Terrain Clearance Research Articles

Objective classification of high‐resolution geophysical data: Empowering the next generation of mineral exploration in Sierra Leone

AbstractIn 2019, a nationwide airborne geophysical survey of Sierra Leone was flown at 150 m nominal line spacing and 50 m nominal terrain clearance. Contractual deliverables included magnetic, radiometric and supporting data streams. The primary aim of the geophysical survey was to provide a national geoscientific benchmark for resource management by the National Minerals Agency and to encourage foreign investment in the rich mining potential of Sierra Leone. Geophysical interpretation is often highly subjective. This can be helpful when an interpreter is skilled in the project area and exploration targets being sought but can be constraining and even dangerous when exploring new areas, and for new commodities. Objective classification tools can challenge interpretive bias and enhance geological understanding, both locally and regionally. We present self‐organizing maps and neural network products generated from the Sierra Leone geophysical dataset which both confirm and challenge the validity of conventional, subjective interpretation workflows. We demonstrate the usefulness of self‐organizing maps as a tool for inferring complementary geological information and show some initial results of convolution neural network classification of a reduced‐to‐equator grid to locate magnetic lineaments in plan and in depth. Moreover, we show initial results of self‐organizing map clustering as a tool for joint analysis of multiphysics datasets and construction of a pseudolithological cluster map.

Interpretation of Aeromagnetic Data of Part of Gwagwalada Abuja Nigeria for Potential Mineral Targets

This study analyzes aeromagnetic data over a section of Gwagwalada in Abuja. The data were obtained from the Nigerian Geological Survey Agency acquired at 100 m terrain clearance. The study area spans longitudes 7.0875 E to 7.1458 E and latitude 8.9625 N to 9.0 N (about 27 km2). The dataset was reduced to the equator (RTE) and downward continued by 50 m. Analytic signal filter was applied on TMI-RTE grid to detect the edges of the magnetic bodies present. The structure was observed to trend NE-SW. The CET lineament map reveals intersections such as junctions and corners on the map. This revealed structure liable for potential mineralization zone. Euler deconvolution technique applied over the transformed dataset ascertain the location and depth of the structure,having a maximum depth of about 421 m and a minimum of about 59 m.Variation in magnetic depth and susceptibility contrast is specified by the gridded SPI depth map.

Structural Exploration of Aeromagnetic Data over Part of Gwagwalada, Abuja for Potential Mineral Targets Using Derivatives Filters

Aeromagnetic data are consistently used for economic interest targeting and geological mapping. Besides solving problems that are concerned with the basement, the method has become a useful tool in exploring minerals,hydrocarbons occurrence,groundwater investigations, and geothermal potentials. This study analyses aeromagnetic data from the Nigerian Geological Survey Agency acquired at 100 m terrain clearance over a section of Gwagwalada in Abuja. The study area spans longitudes 7.0875° E to 7.1458° E and latitude 8.9625° N to 9.0° N (about 27 km2 ). After a reduction to the equator (RTE) transformation, the data is downward continued by 50 m. Different filters are applied to outline area of alteration associated to mineral deposit. Regional geologic structures trend NE - SW.The application of vertical derivatives (FVD and SVD) to the RTE grid enhanced shallow structures which trend NE - SW. Horizontal gradients along the X- and Y- directions enhance geological contacts attributable to blind faults. The Tilt derivative (TD) accentuated fault lines which trend NE - SW.

2-dimensional modeling of high-resolution aeromagnetic data over Yola arm of the upper Benue trough, Northeastern Nigeria

