Evaluation of mineral exploration targets defined by airborne gravity gradiometry through gravity and magnetic modelling: vicinity of the Iron Range Fault, Purcell anticlinorium, southern Canadian Cordillera

2019 ◽  
Vol 56 (5) ◽  
pp. 452-470
Author(s):  
Mike D. Thomas ◽  
Mark Pilkington ◽  
Mike McCuaig
Keyword(s):  
Iron Oxide ◽  
Regional Scale ◽  
Vertical Component ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Canadian Cordillera ◽  
Iron Range ◽  
Vertical Gravity Gradient ◽  
Magnetic Modelling

An airborne gravity gradiometer survey was recently flown over the Iron Range Fault in the Purcell anticlinorium, southern Canadian Cordillera. The fault is commonly associated with iron oxide mineralization having characteristics similar to those of iron oxide Au ± Cu deposits. Drilling near the fault has revealed Au ± Cu–Pb–Zn mineralization. Prominent positive vertical gravity gradient (VGG) anomalies defined by the survey were identified as targets for follow-up exploration. Possible sources of the target anomalies were investigated by modelling gravity, VGG, and magnetic data along several profiles. Modelling of regional-scale profiles of the vertical component of gravity crossing exploration targets provides a regional perspective on the regional geological setting, dominated by the broad Goat River anticline, whose axis closely follows the Iron Range Fault. Modelling indicates that several VGG anomalies are related to Moyie sills, although one anomaly is modelled as a narrow vertical body (120 m wide, 1000 m vertical extent, 40 m deep) just west of the Iron Range Fault. Its apparent high density of 3500 kg/m3 suggests metallic content, making it a choice candidate for follow-up investigation. Drilling at the southern end of this geophysical target intersected a Moyie intrusion, but untested geochemical anomalies in the vicinity encourage follow-up exploration. The densities of modelled units derived from VGG profiles across two other specific targets indicate that Moyie sills represent one target and iron oxide mineralization the other, as supported by magnetic modelling, which also delineated vertical zones of significantly magnetic material along the Iron Range Fault.

Resolution and variance operators of gravity and gravity gradiometry

Geophysics ◽  
1989 ◽  
Vol 54 (7) ◽  
pp. 889-899 ◽  
Author(s):  
D. W. Vasco
Keyword(s):  
Inverse Problem ◽  
Vertical Component ◽  
Maximum Error ◽  
Airborne Gravity ◽  
Gravity Gradient ◽  
Density Structure ◽  
Gravity Gradiometry ◽  
Data Set ◽  
Model Parameter ◽  
Vertical Gravity Gradient

Gravity gradiometry represents a new potential field data set which may better constrain the density structure of the earth. Using singular value (spectral) decomposition of the gravity and gravity gradient kernels, the model parameter resolution and model parameter variance of the two data types are compared using data from the Defense Mapping Agency and a recently acquired collection of airborne gradient measurements from Bell Aerospace Textron’s Gravity Gradient Survey System (GGSS). The GGSS was flown over a portion of southwestern Oklahoma, where the gravitational anomaly from the buried Wichita basement rocks is over 60 mGal. The corresponding maximum vertical gravity gradient was found to be 46.2 Eötvös. The determination of the subsurface density structure is cast as a linear inverse problem and, for comparison, a nonlinear inverse problem. For both the linear and nonlinear inversions, the gravity gradients improve the resolution and result in smaller variances than the vertical component of gravity. The density resolution and variance were computed for a subset of tracks from an airborne gravity gradient survey made in the summer of 1987. For the linear inversion, the resolution of the density is not adequate below the second layer (20 km). Furthermore, the estimated error of the actual gradient observations for a resolution of 0.9 km is 10E, for which the maximum error of the density values is [Formula: see text]. The linearized resolution of the boundary perturbations is better, with most parameters being well resolved. The standard errors for the layer perturbations are less than 1 km for the shallower layer (5.0 km) when using the gradiometer data. For the deeper layer (25.0 km), the maximum error is larger, 4.3 km.

