodology, ordered-assignment, which is significantly more accurate than simply assigning a node to the closest point. I show that the misassignment rate for 4 m spaced nodes can be as low as 3% with the rate decreasing exponentially with node separation. On this basis I conclude that self-assignment will be sufficiently accurate for all but the most high-resolution surveys.

Spectral induced polarisation (SIP) is a valuable geophysical technique for exploring sulfide-bearing metallurgical deposits. Effective application of SIP in mineral exploration requires an understanding of the relationship between SIP parameters – such as chargeability and critical frequency – and sulfide mineral type, content, and particle size. In this study, SIP measurements were performed on artificial samples comprising crushed chalcopyrite, pyrite, and galena mixed with glass beads, as well as on 175 natural rock samples collected from a skarn deposit in Korea. The results demonstrated strong correlations between the chargeability, critical frequency, and sulfide mineral characteristics. The ore-bearing samples exhibited a higher chargeability than the surrounding host rocks, confirming the effectiveness of SIP in identifying sulfide-rich zones. Additionally, large sulfide mineral particles exhibited low critical frequencies, whereas smaller particles exhibited high critical frequencies owing to their increased surface area and enhanced electrochemical interactions. The SIP responses of mixed sulfide mineral samples approximated the average responses of individual mineral components, indicating that SIP anomalies in natural ore deposits reflect the combined effects of multiple sulfide minerals rather than a single dominant phase. Furthermore, SIP results effectively distinguished ore-bearing formations from the surrounding intrusive rocks, enabling more accurate delineation of the mineralised zones. These findings highlight the potential of SIP measurements to optimise mineral exploration strategies, particularly for sulfide-bearing ore deposits.

We acquired two LiDAR surveys in April 2023 and May 2024 before and after a 36 km² seismic survey that included over 1,800 km of line clearing in the Bowen Basin in central Queensland, Australia. The initial LiDAR survey was used to design the seismic survey such that its environmental impact was minimised. Prior to the seismic survey, disturbance was estimated at 3.5% using the tree fractional cover metric for vegetation with heights ≥ 5 m. Comparisons of the first and second surveys showed that the actual disturbance, again estimated using tree fractional cover, was actually -1.2%, i.e. vegetation growth had more than offset the limited areas of disturbance caused by the survey. Disturbance calculations based on identifying individual contiguous areas with a total area greater than 10 m² gave a total disturbance value of just 0.2%. These results are likely a result of a combination of an initial overestimation of the likely disturbance and vegetation growth taking place between the two surveys.