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An Overview of Technologies for Geophysical Vector Magnetic Survey: A Case Study of the Instrumentation and Future Directions

Huan Liu1,2,3, Member, IEEE, Haobin Dong1,2, Jian Ge1,2, and Zheng Liu3, Senior Member, IEEE

1School of Automation, China University of Geosciences, Wuhan Hubei, 430074, China 2Hubei Key Laboratory of Advanced Control and Intelligent Automation for Complex Systems, Wuhan Hubei, 430074, China 3School of Engineering, University of British Columbia Okanagan, Kelowna BC, V1V 1V7, Canada

Magnetic survey techniques have been used in many years in an attempt to better evaluate the likelihood of recoverable hydrocarbon reservoirs by determining the depth and pattern of sedimentary rock formations containing magnetic minerals, such as magnetite. Utilizing airplanes, large area magnetic surveys have been conducted to estimate, for example, the depth to igneous rock and the thickness of sedimentary rock formations. In this case, the vector magnetic survey method can simultaneously obtain the modulus and direction information of the Earth’s magnetic field, which can effectively reduce the multiplicity on data inversion, contribute to the quantitative interpretation of the magnetic body and obtain more precise information and characteristics of magnetic field resource, so as to improve the detection resolution and positioning accuracy of the underground target body. This paper presents a state-of-the-art review of the application situations, the technical features, and the development of the instruments for different application scenarios, i.e., ground, wells, marine, airborne, and satellites, respectively. The potential of multi-survey technique fusion for magnetic field detection is also discussed.

Index Terms—Magnetic survey technique, geophysical exploration, instrumentation, magnetometer, geomagnetic

