Total Magnetic Field Measurements Research Articles

Characterizing electromagnetic interference signals for unmanned aerial vehicle geophysical surveys

The development of a functional unmanned aerial vehicle (UAV) mounted aeromagnetic system requires integrating a magnetometer payload onboard a UAV platform in a manner that preserves the integrity of the total magnetic field measurements. One challenge when developing these systems is accounting for the sources of in-flight magnetic and electromagnetic interference signals that are greater than the resolvability threshold of the magnetometer. Electromagnetic interference generated by the platform has the potential to be mitigated using several techniques such as magnetic shielding, filtering, or compensation and can be attenuated by strategically positioning the magnetometer at a distance from the UAV. The integration procedure and selection of a mitigation strategy can be informed by characterizing the electromagnetic interference generated by the platform. Scalogram analysis is used to characterize the high-frequency electromagnetic signals generated by multirotor UAV electromagnetic motors. A low-sensitivity (7 nT) vector, fluxgate magnetometer is used to measure the electromagnetic interference generated by two unique multirotor UAVs in a controlled laboratory setting. Results demonstrate three spectrally distinct electromagnetic signals, each with unique frequency and amplitude, generated by each UAV platform. The frequency of these electromagnetic interference signals is found to be directly proportional to the applied rotation frequency of the electromagnetic motor. The aforementioned knowledge is applied to UAV field surveys to assess the high-frequency electromagnetic interference signals experienced. This is achieved using a high sensitivity (0.01 nT), scalar optically pumped magnetometer with a 1000 Hz sampling frequency. Our results indicate that adequate sensor placement and preflight evaluation of the platform-sensor interactions provide useful mitigation strategies, which can compensate for electromagnetic interference signals generated by the UAV platform during aeromagnetic surveys.

Airborne Magnetic Surveying With a Drone and Determination of the Total Magnetization of a Dipole

Recent developments of unmanned aerial vehicle (UAV) raise an interest for aerial magnetic measurements closer to the sources than the usual aeromagnetic surveys done with planes. For some years now, the ECA Group has been developing an UAV named IT-180. It was decided to use this UAV to carry a three-component fluxgate magnetometer in order to measure ships’ signature. Some major issues are addressed as the magnetic field of the drone is reduced as low as possible. On the other hand, vector magnetic measurements cannot be performed with a moving sensor and only the total magnetic field can be measured. It is shown how the influence of a carrier and the three main types of errors of the fluxgate magnetometer can be reduced using calibration and compensation procedures, and two different methods are proposed to compute the three components of the magnetic field using the total magnetic field measurements. Finally, those methods are field tested using a calibrated source with a strong vertical magnetization.

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

ABSTRACTRecent 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.

Identification of Magnetic Anomaly at Geothermal Subsurface Area PLTP Sarulla Unit I Pahae Jae, North Sumatera,Indonesia

This research was done entitled Identification of Magnetic Anomalies at Geothermal Subsurface Area at PLTP Sarulla Unit I Pahae Jae, North Tapanuli. This research was conducted to find out the dispersion pattern of earth magnetic anomaly based on its magnetism and to know the rock structure based on susceptibility value in the geothermal area. The measurements of Total magnetic field using Proton Precission Magnetometer (PPM) Elsec 770 type, Global Position System (GPS) for positioning and geological compass for north direction orientation. The data were collected randomly with the number of points obtained as much as 50 measuring point, then data analysis was done by using surfer 11 to get contour map and Mag2DCfor windows maps to get the magnetic anomaly cross section. The results showed that the magnetic anomaly value had the lowest value of 34,4358 nT and the highest value 144,789 nT moving from North toward East. While the results of quantitative interpretation of A’A model show sedimentary rock with k value (0.0035 x 103, 00059 x 103, 00061 x 103), and esite lava with k value (0.0277 x 103, 0.0241 x 103) and basalt with k value (0.2437 x 103).

