THE GEOMAGNETIC GRADIOMETER

Geophysics ◽  
1967 ◽  
Vol 32 (5) ◽  
pp. 877-892 ◽  
Author(s):  
Howard A. Slack ◽  
Vance M. Lynch ◽  
Lee Langan
Keyword(s):  
Magnetic Field ◽  
Vertical Gradient ◽  
Diurnal Variations ◽  
Resolving Power ◽  
Magnetic Intensity ◽  
Geophysical Prospecting ◽  
Total Field ◽  
Optically Pumped ◽  
Magnetic Gradient ◽  
The Difference

The geomagnetic gradiometer is a new geophysical prospecting tool which measures directly the vertical gradient of the earth’s magnetic field and the total field intensity. The system is composed of two simultaneously recording, optically pumped and monitored magnetometer sensors suspended from a helicopter. The sensors are separated vertically by a known distance so that the magnetic gradient can be determined from the difference in total magnetic intensity between the two sensors. Since the gradient is measured directly, the gradiometer allows geophysicists to make better use of LaPlace’s and Euler’s equations. The gradiometer increases the value of magnetic prospecting by: (1) greatly increasing resolving power, (2) discriminating between intrabasement and suprabasement anomalies, and (3) eliminating problems caused by diurnal variations.

A comparison of methods for approximating the vertical gradient of one‐dimensional magnetic field data

Geophysics ◽  
1986 ◽  
Vol 51 (9) ◽  
pp. 1725-1735 ◽  
Author(s):  
J. W. Paine
Keyword(s):  
Magnetic Field ◽  
Fourier Transform ◽  
Field Data ◽  
Total Error ◽  
Vertical Gradient ◽  
Magnetic Data ◽  
Total Field ◽  
One Dimensional ◽  
Convolution Filter ◽  
Principal Factors

The vertical gradient of a one‐dimensional magnetic field is known to be a useful aid in interpretation of magnetic data. When the vertical gradient is required but has not been measured, it is necessary to approximate the gradient using the available total‐field data. An approximation is possible because a relationship between the total field and the vertical gradient can be established using Fourier analysis. After reviewing the theoretical basis of this relationship, a number of methods for approximating the vertical gradient are derived. These methods fall into two broad categories: methods based on the discrete Fourier transform, and methods based on discrete convolution filters. There are a number of choices necessary in designing such methods, each of which will affect the accuracy of the computed values in differing, and sometimes conflicting, ways. A comparison of the spatial and spectral accuracy of the methods derived here shows that it is possible to construct a filter which maintains a reasonable balance between the various components of the total error. Further, the structure of this filter is such that it is also computationally more efficient than methods based on fast Fourier transform techniques. The spacing and width of the convolution filter are identified as the principal factors which influence the accuracy and efficiency of the method presented here, and recommendations are made on suitable choices for these parameters.

GRADIENT MEASUREMENTS IN AEROMAGNETIC SURVEYING

Geophysics ◽  
1965 ◽  
Vol 30 (5) ◽  
pp. 891-902 ◽  
Author(s):  
Peter Hood
Keyword(s):  
Optical Pumping ◽  
Vertical Gradient ◽  
Horizontal Distance ◽  
Total Field ◽  
Highly Sensitive ◽  
Geophysical Information ◽  
The Difference ◽  
Gradient Measurements ◽  
First Vertical Derivative ◽  
Depth Of Burial

The recent development of highly sensitive magnetometers, such as the optical‐pumping varieties, has made feasible the measurement of the first vertical derivative of the total field (∂ΔT/∂h) in aeromagnetic surveys. This is accomplished by using two sensitive magnetometer heads separated by a constant vertical distance, and recording the difference in outputs. The effect of diurnal is thus eliminated in the resultant differential output, and this is an especially desirable feature in northern Canada where the diurnal variation is usually much greater than is found in more southerly magnetic latitudes. Moreover, steeply dipping geological contacts in high‐magnetic latitudes are outlined by the resultant zero‐gradient contour. It is also possible to obtain the depth of burial of the contact from the graph of (∂ΔT/∂h) versus (x∂ΔT/∂x) where x is the horizontal distance measured from the contact. Similar quantitative interpretations may be made for the point pole and dipole. The data reduction necessary to produce a vertical‐gradient map is much simpler than with the total‐field case because no datum levelling is necessary. Since the aircraft track will be available from the main compilation it is only necessary to plot the resultant vertical‐gradient values on the track map and contour. Thus, two maps will be obtained for little more than the price of one but with a greatly increased gain in geophysical information concerning the geometry of the causative bodies. Actually, a first‐derivative map is difficult (and therefore costly) to produce by any other means. The measurement of the vertical gradient would appear to be the main advantage to using hundredth‐gamma magnetometers in aeromagnetic surveys, since those types presently in service are sensitive enough for the effective delineation of total‐field anomalies.