I.INTRODUCTION non-ferrous metals with weaker magnetic intensities, and it agnetic survey techniques are one of the most effective is an effective method for detecting magnetic ore bodies, M methods for geophysical engineering and environmen- especially, provides a scientific basis for the exploration and tal exploration [1]–[4], e.g., unexploded ordnance (UXO) evaluation of mineral resources in the deep earth [31]. The detection [5]–[7], mineral exploration [8]–[11], etc [12]–[17]. marine magnetic survey always adopts a ship equipped with In recent years, the vector magnetic survey techniques have magnetometers to conduct geomagnetic surveys in the ocean developed rapidly. When compared it with the traditional area. It plays a crucial role in the detection of military targets scalar magnetic survey methods [18]–[21], the vector magnetic such as underwater submarines, unexploded weapons, and survey can simultaneously obtain the modulus and direction magnetic obstacles [32]. The traditional approach is mainly information of the Earth’s magnetic field, which can effectively based on total field measurement. In recent years, the three- reduce the multiplicity on data inversion [22]–[24]. Further, it component or full-tensor magnetic gradient based marine can contribute to the quantitative interpretation of the magnetic magnetic survey technique has been employed to obtain more body and obtain more precise information and characteristics geomagnetic information to provide important parameters for of magnetic field resource, and thus improve the detection the naval battlefield [33]. The airborne magnetic survey is resolution and positioning accuracy of the underground target appropriate for scanning large areas during reconnaissance body [25]–[27]. to delimit target areas for detailed ground surveys during The vector magnetic survey technique can be mainly divided the prospecting stage [34]. In particular, it can be used in into ground magnetic survey, wells magnetic survey, marine coalfield studies to map out a broad structural framework magnetic survey, airborne magnetic survey, and satellites mag- over an exploration area with complex terrain both quickly and effectively [35]. The satellite magnetic survey can obtain arXiv:2007.05198v1 [physics.geo-ph] 10 Jul 2020 netic survey, and each technique has its own characteristics. The ground magnetic survey is mainly used for the detection high-quality, global coverage magnetic survey data, conduct of ore bodies distributed horizontally on the surface [28]. all-weather and uninterrupted measurements, and thus carry However, due to the abnormal superposition of different ge- out a series of scientific researches such as the evolution of the ological bodies on the surface, it is difficult to distinguish geomagnetic field and the law of space current movement [36], the depth. Hence, it is often combined with other magnetic [37]. survey techniques [29]. The wells magnetic survey is based In this paper, according to different application scenarios, on the magnetic characteristics of rock ore, measuring three first, the technical characteristics of the vector magnetic mea- orthogonal components of the geomagnetic field, and the surement compared with the scalar magnetic measurement are radial detection range is large [30]. Further, this technique summarized. Second, we elaborate on the application situa- can be employed to find both strong magnetite deposits and tions, the technical features, and the instruments development of the ground vector magnetic survey technique, the wells Corresponding author: Haobin Dong (email: [email protected]). vector magnetic survey technique, the marine vector magnetic 2 survey technique, the airborne vector magnetic survey tech- space weather [19], [40], [60], [61]. For the particularity of nique, and the satellites vector magnetic survey technique, field observations, the fluxgate magnetometer array system respectively. Finally, the advantages and disadvantages of these in the basic station network realizes the Network functions five vector magnetic survey techniques are summarized, and such as communication access, automatic identification, mon- their application prospects and development directions are itoring & management, and data collection for field mobile discussed. observation equipment. According to the temporal and spa- tial distribution characteristics of the geomagnetic field, each II.GROUNDVECTORMAGNETICSURVEYTECHNIQUE three-component fluxgate magnetometer which is arranged in The magnetic field at any point in space is a vector quantity, the basic network channels, is connected to the monitoring which means there is a direction associated with the field as center through the wireless or wired Internet network, and a well as a field strength. The direction of the arrow can be large amount of geomagnetic raw observation data is acquired thought of as the direction of the magnetic field. The length after a period of recording to achieve reliable and real-time of the arrow can be thought of as the strength of the field, i.e. monitoring of the geomagnetic data. This system plays an the longer the arrow, the stronger the field. We can define this important role in the observation platform for short-period length as B, which implies that B represents the strength of geomagnetic. However, the observation interruption will occur the magnetic field. Generally, a single axis measuring device due to some factors such as the observation site, power supply, will change its reading depending on which way the sensitive communication conditions, etc., and its stability needs to be axis is oriented with respect to the direction of the magnetic further strengthened. Besides, the three-component fluxgate field. To obtain a complete representation of magnetic field at magnetometer used in the basic network of the geomagnetic any point in space, one needs not only the value of B, but the station is limited by the working principle, leading to the direction, which can be expressed as the three components, measurement accuracy is lower than 0.1 nT. Hence, a Super- Bx,By and Bz. Some magnetic field sensors measure only conducting Quantum Interference Device (SQUID) could be one component of the magnetic field, e.g., Fluxgates and considered to deal with the three-component magnetic survey Hall effect instruments which are referred to as single axis of geomagnetic stations to obtain a higher sensitivity [62], devices [38], [39]. Other instruments measure only the total [63]. field B, e.g., NMR [40], [41], ESR [42], [43]. Aiming to get more information for engineering geophysical exploration, it is possible to combine three-axis sensors to give three field measurements in a single probe package. These are referred to as three-axis devices [44], [45]. The ground vector magnetic survey using three-axis device is mainly divided into two types of measurement: 1) station- ary measurement and 2) mobile measurement. The purpose is to analyze the geomagnetic characteristics by measuring the Earth’s magnetic field. The main task is to survey ore bodies with magnetic anomalies, geological structural zoning, and geological mapping services [46]–[50]. However, the ground-based mobile three-component magnetic survey has not achieved widespread application, due to the performance limitations of the mobile platform’s vibration and attitude mea- surement unit [51]. Conversely, the stationary measurement has been widely used in many fields such as the basic network of seismic stations, underground magnetic anomaly source detection, and weak magnetic resonance signal acquisition, be- Fig. 1: JESSY SMART ground tensor magnetic gradient cause of its early development and easy implementation [52]– measurement system developed by IPHT [64]. [54]. The basic station network of the China Earthquake Admin- In 2007, the Institute for Photonic Technologies (IPHT) of istration’s geomagnetic station has been using fluxgate magne- Jena, Germany, adopted a low temperature superconducting tometers to detect changes in the geomagnetic field in seismic (LTS) SQUID to develop a ground tensor magnetic gradi- danger zones since 2003 [55]–[58]. This method can be used ent measurement system, named JESSY SMART [64], as to detect earthquake anomaly precursors in time and provide shown in Fig. 1. The JESSY SMART is a novel ground basic information for accurate prediction of earthquake occur- tensor magnetic gradient measurement system for fast three- rence. Meanwhile, the observed geomagnetic field strength, dimensional (3D) geomagnetic mapping of hidden subsurface inclination, and declination can be adopted to study the Earth’s anomalies. It can localize smallest Earth’s magnetic field basic magnetic field and deep underground structures [59]. In gradient inhomogeneities in a depths of up to approximate addition, the high-quality observation data of the ionosphere, 10 meters. The sensors are mounted onto a cart which can the substorm current, and the very-low-frequency (VLF) mag- be moved attached to a motor car or alternatively pushed netic field of the measured region can be used to study the along manually. To achieve the highest detection rates, the 3 system applies the most sensitive sensors presently available, Likewise, Jilin University proposed a novel noise reduction SQUIDs, and it has the following advantages when compared method and developed a corresponding magnetic resonance with conventional geomagnetic methods: 1) highest possible three-component noise reduction device [66], [67]. To be magnetic field resolution (sensor 7 fT/cm); 2) fast mapping specific, a new approach for acquiring weak signals with of measuring area (up to 7 acre and accordingly 3 ha/hour); complex environmental noise and limited detection space was 3) very accurate position and spatial resolution of data in cm designed. To obtain the magnetic resonance signal containing range); 4) measurement in difficult ground, environment, and noise, the receiving coil was arranged in front of the palm climate conditions possible; 5) nondestructive measuring and face, and three reference coils were laid orthogonally nearby a high planning reliability. to collect three-component electromagnetic noise. Further, different component noise correlations were employed to suppress electromagnetic noise in the received signal, which can guarantee a reliable extraction of magnetic resonance signals. However, this method is in the laboratory stage, and its practicability for field application still needs to be further validated.

