Chris Wijns

Theta map: Edge detection in magnetic data

The 3D analytic signal amplitude of a total magnetic intensity (TMI) map, introduced by Roest et al. (1992), is widely used in magnetic interpretation as a means of positioning anomalies directly over their sources. This technique is most important at low magnetic latitudes, where reduction to the pole distorts anomalies to the point where they often become uninterpretable: the reduction operator does not converge if the magnetization and regional field are truly horizontal (Baranov, 1957). Methods have been devised to suppress the artifacts appearing in low-latitude reduction to the pole, but no method can reduce such data without distortion (e.g., Silva, 1986; Hansen and Pawlowski, 1989), which becomes severe for inclinations less than 20°. The amplitude of the analytic signal, denoted by | A |, has the added advantage of being independent of the orientation of magnetization of the source bodies. It reaches a maximum over magnetic contacts, and thus, in theory, can be used to trace the outline of magnetic bodies. In practice, especially in the case of aeromagnetic data at high instrument-source separation, | A | is high over magnetic bodies, but is not sufficient to resolve body edges. This appears to be true even with higher-order analytic signal derivatives (Debeglia and Corpel, 1997, their Figure 11).

Inverse modelling in geology by interactive evolutionary computation

Inverse modelling of geological processes, in the absence of established numerical criteria to act as inversion targets, requires an approach that uses human interaction to assess forward model results. The method of interactive evolutionary computation provides for the inclusion of qualitative geological expertise within a rigorous mathematical inversion scheme, by simply asking an expert user to evaluate a sequence of forward geological models. The traditional numerical misfit is replaced by a human appraisal of misfit. We use this interactive technique to successfully invert a geodynamic model for a conceptual pattern of fault spacing during crustal extension.

Inversion in geology by interactive evolutionary computation

We present the first step in the development of a system that would allow geological models to evolve backwards in time. The method of interactive evolutionary computation provides for the inclusion of geological knowledge and expertise in a rigorous mathematical inversion scheme, by simply asking an expert user to visually evaluate different geological models. We demonstrate the potential of the technique for the cases of folding and faulting.

Interactive inverse methodology applied to stratigraphic forward modelling

Abstract An effective inverse scheme that can be applied to complex 3-D hydrodynamic forward models has so far proved elusive. In this paper we investigate an interactive inverse methodology that may offer a possible way forward. The scheme builds on previous work in linking expert review of alternate output to rapid modification of input variables. This was tested using the SEDSIM 3-D stratigraphic forward-modelling program, varying nine input variables in a synthetic example. Ten SEDSIM simulations were generated, with subtle differences in input, and five dip sections (fences) were displayed for each simulation. A geoscientist ranked the lithological distribution in order of similarity to the true sections (the true input values were not disclosed during the experiment). The two or three highest ranked simulations then acted as seed for the next round of ten simulations, which were compared in turn. After 90 simulations a satisfactory match between the target and the model was found and the experiment was terminated. Subsequent analysis showed that the estimated input values were ‘close’ to the true values.

Reply to the discussion

First, we thank Xiong Li for drawing to the attention of readers our lapse in not referencing Miller and Singh (1994) as the originators of the tilt angle. He also correctly points out that Miller and Singh (1994) first stated the automatic gain control (AGC) nature of the tilt angle. Thurston and Smith (1997) introduce the local wavenumber, from Nabighians (1972) complex analytic signal, as an edge detection and characterisation tool, and are also worthy of reference.

Interactive geophysical inversion using qualitative geological constraints

Numerical inversion of geophysical data does not normally require user interaction apart from the selection of initial inversion parameters. However, such an inversion often returns a single solution based upon default parameters. While this solution will be geophysically correct, assuming convergence of the algorithm, it may not be the most geologically reasonable answer. It is necessary to incorporate human interaction in selecting inversion solutions, this being the most efficient method for adding qualitative geological constraints. An automatic system provides a user-directed search of the space of geophysical solutions. Rankings assigned to numerical inversion results guide a genetic algorithm in advancing towards a conceptual target. Our example uses resistivity and chargeability data from a pole-dipole induced polarisation survey collected during a mineral exploration program. We invert for specific geological features: a defined, conductive top layer, sharp geological boundaries in the resistivity, and greatest depth of resolution of the inversion algorithm. The interactive system is an organised way to investigate the solution space for valid inversion results that emphasise these geological possibilities.

Building a 3D model of lithological contacts and near‐mine structures in the Kevitsa mining and exploration site, Northern Finland: constraints from 2D and 3D reflection seismic data

The Kevitsa mafic-ultramafic intrusion in Northern Finland hosts a large, disseminated nickel-copper sulphide ore body. The Kevitsa intrusion is an active mining and exploration site, for which we ...

3D Traveltime Tomography and Reflection Imaging for Mine Planning and Exploration in the Kevitsa Ni-Cu-PGE Mine, Finland

Summary Kevitsa disseminated Ni-Cu-PGE deposit in northern Finland is hosted by a major high velocity (6000–7500 m/s) gabbroic intrusion. The deposit is currently being mined using open-pit mining method at already over 100 m depth, and the final pit will extend to about 400–500 m depth in about 20 years, hence justifying a 3D seismic survey for its careful planning. The 3D seismic survey was carried out in 2010, before mining activities commenced, and resulted in a reflection volume rich in reflectivity, however, poor near the surface likely due to the survey setup and extremely high bedrock velocities. 3D first break tomography was performed with the main objective of linking near-surface geological information with the reflection seismic volume allowing to find a major low-velocity linear zone in the bedrock within the first 50 m depth. This was associated with a gently-dipping reflector with the same strike at 150 m depth and extending to about 500 m depth. The low-velocity zone striking in NE-SW, and originating from a gently-dipping fracture system, may contribute towards the formation of wedge blocks within the northern side of the planned open pit, which can be critical for its stability in the near future.

Seismic volumetric interpretation of a disseminated copper system in Kevitsa, northern Finland

Improved mining technology and scarcity of near-surface deposits is forcing the mining industry to explore deeper in the search for economic mineralisation. Reflection seismic is one of the few geophysical methods that have sufficient resolution at depth to constrain geological information of an ore deposit at the drilling scale. Reflection seismic methods can be used to reduce drilling costs by focusing the drilling in strategically important areas. Recently introduced seismic volumetric interpretation techniques have advantages over conventional interpretation techniques where the interpretation is done by slicing the volume in 2D planes. Volumetric interpretation is performed in 3D, in real time, by applying various opacity and transparency filters to the seismic volume from different angles, which enables in-depth understanding of the volume. This initial stage of volumetric interpretation is followed by mapping the interfaces and associated structures of exploration interest. A 3D high-resolution seismic dataset was collected to investigate steeply dipping to sub-vertical structures in Kevitsa, northern Finland. Automatic fault extraction using a modified ant-tracking workflow was done on the seismic volume.

Improving resource density models via surface gravity inversion

Density is one of the fundamental physical properties required in a mining operation, underpinning the calculation of ore tonnages and thus metal produced. The resource density model captures this information, but is often based on a relatively sparse collection of density measurements. Gravity data are a direct reflection of the true distribution of subsurface density, and can be used to improve the resource model. The example of the Ravensthorpe nickel laterite mine illustrates the improvement in the resource density model that results from combining high resolution surface gravity with the set of borehole logged density readings.