2-D modeling of high-resolution aeromagnetic data over the area investigated was carried out with the aid of Oasis MontajTM software in order to get the total magnetic intensity map as well as the residual map from where the modeling was carried out. The data was acquired for Nigeria Geological Survey Agency (NGSA) in 2010 by Fugro international of Netherlands. The specifications of the data were terrain clearance of 80 m, flight line spacing of 500 m and a tie line spacing of 5000 m. The area investigated in this work is located between longitude 120 00' - 130 30' E and Latitude 80 00' – 100 00'N. GM-SYS module of the Oasis montaj was used for this research, with depocentres, structural patterns, basement geometry and morphology as well as estimates of the physical character of the cause to an investigated anomaly determined. The models reveal the horst and graben structures of the basement and the various faults that segmented the area into block patterns. The results revealed sediment thickness in the range of 2-5 km for the models which are in agreement with the results obtained by various researchers in the area. Horst and graben were identified in the models which confirmed the rifting nature of the Benue Trough. The authors of this research are of the view that these depocentres, structural patterns and basement geometry may be a promising site for hydrocarbon traps and reservoirs.
 Keywords: Aeromagnetic Data, GM-SYS, Modeling, Horst, Graben.

ОСОБЛИВОСТІ КОНСТРУКЦІЇ ТА ЗНЕШКОДЖЕННЯ ВИБУХОВИХ ПРИСТРОЇВ

The article provides a general analysis of explosives, discloses the concepts of an explosion and its characteristics, gives a description of explosives. Features of the design and destruction of modern explosive devices during mine clearance and the organization of these works are disclosed. The necessity of observing international standards in matters of mine action, which after its ratification became part of Ukrainian national legislation, is examined. In accordance with international standards and the experience of other countries, the main responsibility lies with the National Mine Action Authority The factors that complicate the clearing of the terrain and the destruction of explosive objects are disclosed. Limitations are formulated that are imposed on the ways of clearing settlements located near historical and cultural monuments (churches, cemeteries, etc.) and industrial objects that pose a great danger when they are destroyed, a list of improvised explosive objects is proposed, their unmasking factors are disclosed when they are detected using special equipment and specially trained dogs, in open areas, in rooms and vehicles. Separately investigated issues of localization and destruction of improvised explosive devices. Keywords: explosive objects, explosive, basic properties, terrain clearance, improvised explosive device, explosion classification, international standards.

Method to Create Perceptually Uniform Color Gradients with Optimized Color Differences between each Step for Data Visualization

This paper presents an original method to create optimized color differences between each step of a CIELAB perceptually uniform color gradient. The relevant equations for the necessary color space transformations are discussed, along with the procedure to determine and optimize the color differences between each gradient step. A specific implementation for perceptually uniform color gradients is discussed for safe terrain clearance in manned aviation during a degraded visual environment.

Detection of Sulphide Deposit Using Uranium/Potassium Ratio Map

Potassium (K) response is associated with minerals that can be easily weathered, whereas uranium (U) is usually related to residual clay, oxides and accessory minerals. With a view to interpret for the detection of sulphide deposit based on similar responses exhibited by the various rock units, radiometric data set was interpreted. The study area is located within Latitude +8000’00’’to +8030’00’’ and Longitude +5030’00’’E to +6000’00’’E. The data was collected in most parts of the Nigeria by Fugro Airborne Services Ltd. Johannesburg in collaboration with the Nigerian Geological Survey agency using a light aircraft that was flown in NW-SE direction with a nominal terrain clearance of 80 m and at a speed of 70-80 m/s, with a flight line spacing of 500 m apart. The acquired data was subjected to elemental concentration enhancement using Geosoft Oasis Montaj Software. From the interpreted data, the red coloured portions indicate high uranium and low potassium formations (H-U/K), while the blue coloured portions indicate low uranium and high potassium formations (L-U/K). The regions L-U/K have recorded low U/K ratios, meaning that L-U/K have very low uranium count rates as compared to the count rates of potassium in these regions; this signifies potassium abundance. This also suggests igneous activities that usually culminate to the emplacement of the meta-volcanic lithologies. H-U/K and M-U/K (Moderate U/K formations) are seen to have patches of high U/K ratios within the central portions. This high U/K ratio was therefore, as a result of sulphide deposition.