III.WELLSVECTORMAGNETICSURVEYTECHNIQUE Wells vector magnetic survey technique is used for cen- sus exploration of magnetite deposits or poly-metallic de- posits containing ferromagnetic minerals. It is a geophysical prospecting method for the development and extension of the ground magnetic surveys in space, based on the study of the magnetism of rocks and ore bodies. Besides, this technique has a unique advantage in the exploration of deep magnetic deposits, and it can solve some problems that the surface Fig. 2: JESSY SMART measuring data, geo-referenced and magnetic survey cannot solve, such as whether there are ore embedded and in aerial image blue-red contrast represents bodies at the bottom of the well or beside the well, the depth magnetic anomaly like aircraft bombs and filled craters [64]. position of the ore tail and the top of the mine, etc. The wells magnetic survey can avoid the human interference and Figure 2 shows the mission results for the Localisation the unevenness of shallow magnetic in the ground magnetic of remains of World War II aircraft bombs, munition, and survey, and the vertical resolution is relative high. Because of bomb craters before municipal construction work, in which the these advantages, the three-component magnetic survey in the localization accuracy is up to 5 cm. The magnetic anomalies well has become an effective method for surveying every hole and the corresponding localization of the aircraft bombs can of the magnetite deposit in the general survey [68], [69]. be visualized clearly, which demonstrates the effectiveness of The three-component magnetic survey in the well is carried the system. out along the drilling direction of the borehole. Through In addition, the three-component measurement of the ge- measuring the x, y, and z components of the magnetic field at omagnetic field could also be applied to the acquisition of different depths in the well, the magnetic anomaly components ground weak magnetic resonance signals. Since the weak mag- ∆x, ∆y, and ∆z can be calculated to judge and evaluate the netic resonance signal is easily overwhelmed by high levels blind magnetic ore beside the well, the blind ore at the bottom of electromagnetic noise, we can collect three components of the well, the shape of the ore body, the size of the mineral, of electromagnetic noise by laying orthogonal coils on the etc [70]–[72]. The three components ∆x, ∆y, and ∆z can also ground, and use the correlation of different components to be synthesized into an total intensity anomaly ∆T . Meanwhile, perform reference cancellation to extract reliable magnetic the three-component magnetometer can also be employed to resonance signals. Based on this idea, Costabel and Muller- obtain the depth of each measuring point, and the top angle Petke adopted ”Bradikow” and ”Barnewitz” to implement field and azimuth angle of the corresponding depth. experiments and compared the detection results [65]. The In practical applications, aiming to facilitate the qualitative electromagnetic interference of the Bradikow is dominated by study of magnetic anomalies, ∆T is divided into a vertical the fundamental frequency of 50 Hz, which is generated by the component ∆Z and a radial component ∆H, and the ∆H residents of nearby villages. The electromagnetic interference can be further divided into a horizontal component which is of the Barnewitz is dominated by the fundamental frequency perpendicular to the direction of the ore body and a horizontal of 16.7 Hz, which is caused by the eddy current of the railway component which is parallel to the direction of the ore body. and the power lines 3.5 km away. In this case, the three In this case, we can obtain the relations between the magnetic components of the measured electromagnetic noise field are anomaly strength and the direction of the ore body. Through used for reference denoising, and the best detection results the analysis of the parameters such as the magnetic vertical are obtained. The two key parameters, the initial amplitude E component and the radial component of the magnetic anomaly and the transverse relaxation time T2, got the smallest fitting area, we can infer whether there are ore bodies at the bottom error. of the well or besides the well. 4

chemical processes (reduction/oxidation) are active. Moreover, the United States, Sweden, and Canada have made important contributions to the design of magnetic mea- suring instruments, data processing, etc. For instance, Kuhnke et al. [76] designed a borehole magnetometer, analyzed the possible problems of the magnetometer during operation in the well, and developed different types of instrument systems based on the temperature resistance characteristics. Bosum et al. [77] analyzed the characteristics of borehole magnetic sur- vey data, and Worm [78] conducted rock magnetic simulation studies by combining with KTB borehole magnetic survey data. Leonardi et al. [79] adopted KTB magnetic survey data to study the anisotropy of the crustal structure, and obtained some fractal features. Li et al. [80] studied the magnetic survey data from ground and borehole, and improved the detection resolution by using the joint inversion method. In the early 21st century, Chongqing Geological Instrument Factory developed a high-precision three-component magne- tometer, named GJCX-1 [82]. The steering differences of x and y direction are both less than 100 nT, the steering Fig. 3: Overall view of the KTB drilling system and the difference of z direction is less than 50 nT. Shanghai Institute corresponding module structure [73]. of Geosciences developed a high-precision three-component logging tool, named JCC3-2A, in which the three-axis giant magnetoresistive sensor is used as the magnetic sensitive element, and the three-axis gravity acceleration sensor is used In the mid-1960s, Wang et al. [74] developed China’s as the directional element. The sensitivity of the magnetic first three-component magnetic survey system for transistor component is 40 nT, and the accuracy is better than 100 wells. This system can directly measure the three-component nT [83]. magnetic field value of the vertical coordinate system in the Consequently, the wells three-component magnetic survey well, using a gravity orientation with two degrees of freedom. technique has the advantage of a large radial detection range, When compared with the degree of freedom (DOF) gravity which provides an effective tool for dividing the magnetic rock orientation system developed in Sweden at that time, it has or ore body on the borehole profile, the magnetic minerals obvious advantages such as high measurement accuracy, real- besides or below the exploration well, and the blind ore bodies time measurement, etc. associated with the magnetic minerals. To a certain extent, In the 1990s, the German Continental Drilling Program it is possible to perform depth calibration on the inversion (KTB) has made great progress in data processing and in- of the ground magnetic survey, solving the detection work version interpretation of magnetic surveys in boreholes [75]. that cannot be completed by the ground magnetic survey Fig. 3 shows the overall KTB drilling system [73], in which technique, which implies that the wells vector magnetic survey a combination of a closely spaced surface gravity survey with an effective method for deep ore body detection. However, a high-resolution helicopter aeromagnetic survey as well as due to the limitations of borehole diameter, well temperature, borehole gravity and magnetometer measurements allowed a and the size & volume of the instrument itself, the accuracy detailed 3D modeling of the anomalies at the KTB drill site. of the magnetic sensor used in the wells magnetic survey The models could be constrained by new evidence from a 3D is lower than that of the ground magnetic survey, and the seismic survey and by structural geology and petrophysical overall observation accuracy is also lower than the surface data from drill cores and cuttings. The source body for the magnetic survey by an order of magnitude. Hence, it is of great positive gravity anomaly consist of high-density metabasite. significance to develop a higher-precision three-component The vertical derivative of the Bouguer anomaly does not magnetic measuring instrument in the well. resemble the aeromagnetic anomaly in all areas, indicating that parts of the metabasites are more or less nonmagnetic. IV. MARINEVECTORMAGNETICSURVEYTECHNIQUE Surprisingly and confirming the observation in other deep drill holes into continental crystalline basement rocks, pyrrhotite is Marine magnetic survey is the main approach to obtain the dominant magnetic mineral below a depth of about 300 the information on the distribution and variation character- m. Magnetite mainly occurs in the depth intervals 360-520 m istics of the geomagnetic field in the ocean area, and it and 7300-7900 m. The lower interval causes the anomalous is also one of the important cases of marine engineering vertical gradient of 60 nT/km for the geomagnetic field. The surveys and military hydrographic surveys [84]. In the 1950s, occurrence of strongly magnetic minerals in the borehole down Vacquier et al. [85] adopted a fluxgate magnetometer to to about 3000 m correlates with the lithology, while in the measure the geomagnetic field in the three oceans. Since deeper parts it is more related to fissures and fault zones where the 1980s, the international geoscience community has begun 5