Automated interpretation of horizontal magnetic gradient profile data

Geophysics ◽  
1992 ◽  
Vol 57 (2) ◽  
pp. 288-295 ◽  
Author(s):  
D. L. Marcotte ◽  
C. D. Hardwick ◽  
J. B. Nelson
Keyword(s):  
Vertical Gradient ◽  
Horizontal Gradient ◽  
Two Dimensional ◽  
Total Field ◽  
Profile Data ◽  
Magnetic Gradient ◽  
Inversion Methods ◽  
Automated Interpretation ◽  
A New Technique ◽  
Magnetic Gradients

A new technique has been developed to estimate the vertical magnetic gradient [Formula: see text] along a profile from measurements of the two orthogonal horizontal magnetic gradients [Formula: see text]. In addition, a means of identifying two‐dimensional anomaly sources and the angle of the structure relative to the profile direction is shown to be a function of [Formula: see text]. This angle can be used to correct interpretations from vertical gradient or total field inversion methods which assume source structures oriented perpendicular to the profile direction. A modified Werner deconvolution algorithm for vertical gradient data incorporating these features has been applied to both real and simulated horizontal gradient data.

MAGNETIC SURVEYING USING HORIZONTAL GRADIENT VECTORS

Geophysics ◽  
1977 ◽  
Vol 42 (6) ◽  
pp. 1262-1264 ◽  
Author(s):  
S. Thyssen‐Bornemisza
Keyword(s):  
Magnetic Field ◽  
Unit Vector ◽  
Magnetic Potential ◽  
Horizontal Gradient ◽  
Magnetic Intensity ◽  
Total Field ◽  
Nonlinear Characteristics ◽  
Normal Magnetic Field ◽  
Vertical Gradients ◽  
Airborne Magnetic

It was pointed out some time ago (Bhattacharyya, 1965) that the total intensity anomaly of a magnetic field ΔT in the direction of the normal magnetic field of earth is expressed by the equation, [Formula: see text]Here ΔV denotes the anomaly of the magnetic potential and t the unit vector in the direction of earth’s undisturbed total field. Horizontal and vertical gradients observed along the tracks of airborne magnetic surveys were discussed by several authors (Wickerham, 1954; Glicken, 1955; Hood, 1965; Langan, 1966). These gradients are obtained from the formulas [Formula: see text] [Formula: see text]where the magnetic intensity differences are observed over horizontal and vertical intervals Δx and Δz between two sensors. However, this approach is only valid when the depth h of the causative body or structure is relatively large compared to Δx and Δz; thus in cases of shallow anomalies, the nonlinear characteristics of the anomalous magnetic field would distort the observed gradients and render interpretation of data very difficult.

APPROXIMATION OF RESIDUAL TOTAL‐MAGNETIC‐INTENSITY ANOMALIES

Geophysics ◽  
1964 ◽  
Vol 29 (4) ◽  
pp. 623-627 ◽  
Author(s):  
A. L. Kontis ◽  
G. A. Young
Keyword(s):  
Magnetic Field ◽  
Magnetic Intensity ◽  
Normal Field ◽  
The Difference ◽  
The Magnetic Field

Utilizing magnitude and direction observations of the magnetic field, the anomalous total intensity (FA), the residual total intensity (FR), and the projection of the anomalous force in the direction of the earth’s normal field (T) are computed for two profiles over Plantagenet Bank. It is demonstrated that for the large anomaly over Plantagenet, the difference between T and FR is relatively small, and that in general T is a good approximation of the residual anomaly.

DOWNWARD CONTINUATION USING THE MEASURED VERTICAL GRADIENT

Geophysics ◽  
1971 ◽  
Vol 36 (3) ◽  
pp. 609-612 ◽  
Author(s):  
John Ackerman
Keyword(s):  
Magnetic Field ◽  
Integral Equation ◽  
Direct Method ◽  
Short Note ◽  
Taylor Series Expansion ◽  
Vertical Gradient ◽  
Downward Continuation ◽  
Total Field ◽  
Continuation Problem ◽  
Gradient Measurement

The availability of two independent measured quantities, the total magnetic field and the vertical gradient of the total field, allows a solution to the downward continuation problem not heretofore practical. The method proposed in this short note uses the Laplace equation and a Taylor series expansion but does not require the numerical solution of an integral equation derived from potential theory. Evjen (1937) discussed the use of this technique for the gravitational field; however, he did not advocate the method because of the practical difficulties involved in making the gradient measurement. It is the purpose of my note to point out the utility of this direct method and to discuss the details of its application to airborne gradiometer survey data now being collected.

International geomagnetic reference field—Its evolution and the difference in total field intensity between new and old models for 1965–1980

Geophysics ◽  
1983 ◽  
Vol 48 (12) ◽  
pp. 1691-1696 ◽  
Author(s):  
Norman W. Peddie
Keyword(s):  
Magnetic Field ◽  
Field Intensity ◽  
Upper Atmosphere ◽  
Reference Field ◽  
Total Field ◽  
Main Field ◽  
International Geomagnetic Reference Field ◽  
The Earth ◽  
Crustal Field ◽  
The Difference

The total magnetic field near the surface of the Earth is a sum of several constituent fields. Part of the total field consists of fields that are transient or rapidly varying. These fields are caused, either directly or indirectly, by electric currents in the upper atmosphere and beyond. The part of the total field that is more permanent arises from sources that are located inside the Earth. Evidence suggests that this part has two principal constituents: the main field and the crustal field.