Localized geomagnetic field anomalies in an underground gas storage

To investigate possible piezomagnetic effects associated with imposed tectonic stresses, repeated measurements of total magnetic field in an array (∼120 km × 90 km in extent) were performed across the Hutubi underground gas storage (HUGS). Forty repeated measurement stations were established in the HUGS and its surrounding areas. Proton precession magnetometers with a sensitivity of 0.15 nT @ 1 Hz were used to measure the total magnetic field at each repeated station. We conducted two field measurement surveys, one with a minimum pressure during April 2017 and the other with a maximum pressure during November 2017 in the HUGS. The local magnetic field (LMF) of the HUGS and its surrounding areas was obtained after data processing, which included diurnal variation reduction (DVR), secular variation reduction (SVR) and LMF acquisition. The average of the mean standard deviations (σ) of the geomagnetic DVR values were 0.038 nT and 0.045 nT for the first and second field surveys respectively. The amplitude of the LMF varied between about 10 nT and 90 nT in the local area. Negative anomalies with a maximum amplitude of −2.1 nT was observed over a seven-month period (April 2017–November 2017) during the loading process with a constantly increasing pressure in the HUGS. The wide range of negative LMF anomalies shows a negative correlation with the increasing pressure in the HUGS. This result constitutes powerful field observational evidence for the existence of piezomagnetic effects under middle-scale experimental conditions.

Evaluation and archaeological potential of geophysical investigations of the Roman fort and baths in Northern Muntenia, România

The Roman fort from Sfârleanca is one of the most representative archaeological sites dating from the Roman period (2nd century A.D.) in Northern Muntenia. The existing natural and anthropic features of the environment required the application of geophysical methods in order to outline the spatial pattern of the buried remains, to define the geometry of the anthropogenic settlements and to obtain detailed information about different archaeological materials without digging. During the survey, two different geophysical methods have been employed: total magnetic field measurements and electrical resistance mapping using Twin-probe array. The instrument consists of GSM19W Overhauser magnetometers with GPS, in base-rover system, and a twin-probe array LGM 4-Point light hp. The measurements were used to draw primary maps of the physical parameters (total magnetic field strength/intensity, electrical resistance), and also processed maps (filtering, derivative). The magnetic results obtained by interpreting the anomalies yielded information about the limits of the fort, about the internal organization of the military structure (its axial road, partially its secondary road, the localization and the shape of its constructions) and at the bath and heating installation. A previously unknown element is the possible water supply pipe made of ceramic material highlighted by the mathematical modelling of the data obtained by the magnetic investigations. Electrical resistance results provide complementary information to the magnetic survey concerning the limits of the baths and the remains of the fort structure. This paper brings to light geophysical investigations into this Roman fort and baths, extending the picture produced by previous archaeological excavations that only dealt with a small part of the site. It indicates the importance of using geophysical methods in preliminary archaeological research and the advantages of combining total magnetic field measurements and electrical resistance mapping when investigating an archaeological site characterized by a number of environmental difficulties.

Magnetic characteristics of fracture zones and constraints on the subsurface structure of the Colima Volcanic Complex, western Mexico

Detailed magnetic anomaly surveys over the central and southern sector of the Colima rift, western Mexico, are used to investigate the subsurface structure and faults and/or fractures in the volcanic terrains formed by activity in the Colima volcanic complex (CVC). The CVC is located within the large north-south Colima rift in western Mexico. The Colima rift is a major active tectonic structure, trending perpendicular to the Middle America Trench and related to subduction of the Rivera and Cocos plates. Volcanic activity in the CVC has migrated southward toward the trench. Analyses of faults and recent deformation in the CVC and Colima rift are of major interest in volcano-tectonic studies and for hazard assessment. Structural analyses and fault mapping, however, are difficult because young volcanic and pyroclastic rocks obscure structural features and stratigraphy. Most of the southern Colima rift is covered by volcanic avalanches and volcaniclastic units, which have resulted in resurfacing of the volcanic terrains. Here we show that magnetic anomalies permit identification of faults and mapping of volcano-sedimentary and volcanic units. Total magnetic field measurements spaced every 0.5 km along 8 profiles, with an overall length of 284.5 km and covering the CVC sector of the Colima rift, have been obtained. We recognize fractures and fault zones of local and regional character from their characteristic magnetic anomaly response. Large mapped structures include the north-south Montitlan, northeast-southwest La Lumbre, and east-west La Escondida faults, which can be traced across the area from the magnetic profiles. Fault magnetic anomalies are modeled by lateral contrasts in terms of step models assuming thin dipping elongated zones along the fault planes. The study shows that faults in the CVC volcanic terrain can be investigated by magnetic surveying.

