High-resolution unmanned aerial vehicle aeromagnetic surveys for mineral exploration targets

Recent advancements in geophysical exploration have been realized through reliably integrating unmanned aerial vehicle platforms with lightweight, high-resolution magnetometer payloads. Unmanned aerial vehicle aeromagnetic surveys can provide a contemporary data product between the two end-members of coverage and resolution attained using manned airborne and terrestrial magnetic surveys. This new data product is achievable because unmanned aerial vehicle platforms can safely traverse with magnetometer payloads at flight elevations closer to ground targets than manned airborne surveys, while also delivering an increased coverage rate compared to walking conventional terrestrial surveys. This is a promising new development for geophysical and mineral exploration applications, especially in variable terrains. A three-dimensional unmanned aerial vehicle aeromagnetic survey was conducted within the Shebandowan Greenstone Belt, northwest of Thunder Bay, Ontario, Canada, in July 2017. A series of two-dimensional grids (∼500 m × 700 m) were flown at approximate elevations of 35, 45 and 70 m above ground level using a Dà-Jiāng Innovations multi-rotor unmanned aerial vehicle (S900) and a GEM Systems, Inc., Potassium Vapour Magnetometer (GSMP-35U). In total, over 48 line-km of unmanned aerial vehicle aeromagnetic data were flown with a line spacing of 25 m. The collected aeromagnetic data were compared to a regional heliborne aeromagnetic survey flown at an elevation of approximately 85 m above the terrain, with a line spacing of 100 m, as well as a follow-up terrestrial magnetic survey. The first vertical derivative of the gathered unmanned aerial vehicle total magnetic field data was calculated both directly between each of the different flight elevations, and indirectly by calculating the values predicted using upward continuation. This case study demonstrates that low flight elevation unmanned aerial vehicle aeromagnetic surveys can reliably collect industry standard total magnetic field measurements at an increased resolution when compared to manned airborne magnetic surveys. The enhanced interpretation potential provided by this approach also aided in delineating structural controls and hydrothermal fluid migration pathways (a pair of adjacent shear zones) related to gold mineralization on site. These structural features were not clearly resolved in the regional manned airborne magnetic data alone, further demonstrating the utility of applying high-resolution unmanned aerial vehicle aeromagnetic surveys to mineral exploration applications. The conclusions and interpretations drawn from the unmanned aerial vehicle aeromagnetic data, coupled with historical data, were applied to make a new gold mineralization discovery on the site, assayed at 15.7 g/t.     

Aeromagnetic surveying using a simulated unmanned aircraft system

Carleton University and Sander Geophysics are developing an unmanned aircraft system (UAS) for aeromagnetic surveying. As an early indication of the expected performance of the unmanned aircraft system, a simulated unmanned aircraft system (sUAS) was built. The simulated unmanned aircraft system is a T‐shaped structure configured as a horizontal gradiometer with two cesium magnetometers spaced 4.67 m apart, which is the same sensor geometry as planned for the unmanned aircraft system. The simulated unmanned aircraft system is flown suspended beneath a helicopter. An 8.5 km² area in the Central Metasedimentary Belt of the Grenville Province, near Plevna, Ontario, Canada, was surveyed with the simulated unmanned aircraft system suspended 50 m above ground. The survey site was chosen on the basis of its complex geological structure. The total magnetic intensity (TMI) data recorded were compared to that obtained during a conventional fixed‐wing survey and a ground survey. Transverse magneto‐gradiometric data were also recorded by the simulated unmanned aircraft system. The simulated unmanned aircraft system total magnetic intensity data have a higher resolution than the conventional fixed‐wing data and were found to have a similar resolution to that of the ground survey data. The advantages of surveying with the simulated unmanned aircraft system were: (1) the acquisition of a detailed data set free of gaps in coverage at a low altitude above the terrain and (2) substantial saving of time and effort. In the survey site, the 4.67 m simulated unmanned aircraft system gradiometer measured the transverse magnetic gradient reliably up to an altitude of 150 m above ground.     