Fig. 4: Marine vector magnetometer [81]. (a) G-882; (b) SeaSPY2; (c) SeaQuest. to implement seafloor geomagnetic observations, deploying integrations are also available, as are horizontal transverse dozens of seafloor observation stations to conduct oceanic gradiometers. This system’s absolute accuracy is up to 0.2 and geomagnetic surveys, and carrying out research on un- nT, and its operating range is 18000 nT ∼ 120000 nT. In derground deposits and geodynamics. Afterwards, the earth- addition, the gradient magnetic measurement can be achieved based electromagnetic measurement method based on artificial by combining four independent SeaSPY2 magnetometers. As excitation appeared, e.g., electromagnetic method of ocean shown in Fig. 4 (c), the array-type marine magnetometer controlled source [86]. SeaQuest can obtain more accurate parameter information Up to now, the marine magnetic survey mainly employs an and provide a more reliable basis for detecting underwater optical pump magnetometer or an Overhauser magnetometer anomalies. for the underwater magnetic survey [87]–[89]. To suppress the To further identify the magnetic anomalies caused by short- influence of waves, the magnetic probes are usually measured polarity events or other local magnetic bodies, Blakely et at a certain depth below the surface of the ocean, using a drag- al. [94] developed a magnetic vector measurement system. and-drop measurement approach. In 1997, Seama et al. [90] Besides, Horner and Gordon [95] adopted spectral analysis developed a vector magnetometer for deep tow detection. To methods to identify anomalies in the seabed magnetic bands, determine the position of the tow body in the deep ocean, an and they found that the aeromagnetic survey data had better inertial navigation system and GPS, short baseline acoustic results than shipboard total field survey data. Hence, the devel- measurement and pressure measurement were adopted. Gee opment of on-board small AUVs for the autonomous marine and Cande [91] developed a vector magnetometer system with magnetic surveys is gradually becoming the new direction of a speed of 10 ∼ 12 knots. The test results show that the device future marine magnetic survey technique. can determine the horizontal and vertical components of 30 nT In 1995, the Woods Hole Oceanographic Institute developed ∼ 50 nT, which is of great significance for magnetic survey the world’s first unmanned submersible autonomous benthic applications in low latitude areas. explorer (ABE) for marine magnetic survey [96], which is American Geometrics Company is at the international ad- mainly composed of conductivity probe, temperature probe, vanced level in the research and development of marine depth gauge, camera and magnetometer, as shown in Fig. 5 (a). magnetometers, and the G-882 Cesium optical pump marine The ABE is a robotic underwater vehicle used for exploring magnetometer developed by it is shown in Fig. 4 (a). This the ocean to depths of 4,500 meters. It was the first AUV very high-resolution Cesium vapor marine magnetometer is used by the U.S. scientific community. ABE is often used in low in cost, small in size, and offers flexibility for professional tandem with Alvin or Jason surveying large swaths of ocean surveys in shallow or deep water. It can directly interface to all floor to determine the best spots for close-up exploration. To major side-scan manufacturers for tandem tow configurations. be specific, ABE is designed to perform a pre-programmed Further, it is easily deployed and operated by one person be- set of maneuvers, using its five thrusters to move in any cause of its small and lightweight characteristics. This system’s direction, hover, and reverse. The AUV excels at surveys absolute accuracy is better than 3 nT, and its operating range of the shape of the seafloor, its chemical emissions, and its is 20000 nT ∼ 100000 nT [92]. magnetic properties. ABE is particularly valuable in rugged The SeaSPY2 marine magnetometer developed by the Cana- terrain. On-board sensors tell the vehicle how deep it is and dian Marine Magnetics Company is shown in Fig. 4 (b). how far it is off the ocean floor, and the AUV calculates its The SeaSPY2 is a reliable high sensitivity magnetometer with horizontal position by contacting a system of acoustic beacons unmatched absolute accuracy. It is the only mag that has been (transponders) set out in fixed locations. integrated inside an unmanned aerial vehicle (AUV) and with In 2012, Pei et al. [97] developed a small unmanned subma- a glider [81]. It is also the only longitudinal gradiometer on rine, and obtained satisfactory results in practical applications. the market. Side-scan and remote operated vehicle (ROV) In the same year, American Geometrics and Delaware Uni- 6