Integrated Geophysical Studies in the East-Indian Geothermal Province

Integrated geophysical surveys using vertical electrical sounding (VES), very low frequency (VLF) EM, radiation counting, total magnetic field and self-potential (SP) measurements are carried out to characterize the geothermal area around a hot spring in the Nayagarh district, Orissa, India that lies in the East Indian geothermal province. The study was performed to delineate the fracture pattern, contaminated groundwater movement and possible heating source. VES interpretations suggest a three- to four-layer structure in the area. Resistivity survey near the hot spring suggests that weathered and fractured formations constitute the main aquifer system and extend to 60 m depth. Current flow measured at various electrode separations normalized by the applied voltage suggests that fractures extend to a greater depth. Detailed VLF study shows that fractures extend beyond 70 m depth. VLF anomaly has also very good correlation with the total magnetic field measured along the same profiles. Study results suggest that a gridded pattern of VLF survey could map the underground conductive fracture zones that can identify the movement of contaminated groundwater flow. Therefore, precautionary measures can be taken to check further contamination by delineating subsurface conducting structures. Self potential (SP) measured over the hot spring does not show a large anomaly in favor of the presence of a sulphide mineral body. A small positive (5–15mV) SP anomaly is measured which may be streaming potential due to subsurface fluid flow. A high radiation is measured about four kilometers from the hot spring, suggesting possible radiogenic heating. However, the exact nature of the heating source and its depth is not known in the area. Deep resistivity followed by a magneto-telluric survey could reveal the deeper structures.

Magnetic and electric field monitoring of Taal volcano, Philippines Part I: Magnetic measurements

We have started electromagnetic (EM) monitoring of Taal volcano, Philippines. Taal is a basaltic-andesite volcano which has many eruption histories and is close to Metro Manila. On Taal, various geophysical and geochemical observations have been conducted by the government basis, but not the EM monitoring. A cooperative project for EM studies between PHIVOLCS and EMSEV/IUGG has been started. Measurements of total magnetic field (TMF), self-potential (SP), ground temperature and carbon dioxide degassing were made during the first survey in January 2005. Distinct anomalies in TMF and SP were found in the geothermal areas in Taal. Repeated survey in February detected changes in TMF up to a few tens of nT and enhancement of positive SP anomaly during the one and half-months period, when the surface geothermal activities were high. We believe that the combination of mechanical and electromagnetic measurements is uniquely effective in deciphering the volcanic activity. (Communicated by Seiya UYEDA, M.J.A.)

자기이상 역산에 의한 마샬제도 북서쪽 세 해저산의 고지자기 해석

Total magnetic field measurements were performed to study paleomagnetism of three seamounts (OSM7, OSM8-1, and OSM8-2) to the northwest of the Marshall Islands in the western Pacific. The study area is located at the Ogasawara Fracture Zone which is a boundary between the Pigafetta and East Mariana basins. The magnetic parameters and paleopoles of three seamounts were derived from inversion of the measured magnetic field. The goodness-of-fit ratio of OSM7 is too low to be included to the estimation of parameters. The complex magnetic anomalies of center, scarcity of flank rift zones and steep slope at OSM7 suggest that the multiple intrusions of magma converge into the center of volcanic edifice. Inclination calculated from the magnetic anomalies of OSM8-1 and OSM8-2 is , and the paleolatitude calculated from the inclination is . The corresponding paleopoles for OSM8-1 and OSM8-2 are , respectively. In comparison with the apparent polar wander path (APWP) of the Pacific plate, the paleopoles are close to 129-Ma pole. The paleopoles and paleolatitudes of OSM8-1 and OSM8-2 suggest that they were formed at similar time and location. The seamounts have drifted northward about from the paleolatitude to present latitude of seamounts.

