Overhauser Magnetometer Research Articles

Estimation and Utilization of the Geomagnetic Field Inhomogeneities Using the Relaxation Characteristics of the FID Signal in an Overhauser Magnetometer

Estimation and Utilization of the Geomagnetic Field Inhomogeneities Using the Relaxation Characteristics of the FID Signal in an Overhauser Magnetometer

Construction and Experimental Verification of an Integrated Dual-Mode Overhauser Magnetometer for Marine Magnetism Survey

Construction and Experimental Verification of an Integrated Dual-Mode Overhauser Magnetometer for Marine Magnetism Survey

A fast tracking method for magnetic abnormalities using distributed Overhauser magnetometer system based on genetic algorithm.

Magnetic anomaly detection technologies have been widely used for tracking moving targets. In this paper, we present a fast-tracking method for magnetic abnormalities using a distributed Overhauser magnetometer system based on the genetic algorithm. Our proposed framework of the Overhauser magnetometer system employs multiple sensors to eliminate background interference, and the genetic algorithm efficiently solves magnetic anomaly data without requiring the derivation of the objective function. Test platforms were built to evaluate the distributed Overhauser magnetometer system and the genetic algorithm. Results from the natural outdoor magnetism laboratories showed that the noise of our presented magnetometers was below 0.134 nT. The optimal factors for solution precision and effectiveness in the genetic algorithm were obtained from the simulation. Moreover, the outdoor tracking experiments indicated that the proposed method could accurately and quickly detect the moving ferromagnetic object within 6.9% maximum positioning error in 0.55m, and the tracking precision of the object velocity can get 5.88% maximum error in 4.33 km/h.

Integrated geophysical investigation for lead and zinc mineralization in Wase, middle Benue Trough, Nigeria

There is an increasing demand for solid industrial minerals worldwide, as countries seek to support their economic growth. Wase, situated in Plateau State, is notable for lead and zinc mineralisation due to tectonic and hydrothermal activities in the region. The study utilises geophysical surveys to map mineralised zones of lead-zinc occurrence. These include ground magnetic survey, very low frequency electromagnetic, electrical resistivity tomography, and induced polarisation (IP). The field data were acquired using a GEMSys Overhauser magnetometer with VLF option and ABEM LS 2 Terrameter. twenty-nine (29) magnetic profiles with 50m spacing and a total length of 24.0 km were obtained and fifteen (15) very low frequency electromagnetic (VLF-EM) profiles with 10 m spacing in the E-W direction. A total length of 12.5 km was acquired across target areas of the magnetic profiles. The multiple-gradient electrode array of the ERT/IP surveys was used for data acquisition, and RES2DINV software by Geotomo was used for the inversion of the acquired data. High chargeability values corresponding to low resistivity values characterised zones with suspected lead-zinc mineralisation. Following this preliminary finding, we advise that further trenching, pitting, or core drilling be carried out to ascertain ore grade, possible beneficiation, and estimate of the reserve.

Suppressing the Saturated Negative Effects to Recover the Effective Sensing FID Signal From an Overhauser Magnetometer via Segmented Linear Regression

The free induction decay (FID) signal of Overhauser geomagnetic sensors may suffer from saturation distortion in the mobile magnetic survey, which results in the loss of signal envelope information. Aiming to suppress the negative effects of saturation, this article proposes a recovery method for the FID signal from saturation based on segmented linear regression. The influences of the signal-to-noise ratio (SNR) and the saturation level on the recovery effect of the proposed method were investigated. Numerical simulations show that the proposed method has a good recovery performance under low SNR and high saturation levels. Furthermore, we set up an Overhauser geomagnetic sensor experimental test platform and conducted extensive experiments. The experimental results show that when the FID signal was partially saturated with an SNR of 20.47 dB, the initial amplitude and the relaxation time recovery error were reduced by a factor of 7.30 and 6.76, respectively. Even if the FID signal is fully saturated, the proposed method can still obtain the initial amplitude and the relaxation time accurately.