Fig. 5: Photographs of the unmanned underwater vehicle [93]. (a) Unmanned underwater vehicle ABE; (b) Unmanned underwater magnetic detection vehicle G-880AUV; (c) Module design schematic of G-880AUV. versity jointly developed the G-880AUV underwater magnetic and plays an important role in geophysical exploration [101], detection unmanned submarine, as shown in Fig. 5 (b) and (c), [102]. Before the 21st century, the airborne magnetic survey which was successfully applied to underwater UXO detection. technique is mainly based on the measurement of total field The G-880AUV has the capability of descending close to the strength or gradient. As the continuous development of mag- seabed and maintaining a set elevation track conforming to netic exploration theories and methods, the airborne survey the shape of the seabed in ”terrain following mode.” In terrain technique has been promoted from the original total field following mode, the vehicle uses the precise altitude data from strength or gradient measurement to vector (three-component the DVL to measure the distance to the bottom below the AUV. or gradient tensor) measurement [103], [104]. Control algorithms within the vehicle automatically adjust the position of the AUV based on the commanded terrain altitude offset and in response to changes in the seabed geometry. Typical terrain following performance of the vehicle is to maintain position within ± 10 cm of the set point altitude over a flat seafloor. Precise terrain following improves data quality and survey efficiency by helping to maintain even survey swath coverage. Later this year, the China Shipbuilding Industry Corporation 715-Research Institute developed the RS-YGB6A marine optical pump magnetometer [98]. The resolution is up to 0.001 nT, the operating range is 35000 nT ∼ 70000 nT. This system is stable and reliable, and it has been widely used in marine engineering such as pipeline detection, underwater obstacle investigation, etc. In 2014, the 715-research institute developed the RS-HC3 ocean tensor magnetic gradiometer Fig. 6: Aero three-component magnetic measurement system which dynamic range is -100000 nT ∼ 100000 nT, and this developed by AGRS [105] system also has got a preferable result. At present, the vector magnetometers commonly used in V. AIRBORNEVECTORMAGNETICSURVEYTECHNIQUE geophysical engineering mainly include fluxgate magnetome- The resource conditions and environmental conditions in ters and superconducting magnetometers [59], [106]. Since the many areas of the earth are complex, such as swamps, forests, vector magnetic survey data includes the magnetic field value deserts, and mountain areas, where are inconvenient for per- and the corresponding direction, the data obtained from the air- sonnel equipment to access [99], [100]. The airborne electro- borne magnetic survey needs to be corrected to the geographic magnetic detection system can overcome the impact of the coordinate system for further processing and interpretation. complex ground environment. It has the characteristics of low Hence, it is necessary to obtain the system attitude and cost, fast survey speed, wide survey area, and strong versatility, orientation during flight, and the final magnetic measurement 7

Fig. 7: HTS full-tensor magnetic gradiometer developed by CSIRO [113]. (a) The magnetic probe’s structure; (b) The magnetic probe; (c) The overall system; (d) System experiment on ground. accuracy depends on the accuracy of the magnetometer and magnetic survey technique: aeronautical full-tensor magnetic the accuracy of the attitude measurement system [107], [108]. gradient measurement has become a hot spot for geophysicists In 2003, the Australia BHP Billiton Petroleum Company in various countries. This technique refers to measure the developed a fluxgate aviation three-component magnetic mea- spatial change rate of the three field components of the surement system, and conducted flight tests in the Rocky Strip geomagnetic field vector along three mutually orthogonal iron-bearing construction area of Western Australia [109]. axes, and there are nine elements in total [2]. Its outstanding After attitude correction, three-component geomagnetic field advantages include the invariant contour plot calculated from data and attitude were obtained, in which the corrected noise the gradient tensor is easy to explain, the dipole tracking level is 50 nT ∼ 100 nT. In 2011, Munschy et al. [110] algorithm can be used to accurately determine the depth installed two fluxgate magnetometers on the tail of the Maule and horizontal position of the magnetic dipole, etc. Hence, MX7 small aircraft, and conducted flight tests in the Vosges the high-precision 3D positioning of underground magnetic area. After magnetic compensation, the magnetic field data geological bodies and ore bodies can be realized through full- of two horizontal components were obtained, and the vertical tensor magnetic gradient measurement data, and their spatial component was calculated by combining the measurement distribution information can be obtained [4]. results of the total field. After mapping the magnetic field In 2003, the U.S. Geological Survey (USGS) conducted distribution in this area, it is basically consistent with the proof-of-principle tests of the tensor magnetic gradiometer actual geological conditions. In the same year, the China system at the Strategic Environmental Research and Develop- Aero Geophysical Survey and Remote Sensing Center (AGRS) ment Program’s (SERDP) UXO Standardized Test Site at the developed an airborne three-component magnetic survey sys- Yuma Proving Ground (YPG), Arizona [114]. The objective tem, which is composed of a fluxgate magnetometer and of these tests was to assess the potential of this system for an INS/GPS strap-down inertial navigation module [105], UXO applications and to help us formulate design parameters [111], [112]. In addition, a verification test was conducted by for an improved tensor magnetic gradiometer system designed mounting this system to a fixed-wing aircraft [98], as shown specifically for UXO. Although the current system was not in Fig. 6, in which a satisfactory result was obtained. designed with UXO applications in mind, it detected the At the beginning of the 21st century, another airborne vector majority of the buried targets in the Calibration Area at YPG. 8

Fig. 8: Setup of the first-generation airborne SQUID system developed by IPHT [117].