Geophysical methods used in the discovery of the Kitumba iron oxide copper gold deposit

2015 ◽  
Vol 3 (2) ◽  
pp. SL15-SL25 ◽  
Author(s):  
Thomas R. H. Woolrych ◽  
Asbjorn N. Christensen ◽  
Darcy L. McGill ◽  
Tom Whiting
Keyword(s):  
Iron Oxide ◽  
Structural Information ◽  
Geophysical Methods ◽  
3D Models ◽  
Induced Polarization ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Data Sets ◽  
Geologic Mapping ◽  
Copper Gold

A range of geophysical techniques has been used at various stages of the discovery and delineation of the Kitumba deposit in Central Zambia. Early era magnetics, geologic mapping, artisanal Cu plays, and the application of an iron oxide copper gold (IOCG) exploration model led explorers to the area in the 1990s. An airborne gravity gradiometer (AGG) survey was flown in 2004, and it highlighted key regional elements considered to be prerequisite for prospective IOCG mineralization. The AGG survey accurately delineated the spatial extents of two target areas referred to as the Kitumba and Mutoya systems. Gravity, radiometric, and magnetic data sets acquired as part of the AGG survey have mapped geologic and structural information as well as the extent of the IOCG alteration system. Significant uranium anomalism in the radiometric data was identified at Kitumba upon which the discovery hole S36-001 was sited. In 2012, a 3D direct current resistivity and induced polarization survey was conducted over Kitumba. The survey results provided 3D models of induced polarization chargeability anomalism and allowed successful delineation of sulfide material within the known deposit. The survey also provided an enhanced understanding of the 3D geometry of the mineralization. This improved understanding allowed a refocusing of drilling activities to best target extensions to existing mineralization.

Application of high resolution airborne geophysics to epithermal gold exploration in Northeast Queensland and Coromandel, New Zealand

1989 ◽  
Vol 20 (2) ◽  
pp. 99 ◽  
Author(s):  
S.S. Webster ◽  
R.W. Henley
Keyword(s):  
High Resolution ◽  
Regional Scale ◽  
Cost Effective ◽  
Gold Deposits ◽  
Ore Deposits ◽  
Magnetic Data ◽  
Mapping Technique ◽  
Epithermal Gold ◽  
Airborne Geophysics ◽  
Airborne Geophysical Data

High resolution airborne geophysical data over broad areas have been found to optimize exploration for epithermal gold deposits in differing geological environments.Genetic exploration models may be tested in favourable sites by the recognition of geophysical signatures. These signatures reflect structural, lithological and alteration patterns arising from controls on ore deposits and can be applied at regional or detailed scales, using the same data set.At regional scale (e.g. 1:100,000) the magnetic data reflect the regional tectonics and divide the area into domains for the application of appropriate genetic models. At prospect scale (e.g. 1:25,000) the radiometric data allow the extrapolation of poorly outcropping geology to provide a cost-effective mapping technique. The magnetic data can be used to supplement this interpretation or can be used to target deeper sources for direct investigation by drilling.

scholarly journals Interpretation of airborne gravity gradiometry and magnetic data using cross-gradients

2012 ◽  
Vol 2012 (1) ◽  
pp. 1-4
Author(s):  
Carlos Cevallos ◽  
Mark Dransfield ◽  
Jacqueline Hope ◽  
Heather Carey
Keyword(s):  
Magnetic Data ◽  
Airborne Gravity ◽  
Gravity Gradiometry ◽  
Airborne Gravity Gradiometry

scholarly journals Seismic detection of rockslides at regional scale: examples from the Eastern Alps and feasibility of kurtosis-based event location

2018 ◽  
Vol 6 (4) ◽  
pp. 955-970 ◽  
Author(s):  
Florian Fuchs ◽  
Wolfgang Lenhardt ◽  
Götz Bokelmann ◽  
Keyword(s):  
Large Scale ◽  
Seismic Velocity ◽  
Regional Scale ◽  
Vertical Component ◽  
Mean Amplitude ◽  
Velocity Model ◽  
Automatic Processing ◽  
Eastern Alps ◽  
Seismic Records ◽  
Seismic Networks