JOM-4S Overhauser Magnetometer and Sensitivity Estimation.

The Overhauser magnetometer is a scalar quantum magnetometer based on the dynamic nuclear polarization (DNP) effect in the Earth’s magnetic field. Sensitivity is a key technical specification reflecting the ability of instruments to sense small variations of the Earth’s magnetic field and is closely related to the signal-to-noise ratio (SNR) of the free induction decay (FID) signal. In this study, deuterated 15N TEMPONE radical is used in our sensor to obtain high DNP enhancement. The measured SNR of the FID signal is approximately 63/1, and the transverse relaxation time T2 is 2.68 s. The direct measurement method with a single instrument and the synchronous measurement method with two instruments are discussed for sensitivity estimation in time and frequency domains under different electromagnetic interference (EMI) environments and different time periods. For the first time, the correlation coefficient of the magnetic field measured by the two instruments is used to judge the degree of the influence of the environmental noise on the sensitivity estimation. The sensitivity evaluation in the field environment is successfully realized without electrical and magnetic shields. The direct measurement method is susceptible to EMI and cannot work in general electromagnetic environments, except it is sufficiently quiet. The synchronous measurement method has an excellent ability to remove most natural and artificial EMIs and can be used under noisy environments. Direct and synchronous experimental results show that the estimated sensitivity of the JOM-4S magnetometer is approximately 0.01 nT in time domain and approximately 0.01 nT/ in frequency domain at a 3 s cycling time. This study provides a low-cost, simple, and effective sensitivity estimation method, which is especially suitable for developers and users to estimate the performance of the instrument.

Geophysical study of subsurface structures of the area surrounding the new port of Jarjob in El-Negila-Marsa Matruh using magnetic data

ABSTRACT The purpose of the current paper is to investigate the subsurface geological active structures, through the analysis of the magnetic data, to conduct a scientific evaluation of the infrastructure of the area surrounding the new port of Jarjob that located on the northern coast about 60 km from the city of Marsa Matruh towards the city of Salloum and sited in the vicinity of the proposed area. The Magnetic data comprise the RTP aeromagnetic map as well as the resulting map from the geomagnetic survey for the part that was chosen in the proposed area. Two Overhauser Magnetometers are used to make a detailed comprehensive land magnetic survey, while the aeromagnetic map is obtained with authorisation from the General Petroleum Company. The necessary reduction-filtering phases like the daily variation, regional gradient, and the time variation are applied to obtain the total intensity anomaly map. Resulting total land magnetic intensity map is then reduced to the north magnetic pole and followed by the application of filtering linear wavelength technique to RTP aeromagnetic map and RTP land magnetic map to isolated the shallower residual components using different grid intervals. The obtained major tectonic trends of the study area show that the regional structures are controlled by the regional deeper structure taken NE-SW and E-W directions. The average calculated depth to the basement surface ranges from 2.6 to 3.2 km. So, the strength and solidity of the infrastructure of the port area, especially land magnetic survey area being studied that occupies a privileged place, which overlooks directly on the port coast and close to the main building, makes it a candidate for the establishment of industrial and economic projects and logistical services in the port.

Vector Overhauser magnetometer POS-4: experience and prospects of application

The results of practical use of a POS-4 vector magnetometer, developed by the Research Laboratory of Quantum Magnetometry, UrFU (Yekaterinburg) and based on POS Overhauser sensors, are presented. Continuous measurements by POS-4 have been carried out at the Paratunka observatory (IKIR FEB RAS, Kamchatka) since 2015, were done at the Saint Petersburg observatory (GC RAS / IZMIRAN SPb Branch, Leningrad Region) in 2017-2018 and have been performed at the Arti observatory (Institute of Geophysics, UB RAS, Sverdlovsk Region) since 2020. On the new high-latitude observatory White Sea (IAGA code WSE, GC RAS / MSU, Nikolai Pertsov White Sea Biological Station , Karelia), POS-4 is used as a main variometer for magnetic measurements. In April 2019, the magnetometer was successfully used for field measurements on ice during the TRANSARCTIC expedition in the Barents Sea (AARI, Roshydromet). At the beginning of 2021 IZMIRAN started testing two POS-4 magnetometers at the Moskow observatory. According to the results of field and observatory measurements it was possible to identify the advantages and disadvantages of the magnetometer and provide the information for its developers for further modernization in order to improve its efficiency and reliability. Many years of experience in POS-4 application determine the areas where its scientific and applied usage will provide important results, for example, for magnetic measurements in the Arctic regions or for monitoring of active zones around volcanoes.