Impact of three-dimensional attitude variations of an unmanned aerial vehicle magnetometry system on magnetic data quality

Optically pumped vapour magnetometers have an orientation dependency in measuring the scalar component of the ambient magnetic field which leads to challenges for integration with mobile platforms. Quantifying the three-dimensional attitude variations (yaw, pitch and roll) of an optically pumped vapour magnetometer, while in-flight and suspended underneath a rotary unmanned aerial vehicle, aids in the successful development of reliable, high-resolution unmanned aerial vehicle magnetometry surveys. This study investigates the in-flight three-dimensional attitude characteristics of a GEM Systems Inc. GSMP-35U potassium vapour magnetometer suspended 3 m underneath a Dà-Jiāng Innovations S900 multi-rotor unmanned aerial vehicle. A series of unmanned aerial vehicle-borne attitude surveys quantified the three-dimensional attitude variations that a simulated magnetometer payload experienced while freely (or semi-rigidly) suspended underneath the unmanned aerial vehicle in fair weather. Analysis of the compiled yaw, pitch and roll data resulted in the design of a specialized semi-rigid magnetometer mount, implemented to limit magnetometer rotation about the yaw axis. A subsequent unmanned aerial vehicle-borne magnetic survey applying this specialized mount resulted in more than 99% of gathered GSMP-35U magnetic data being within industry standards. Overall, this study validates that maintaining magnetometer attitude variations within quantified limits (±5° yaw, ±10° pitch and roll) during flight can yield reliable, continuous and high-resolution unmanned aerial vehicle-borne magnetic measurements.     

A geophysical survey on the archaeological site of Mugardos (NW Iberian peninsula)

Archaeological indications near Mugardos (Ferrol, NW Spain) suggest the existence of a Roman settlement. In fact, in the area were found pavements, walls with north-south and east-west orientations and some structures that endured heating. These remains are covered by soil, more than 1 m thick, and lie over schists. In order to determine the archaeological potential of the area and to delimit future excavations a geophysical survey, consisting of a joint resistivity and magnetic survey, was planned and carried out. The square array of electrodes was used and the data are discussed as apparent square array resistivity maps and azimuthal inhomogeneity ratio (AIR) maps. The magnetic survey included total field measurements using sensor heights of 0.30 and 2.30 m above the ground, so that a magnetic gradient could be computed.A combined interpretation of both resistivity and magnetic data is discussed. Later excavations have confirmed the geophysical interpretation.     

特殊区域旋翼无人机航磁测量研究

为解决地面人员难以到达的特殊区域大比例尺磁测数据获取问题,本文提出一种基于三轴磁通门传感器,采用旋翼无人机进行航空磁测的方法.项目组研制适合旋翼无人机搭载的航磁数据记录仪,并完成旋翼无人机、三轴磁通门传感器、航磁数据记录仪的集成工作,开展了三轴磁通门传感器安装点位噪声水平试验、地面空中对比试验、磁性体识别试验、动态飞行一致性试验、区域飞行试验.旋翼无人机航磁系统具有操作简单、设备轻便、效率高的特点,能在地面人员难以达到的区域开展低空航磁测量.中国四川省黑竹沟地区是地磁异常区域,项目组在该区域内的沟壑、峡谷、湖泊等实地开展了野外应用测试,完成了这些特殊区域的航磁调查工作.     

THE DETECTION OF BURIED PLACER DEPOSITS BY GROUND MAGNETIC SURVEY1

A method is proposed to detect buried magnetite concentrations within river sediments. Model studies show that relatively small (thickness 1 m, width 10 m) plate-shaped volumes with magnetite contents of 0.5% within a non-magnetic matrix can be detected by a detailed ground magnetic survey using a sensitive (0.1 nT) magnetometer. Tests along lines perpendicular to the river but at a different angle to the bedrock trend (or the aeromagnetic anomaly trend) reveal that essentially all total field and vertical’gradient’anomalies detected in the profiles are elongated in the river and/or valley direction. This shows that the anomaly sources are (buried) volumes of higher magnetite content within the river sediments. Further evidence for this is (1) that higher intensity total field and vertical’gradient’anomalies occur in an area where the river cuts through strongly magnetic serpentinites, (2) the absence of high frequency anomalies over unsorted glacial till, and (3) some features on power spectra. Relatively large volumes of high magnetite concentrations within the river sediments may well contain the highest concentration of heavy economic minerals such as gold. Consequently, the areas of greatest interest in the exploitation of buried placers may be selected from maps obtained by detailed magnetic surveys either on the ground or at low altitude by helicopter.     