Square-wave processing of MEGATEM data

The recording of raw or streamed EM survey data, as done by CGG during MEGATEM surveys, allows for the reprocessing of the acquired EM data, including square-wave processing. During the latter, the recorded EM response to the actual half-sine waveform is replaced by the EM response to a square-wave, derived via deconvolution/convolution in the frequency domain. This makes the on- and early-time information more accessible for data modelling, including 1D inversions and conductivity-depth transformations. Square-wave EM data can also be corrected for survey height, transmitter-receiver offset and transmitter attitude. That correction allows for the interpretation of early-time EM response grids, which generally offer better spatial resolution than derived conductivity-depth slices. The advantages of square-wave processing are demonstrated on a MEGATEM data set acquired in 2013 in South America. With survey terrain clearance ranging from 100 – 1600 m, due to the rugged topography, early-time grids of elevation-corrected square-wave data outlined the shallow conductivity structure, whereas early-time grids of the original half-sine data mostly reflected the variable system elevation. Further, derived conductivity-depth sections of the square-wave data show more lateral continuity than the sections derived from the original half-sine data. These results show that the early-time information of square-wave is more accessible than in the original data, facilitating interpretation of shallow conductivity structures.

Rates and causes of accidents for general aviation aircraft operating in a mountainous and high elevation terrain environment

BackgroundFlying over mountainous and/or high elevation terrain is challenging due to rapidly changeable visibility, gusty/rotor winds and downdrafts and the necessity of terrain avoidance. Herein, general aviation accident rates and mishap cause/factors were determined (2001–2014) for a geographical region characterized by such terrain. MethodsAccidents in single piston engine-powered aircraft for states west of the US continental divide characterized by mountainous terrain and/or high elevation (MEHET) were identified from the NTSB database. MEHET-related-mishaps were defined as satisfying any one, or more, criteria (controlled flight into terrain/obstacles (CFIT), downdrafts, mountain obscuration, wind-shear, gusting winds, whiteout, instrument meteorological conditions; density altitude, dust-devil) cited as factors/causal in the NTSB report. Statistics employed Poisson distribution and contingency tables. ResultsAlthough the MEHET-related accident rate declined (p<0.001) 57% across the study period, the high proportion of fatal accidents showed little (40–43%) diminution (χ2=0.935). CFIT and wind gusts/shear were the most frequent accident cause/factor categories. For CFIT accidents, half occurred in degraded visibility with only 9% operating under instrument flight rules (IFR) and the majority (85%) involving non-turbo-charged engine-powered aircraft. For wind-gust/shear-related accidents, 44% occurred with a cross-wind exceeding the maximum demonstrated aircraft component. Accidents which should have been survivable but which nevertheless resulted in a fatal outcome were characterized by poor accessibility (60%) and shoulder harness under-utilization (41%). ConclusionDespite a declining MEHET-related accident rate, these mishaps still carry an elevated risk of a fatal outcome. Airmen should be encouraged to operate in this environment utilizing turbo-charged-powered airplanes and flying under IFR to assure terrain clearance.

Identifying potential mineralisation targets through airborne geophysics - The Western Papua New Guinea Case study

Over the last few years exploration activity in Papua New Guinea (PNG) has declined to alarming lows that the Mineral Resources Authority (MRA) has introduced initiatives to acquire new geo-scientific datasets to enhance exploration in the country. One such initiative has been the 30,000 plus line kilometre airborne magnetic and radiometric survey over the western part of the country. The airborne survey was strategically located between the large Ok Tedi porphyry copper-gold mine and the advanced Frieda River porphyry copper-molybdenum prospect. The airborne datasets were acquired at a north-south line spacing of 500m with a Tie-line spacing of 5km and at nominal terrain clearance of 100m. The main aim of the airborne geophysical survey was to identify magnetic and radiometric anomalies that may be further investigated for their mineral potential. Preliminary results of the airborne survey show large deep-seated NW-trending fault systems cross-cut by north-east trending transfer structures that may have contributed to mineral deposition in the region. Several magnetic and radiometric anomalies located within the periphery of existing prospects also have the potential to substantially increase resource values, while isolated anomalies may indicate new targets for further exploration. While this paper highlights some of the interesting magnetic and radiometric features defined from the WPAGS, the release of the datasets now ensures investors have an extra layer of information to build on the mineral potential of the surveyed area. It is anticipated that the results from the survey will not only boost interest to explore in the area, but also drive exploration activities in the area to new heights, hopefully resulting in the identification of new mineral prospects.

