Predictive Geologic Mapping and Mineral Exploration

Electromagnetic exploration methods have important applications for geologic mapping and mineral exploration in igneous and metamorphic terranes. In such cases, the earth is often largely resistive and the most important interaction is between a conductor of interest and a shallow, thin, horizontal sheet representing glacial tills and clays or the conductive weathering products of the basement rocks (both of which are here termed the “conductive overburden”). To this end, we have developed a theory from which the step and impulse responses of a sphere interacting with conductive overburden can be quickly and efficiently approximated. The sphere model can also be extended to restrict the currents to flow in a specific orientation (termed the dipping-sphere model). The resulting expressions are called semianalytical because all relevant relations are developed analytically, with the exception of the time-convolution integrals. The overburden is assumed to not be touching the sphere, so there is no galvanic interactions between the bodies. We make use of the dipole sphere in a uniform field and thin sheet approximations; however, expressions could be obtained for a sphere in a dipolar (or nondipolar) field using a similar methodology. We have found that there is no term related to the first zero of the relevant Bessel function in the response of the sphere alone. However, there are terms for all other zeros. A test on a synthetic model shows that the combined sphere-overburden response can be reasonably approximated using the first-order perturbation of the overburden field. Minor discrepancies between the approximate and more elaborate numerical responses are believed to be the result of numerical errors. This means that in practice, the proposed approach consists of evaluating one convolution integral over a sum of exponentials multiplied by a polynomial function. This results in an extremely simple algorithmic implementation that is simple to program and easy to run. The proposed approach also provides a simple method that can be used to validate more complex algorithms. A test on field data obtained at the Reid Mahaffy site in Northern Ontario shows that our approximate method is useful for interpreting electromagnetic data even when the background is thick. We use our approach to obtain a better estimate of the geometry and physical properties of the conductor and evaluate the conductance of the overburden.

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Airborne resistivity data leveling

Geophysics ◽

10.1190/1.1444542 ◽

1999 ◽

Vol 64 (2) ◽

pp. 378-385 ◽

Cited By ~ 18

Author(s):

Haoping Huang ◽

Douglas C. Fraser

Keyword(s):

Frequency Domain ◽

Mineral Exploration ◽

Primary Field ◽

Geologic Mapping ◽

Resistivity Data ◽

High Magnitude ◽

Geological Features ◽

Absolute Measure ◽

Tie Lines ◽

Automated Technique

Helicopter‐borne frequency‐domain electromagnetic (EM) data are used routinely to produce resistivity maps for geologic mapping, mineral exploration, and environmental investigations. The integrity of the resistivity data depends in large part on the leveling procedures. Poor resistivity leveling procedures may, in fact, generate false features as well as eliminate real ones. Resistivity leveling is performed on gridded data obtained by transformation of the leveled EM channel data. The leveling of EM channel data is often imperfect, which is why the resistivity grids need to be leveled. We present techniques for removing the various types of resistivity leveling errors which may exist. A semi‐automated leveling technique uses pseudo tie‐lines to remove the broad flight‐based leveling errors and any high‐magnitude line‐based errors. An automated leveling technique employs a combination of 1-D and 2-D nonlinear filters to reject the rest of the leveling errors including both long‐and short‐wavelength leveling errors. These methods have proven to be useful for DIGHEM helicopter EM survey data. However, caution needs to be exercised when using the automated technique because it cannot distinguish between geological features parallel to the flight lines and leveling errors of the same wavelength. Resistivity leveling is not totally objective since there are no absolutes to the measured frequency‐domain EM data. The fundamental integrity of the EM data depends on calibration and the estimate of the EM zero levels. Zero level errors can be troublesome because there is no means by which the primary field can be determined absolutely and therefore subtracted to yield an absolute measure of the earth’s response. The transform of incorrectly zero‐leveled EM channels will yield resistivity leveling errors. Although resistivity grids can be leveled empirically to provide an esthetically pleasing map, this is insufficient because the leveling must also be consistent across all frequencies to allow resistivity to be portrayed in section. Generally, when the resistivity looks correct in plan and section, it is assumed to be correct.