新疆西天山东段航磁异常特征及找矿效果

介绍了高精度航磁调查在西天山东段的应用效果.在高精度航磁图上,展现出多个不同磁场背景及磁异常特征区,它们是不同构造、岩浆活动、地层及岩性分布的综合反映.该区不仅区域磁场特征明显,而且局部异常信息也极为丰富.在最新的高精度航磁图上,已知铁矿异常特征明显,为矿区外围勘探和同类地区寻找新的铁矿提供了依据.利用高精度航磁资料,新发现了五十余处铁矿异常,在2007年进行了地面查证的5处异常中,有4处见矿.这些新的铁矿异常的成功发现不仅带来了巨大的经济效益,也为今后利用高精度航磁测量在类似地区进行找矿积累了经验.     

岷县—漳县6.6级地震极震区场地覆盖层厚度探测及三维地形模型建立

2013年7月22日岷县漳县MS6.6地震的震后科学考察发现,极震区局部场地覆盖层厚度和地形条件对地震动有明显放大效应。利用伺服速度地震仪对此次地震极震区场地进行高密度地脉动观测,获得该区不同场地的卓越频率,运用频谱分析法得到该区场地覆盖层厚度分布。基于在该区进行的无人机航飞遥测,获取地形地貌数据,通过制作表层和基岩层DEM数据,由Rhino进一步处理生成地形模型,实现真实地形在通用有限元软件中的三维模型的建立。     

Geomagnetic field mapping from a satellite: Spatial power spectra of the geomagnetic field at various satellite altitudes relative to natural noise sources and instrument noise

For a low-level geomagnetic satellite survey, for which the motion of the satellite converts spatial variation into temporal variation, the limit on accuracy may well be background temporal fluctuations. The sources of the temporal fluctuations are current systems external to the Earth and include currents induced in the Earth due to these sources. The internal sources consist primarily of two components, the main geomagnetic field with sources in the Earth's core and a crustal geomagnetic field.Power spectra of the vertical geomagnetic field internal component that would be observed by a spacecraft in circular orbit at various altitudes, due to satellite motion through the spatially varying geomagnetic field, are compared to power spectra of the natural temporal fluctuations of the geomagnetic field vertical component (natural noise) and to the power spectrum for typical fluxgate magnetometer instrument noise. The natural noise is shown to be greater than this typical instrument noise over the entire frequency range for which useful measurements of the geomagnetic field may be made, for all geomagnetic latitudes and all times. Thus there would be little benefit in reducing the instrument noise below the typical value of 10^?4 gamma² Hz^?1 plus a 1/f component of 10 milligamma rms decade^?1.For a given satellite altitude, there is a maximum frequency above which the natural noise is greater than the power spectrum of the crustal geomagnetic field vertical component. Below this maximum frequency, the situation is reversed. This maximum frequency depends on geomagnetic latitude (and to a lesser extent on time of day and season of year), being lower in the auroral zone than at lower latitudes. The maximum frequency is also lower at higher satellite altitudes. The maximum frequency determines the spatial resolution obtainable on a magnetic field map. The spatial resolution (for impulses) obtainable at low latitudes for a 100-km satellite altitude (possibly achievable by tethering a small satellite at this altitude to a space vehicle at a higher altitude) is 60 km, while at the auroral zone the obtainable spatial resolution is 100 km. At the higher satellite altitude of 300 km the obtainable spatial resolution is 230 km at low latitudes and 530 km at the auroral zone. At 500-km satellite altitude, the obtainable spatial resolution is 500 km at low latitudes, while maps cannot be made at all for the auroral zone unless the data are selected for “quiet” days.For the lower satellite altitudes, greater spatial resolution can be obtained than at higher altitudes. Furthermore since the crustal geomagnetic field power spectrum is larger at lower altitudes, the relative error due to the natural noise is less than for higher altitudes.     