Application of the airborne electromagnetic method for Banded Iron-Formation mapping in the Hamersley Province, Western Australia

Stratigraphy of the Hamersley Province in Western Australia, featuring alternating units of banded iron-formations and shales with contrasting electrical properties in a mostly gently undulating shallow dipping layered geometry, is particularly favourable to airborne time-domain electromagnetic mapping techniques. Manipulated vertical cross-sections of modelled conductivity obtained from laterally constrained 1D inversion of SkyTEM304 data enable the exploration geologist to interpret weathering profiles, shallow dipping stratigraphy and steep structures, all of which are crucial aspects of bedded iron ore deposits genesis models. Five potential interpretation pitfalls have nevertheless been encountered. Occasional obvious artefacts can be present in inverted models but their causative sources should be easily identifiable in the measured channel data. The inverted models coherence is generally compromised when acquisition system terrain clearance cannot be maintained below approximately 80 m. The in-loop setup and processing routines have inherent limitations over steep stratigraphy. Complex stratigraphy geometries can result in off line responses to be artificially incorporated in the 1D inverted conductivity cross-sections. Lastly, lateral interfaces between fresh and altered rocks could easily be misinterpreted as fault contacts.

An Exploratory Evaluation of General Aviation Pilot Performance and Preferences Using Synthetic Vision Displays for Approaches and Missed Approaches in Flat and Challenging Terrain

Eight instrument-rated General Aviation pilots completed CAT I SA approaches to Oklahoma City (KOKC) and localizer approaches to Aspen (KASE), the latter with missed approaches. Approaches to KOKC were conducted in two levels of runway visual range (RVR) with two decision heights (DH) using a head-up or head-down display with Synthetic Vision (SV) imagery. The lower DH led to significantly more completed approaches while the higher DH regularly caused pilots to execute missed approaches. RVR was not a significant factor for those values examined. Although SV was expected to be more beneficial for missed approaches in mountainous terrain (KASE) than for initial approaches, terrain clearance achieved was not influenced by display format. Cross-track and glide-slope root-mean-square error for the approaches to KOKC did not differ reliably between display conditions. Pilots uniformly believed they could successfully and comfortably complete approaches in lower visibilities if SV equipped as compared with other displays.

Gamma-ray spectrometry of granitic suites of the Paranaguá Terrane, Southern Brazil

The Paranaguá Terrane, located in the coastal portion of the states of Santa Catarina, Paraná and São Paulo in Southern Brazil is a crustal segment constituted mainly by an igneous complex, with a variety of granitic rocks inserted into the Serra do Mar ridge. The average altitude is approximately 1200m above sea level, with peaks of up to 1800m. Due to the difficulty of accessing the area, a shortage of outcrops and the thick weathering mantle, this terrane is understudied. This research aims to evaluate the gamma-ray spectrometry data of the granitic suites of the Paranaguá Terrane, in correspondence with the geological, petrographical, lithogeochemical, relief and mass movement information available in the literature. Aerogeophysical data were acquired along north–south lines spaced at 500m, with a mean terrain clearance of 100m. These data cover potassium (K, %), equivalent in thorium (eTh, ppm) and equivalent in uranium (eU, ppm). After performing a critical analysis of the data, basic (K, eU, eTh) and ternary (R–K/G–eTh/B–eU) maps were generated and then superimposed on the digital elevation model (DEM). The investigation of the radionuclide mobility across the relief and weathering mantle consisted of an analysis of the schematic profiles of elevation related with each radionuclide; a comparison of the K, eU and eTh maps with their 3D correspondents; and the study of mass movements registered in the region. A statistical comparison of lithogeochemical (K, U, Th) and geophysical (K, eU, eTh) data showed consistency in all the granitic suites studied (Morro Inglês, Rio do Poço and Canavieiras–Estrela). Through gamma-ray spectrometry, it was possible to establish relationships between scars (from mass movements) and the gamma-ray responses as well as the radionuclide mobility and the relief and to map the granitic bodies.

