Ian N. MacLeod

Bio: Ian N. MacLeod is an academic researcher. The author has contributed to research in topics: Magnetization & Remanence. The author has an hindex of 5, co-authored 7 publications receiving 360 citations.

3-D analytic signal in the interpretation of total magnetic field data at low magnetic latitudes

Abstract: The interpretation of magnetic field data at low magnetic latitudes is difficult because the vector nature of the magnetic field increases the complexity of anomalies from magnetic rocks. The most obvious approach to this problem is to reduce the data to the magnetic pole (RTP), where the presumably vertical magnetisation vector will simplify observed anomalies. However, RTP requires special treatment of north-south features in data observed in low magnetic latitudes due to high amplitude corrections of such features. Furthermore, RTP requires the assumption of induced magnetisation with the result that anomalies from remanently and anisotropically magnetised bodies can be severely disturbed. The amplitude of the 3-D analytic signal of the total magnetic field produces maxima over magnetic contacts regardless of the direction of magnetisation. The absence of magnetisation direction in the shape of analytic signal anomalies is a particularly attractive characteristic for the interpretation of magnetic field data near the magnetic equator. Although the amplitude of the analytic signal is dependent on magneti­sation strength and the direction of geological strike with respect to the magnetisation vector, this dependency is easier to deal with in the interpretation of analytic signal amplitude than in the original total field data or pole-reduced magnetic field. It is also straightforward to determine the depth to sources from the distance between inflection points of analytic signal anomalies.

Inversion of Magnetic Data from Remanent and Induced Sources

Abstract: Magnetic field data are of fundamental importance in many areas of geophysical exploration with 3D voxel inversion being a common aid to their interpretation. In the majority of voxel based inversions it is assumed that the magnetic response arises entirely from magnetic induction. However, in the last decade, several studies have found that remanent magnetization is far more prevalent than previously thought. Our experience with numerous minerals exploration projects confirms that the presence of non-induced magnetization is the rule rather than the exception in base metal exploration. In this work we show that failure to accomodate for remanent magnetization in 3D voxel-based inversion can lead to misleading interpretations. We present a technique we call Magnetization Vector Inversion (MVI), which incorporates both remanent and induced magnetization without prior knowledge of the direction or strength of remanent magnetization. We demonstrate our inversion using model studies and field data. Successful application to numerous minerals exploration surveys confirms that incorporating remanent magnetization is essential for the correct interpretation of magnetic field data.

Constrained voxel inversion using the cartesian cut cell method

Abstract: Voxel inversion is a well-established method for constructing a physical property model from geophysical data. However, a limitation on Cartesian voxel inversion is that the voxel earth model is re...

A new, rapid, automated grid stitching algorithm

Abstract: Regional compilations of gridded geophysical data from disparate individual surveys are playing an ever more important role in resource exploration. A key processing step in such compilations is the merging of overlapping grids to create a single grid. Traditional methods of connecting grids together can produce smooth final products but the process is time-consuming and has difficulty with differences that involve both long and short wavelength errors. A novel, completely automated method addresses several main challenges, such as determining how to select a path along which overlapping grids can be joined. The technique uses Fourier analysis to deconstruct the errors along a suture path, into a sum of functions with different spatial wavelengths, and applies corrections that propagate smoothly into the grids by a distance proportional to the individual wavelengths. The result is an almost seamless grid that minimises distortion from the correction process.

Quantitative Magnetization Vector Inversion

Abstract: Modelling of magnetic rock properties from magnetic field observations has been an important practice in resource exploration for decades. However, the application of this practice has been limited by conventional thinking that assumes rock magnetization is dominated by induced magnetization such that magnetization direction is aligned with the geomagnetic field. Convention has also accepted that we are unable to model for magnetic remanence without a-priori knowledge of remanence direction and strength. Recent practical successes in directly modelling magnetization vector direction and strength using Magnetization Vector Inversion (MVI) have challenged these conventions, and MVI modelling is proving useful in practical exploration scenarios. The addition of new information, namely the direction and amplitude of magnetization, demands new thinking and approaches to understanding what this information means, and how to use the modelled direction of magnetization in practical situations. This paper presents a new statistical and quantitative approach to define and discriminate different magnetization domains within a full 3D MVI voxel model. Our studies show that modelled vector direction is meaningful even without prior knowledge of remanence (and other) magnetization characteristics. We also demonstrate that reasonable magnetization direction can be recovered from both weakly and strongly magnetized source rocks.