Its performance rivaled that of the cesium vapor companion tensor components allowing the user to discriminate between system, and it showed true tensor gradiometric performance harmless metallic debris or shrapnel and a potentially lethal over a selected target. Jilin University constructed a magnetic UXO [113]. It uses multiple CSIRO’s HTS planar SQUID full-tensor gradiometer, which utilizes four fluxgates arranged gradiometers configured unique geometry to determine the on a planar cross structure, and a single, triaxial, spherical magnitude of the full-tensor. Referencing magnetometers are feedback coil assembly [115]. In this arrangement, one of used to improve common mode rejection. The UXOMAG’s the fluxgates is used as a reference, controlling the currents high sensitivity has the potential to detect 40 mm calibre UXO through the feedback coils. Since the fluxgates are working in at a distance of up to 4 m away. the near-zero magnetic field environment, the magnetic tensor gradiometer is stable and of an improved accuracy. However, due to the low sensitivity of the fluxgate mag- netometer, the baseline distance between the sensors is very large when performing gradient measurement, and thus, the requirements on the system structure are relative high, and the measurement accuracy is difficult to guarantee. Consider for this case, the SQUID based vector magnetic sensor with higher sensitivity has become the first choice for the magnetic gradient tensor measurement system. In 2005, the U.S. Oak Ridge National Laboratory (ORNL) developed an aeronautical full-tensor magnetic gradient mea- surement system which is composed of high temperature su- perconducting (HTS) SQUIDs and an aeronautical geophysical platform [116], and implemented a ground experiment for this Fig. 9: Setup of the second-generation airborne SQUID system system. The system adopts eight HTS-SQUID sensors which developed by IPHT [117]. are all fixed on a bracket, forming a full-tensor probe through a reasonable layout, and the equivalent dipole position can be There are also some other SQUID based full-tensor mag- determined by a single movement measurement. netic gradient measurement systems, e.g., the prototype devel- Likewise, the Commonwealth Scientific and Industrial oped by Jilin University [118]. For this system, six first-order Research Organization (CSIRO) developed an HTS-SQUID planar-type SQUID gradiometers mounted on the six sloping based areo full-tensor magnetic gradient measurement system, facets of a hexagonal pyramid, which can form the probe namely, UXOMAG, as shown in Fig. 7. The UXOMAG and determine all independent components of the magnetic is a mobile magnetic tensor gradiometer prototype system gradient tensor with the help of triaxial SQUID magnetometers capable of not only detecting, but also locating and classifying placed near the probe. The gradiometer sensitivity is about 100 UXO. This system can measure all five independent magnetic pT/m. The Shanghai Institute of Microsystem and Information 9

Technology (SIMIT) and the Institute of Remote Sensing and Digital Earth (RADI) from the Chinese Academy of Sciences jointly developed a prototype of HTS-SQUID based areo full- tensor magnetic gradient measurement system, and conducted a flight measurement test in Inner Mongolia of China. The gradiometer sensitivity is up to 50 pT/m. However, these two systems are still in the experimental stage. Up to now, the most commonly used and accepted practical aero full tensor magnetic gradient system is developed by IPHT. The IPHT won the ”Outstanding Achievement Award” in the category Mining Research for their ground breaking research in the field of world’s first full-tensor airborne mag- netic gradiometer, known by its synonym ”JESSY STAR”. Fig. 8 illustrates the first-generation system, and Fig. 9 shows the second-generation system [117]. The JESSY STAR system has surpassed all conventional surveying technology in perfor- mance and, in addition, provided the exploration community with magnetic data never measured before. It also utilizes the ultimate sensitivity of LTS-SQUID sensors [119]. The JESSY STAR system include six gradiometers and three magnetome- ter channels, to compensate for sensor motion noise. Further, high spatial resolution is provided by a differential GPS system using SBAS and an inertial unit (INU), which are synchronized with the magnetic gradient data, The INU data is used for mo- tion compensation. The data acquisition system, incorporate Fig. 10: Illustration of the Ørsted spacecraft in orbit [131]. a small-sized 15 channels 24 bit analogue digital converter unit. To date, JESSY STAR has flown hundreds of hours in magnitude to better than 2 nT and each component to better both helicopter mode and fixed wing plane configuration, and than 6 nT [133]. Two star cameras, a high-accuracy sun it has transformed exploration from ”flying blind” to ”seeing sensor and a pitch axis gyro provided the 10-20 arc-second clearly”. This new generation system is capable of detecting attitude measurements necessary to achieve this accuracy. The minerals and precious metals that were deemed ”undetectable” magnetometers were located at the end of a boom to eliminate before. the effect of spacecraft fields. An optical system measured the attitude of the vector magnetometer and sun sensor (at the VI.SATELLITES VECTOR MAGNETIC SURVEY TECHNIQUE end of the boom) relative to the star cameras (on the main spacecraft) [134]. The data are available in several formats In 1958, the Soviet Union launched the first satellite from the National Space Science Data Center and are under- SPUTNIK-3 to measure the geomagnetic field, which fore- going analysis by a team of investigators [135]. In addition, shadowed the beginning of the satellites magnetic survey An et al. [136], [137] employed the method of spherical cap technique [120], [121]. The fluxgate magnetometer was in- harmonic analysis to derive a spherical cap model based on stalled on this satellite, however, since the direction of the the MAGSAT data set, and described the three-dimensional magnetometer cannot be accurately determined, only total field structure over Asia and Europe region, respectively. strength data were obtained. Since then, a series of mag- netic survey satellites carried total field magnetometers were launched by the Soviet Union, including proton precession magnetometers or optical pump magnetometers. Likewise, they cannot be called vector magnetic survey. With the devel- opment of space magnetic field measurement technology, there have been numerous professional satellite programs dedicated to the mapping of the Earth’s intrinsic geomagnetic, such as the U.S. MAGSAT satellite program [122], [123], Denmark’s Ørsted satellite program [124]–[126], Germany’s CHAMP satellite program [127], [128], European Space Agency’s (ESA’s) Swarm [129], and Argentina’s SAC-C satellite pro- gram [130]. Fig. 11: Photograph of the CHAMP satellite [138]. The MAGSAT spacecraft was launched into a twilight, sun- synchronous, orbit with inclination 96.76o, perigee 352 km The Ørsted satellite, named after the Danish scientist Hans and apogee 561 km [132]. The Cesium vapor scalar and Christian Ørsted, is the first satellite mission since MAGSAT fluxgate vector magnetometers together measured the field designed for high-precision mapping of the Earth’s magnetic 10