Suitability of magnetometry to detect clandestine buried firearms from a controlled field site and numerical modeling

The detection of buried firearms remains a critical issue in law enforcement. We assess the suitability of magnetic gradiometers to detect buried rifles and handguns at multiple depths using numerical modeling and field investigations. Our simulation is based on a simple approach to characterize handguns and rifles as long magnetic dipoles with the firearm characterized by its magnetization, length, centre, azimuth and plunge which allows us to calculate their total magnetic field and gradient anomalies. We compare these synthetic data to field gradiometer data collected with a Gem Systems GSM-19GW Overhauser magnetometer at a field site near Toronto, Canada, where six firearms are buried. Our field magnetometer consists of two sensors with a relative vertical sepration of 0.55 m. We measure the largest anomaly (+/−20 nT) for a rifle at 0.6 m depth, and the smallest anomaly (+/−2 nT) for a handgun buried at 1.8 m depth. The measured anomalies spatially coincide with the locations of weapons while dipole anomalies align along the orientation of the firearms. Our modeling results show that vertically buried weapons produce significantly stronger anomalies than horizontal ones, and even slight tilts enhance the anomalies. We recommend a 0.25 m grid spacing to search for weapons using magnetometry. Our study shows that a range of firearms buried up to 1.8 m can be detected, suggesting that gradient magnetometers are useful tools in forensic weapon searches.

Efficient Performance Optimization for the Magnetic Data Readout From a Proton Precession Magnetometer With Low-Rank Constraint

Proton precession magnetometer (PPM) is a potential device to measure the low-frequency geomagnetic field in the time domain. Nevertheless, the uncontrollable noise such as power frequency noise, random noise, etc., severely affects the detection results. In this paper, aimed to boost the performance of the PPM, a novel real-time processing algorithm for the optimization of PPM output strategy based on the low-rank constraint (LRC) is proposed, and a new efficient framework, dubbed LRC-based geomagnetic data readout optimizer (LRC-GDRO), is implemented. This method aims to extract the dominant principal components of the pure magnetic signal and identify the singular values of noise. Thereby, the relative ideal signal without external noise, which still contains the ambient field information is yielded. Moreover, we develop a prototype device and compare the test results before and after using the proposed method. Consequently, compared with traditional PPM, the performance of the PPM using the proposed method has been significantly improved with a measurement accuracy of ± 0.2 nT. Furthermore, the experimental results reveal that the LRC-optimized PPM is comparable to a very high-accuracy Overhauser magnetometer in terms of 12 h geomagnetic field observation.

Implement of Overhauser magnetometer coordinate transformation software

Two kinds of coordinate projections of Overhauser magnetometer coordinate transformation are studied in this paper. On this basis, the MFC program of Visual C++ is programmed independently. The program implements accurate conversion of Gauss-Krüger projection coordinates and UTM projection coordinates to the WGS-84 geodetic coordinates. It is verified by the examples that both forward projection calculation errors are less than 1.0m. The error of Gauss inverse calculation is less than 10-3 degrees, while the error of UTM projection coordinate inverse calculation is less than 10-5 degrees. This research provides assistance for magnetic survey workers to generate coordinate images, and is also the theoretical basis for the corresponding function of magnetometer PC software.

Apparatus and method for efficient sampling of critical parameters demonstrated by monitoring an Overhauser geomagnetic sensor.