基于无人机影像的九寨沟地震建筑物震害定量评估

利用2017年8月8日九寨沟7.0级地震震后获取的无人机影像,结合地面震害调查资料,分析各类建筑物震害特征,建立建筑物震害无人机遥感解译标志;选取地震灾区漳扎镇(部分区域)和荷叶寨2个区域作为研究区,进行了无人机遥感建筑物震害提取,基于遥感震害指数进行了震害定量评估,并与现场建筑物震害调查统计结果进行了比较验证。结果显示,遥感解译建筑物震害与实际震害程度相吻合,表明利用震后快速获取的高分辨率无人机影像,可以较为准确地识别建筑物震害,进而为地震灾害定量评估和应急救援辅助决策提供重要参考。     

Research Note: First results of a low‐altitude unmanned aircraft system gamma survey by comparison with the terrestrial and aerial gamma survey data

In order to increase the efficiency of geological survey in difficult landscape‐morphological conditions, the authors have developed low‐altitude gamma technology surveys based on a multi‐rotor unmanned aircraft system, also known informally as Drones. These results have been compared against those from standard terrestrial and aerial gamma surveys. The successful results obtained at the reference site show that the developed methodology enables compilation of high‐quality data in difficult landscape conditions. These results yield the same level of information content as standard terrestrial gamma surveys. The exact unmanned aircraft system altitude over the earth must be maintained for carrying out high‐quality surveys. It was found that a CsI(Tl) detector with sizes 80 × 80 mm (vol. 0.4 dm³) enables obtaining of high‐quality radiometric data at altitudes of up to 70 m, at a speed of at least 20 km/h. The authors believe that low altitude ‘quasi‐terrestrial’ gamma surveys with a terrain drape may in the future replace terrestrial gamma surveys since they can offer better effectiveness at a lower cost.     

Crustal Structure of Turkey from Aeromagnetic, Gravity and Deep Seismic Reflection Data

In this paper, aeromagnetic and gravity anomalies obtained from the General Directorate of Mineral Research and Exploration were subjected to upward continuation to 3?km from the ground surface to suppress shallow effects and to expose only regional, deep sources. Then, a reduction to pole (RTP) map of aeromagnetic anomalies was produced from the 3?km upward continued data. A sinuous boundary to the south of Turkey is observed in the RTP map that may indicate the suture zone between the Anatolides and African/Arabian Plates in the closure time of the Tethys Ocean. The sinuous boundary can be correlated with the recent palaeo-tectonic maps. The southern part of the sinuous boundary is quite different and less magnetic in comparison with the northern block. In addition, maxspots maps of the aeromagnetic and gravity anomalies were produced to find out and enhance the boundaries of tectonic units. Crustal thickness, recently calculated and mapped for the western Turkey, is also extended to the whole of Turkey, and the crustal thicknesses are correlated with the previous seismological findings and deep seismic sections. The average crustal thickness calculations using the gravity data are about 28?km along the coastal regions and increase up to 42?km through the Iranian border in the east of Turkey. Density and susceptibility values used as parameters for construction of two-dimensional (2D) gravity and magnetic models were compiled in a table from different localities of Turkey. 2D models indicate that all of the anomalous masses are located in the upper crust, and this could be well correlated with the earthquakes which occurred at shallow depths.     