Fig. 12: Illustration of the Swarm satellite in orbit [139].

field [140], [141]. It was launched with a Delta-II rocket from consists of the three identical Swarm satellites (A, B, and Vandenberg Air Force Base into a near-polar orbit. As the C), which were launched into a near-polar orbit [151]. All first satellite of the ”International Decade of Geopotential Re- the three Swarm satellites are equipped with the following search”, the satellite and its instrumentation have been a model set of identical instruments, including an absolute scalar for other present and forthcoming missions like CHAMP and magnetometer, a vector field magnetometer, a star tracker, an Swarm [131]. The Ørsted spacecraft is composed of two main electric field instrument, a GPS receiver, a laser retro-reflector, instruments, including an Overhauser magnetometer and a and an accelerometer [152], [153], as shown in Fig. 12. The compact spherical coil based triaxial fluxgate magnetometer, absolute scalar magnetometer is an optically-pumped meta- as illustrated in Fig. 10. The objective of the Overhauser stable helium-4 magnetometer, developed and manufactured magnetometer is to measure magnetic field scalar values with by CEA-Leti under contract with the French Space Agency. an absolute measurement error better than 0.5 nT, an dynamic It provides scalar measurements of the magnetic field to range from 16000 nT to 64000 nT, and a sampling rate calibrate the vector field magnetometer [154]. The vector of 1 Hz [142]. The fluxgate magnetometer which measures field magnetometer is the mission’s core instrument, which magnetic field vectors at an absolute measurement error less is developed and manufactured at the Technical University of than 1 nT, an dynamic range from -65536 nT to 65536 nT, and Denmark [155]. It makes high-precision measurements of the a resolution better than 0.25 nT. Further, the collected vector magnitude and direction of the magnetic field. The orientation data is calibrated using the absolute intensity measured by of the vector is determined by the star-tracker assembly, which the Overhauser magnetometer. After calibration, the agreement provides attitude data. The vector field magnetometer and the between the two magnetometers is better than 0.33 nT [143]. star-trackers are both housed on an ultra-stable structure called The CHAMP is a German small satellite mission for geosci- an optical bench, halfway along the satellite’s boom. entific and atmospheric research and applications, managed by GeoForschungsZentrum (GFZ) [144], [145]. The satellite was built by the German space industry with the intent to foster high-tech capabilities, especially in the East-German space industry, and it was launched from the cosmodrome Plesetsk (north of Moscow) aboard a Russian COSMOS launch vehi- cle [146]. Fig. 11 shows a photograph of the CHAMP satel- lite. The payload includes an accelerometer, a magnetometer instrument assembly system, a GPS receiver, a laser retro reflector, and an ion drift meter. The magnetometer instrument assembly system is a boom-mounted package consisting of an Overhauser scalar magnetometer (the measurement range is 16000 nT ∼ 64000 nT, the resolution is 0.1 nT, the absolute accuracy is 0.5 nT, and the sampling rate is 1 Hz.) [147], two fluxgate vector magnetometers (the measurement range is -64000 nT ∼ 64000 nT and the resolution is 1 nT ∼ 2 nT) [148], and two-star imagers to provide attitude information for fluxgates [149]. Fig. 13: Artist’s view of the deployed SAC-C spacecraft [156]. The Swarm satellite is constructed for the first constellation mission of the European Space Agency (ESA) for geomag- The SAC-C satellite (illustrated in Fig. 13) is an interna- netic observation [150]. This mission is operated by ESA’s tional cooperative mission between the National Aeronautics European Space Operations Centre (ESOC), in Germany, via and Space Administration (NASA), the Argentine Commission the primary ground station in Kiruna, Sweden. This mission on Space Activities (CONAE), the French Space Agency, the 11