A Super-High-Resolution HVEM (H-1500) Newly Installed in NIRIM

1990 ◽  
Vol 48 (1) ◽  
pp. 142-143
Author(s):  
S. Horiuchi ◽  
Y. Matsui
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Chromatic Aberration ◽  
Inorganic Materials ◽  
Electron Gun ◽  
Resolving Power ◽  
National Budget ◽  
Voltage Electron ◽  
Specimen Holder ◽  
The Magnetic Field

A new high-voltage electron microscope (H-1500) specially aiming at super-high-resolution (1.0 Å point-to-point resolution) is now installed in National Institute for Research in Inorganic Materials ( NIRIM ), in collaboration with Hitachi Ltd. The national budget of about 1 billion yen including that for a new building has been spent for the construction in the last two years (1988-1989). Here we introduce some essential characteristics of the microscope.(1) According to the analysis on the magnetic field in an electron lens, based on the finite-element-method, the spherical as well as chromatic aberration coefficients ( Cs and Cc ). which enables us to reach the resolving power of 1.0Å. have been estimated as a function of the accelerating As a result of the calculaton. it was noted that more than 1250 kV is needed even when we apply the highest level of the technology and materials available at present. On the other hand, we must consider the protection against the leakage of X-ray. We have then decided to set the conventional accelerating voltage at 1300 kV. However. the maximum accessible voltage is 1500 kV, which is practically important to realize higher voltage stabillity. At 1300 kV it is expected that Cs= 1.7 mm and Cc=3.4 mm with the attachment of the specimen holder, which tilts bi-axially in an angle of 35° ( Fig.1 ). In order to minimize the value of Cc a small tank is additionally placed inside the generator tank, which must serve to seal the magnetic field around the acceleration tube. An electron gun with LaB6 tip is used.

Effect of Magnetic Fields on the Resistance and Microstructure of Al-Cu Alloy during Solidification Processing

2011 ◽  
Vol 287-290 ◽  
pp. 2916-2920
Author(s):  
Chun Yan Ban ◽  
Peng Qian ◽  
Xu Zhang ◽  
Qi Xian Ba ◽  
Jian Zhong Cui
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Turning Points ◽  
Probe Method ◽  
Solidification Processing ◽  
Ac Magnetic Field ◽  
Dc Magnetic Field ◽  
Cu Alloy ◽  
The Difference ◽  
Good Agreement

The resistance of Al-21%Cu alloy under no magnetic field, DC magnetic field and AC magnetic field from liquid to solid was measured by a four-probe method. The difference of resistance versus temperature curves (R-T curves) was analyzed. It is found that the R-T curves of Al-21%Cu alloy are monotone decreasing and have two obvious turning points. Under DC magnetic field, the liquidus and solidus temperatures of the alloy both decrease, while under AC magnetic field, the liquidus and solidus temperatures both increase. There is a good agreement between the microstructure of quenching sample and R-T curves. The mechanism of the effect of magnetic fields was discussed.

scholarly journals First in-orbit results of the vector magnetic field measurement of the High Precision Magnetometer onboard the China Seismo-Electromagnetic Satellite

2019 ◽  
Vol 71 (1) ◽  
Author(s):  
Bin Zhou ◽  
Bingjun Cheng ◽  
Xiaochen Gou ◽  
Lei Li ◽  
Yiteng Zhang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Precision ◽  
Magnetic Field Measurement ◽  
Vector Data ◽  
Magnetic Field Data ◽  
Night Side ◽  
The Difference ◽  
The Magnetic Field ◽  
Ground Calibration ◽  
Data Calibration

Abstract The High Precision Magnetometer (HPM) is one of the main payloads onboard the China Seismo-Electromagnetic Satellite (CSES). The HPM consists of two Fluxgate Magnetometers (FGM) and the Coupled Dark State Magnetometer (CDSM), and measures the magnetic field from DC to 15 Hz. The FGMs measure the vector components of the magnetic field; while the CDSM detects the magnitude of the magnetic field with higher accuracy, which can be used to calibrate the linear parameters of the FGM. In this paper, brief descriptions of measurement principles and performances of the HPM, ground, and in-orbit calibration results of the FGMs are presented, including the thermal drift and magnetic interferences from the satellite. The HPM in-orbit vector data calibration includes two steps: sensor non-linearity corrections based on on-ground calibration and fluxgate linear parameter calibration based on the CDSM measurements. The calibration results show a reasonably good stability of the linear parameters over time. The difference between the field magnitude calculated from the calibrated FGM components and the magnitude directly measured by the CDSM is just 0.5 nT (1σ) when the linear parameters are fitted separately for the day- and the night-side. Satellite disturbances have been analyzed including soft and hard remanence as well as magnetization of the magnetic torquer, radiation from the Tri-Band Beacon, and interferences from the rotation of the solar wing. A comparison shows consistency between the HPM and SWARM magnetic field data. Observation examples are introduced in the paper, which show that HPM data can be used to survey the global geomagnetic field and monitor the magnetic field disturbances in the ionosphere.

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