Guidelines for radioelement mapping using gamma ray spectrometry data

Abstract: FOREWORD Gamma rays are the most penetrating radiation from natural and man-made sources, and gamma ray spectrometry is a powerful tool for the monitoring and assessment of the radiation environment. Gamma ray surveys are carried out from aircraft, field vehicles, on foot, in boreholes, on the sea bottom and in laboratories. Ground and airborne gamma ray measurements cover large areas of the earth's surface, and many national and regional radiometric maps have been compiled and published. Standardized maps of terrestrial radiation and radioelement concentrations can be compared and regionally unified, showing general regional trends in radionuclide distribution and making the radiological assessment of the environment possible. Radiometric surveys and maps are applicable in several fields of science. They retain their geological and geophysical information for mineral prospecting, geochemical mapping and structural geology, and enable the comparison of geological features over large regions. Although the gamma ray method was originally developed for geoscience, it has also been successfully applied in emergency situations for mapping the contamination from nuclear fallout and for the location of lost radioactive sources. The use of modern computer data processing has enabled the introduction of new interpretation methods and the achievement of greater reliability in solving geological and environmental problems. The purpose and scope of this report is to introduce the theoretical background of gamma ray spectrometry in its application to the radiation environment, and to emphasize and illustrate new procedures in this field based on current knowledge. The report introduces the principles of radioactivity, contemporary radiation units, and the fundamentals of gamma ray spectrometry and its application to airborne, ground, car-borne, borehole and laboratory measurements. Examples of the use of gamma ray spectrometry in environmental studies and geological mapping illustrate the conditions, requirements and procedures for data acquisition, processing and reporting using this method. For many years the International Atomic Energy Agency (IAEA) has been involved in the treatment of exploration data and their multiple applications related to mineral exploration, agricultural, and environmental assessment. Recognizing that techniques and methods are rapidly progressing in this field, the IAEA invited a group of specialists to write a comprehensive report on radioelement mapping using gamma ray spectrometry. The authors of this publication are known and recognized specialists from four countries involved in mapping and data treatment using spectrometry. They all have extensive experience in the application and use of gamma ray spectrometry for radioelement mapping. This TECDOC is one …

Artificial Neural Networks for Mineral-Potential Mapping: A Case Study from Aravalli Province, Western India

Abstract: This paper describes a GIS-based application of a radial basis functional link net (RBFLN) to map the potential of SEDEX-type base metal deposits in a study area in the Aravalli metallogenic province (western India). Available public domain geodata of the study area were processed to generate evidential maps, which subsequently were encoded and combined to derive a set of input feature vectors. A subset of feature vectors with known targets (i.e., either known mineralized or known barren locations) was extracted and divided into (a) a training data set and (b) a validation data set. A series of RBFLNs were trained to determine the network architecture and estimate parameters that mapped the maximum number of validation vectors correctly to their respective targets. The trained RBFLN that gave the best performance for the validation data set was used for processing all feature vectors. The output for each feature vector is a predictive value between 1 and 0, indicating the extent to which a feature vector belongs to either the mineralized or the barren class. These values were mapped to generate a predictive classification map, which was reclassified into a favorability map showing zones with high, moderate and low favorability for SEDEX-type base metal deposits in the study area. The method demarcates successfully high favorability zones, which occupy 6% of the study area and contain 94% of the known base metal deposits.

A combined analytic signal and Euler method (AN-EUL) for automatic interpretation of magnetic data

Abstract: We present a new automatic method of interpretation of magnetic data, called AN-EUL (pronounced “an oil”). The derivation is based on a combination of the analytic signal and the Euler deconvolution methods. With AN-EUL, both the location and the approximate geometry of a magnetic source can be deduced. The method is tested using theoretical simulations with different magnetic models placed at different depths with respect to the observation height. In all cases, the method estimated the locations and the approximate geometries of the sources. The method is tested further using ground magnetic data acquired above a shallow geological dike whose source parameters are known from drill logs, and also from airborne magnetic data measured over a known ferrometallic object. In both these cases, the method correctly estimated the locations and the nature of these sources.

Geophysics for the Mineral Exploration Geoscientist

Abstract: Providing a balance between principles and practice, this state-of-the-art overview of geophysical methods takes readers from the basic physical phenomena, through the acquisition and processing of data, to the creation of geological models of the subsurface and data interpretation to find hidden mineral deposits. Detailed descriptions of all the commonly used geophysical methods are given, including gravity, magnetic, radiometric, electrical, electromagnetic and seismic methods. Each technique is described in a consistent way and without complex mathematics. Emphasising extraction of maximum geological information from geophysical data, the book also explains petrophysics, data modelling and common interpretation pitfalls. Packed with full-colour figures, also available online, the text is supported by selected examples from around the world, including all the major deposit types. Designed for advanced undergraduate and graduate courses in minerals geoscience, this is also a valuable reference for professionals in the mining industry wishing to make greater use of geophysical methods. In 2015, Dentith and Mudge won the ASEG Lindsay Ingall Memorial Award for their combined effort in promoting geophysics to the wider community with the publication of this title.