A unified trajectory optimization framework for lunar ascent

This paper presents a unified trajectory optimization framework for lunar ascent. Compared with prevailing and emerging studies on lunar ascent trajectory optimization with simplified lunar ascent process and simple constraints, our method formulates in detail the lunar ascent process with complex constraints in a unified manner. The kinematics and dynamics model of lunar ascent with mission-specific constraints explicitly expressed through equalities or inequalities form the fuel-optimal lunar ascent trajectory optimization problem. A proper direct trajectory optimization method is chosen to transcribe the original trajectory optimization problem into a nonlinear programming (NLP) problem solved by a highly efficient NLP solver. The homotopy-based backtracking initial value strategy is designed to enhance convergence of the solving process. First, a two-phase trajectory optimization problem including vertical-rise phase and orbit-insertion phase is solved in the proposed unified framework. Subsequently, to obtain terrain clearance, we directly incorporate terrain description into the lunar ascent problem to obtain the optimal lunar ascent trajectory. Simulation results indicate that the proposed unified trajectory optimization framework has enough adaptability to efficiently handle complex lunar ascent scenarios. The proposed framework may benefit future autonomous lunar ascent missions.

Aeromagnetic survey using an unmanned autonomous helicopter over Tarumae Volcano, northern Japan

Unmanned aerial vehicles (UAVs) have recently received attention in various research fields for their ability to perform measurements, surveillance, and operations in hazardous areas. Our application is volcano surveillance, in which we used an unmanned autonomous helicopter to conduct a dense low-altitude aeromagnetic survey over Tarumae Volcano, northern Japan.In autonomous flight, we demonstrated positioning control with an accuracy of ~10 m, which would be difficult for an ordinary crewed vehicle. In contrast to ground-based magnetic measurement, which is highly susceptible to local anomalies, the field gradient in the air with a terrain clearance of 100 to 300 m was fairly small at 1 nT/m. This result suggests that detection of temporal changes of an order of 10 nT may be feasible through a direct comparison of magnetic data between separate surveys by means of such a system, rather than that obtained by upward continuation to a common reduction surface. We assessed the temporal magnetic changes in the air, assuming the same remagnetising source within the volcano that was recently determined through ground surveys. We conclude that these expected temporal changes would reach a detection level in several years through a future survey in the air with the same autonomous vehicle.

Results from FALCON® Airborne Gravity Gradiometer surveys over the Kauring AGG Test site

The Kauring Test Site in Western Australia was established in 2009 to provide a public benchmarking and comparison venue for new and existing airborne gravity and airborne gravity gradiometry (AGG) technology. Fugro Airborne Surveys flew the fixed-wing FALCON® AGG system over the Kauring AGG Test Site over three periods in July 2011, November 2011 and February 2012. Comparison between the FALCON AGG survey data and the high resolution ground gravity data over the Kauring AGG Test site indicates that the FALCON vertical gravity gradient, GDD, has an error of +/- 5.6 Eo, and that the FALCON vertical gravity, gD, has an error of +/- 0.18 mGal,. Comparison between the digital elevation model (DEM) derived from the fixed wing FALCON survey laser scanner data, and a high-resolution third party DEM, indicates that the error of the vertical position of the FALCON differential GPS is less than 0.5m. At 60m terrain clearance this corresponds to a subsequent error in AGG terrain correction of less than 2 eotvos. This terrain correction error is well within the Kauring AGG Test Site FALCON survey noise envelope of 5.6 Eo.

Unsupervised soft clustering of high resolution airborne geophysical and satellite data suites from the Sperrgebiet, Karas region, Southern Namibia, to enhance lithology mapping