Abstract. Seismic records can provide detailed insight into the mechanisms of gravitational mass movements. Catastrophic events that generate long-period seismic radiation have been studied in detail, and monitoring systems have been developed for applications on a very local scale. Here we demonstrate that similar techniques can also be applied to regional seismic networks, which show great potential for real-time and large-scale monitoring and analysis of rockslide activity. This paper studies 19 moderate-sized to large rockslides in the Eastern Alps that were recorded by regional seismic networks within distances of a few tens of kilometers to more than 200 km. We develop a simple and fully automatic processing chain that detects, locates, and classifies rockslides based on vertical-component seismic records. We show that a kurtosis-based onset picker is suitable to detect the very emergent onsets of rockslide signals and to locate the rockslides within a few kilometers from the true origin using a grid search and a 1-D seismic velocity model. Automatic discrimination between rockslides and local earthquakes is possible by a combination of characteristic parameters extracted from the seismic records, such as kurtosis or maximum-to-mean amplitude ratios. We attempt to relate the amplitude of the seismic records to the documented rockslide volume and reveal a potential power law in agreement with earlier studies. Since our approach is based on simplified methods we suggest and discuss how each step of the automatic processing could be expanded and improved to achieve more detailed results in the future.

Magnetic Field Variations in Alaska: Recording Space Weather Events on Seismic Stations in Alaska

2020 ◽  
Vol 110 (5) ◽  
pp. 2530-2540 ◽  
Author(s):  
Adam T. Ringler ◽  
Robert E. Anthony ◽  
David C. Wilson ◽  
Abram C. Claycomb ◽  
John Spritzer
Keyword(s):  
Magnetic Field ◽  
Ground Motion ◽  
Space Weather ◽  
Vertical Component ◽  
Environmental Data ◽  
Magnetic Data ◽  
Seismic Stations ◽  
Weather Events ◽  
S Period ◽  
Magnetic Field Variations

ABSTRACT Seismometers are highly sensitive instruments to not only ground motion but also many other nonseismic noise sources (e.g., temperature, pressure, and magnetic field variations). We show that the Alaska component of the Transportable Array is particularly susceptible to recording magnetic storms and other space weather events because the sensors used in this network are unshielded and magnetic flux variations are stronger at higher latitudes. We also show that vertical-component seismic records across Alaska are directly recording magnetic field variations between 40 and 800 s period as opposed to actual ground motion during geomagnetic events with sensitivities ranging from 0.004 to 0.48  (m/s2)/T. These sensitivities were found on a day where the root mean square variation in the magnetic field was 225 nT. Using a method developed by Forbriger (2007, his section 3.1), we show that improving vertical seismic resolution of an unshielded sensor by as much as 10 dB in the 100–400 s period band using magnetic data from a collocated three-component magnetometer is possible. However, due to large spatial variations in Earth’s magnetic field, this methodology becomes increasingly ineffective as the distance between the seismometer and magnetometer increases (no more than 200 km separation). A potential solution to this issue may be to incorporate relatively low-cost magnetometers as an additional environmental data stream at high-latitude seismic stations. We demonstrate that the Bartington Mag-690 sensors currently deployed at Global Seismographic Network sites are not only acceptable for performing corrections to seismic data, but are also capable of recording many magnetic field signals with similar signal-to-noise ratios, in the 20–1000 s period band, as the observatory grade magnetometers operated by the U.S. Geological Survey Geomagnetism Program. This approach would densify magnetic field observations and could also contribute to space weather monitoring by supplementing highly calibrated magnetometers with additional sensors.