The polarization frequency of free radical solution in Overhauser geomagnetic sensor determines the quality of the Larmor precession signal generated by the sensor. To obtain the polarization frequency accurately, a test apparatus was designed in this paper, which can overcome existing problems in the presently used apparatuses, such as lower resolution, complex operation, etc. The proposed apparatus adopts a high-resolution direct digital synthesis as the controllable radio frequency (RF) signal source. Meanwhile, an analog-to-digital converter synchronization acquisition technology combined with a normalization approach is proposed, which effectively solves the problem of the uneven amplitude-frequency characteristic in the range of 50 MHz-100 MHz. Moreover, the apparatus is integrated by adopting the RF power and applying the weak signal amplifier as an auxiliary measurement channel. The equivalent circuit of the sensors resonant cavity was simulated, and the efficiency curve of the adjustable capacitors to the resonant frequency and the quality factor were obtained. The simulated results were further verified by testing the resonance cavity characteristics of a commercial Overhauser geomagnetic sensor under different conditions. Furthermore, the relationship between the polarization degree of the free radical solution and RF excitation power and time were also obtained. The testing methods and results were given, and the experimental data were analyzed. Finally, the experimental results demonstrate that the proposed apparatus can measure the polarization frequency of the free radical solution, the bandwidth, and the quality factor, accurately. Furthermore, it can be used for the determination of the polarization power and time during the design process for an Overhauser magnetometer.

Integrating gravity and magnetic field data to delineate structurally controlled gold mineralization in the Sefwi Belt of Ghana

Gravity and magnetic surveys were used to delineate potential gold mineralization zones in the Sefwi belt of Ghana. The study area is an intrusive dominated area that hosts pockets of small scale mining operations locally referred to as Galamsey. These Galamsey operations are not guided by a scientific approach to back the trend of gold mineralization which is conventionally mined. The study aimed at mapping lithological units, structural setting and relating Galamsey sites to delineate potential zones of gold mineralization. A Scintrex CG5 gravimeter and GEM's Overhauser magnetometer were used for gravity and magnetic data acquisition respectively. The magnetic data were corrected and enhancing filters such as reduction to the pole (RTP), analytical signal and first vertical derivative were applied using Oasis montaj 7.1. Gravity data were also reduced to the geoid using the Oasis montaj software to produce a complete Bouguer anomaly map. The regional/residual separation technique produced a residual gravity map. The RTP and analytical signal filters from the magnetic data and residual gravity anomaly map from the gravity data helped in mapping belt type (Dixcove) Birimian granitoids and mafic intrusive unit, interpreted as gabbro. The first vertical derivative filter was useful in mapping NE/SW minor faults and crosscutting dykes largely concentrated in the belt type Birimian granitoids. All the three mapped Galamsey sites fell on a minor fault and are associated with the belt type granitoids which were used in delineating four potential zones of gold mineralization.

Research on an Improved Resonant Cavity for Overhauser Geomagnetic Sensor

Resonant cavity is an important component of Overhauser magnetometer sensor. Its function is to make the working substance generate dynamic nuclear polarization effect in the sensor. An alternative design of resonant cavity based on the birdcage coil structure is proposed to make the resonant frequency in accordance with the polarization frequency of the working substance. The structure and equivalent circuit of resonant cavity are analyzed. The model of resonant cavity is established, and S-parameter is calculated. The simulating results show that resonant frequency of the resonant cavity is about 60 MHz, which is consistent with the polarization frequency of the working substance in the sensor, and Q-factor of the resonant cavity is about 90. A dual-coil structure for pick-up coil and series resonant circuit are designed for reducing interference. In addition, output signal experiment and simultaneous comparison testing of sensor prototype are conducted. Overall, this design has the advantages of high performance and strong anti-interference. The resolution, uncertainty, and range of the sensor reach to 0.005 nT, 0.2 nT, and 20 000–100 000 nT, respectively.

