GRADIENT MEASUREMENTS IN GROUND MAGNETIC PROSPECTING

Geophysics ◽  
1965 ◽  
Vol 30 (3) ◽  
pp. 403-410 ◽  
Author(s):  
Peter Hood ◽  
D. J. McClure
Keyword(s):  
Contact Point ◽  
Vertical Component ◽  
Vertical Gradient ◽  
Horizontal Distance ◽  
Total Field ◽  
Gradient Measurements ◽  
Ground Magnetic ◽  
Vertical Contact ◽  
First Vertical Derivative ◽  
Depth Of Burial

The development of electronic magnetometers, i.e., the proton‐precession and fluxgate instruments, for use in ground magnetic surveys has permitted the measurement of the first‐vertical derivative of the total field, or of the vertical component of that field, with negligible addition to the total cost of the survey. The gain in information is, however, significant. Curves for the vertical gradient over a vertical contact, point pole, and finite dipole are presented. The vertical contact is outlined by the zero contour for the vertical gradient of the vertical component, and the depth of burial is half the horizontal distance between the positive and negative maxima. The depth of burial of the point pole and finite dipole is approximately equal to the horizontal distance between the negative half‐maximum points on the vertical‐gradient curves.

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.

scholarly journals Evaluation of mafic dyke swarm continuity under the Joaquina dune field with ground magnetic survey

2017 ◽  
Vol 47 (2) ◽  
pp. 237-247 ◽  
Author(s):  
George Caminha-Maciel ◽  
Marcia Ernesto ◽  
Welitom R. Borges ◽  
Junior Bresolin ◽  
Reginaldo Lemos
Keyword(s):  
Vertical Gradient ◽  
Crystalline Basement ◽  
Dyke Swarm ◽  
Magnetic Survey ◽  
The North ◽  
Magnetic Signature ◽  
Mafic Dyke Swarm ◽  
Gradient Measurements ◽  
Ground Magnetic ◽  
North And South

ABSTRACT: A ground magnetic survey in a Central-East area of the Santa Catarina Island tested the continuity of the Cretaceous mafic dykes beneath the aeolic sediments of the Joaquina plain. Vertical gradient measurements taken in 1880 stations did not detect any magnetic anomaly related to subsurface dykes. Four magnetic profiles located to the north and south of the main area showed the magnetic signature of various dykes some of them already mapped (north profiles), but also some in subsurface (south profiles). These results suggest that the dykes probably were shallow and truncated, and were already eroded along with the crystalline basement.

Calculation of the magnetic gradient tensor from total field gradient measurements and its application to geophysical interpretation

Geophysics ◽  
1988 ◽  
Vol 53 (7) ◽  
pp. 957-966 ◽  
Author(s):  
J. Bradley Nelson
Keyword(s):  
Signal To Noise Ratio ◽  
Depth Estimation ◽  
Vertical Gradient ◽  
Rate Of Change ◽  
The Body ◽  
Total Field ◽  
Gradient Tensor ◽  
Field Gradients ◽  
Contour Plots ◽  
Gradient Measurements

The very low inherent noise levels of superconducting quantum interference device (SQUID) sensors have led to proposals for the use of airborne SQUID magnetic gradiometers as geophysical interpretation tools. The quantity measured by such systems will be the gradient tensor, the spatial rate of change of the vector components of the magnetic field. By contrast, existing airborne gradiometers measure the spatial rate of change of the magnitude of the total field. This work describes a technique whereby the gradient tensor can be calculated from measurements of either the vertical or horizontal total field gradients throughout a plane. The signal‐to‐noise ratio of the calculated tensor components is essentially the signal‐to‐noise ratio of the original total field gradient measurements. The resulting tensor components may be upward or downward continued with standard techniques. Two advantages of using the tensor gradients instead of the total field gradients have been determined. Because the tensor components are not a function of the direction of the Earth’s field, contour plots do not suffer the skewing problems that total field or vertical gradient plots do. Thus, tensor gradient contour plots may be easier to interpret or may enhance the information obtained from total field or vertical gradient maps. In addition, the dipole‐tracking algorithm developed by Wynn et al. (1975) has been shown to be quite successful in determining the depth and horizontal location of block‐shaped bodies. The error in depth estimation is a strong inverse function of the ratio of the closest point of approach to largest dimension of the body. However, if the smallest separation is more than twice the largest dimension of the body, errors in depth estimation are less than 10 percent. Because the tensor components are calculated on a horizontal plane, they can be upward continued to meet this condition.