The spectrum of GPS measurement errors and the accuracy of airborne gravity measurements

Geophysics ◽  
1990 ◽  
Vol 55 (8) ◽  
pp. 1101-1104 ◽  
Author(s):  
D. R. Bower ◽  
J. Kouba ◽  
R. J. Beach
Keyword(s):  
Measurement Errors ◽  
Regional Scale ◽  
Vertical Motion ◽  
Airborne Gravity ◽  
Width Ratio ◽  
Noise Power ◽  
Power Spectral ◽  
Gps Measurement ◽  
Gravity Measurements ◽  
Allowable Error

Recent observations (Georgiadou and Kleusberg, 1987; Kleusberg et al., 1989) suggest that errors in GPS carrier phase observations at frequencies within the gravity passband of airborne gravity systems may be due mainly to multipath interference. Further, the power spectral density (PSD) of these errors has been found to fall off rapidly with increase in frequency throughout the anticipated gravity passband, in the manner of a red spectrum rather than a white (which remains constant). It is shown that this implies a much greater allowable error in GPS‐derived altitude reference than would be the case if the PSD of altitude errors (1) was white, (2) had the same shape as that of typical aircraft vertical motion, or (3) was dominated by a sinusoidal wave located near the high frequency limit of the gravity passband. This enhances the feasibility of airborne gravity for regional scale surveys and perhaps explains why actual measurements have been better than predicted. For example, given a uniform [Formula: see text] distribution of spectral noise power and a speed to grid‐width ratio of 60 per hour, an rms altitude error as large as 12 cm will still allow the computation of acceleration correction with an accuracy of 2 mGal. For the same conditions, the allowable rms altitude error given a white distribution of spectral noise power is 1.5 cm.

Metamorphic amphiboles in the Ironwood Iron-Formation, Gogebic Iron Range, Wisconsin: Implications for potential resource development

2020 ◽  
Vol 105 (8) ◽  
pp. 1259-1269
Author(s):  
Carlin J. Green ◽  
Robert R. Seal ◽  
Nadine M. Piatak ◽  
William F. Cannon ◽  
Ryan J. McAleer ◽  
...  
Keyword(s):  
Objective Assessment ◽  
Regional Scale ◽  
Resource Development ◽  
The United States ◽  
Contact Metamorphism ◽  
Iron Formation ◽  
Detailed Knowledge ◽  
Lower Grade ◽  
Iron Range ◽  
Wide Range

Abstract The Paleoproterozoic Ironwood Iron-Formation, a Superior-type banded iron formation located in the western Gogebic Iron Range in Wisconsin, is one of the largest undeveloped iron ore resources in the United States. Interest in the development of this resource is complicated by potential environmental and health effects related to the presence of amphibole minerals in the Ironwood, a consequence of Mesoproterozoic contact metamorphism. The presence of these amphiboles and their contact metamorphic origin have long been recognized; however, recent interest in this resource has highlighted the lack of detailed knowledge on their distribution, mineral chemistry, and morphology. Optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis were utilized to investigate the origin, distribution, morphology, and chemistry of amphiboles in the Ironwood. Amphibole is present in the western portion of the study area due to regional-scale contact meta-morphism associated with the intrusion of the 1.1 Ga Mellen Intrusive Complex. Locally amphibole is also present, adjacent to diabase and/or gabbro dikes and sills in the lower-grade Ironwood in the eastern portion of the study area. In both localities, amphiboles in the Ironwood most commonly developed in massive and prismatic habits, and locally assumed a fibrous habit. Fibrous amphiboles were recognized locally in the two potential ore zones of the Ironwood but were not observed in the portion likely to be waste rock. Massive and prismatic amphiboles show a wide range of Mg# [molar Mg/(Mg+Fe2+)] values (0.06 to 0.87), whereas Mg# values of fibrous amphiboles are restricted from 0.14 to 0.35. Factors that influenced the compositional variability of amphiboles in the Ironwood may have included temperature of formation, morphology, bulk chemistry of the iron formation, and variations in prograde and retrograde metamorphism. The presence of amphiboles in the Ironwood is a known issue that will need to be factored into any future mine plans. This study provides an objective assessment of the distribution and character of amphiboles in the Ironwood to aid all decision-makers in any future resource development scenarios.

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