Magnetic survey of archaeological kiln sites with Overhauser magnetometer: A case study of buried Sue ware kilns in Japan

We conducted magnetic surveys using an Overhauser magnetometer, a fast-sampling, high-accuracy magnetometer, at two archaeological sites in central Japan to detect magnetic anomalies associated with buried climbing Sue ware kilns in suburban hill areas. Our magnetic survey was conducted in two stages, a preliminary sweeping survey followed by a detailed mapping survey. The mapping survey at both sites revealed magnetic anomalies of the order of 10–100 nT that were clearly distinguishable from the low background signals due to the basement rocks and soils. After the magnetic surveys, archaeological excavations were conducted on the basis of these magnetic anomalies, where two kilns were successfully discovered. The shape, dimensions, and magnetization of the kilns are consistent with the amplitude and distribution of the magnetic anomalies. These results highlight that the combination of quick sweeping and detailed mapping magnetic surveys is useful for detecting buried kilns in hillslope areas.

Short-Time and High-Precision Measurement Method for Larmor Frequency of Marine Overhauser Sensor

The measurement accuracy of the Larmor frequency of the Overhauser magnetic sensor directly determines the magnetic accuracy of the Overhauser magnetometer. We propose a multichannel interpolation method for marine magnetic measurements, addressing specifically the deficiencies in traditional measurement methods. The quantization error of the count value of the standard signal is greatly reduced using the delay chain interpolation technique. Also the influence arising from the phase error is weakened through channel expansion. In addition, we also design a test platform and conduct two sets of field contrast tests, the test results showing that the proposed method not only improves the precision of the frequency measurement by about three times but also improves the resolution and lowers the noise level of the system.

Construction of an Overhauser magnetic gradiometer and the applications in geomagnetic observation and ferromagnetic target localization

The proton precession magnetometer with single sensor is commonly used in geomagnetic observation and magnetic anomaly detection. Due to technological limitations, the measurement accuracy is restricted by several factors such as the sensor performance, frequency measurement precision, instability of polarization module, etc. Aimed to improve the anti-interference ability, an Overhauser magnetic gradiometer with dual sensor structure was designed. An alternative design of a geomagnetic sensor with differential dual-coil structure was presented. A multi-channel frequency measurement algorithm was proposed to increase the measurement accuracy. A silicon oscillator was adopted to resolve the instability of polarization system. This paper briefly discusses the design and development of the gradiometer and compares the data recorded by this instrument with a commonly used commercially Overhauser magnetometer in the world market. The proposed gradiometer records the earth magnetic field in 24 hours with measurement accuracy of ± 0.3 nT and a sampling rate of 3 seconds per sample. The quality of data recorded is excellent and consistent with the commercial instrument. In addition, experiments of ferromagnetic target localization were conducted. This gradiometer shows a strong ability in magnetic anomaly detection and localization. To sum up, it has the advantages of convenient operation, high precision, strong anti-interference, etc., which proves the effectiveness of the dual sensor structure Overhauser magnetic gradiometer.

The potential of rotary-wing UAV-based magnetic surveys for mineral exploration: A case study from central Sweden

Unmanned aerial vehicle (UAV)-based geophysical surveys are attractive for land mineral exploration and are gradually opening extraordinary opportunities in providing high-resolution definition of geologic structures and for direct targeting of mineral deposits. There are, however, challenges such as electromagnetic noise from the UAV, limited load capacity, and short flight times. If these are overcome, there will be a new era in using UAV-based geophysical systems for mineral exploration and for a number of mining-related purposes. In this study, we have tested the potential of rotary-wing UAV systems, given their flight flexibility and robustness for direct targeting of iron-oxide deposits in central Sweden. A walking-mode high-precision Overhauser magnetometer was reassembled so that it could be lifted by the rotary-wing system. Successful backyard tests were performed, but during the real experiment several issues related to high UAV noise level and extreme magnetic field from the mineralization delayed data acquisition. At the end, within nearly three hours and 10 sorties, approximately 20 km-line total-field magnetic data were collected covering an area of about 2 km2. Flight lines were designed perpendicular to the strike of the mineralization to maximize data sampling. Two distinct mineralized zones, magnetite- and hematite-rich and only 50–100 m apart, are notable in the magnetic data due to the fine sampling spacing provided by the UAV survey. Historical low-altitude (30 m above the ground) fixed-wing aeromagnetic data are available from the study area and are compared with the UAV data. Both data sets are consistent in delineating the mineralization, therefore demonstrating the potential of UAV-based surveys for mineral exploration in geologically and logistically challenging areas.