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Statistical analysis of airborne gamma‐ray data for geologic mapping purposes: Crixas‐Itapaci area, Goias, Brazil

Geophysics ◽

10.1190/1.1442592 ◽

1989 ◽

Vol 54 (10) ◽

pp. 1326-1332 ◽

Cited By ~ 7

Author(s):

A. C. B. Pires ◽

N. Harthill

Keyword(s):

Statistical Analysis ◽

Gamma Ray ◽

Mineral Exploration ◽

Aerial Survey ◽

Geologic Mapping ◽

Good Correspondence ◽

Gamma Ray Spectrometer ◽

Data Points ◽

Spectrometer Data ◽

Mode Technique

Q‐mode factor analysis, K‐means clustering, and G‐mode clustering were used on digitized gamma‐ray spectrometer data from an aerial survey of the Crixas‐Itapaci area, Goias, Brazil. The data points including seven variables—eU, eTh, K, total count, U/Th, U/K, and Th/K—were digitized for a 2 km square grid. For the northwest corner of the area the data were gridded at 1 km. The Q‐mode classification method supplied results that do not show a good correspondence with the known geology. The K‐means clustering procedure barely identified the main lithologic features of the area. The G‐mode technique produced results that correlate well with the known geology and identified the greenstone belts present in the area by discriminating their ultramafic and mafic components from adjacent felsic rocks. Statistical analysis of aerial gamma‐ray spectrometer data can be very helpful in mapping geologic units in poorly known areas. It can also be used for mineral exploration purposes if mineralization is known to be associated with lithologies that can be identified by the techniques used in this study.

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Composite color images of aerial gamma‐ray spectrometric data

Geophysics ◽

10.1190/1.1441502 ◽

1983 ◽

Vol 48 (6) ◽

pp. 722-735 ◽

Cited By ~ 27

Author(s):

Joseph S. Duval

Keyword(s):

Gamma Ray ◽

Color Image ◽

Mineral Exploration ◽

Color Images ◽

South Texas ◽

Geologic Mapping ◽

Partial Synthesis ◽

Data Set ◽

Spectrometric Data ◽

Sample Data

Aerial gamma‐ray data provide estimates of the apparent surface concentrations of potassium (K), equivalent uranium (eU), and equivalent thorium (eTh). These data can be expressed as nine radiometric parameters: K, eU, eTh, eU/eTh, eU/K, eTh/K, eTh/eU, K/eU, and K/eTh. The U.S. Geological Survey (USGS) has developed a technique which combines any three of these parameters to form a composite color image. The color image provides a partial synthesis of the radiometric data that can be used to aid geologic mapping and mineral exploration. The sample data set, from the Freer area in south Texas, illustrates the use of the color images.

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On: “An Automatic Method of Compilation and Mapping of High‐Resolution Aeromagnetic Data,” by B. K. Bhattacharyya (GEOPHYSICS, August 1971, p. 695–716)

Geophysics ◽

10.1190/1.1440281 ◽

1972 ◽

Vol 37 (3) ◽

pp. 544-544

Author(s):

M. S. Reford

Keyword(s):

High Resolution ◽

Mineral Exploration ◽

Automatic Method ◽

Aeromagnetic Data ◽

Geologic Mapping ◽

Valuable Contribution ◽

Compilation Techniques

The use of high‐resolution aeromagnetic surveys for detailed geologic mapping and mineral exploration is not yet common, nor are the flying and compilation techniques as standardized as those of conventional or lowsensitivity aeromagnetic surveys. Dr. Bhattacharyya has made a valuable contribution by presenting particularly interesting results and describing the techniques in some detail. But there are some points in his comparisons between high‐resolution and conventional surveys which could be misleading.