Aeromagnetic data and geological structure of continental China: A review

We review the latest aeromagnetic geological data of continental China. We discuss the latest achievements in geological mapping and the newly detected features based on aeromagnetic data. Using aeromagnetic data collected for more than 50 years, a series of 1:5000000 and 1:1000000 aeromagnetic maps of continental China were compiled using state-of-the-art digital technology, and data processing and transformation. Guided by plate tectonics and continental dynamics, rock physical properties, and magnetic anomalies, we compiled maps of the depth of the magnetic basement of continental China and the major geotectonic units, and presented newly detected geological structures based on the aeromagnetic data.     

The application of rapid potential field methods for the targeting of IOCG mineralisation based on physical property data,Great Bear magmatic zone,Canada

New techniques for the integration of gravity and aeromagnetic data through rock physical property relationships are tested for the rapid location of potential iron oxide–copper–gold (IOCG) exploration targets across the Great Bear magmatic zone (GBMZ). These techniques are based on the recognition of coincident or near-offset magnetic and gravity anomalies associated with IOCG deposits, resulting from their intense and combined iron-oxide and alkali alteration. In the GBMZ, the NICO (Au–Co–Bi–Cu) deposit is an atypical magnetite-group IOCG hosted within an intensely hydrothermally-altered metasedimentary sequence. Prospectivity maps for the NICO area are derived from the integration of high-resolution aeromagnetic and surface gravity data with physical property measurements of magnetic susceptibility and density. Method 1 combines the pseudo-gravity (derived from the aeromagnetic data) and Bouguer gravity data to determine the locations of superimposed high anomalies. Method 2 estimates the apparent susceptibility and density subsurface distribution based on vertical prism models of the magnetic and gravity data. The apparent susceptibility data are transformed to apparent density using physical property relationships. The two apparent density datasets are then combined to determine the location of coincident magnetic and gravity derived anomalies. The results of these methods are supported by, and compared with, similar prospectivity maps generated through weights of evidence techniques. At NICO, the prospectivity maps accurately locate the deposit and nearby occurrences. Despite the lower resolution of the regional gravity data, application of the techniques across the GBMZ is successful at locating mineralised IOCG systems including the magnetite-group systems at Grouard, Fab, DeVries and Cole Lake, the magnetite to hematite-group systems of the Port Radium-Echo Bay district, and has highlighted possible targets for future exploration.     

中国航磁异常管理信息系统开发与建设

航磁异常管理信息系统是采用GIS可视化技术及数据库技术在MAPGIS软件系统平台上建立的信息管理系统，它能够完成航磁异常的可视化检索、统计、属性编辑和图形显示，并实现检索结构数据的自动成图。首次建立的航磁异常信息数据库整理录入完成中国国土资源航空物探遥感中心资料室保存的20世纪80年代以前全国大部分少、市、自治区的1万多份航磁查证异常登记表和检查卡片的相关信息：航磁异常属性记录约11350条、地检异常属性记录约12870条、见矿异常属性记录约1120条，扫描及矢量化地检异常图件约7270份。本系统通过建立异常属性数据库与航磁地检异常矢量图文件、全国省级航磁数据库文件、全国航磁测量工作程度图及全国1：50万数字地理底图数据库之间合理的逻辑关系，实现了多种信息基于GIS平台与技术的统一管理和应用。     

新型便携式行业级无人机精灵4RTK定位精度分析

本文针对新型便携式行业级无人机精灵4RTK开展了实测数据的定位精度分析,从有、无控制点情况下的绝对定位精度和无控制点情况下的相对定位精度2方面入手,详细计算后者水平距离和高程差的测量误差,探讨网络RTK技术的无控制点情况在活动构造中的应用。结果表明,无人机精灵4RTK在天气较晴朗、飞行高度100 m、镜头角度正射向下、旁向和航向重叠率均为70%等实测条件下,有控制点情况下水平位置和高程测量误差均<4.5 cm,无控制点情况下水平位置测量误差<0.60 m、高程测量误差<1.90 m;无控制点情况下,当实际水平距离<300 m时,水平距离测量误差<0.100 m,当高程差<2.8 m时,高程差测量误差<0.100 m;以复合运动性质的发震断层为例,初步探讨认为无人机精灵4RTK的网络RTK技术在无控制点情况下提取活动构造的定量参数时,其水平位移量精度能够达到厘米级,垂直位错量精度可能达不到厘米级,当垂直位错量小于8.0 m时,精度能够达到0.157 m。     