Brazilian Space Agency, the Danish Space Research Institute, VIII.CONCLUSIONSAND EXPECTATIONS and the Italian Space Agency [157]–[159]. The SAC-C was de- According to the investigations mentioned above, it can veloped through the partnership of its senior partners, CONAE be realized that each magnetic survey method has its unique and NASA with contributions from Brazil, Denmark, France, superiority and defect, and each single method has difficulty and Italy. It was the third satellite launched by CONAE and to meet the requirement of modern geophysical detection was the first operational Earth observation, designed to meet applications. With the development of the magnetic survey the requirements of socio-productive areas of our country, technique and instrument system, the future magnetic survey including agriculture, hydrology, coastlines, geology, health will change from the traditional work mode, i.e., measuring emergencies, etc. This satellite is equipped with a vector the total magnetic field intensity or its gradient to the vector magnetometer and a star imager, further included are the information of the magnetic field, then to multi-parameter associated support electronics to control the sensors and the measurements. The magnetic survey application field will boom deployment, a power control unit and a command and change from the traditional ground magnetic survey method data handling unit. The geomagnetic measurement system on with low efficiency and shallow detection depth to high SAC-C represents essentially the same compact spherical coil efficient airborne magnetic survey, deep well magnetic survey based vector magnetometer as those flown on the Ørsted and abysmal sea magnetic survey, then gradually to the joint and CHAMP missions [160]–[162]. Besides, a scalar Helium detection and interpretation combining with the five kinds magnetometer is placed on the tip of the boom and associated of magnetic survey methods mentioned above. Through the support electronics completes the magnetic mapping payload. combination of a variety of detection methods to realize advan- The magnetic field measurements have a resolution of 1 nT at tageous complementarities, further improvement of detection scalar and 2 nT at vector [163]. accuracy and inversion resolution for Earth’s magnetic field will be implemented. VII.DISCUSSION

Throughout the development of geophysical vector mag- ACKNOWLEDGMENT netic survey techniques, the comparison of the application scenarios, the technical characteristics, and the representative This work is partly supported by the National Natural instrumentation system for the aforementioned five techniques Science Foundation of China under Grant Nos. 41904164 are illustrated before. As the basic method of the vector and 41874212, the Foundation of Wuhan Science and Tech- magnetic survey technique, the ground vector magnetic survey nology Bureau under Grant No. 2019010701011411 and development is the earliest and the technique is the most 2017010201010142, the Foundation of National Key R&D mature. The accuracy of the instrument system for the ground Program of China under Grant No. 2018YFC1503702, the vector magnetic survey is high and the working performance Foundation of Science and Technology on Near-Surface is reliable. Nevertheless, there are also some problems, e.g., Detection Laboratory under Grant Nos. 6142414180913, the detection depth is shallow and working efficiency is low. TCGZ2017A001, and 614241409041217, and the Fundamen- The wells vector magnetic survey is one useful tool for the tal Research Funds for the Central Universities, China Univer- Earth’s deep mineral resources exploration. It can finish the sity of Geosciences (Wuhan) under Grant No. CUG190628. detection work that the ground vector magnetic measurement can’t do. However, due to the limit of the bore diameter and the REFERENCES high temperature in the borehole, the detection accuracy of the instrument system is lower than the ground magnetic instru- [1] S. Maus, U. Barckhausen, H. Berkenbosch, N. Bournas, J. Brozena, V. Childers, F. Dostaler, J. Fairhead, C. Finn, R. Von Frese et al., ment. The marine vector magnetic survey has an irreplaceable “Emag2: A 2–arc min resolution earth magnetic anomaly grid com- effect in the application of military or other ocean engineering. piled from satellite, airborne, and marine magnetic measurements,” Its working mode changes from the method of ship drag- Geochemistry, Geophysics, Geosystems, vol. 10, no. 8, 2009. and-drop to small unmanned underwater vehicle, which can [2] H. Liu, X. Wang, J. Bin, H. Dong, J. Ge, Z. Liu, Z. Yuan, J. Zhu, and X. Luan, “Magnetic gradient full-tensor fingerprints for metallic objects measure a more precise underwater magnetic anomaly. The detection of a security system based on anisotropic magnetoresistance airborne vector magnetic survey has the superiority of fast sensor arrays,” AIP Advances, vol. 10, no. 1, p. 015329, 2020. detection, high efficiency and strong practicality to the com- [3] A. Y. Denisov, V. A. Sapunov, and B. Rubinstein, “Broadband mode in proton-precession magnetometers with signal processing regression plex geophysical environment, etc. However, the detection methods,” Measurement Science and Technology, vol. 25, no. 5, p. accuracy is relatively low because of magnetic interference, 055103, 2014. the posture change of the aircraft carrier, etc. The study [4] H. Liu, X. Wang, H. Wang, J. Bin, H. Dong, J. Ge, Z. Liu, Z. Yuan, J. Zhu, and X. Luan, “Magneto-inductive magnetic gradient tensor of magnetic compensation and data processing methods for system for detection of ferromagnetic objects,” IEEE Magnetics Letters, aero three-component and full-tensor magnetic gradient is vol. 11, no. 1, p. 8101205, 2020. also needed. The satellites vector magnetic survey can obtain [5] S. Chen, S. Zhang, J. Zhu, and X. Luan, “Accurate measurement of characteristic response for unexploded ordnance with transient high-quality and whole global magnetic field data. It has electromagnetic system,” IEEE Transactions on Instrumentation and the advantages of high detection efficiency, wide detection Measurement, vol. 69, no. 4, pp. 1728–1736, 2020. range, etc., accelerating and facilitating the investigations on [6] J. Ge, C. Lu, H. Dong, H. Liu, Z. Yuan, and Z. Zhao, “The detection technology of near-surface uxo based on magnetic gradient method and space measurement technique and the evolution of the Earth’s overhauser sensor,” Chinese Journal of Scientific Instrument, vol. 36, magnetic field. no. 5, pp. 38–50, 2016. 12

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