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Aeromagnetic gradiometer program of the Geological Survey of Canada

Geophysics ◽

10.1190/1.1442726 ◽

1989 ◽

Vol 54 (8) ◽

pp. 1012-1022 ◽

Cited By ~ 41

Author(s):

Peter J. Hood ◽

Dennis J. Teskey

Keyword(s):

Mineral Exploration ◽

Geological Survey ◽

Geologic Mapping ◽

Aeromagnetic Survey ◽

R And D ◽

The Past ◽

Mapping Tool ◽

The World ◽

Computer Scientists ◽

Color Pixel

During the past two decades, the Geological Survey of Canada Aeromagnetic Survey Group, consisting of geophysicists, electronic engineers, technicians, and computer scientists, developed the aeromagnetic gradiometer technique for mineral exploration. The same group ran the aeromagnetic survey program in Canada, perhaps the largest such continuing aeromagnetic survey program in the world. In 1973, fabrication commenced on an inboard vertical gradiometer system on the GSC Queenair aeromagnetic survey aircraft. During the period 1978–1981, a number of experimental gradiometer surveys were carried out by the Geological Survey of Canada to demonstrate the efficacy of the aeromagnetic gradiometry technique as a geologic mapping tool in mineral exploration programs. Because of a need for aeromagnetic gradiometer surveys in the topographically rugged Gaspé Peninsula of Quebec, the GSC began in 1983 to foster the development of helicopter‐borne gradiometer systems through R and D contracts. Four companies responded and built towed‐boom helicopter gradiometer systems which have now been used in surveys in four eastern provinces. It is clear that the aeromagnetic gradiometer technique combined with VLF EM is an excellent geophysical tool to improve the accuracy of detailed geologic mapping for mineral exploration programs. VLF EM is an inexpensive add‐on that materially improves the geologic mapping capability of the airborne system. The product in color pixel form is in essence a pseudogeologic map and it is presently being employed as such.

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Geochemical mapping in Georgia, USA: a tool for environmental studies, geologic mapping and mineral exploration

Journal of Geochemical Exploration ◽

10.1016/s0375-6742(99)00079-5 ◽

1999 ◽

Vol 67 (1-3) ◽

pp. 345-360 ◽

Cited By ~ 14

Author(s):

Mark D. Cocker

Keyword(s):

Mineral Exploration ◽

Environmental Studies ◽

Geochemical Mapping ◽

Geologic Mapping

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Identification of intrusive lithologies in volcanic terrains in British Columbia by machine learning using random forests: The value of using a soft classifier

Geophysics ◽

10.1190/geo2019-0461.1 ◽

2020 ◽

Vol 85 (6) ◽

pp. B249-B258

Author(s):

Stephen Kuhn ◽

Matthew J. Cracknell ◽

Anya M. Reading ◽

Stephanie Sykora

Keyword(s):

Machine Learning ◽

British Columbia ◽

Random Forests ◽

Learning Algorithm ◽

Mineral Exploration ◽

Training Data ◽

Supervised Machine Learning ◽

Data Sampling ◽

Geologic Mapping ◽

Rock Types

Identifying the location of intrusions is a key component in exploration for porphyry Cu ± Mo ± Au deposits. In typical porphyry terrains, in the absence of outcrop, intrusions can be difficult to discriminate from the compositionally similar volcanic and volcanoclastic sedimentary rocks in which they are emplaced. The ability to produce lithological maps at an early exploration stage can significantly reduce costs by assisting in planning and prioritization of detailed mapping and sampling. Additionally, a data-driven strategy provides opportunity for the discovery of intrusions not identified during conventional mapping and interpretation. We used random forests (RF), a supervised machine-learning algorithm, to classify rock types throughout the Kliyul porphyry prospect in British Columbia, Canada. Rock types determined at geochemical sampling sites were used as training data. Airborne magnetic and radiometric data, geochemistry, and topographic data were used in classification. Results were validated using First Quantum Minerals’ geologic map, which includes additional detail from targeted location and transect mapping. The petrophysical and compositional similarity of rock types resulted in a noisy classification. Intrusions, particularly the more discrete, were inconsistently predicted, likely due to their limited extent relative to data sampling intervals. Closer examination of class membership probabilities (CMPs) identified locations where the probability of an intrusion being present was elevated significantly above the background. Indeed, a large proportion of mapped intrusions correspond to areas of elevated probability and, importantly, areas were highlighted as potential intrusions that were not identified in geologic mapping. The RF classification produced a reasonable lithological map, if lacking in resolution, but more significantly, great benefit comes from the insights drawn from the RF CMPs. Mapping the spatial distribution of elevated intrusion CMP, a soft classifier approach, produced a map product that can target intrusions and prioritize detailed mapping for mineral exploration.