Opening of the Gulf of Aden and Afar by progressive tearing

A new detailed aeromagnetic survey of the Republic of Djibouti and surrounding area reveals a wealth of new information which can be correlated with other data, in particular geologic and tectonic maps. Oceanic magnetic anomalies are identified from the Gulf of Aden westward to the Ghoubbet-Asal rift in Afar. Identification of the anomalies, together with a reinterpretation of earlier magnetic profiles, indicates that rifting started earlier in the east and provides clear evidence for the westward propagation of a crack through the lithosphere at an approximate velocity of 3 cm y^?1. The crack tip is now thought to lie somewhere close to lake Asal and should continue its motion further to the northwest. Some first consequences of this non-rigid model of plate opening are discussed and independent support from a number of sources is obtained.     

龙门山和相邻地域航磁场特征与汶川大地震

本文应用化极、水平及垂向导数、向上延拓、视磁化强度填图及磁性界面反演等方法处理了龙门山及相邻地域最新的航空磁测数据,分析了龙门山及相邻地域的航磁异常展布特征.研究结果表明:1)龙门山造山带与其东、西两侧可划分为三个磁异常区:松潘—甘孜磁异常区、龙门山负磁异常带、四川盆地磁异常区;三个区、带的地壳介质磁性结构存在明显差异.2)根据该区航空磁异常场的分布特征分别研究了,松潘—甘孜地块、龙门山造山带和四川地块的磁场特征.3)除识别前人识别的断层外,还推断鲜水河ES延伸甘洛—雷波北断裂作为四川盆地与滇西的界带.4)航空磁异常,磁性体上、下界面及磁源体深度的空间分布特征与汶川MS8.0大地震及芦山地震发生相关.     

THE APPLICATION OF MINIATURE UNMANNED AERIAL VEHICLE IN 25 NOVEMBER 2016 ARKETAO MW6.6 EARTHQUAKE

In order to complete the field investigation to the 25 November 2016 Arketao M_W6.6 earthquake, ultra-low altitude remote-sensing data were obtained from miniature unmanned aerial vehicle. The surface rupture surveying has important significance for earthquake research. This paper selects the macro-epicenter of Arketao as the study area. The pictures were obtained with DJI Phantom 3 professional input into the software, the Digital Elevation Model (DEM), Digital Orthophoto Map (DOM) were acquired based on photogrammetry method using the overlapped optical remote-sensing images of UAV. Using these data, we can identify surface ruptures that have vertical dislocation. We selected six feature points and drew the elevation profile. In the elevation profile map, we chose smooth part of the surface rupture sides and obtained the trend line. A stable point in the surface rupture was selected and the abscissa of the point was taken into the equation of two straight lines. Then subtracting the results of the two equations, we can get the vertical dislocation of the surface rupture. On this basis, we chose six feature points and determined their vertical dislocation, which are between 4.4cm and 10.4cm. What's more, taking Bulungkou Xiang in Xinjiang Uygur Autonomous Region for example, we speculated some surface ruptures that have vertical dislocation. It can provide a new method for identifying surface rupture in the field. In addition, we get DEM data of the Bulunkou area where ambient conditions are very poor, by using miniature unmanned aerial vehicle and taking 255 photos. Putting those photos into the EasyUAV software, we got the area digital elevation of 2cm resolution. Comparing these data with RTK data, we summarized some practical problems and solutions in the practical operation and evaluated the accuracy of miniature unmanned aerial vehicle data. The Pearson Correlation Coefficient is 0.996 6. In terms of absolute elevation, the average result of UAV and RTK differs by 156.96m. In terms of relative elevation, the average result of UAV and RTK differs by 9.74m. Compared with the previous test of Pishan County, there is a notable divergence in the results. It shows that the data accuracy will be affected to some extent in the cold weather in high elevations. The specific impact needs further exploration.