The low cost Proton Precession Magnetometer developed at the Indian Institute of Geomagnetism

Proton magnetometers are the oldest scalar magnetometers. The first commercial units were produced in early 1960s as portable instruments. In continuation airborne instruments appeared with optimized speed of readings and sensitivity, large sensors etc. Later development of Overhauser and optically pumped magnetometers has eliminated Proton magnetometers from airborne surveys. However they remain very popular in various ground surveys and observatories. With this primary purpose of generating the ground based magnetic data, the Indian Institute of Geomagnetism (IIG) for the last 3 decades have been developing low cost Proton Precession Magnetometers (PPM). Beginning with the 1 nT PPM which has undergone several changes in design, the successor PM7 the advanced version has been successfully developed by the institute and is installed at various observatories of the institute. PM7 records the total field `F' with accuracy of 0.1 nT and a sampling rate of 10 seconds/sample. This article briefly discusses the design and development of this IIG make PM7 and compares the data recorded by this instrument with one of the commercially available Overhauser magnetometer in the world market. The quality of data recorded by PM7 is in excellent agreement with the Overhauser. With the available quality of data generated by this instrument, PM7 is an affordable PPM for scientific institutions, schools and colleges intending to carry out geomagnetic studies. The commercial cost of PM7 is ≈ 20% of the cost of Overhauser available in market.

Magnetic observations at Geophysical Observatory Paratunka IKIR FEB RAS: tasks, possibilities and future prospects

Continuous magnetic measurements at Geophysical Observatory “Paratunka” (PET) of IKIR FEB RAS are performed since 1967. In the new millennium analogue magnetometers were modernized to digital, the technologies of absolute observations were changed, the data processing was completely transferred to computers, and the status of INTERMAGNET observatory was obtained. Currently, the observatory uses the following magnetometers: (a) for absolute observations – DIflux LEMI-203 (theodolite 3T2KP) and Mag-01 (theodolite Wild-T1), Overhauser magnetometers POS-1 and GSM-19W; (b) for variation measurements – fluxgate magnetometers FGE-DTU, FRG-601 and MAGDAS (installed under international agreements of IKIR), vector magnetometers dIdD GSM-19FD and POS-4 with Overhauser sensors and coil systems, scalar magnetometer GSM-90 and induction magnetometer STELAB. During Spring-Autumn season dIdD also is installed at remote station “Karymshina” at distance of 15 km from Observatory. There is monitoring system for monitoring of conditions in which magnetic observations are performed, including the semi-professional weather stations Davis Vantage Pro2 and WS2000 and a network of digital temperature sensors DS19B20 located at various points in magnetic pavilions and outdoor. All measurements are synchronized with the UTC. The results of observations are collected by the IKIR data server from the recorders and loggers, including in real-time. Specialized software was developed (based on MATLAB and Octave packages), which allows automatic and semi-automatic processing of data, the comparison of the results from different magnetometers and presenting final data in formats, defined by international standards, including INTERMAGNET. Significant efforts of observatory staff are direct to archive (raw) magnetic data, a significant part of which has not been entirely processed, is not presented in international data centers and is still not available to the scientific community. Digital images of analog magnetograms have been obtained, the digitization of which allowed to fill the gaps in the hourly data available in the WDC for 1991, 1996-1997. The state of the set of the instruments and databases shows that the observatory “Paratunka” can solve most of the problems that arise in the geomagnetic field researches, including the provision of data for the development of regional and global field models, ground support of magnetometers located on satellites, support of the surface (sea) and aeromagnetic surveys and drilling, providing the data for researches of processes in the lithosphere, magnetosphere and ionosphere over a wide frequency range, and also for a number of applied problems.