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Optimum second vertical derivatives in geologic mapping and mineral exploration

Geophysics ◽

10.1190/1.1441320 ◽

1982 ◽

Vol 47 (12) ◽

pp. 1706-1715 ◽

Cited By ~ 21

Author(s):

V. K. Gupta ◽

N. Ramani

Keyword(s):

Power Spectrum ◽

High Frequency ◽

Mineral Exploration ◽

Frequency Noise ◽

Second Derivative ◽

Geologic Mapping ◽

Northern Ontario ◽

Near Surface ◽

Vertical Derivative ◽

Gravity Interpretation

The second vertical derivative is often used in gravity interpretation to enhance localized near‐surface features. Since high‐frequency noise is amplified considerably in a true second derivative map, some smoothing is always necessary. One of the methods available to the interpreter is the optimum second derivative technique which utilizes Wiener theory to design filters from an analysis of the power spectrum to suit the degree of noise in the data. The power spectrum of the Bouguer gravity field obtained from over 5100 gravity stations covering an area of approximately [Formula: see text] in Archean greenstone belts of northern Ontario was computed. The radially averaged spectrum decreased monotonically with frequency and flattened at the high‐frequency end. The spectrum, separated into its signal and noise components, was used to design an optimum second vertical derivative filter which has a peak at 0.35 cycles/grid interval with low‐ and high‐frequency cut‐off at 0.12 and 0.44 cycles/grid intervals, respectively. The resultant second derivative map corresponds remarkably well with the known surface geology. In most of the mapped regions the zero contours coincide with the lithological boundaries; positive and negative anomalies match surface exposures of the mafic and felsic rock units, respectively. In the central part of the area which covers the Birch‐Uchi greenstone belt, where the geology has been well mapped, the second derivative map is quite successful in delineating the successive groups of mafic to felsic metavolcanic rocks that represent the product of cyclic volcanism. The distribution of the mineral deposits in the study area can be related to the positive or negative second derivative anomalies. For example, most of the known gold and silver mineralization occurring in mafic metavolcanics is associated with positive second derivative anomalies. Similarly, the polymetallic Zn‐Cu‐Ag deposits are mainly located in the negative second derivative anomalies caused by felsic metavolcanics. A properly designed second derivative map can thus be an important supplement to geologic mapping in the identification of lithological units, in the study of structure, and as an indirect tool in regional mineral exploration.

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Geophysics in Australian mineral exploration

Geophysics ◽

10.1190/1.1441888 ◽

1985 ◽

Vol 50 (12) ◽

pp. 2637-2665 ◽

Cited By ~ 11

Author(s):

Robert J. Smith

Keyword(s):

Gold Deposit ◽

Genetic Model ◽

Mineral Exploration ◽

Mine Planning ◽

Sulfide Deposit ◽

Geologic Mapping ◽

Case Histories ◽

Color Displays ◽

Regional Area ◽

Lead Zinc

I review a variety of recent case histories illustrating the application of geophysics in mineral exploration in Australia. Geophysics is now an integral part of most programs. Examples are given of contributions by geophysics to all stages of mineral exploration, from regional area selection through to mine planning and development. Specific case histories summarized are as follows: (a) Olympic Dam copper‐uranium‐gold deposit, discovered using a conceptual genetic model and regional geophysical data; (b) Ellendale diamondiferous kimberlites, illustrating the use of low level, detailed airborne magnetics; (c) Ranger uranium orebodies, discovered by detailed airborne radiometric surveys; (d) geologic mapping near Mary Kathleen with color displays of airborne radiometric data; (e) mapping of lignite in basement depressions of the Bremer Basin, near Esperance, with INPUT; (f) White Leads, a lead‐zinc sulfide deposit discovered with induced polarization (IP) and TEM, near Broken Hill; (g) Hellyer, a lead‐zinc‐silver‐gold deposit discovered with UTEM; (h) application of geophysical logging near Kanmantoo; (i) Cowla Peak, a subbituminous steaming coal deposit mapped with ground TEM; and (j) Cook Colliery, where high‐resolution seismic reflection methods have successfully increased the workable reserves.

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