A Novel Method to Search for the Wheel–Rail Contact Point

2019 ◽  
Vol 19 (11) ◽  
pp. 1950142 ◽  
Author(s):  
Hanyun Liu ◽  
Zhiwu Yu ◽  
Wei Guo ◽  
Lizhong Jiang ◽  
Chongjie Kang
Keyword(s):  
Minimum Distance ◽  
Lateral Displacement ◽  
Contact Point ◽  
Normal Contact ◽  
Novel Method ◽  
Maximum Penetration ◽  
Vertical Contact ◽  
Judgment Condition ◽  
Searching Method ◽  
Root Contact

This paper proposed the normal contact searching method (NCSM), a novel method to search for the wheel–rail contact point, which utilizes the normal maximum penetration distance between wheel and rail as the judgment condition. The contact point found by the NCSM can better represent the center of the wheel–rail contact patch which is considered more reasonable than the commonly used vertical contact searching method (VCSM), the latter adopts the vertical minimum distance to determine the wheel–rail contact point. The differences between these two methods are analyzed and compared for the same contact point situation and with same motion parameters. The results show that, for the Chinese LMA wheelset and CHN60 rail profiles, these two methods have slight differences for the same contact point situation. For a wheelset with a lateral displacement less than 7.0[Formula: see text]mm and with no yawing, the NCSM’s contact point is very close to VCSM’s, so both methods are suitable for the dynamic calculation. For a wheelset with a lateral displacement greater than 7.0[Formula: see text]mm or with yawing, an unreasonable jump occurs at the wheel–rail contact point and wheelset angle root contact by applying VCSM, while the NCSM has only small discrete jump on the wheelset tread contact. In this case, the NCSM instead of VCSM should be used in the dynamic analysis.

3D correlation imaging of magnetic total field anomaly and its vertical gradient

2011 ◽  
Vol 8 (2) ◽  
pp. 287-293 ◽  
Author(s):  
Lianghui Guo ◽  
Lei Shi ◽  
Xiaohong Meng
Keyword(s):  
Vertical Gradient ◽  
Total Field

On: “Gravity vertical gradient measurements for the detection of small geologic and anthropogenic forms” by Z. J. Fajklewicz (GEOPHYSICS, October 1976, p. 1016–1030)

Geophysics ◽  
1977 ◽  
Vol 42 (4) ◽  
pp. 872-873
Author(s):  
Stephen Thyssen‐Bornemisza
Keyword(s):  
Research Work ◽  
Vertical Gradient ◽  
Gravity Gradient ◽  
Wind Gust ◽  
Near Surface ◽  
New Methods ◽  
Vertical Gravity Gradient ◽  
Surface Variation ◽  
Gradient Measurements ◽  
Vertical Gravity

In his paper, Fajklewicz discusses the improvement of vertical gravity gradient measurements arising from a very stable tower apparently not affected by wind gust vibration and climatic changes. Further, the lower plate where the gravity meter is resting can be changed in position to avoid possible disturbances from surface and near‐surface variation, and new methods for correcting and interpreting observed gradients over the vertical interval of about 3 m are presented. Some 1000 field stations were observed, including research work and industrial application.

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.

LONG JUMP: MECHANICAL AND ENERGETIC ASPECTS

2004 ◽  
Vol 04 (04) ◽  
pp. 535-557 ◽  
Author(s):  
T. K. KARALIS
Keyword(s):  
Contact Point ◽  
Reaction Force ◽  
Vertical Component ◽  
Centre Of Mass ◽  
Force Vector ◽  
Direct Measurements ◽  
Long Jump ◽  
The Mean ◽  
Vector Formula ◽  
Mass Position

The use of variation techniques is applied to investigate jumper's posture at take-off, resulting in maximum distance of jump. Explicit expressions have been derived between: (i) the take-off angle ϕT (formed by the line connecting the contact point of the leg driving the jump with the ground to the centre of mass position and the horizontal), (ii) the ratio [Formula: see text] of the mean vertical component of the ground reaction force vector to athlete's weight at take-off, (iii) the time spent for the mid-support/take-off phase TT, and (iv) the change in the vertical component of the displacement of the centre of mass compared with the take-off foot ΔyT, measured between two extreme postures, i.e. the mid-support and the take-off phase. The method is illustrated by calculating the state vector [Formula: see text] at take-off, in connection with the take-off angle ϕT. The results are compared to direct measurements from real long jumps.

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