The Karas region is the southernmost region of Namibia extending between the Orange River in the south and latitude 25S in the north and from the South Atlantic Ocean in the west to the eastern border at longitude 20E. The geology of the Karas region is characterized by the pan-African Gariep Fold Belt in the west, the Meso-Proterozoic Namaqua Metamorphic Belt in the central area and south, and the Kalahari Craton in the northeast. The airborne magnetic and radiometric data were acquired with a flight line spacing of 200 m above ground and terrain clearance of 80 m. ASTER and Landsat imagery which completely cover the Karas Region were processed to match for integration with the airborne geophysical data. A novel data integration and classification tool recently developed (e.g. Paasche and Eberle 2011) has been used for automated and objective information extraction. The integrated data, i.e. a large set of multi-element vector samples - each of them carrying magnetic, radiometric, satellite band/hyperspectral and positioning information - has been grouped into a number of classes to provide a pseudo-lithology map. Spatial resolution of the pseudo-lithology map is considerably higher than that of conventional geological maps. High resolution data are thus considered a pre-requisite to identify new mineral exploration targets from this innovative integration technology. A few examples from different geological settings of the Karas region are discussed and compared with conventional mapping products, field trip evidence and geophysical ground truthing. Reference Paasche, H. and Eberle, D. 2011, doi:10/1071/EG11014

Extracting information from ZTEM data with 2D inversions

SummaryA 2D Occam inversion algorithm for modeling ZTEM data is described that takes into account topography and EM receiver terrain clearance. The responses of hills and depressions are shown for synthetic models. These results show that hills and depressions produce responses that could be confused with the responses of conductors and resistors, respectively.The analysis of ZTEM data acquired at different flightline directions at the Forrestania test site, WA, shows great consistency and repeatability of the ZTEM data. 2D inversion results indicate that the derivation of pseudo tipper profiles, perpendicular to the survey flightline direction, from across-line data contain valuable information, especially where the local strike is not perpendicular to the flight-line direction.

Aeromagnetic anomalies over faulted strata

High-resolution aeromagnetic surveys are now an industry standard and they commonly detect anomalies that are attributed to faults within sedimentary basins. However, detailed studies identifying geologic sources of magnetic anomalies in sedimentary environments are rare in the literature. Opportunities to study these sources have come from well-exposed sedimentary basins of the Rio Grande rift in New Mexico and Colorado. High-resolution aeromagnetic data from these areas reveal numerous, curvilinear, low-amplitude (2–15 nT at 100-m terrain clearance) anomalies that consistently correspond to intrasedimentary normal faults (Figure 1). Detailed geophysical and rock-property studies provide evidence for the magnetic sources at several exposures of these faults in the central Rio Grande rift (summarized in Grauch and Hudson, 2007, and Hudson et al., 2008). A key result is that the aeromagnetic anomalies arise from the juxtaposition of magnetically differing strata at the faults as opposed to chemical processes acting at the fault zone. The studies also provide (1) guidelines for understanding and estimating the geophysical parameters controlling aeromagnetic anomalies at faulted strata (Grauch and Hudson), and (2) observations on key geologic factors that are favorable for developing similar sedimentary sources of aeromagnetic anomalies elsewhere (Hudson et al.).

A high-resolution aeromagnetic field test in Friuli: towards developing remote location of buried ferro-metallic bodies

High Resolution AeroMagnetic surveys (HRAM) are a novel tool experimented in several countries for volcano and earthquake hazard re-assessment, ground water exploration and mitigation, hazardous waste site characterization and accurate location of buried ferrous objects (drums, UXO, pipelines). The improvements achieved by HRAM stem from lower terrain clearance coupled with accurately positioned, real-time differential navigation on closely spaced flight grids. In field cultural noise filtering, advanced data processing, imaging and improved interpretation techniques enhance data information content. Development of HRAM approaches might also contribute to mitigate environmental hazards present throughout the Italian territory. Hence an HRAM field test was performed in July 2000 in Friuli, North-Eastern Italy to assess the capabilities and limitations of HRAM over a buried pipeline and a domestic waste site. A Cesium magnetometer in towed bird configuration was used on two separate grids. Profile line spacing was 50-100 m and bird nominal ground clearance was set to 50 m. Microlevelled total field magnetic anomaly data forms the basis for subsequent advanced processing products including 3D analytic signal, maximum horizontal gradient of pseudo-gravity and 3D Euler Deconvolution. The magnetic signatures we detected and enhanced over the environmental test site area in Friuli are also compared with similar but more extensive HRAM signatures recently observed in other countries.