Removal of DC power-line magnetic-field effects from airborne total magnetic-field measurements

Removal of DC power line magnetic field effects from airborne total magnetic field measurements in Gavle area, Sweden

Crustal magnetization of the Reykjanes Ridge and implications for its along‐axis variability and the formation of axial volcanic ridges

We have inverted the sea surface total magnetic field measurements of the Reykjanes Ridge, between 57°N and 63°N, for crustal magnetization intensity assuming a uniformly thick magnetic source layer and direction of magnetization corresponding to the geocentric axial dipole field. This section of the north Mid‐Atlantic Ridge (10 mm/yr half spreading rate) is unique in that it lies within the influence of the Iceland mantle plume, which causes it to spread obliquely and in the north to resemble morphologically a fast‐spreading ridge. On a regional scale the axial magnetization anomalies of the Reykjanes Ridge can be divided into three regimes from south to north. These regimes correspond to sections of the ridge with different magmatic and tectonic styles which are believed to represent the effects of a pulsing plume beneath Iceland. The axis south of 59°10′N has the highest magnetization anomaly with the greatest variability and exhibits the characteristics of a slow‐spreading ridge unaffected by a plume. The axis between 59°10′N and 60°50′N has the lowest magnetization anomaly on average and is where we presume the present day plume front is located. The absence of large fault scarps and low magnetization are consistent with the idea of a midcrustal magma reservoir, where mixing causes the magma to become less highly fractionated and produces less magnetic lavas. Finally, a moderate increase in the magnetization anomaly and degree of faulting occurs north of 60°50′N, signaling a gradual recovery in the crustal production and underlying structure after the passage of the plume front. On the local scale, with a few exceptions, the loci of high magnetization anomalies coincide with the axial volcanic ridges. Notable lateral offsets in axial magnetization and decay in magnitude were found at 57°50′N, 58°05′N, and 59°12′N. Comparisons with seismic layer 2A show that variations in the magnetic source layer thickness may account for as much as half of the observed variability in the along‐axis magnetization. There are also indications that the relative age of the extrusives and the degree of faulting and fissuring may be factors affecting the magnetization intensity of volcanic edifices on the seafloor.

GROUNDWATER EXPLORATION IN HARD‐ROCK TERRAIN—A CASE HISTORY*

AbstractThe utility of electrical resistivity and magnetic measurements to locate suitable wellsites in hard‐rock terrains intruded by dykes is illustrated. The example is taken from a highly hilly terrain of Karnataka State situated in the Western Ghat hills of Peninsular India. Electrical resistivity profiles and soundings, S.P., and total magnetic field measurements are discussed. Of the fifteen recommended sites, nine have been drilled, eight of which are reported to yield between 6–12 m3/hr (1500 to 3000 gals./hr), which is considered satisfactory in this region.

Part 6. Present plate boundary seismic, volcanic and kinematic processes: Magnetics of the Rabaul caldera

The Rabaul caldera in East New Britain is a collapse caldera of about 10 km diameter within which some active volcanism remains. Total magnetic field measurements show large anomalies associated with volcanic centres on the East side of the caldera, but only a small one on Vulcan which is on the West side. This correlates with the contrast between the basalts and andesites erupted on the East side, and the dacite pumice and ash of Vulcan. The absence of a large anomaly field on Vulcan may also be due to its structure, being an ash cone lacking in solid lave flows. A programme of repeat measurements on two potentially active craters on the East side has been mounted in order to detect possible anomaly field changes due to thermal changes.

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A latitude survey of the equatorial electrojet with rocket-borne magnetometers

Latitudinal cross section of ionospheric current density profile of equatorial electrojet from rocketborne magnetometers

Total Magnetic Field Measurements between New Zealand and Antarctica

Total Magnetic Field Measurements between New Zealand and Antarctica

Computation of vertical magnetic anomalies from total magnetic field measurements

Magnetic measurements for geophysical work have been largely confined to measurement of the vertical field. Recently an instrument that measures only the total magnetic field has been invented and used with considerable success. Its operation is simpler and faster than the vertical magnetometer and its accuracy is as good as the best of the available vertical instruments. Since the instrument responds only to the total field, its readings are completely independent of orientation or leveling. It is shown that the readings of this instrument give the component of the magnetic anomalies along the direction of dip of the Earth's field. Since this “slant” anomaly is not directly comparable with ordinary measurements of the vertical and horizontal anomalies, or easily interpretable, a method of computing the equivalent vertical and horizontal anomalies has been devised. It is shown that the horizontal and vertical anomalies can be expressed as surface integrals over the surface of measurement of the “slant” anomaly. Means of evaluating these integrals are discussed.