Vertical Magnetic Intensity Research Articles

Exploration for iron ore in Agbado-Okudu, Kogi State, Nigeria

This report details the result of geophysical exploration for iron ore; which involved vertical magnetic intensity (∆Z) and gravity measurements, to delineate the geometry and depth extent of the deposit and acquiring quantitative and qualitative information for pre-drilling purposes in Agbado-Okudu. It is located about 3 km from Jakura along Lokoja-Jakura marble quarry and within low latitude precambrian basement complex district of Kogi State, Nigeria. A total of 517 magnetic measurement points along 16 traverses and 330 gravity reading along 11 profiles on the deposit in northeast–southwest azimuth were undertaken. The magnetic and gravity data enhancement involved linear regression curve fitting and fast Fourier transform, which were used to construct residual magnetic (RM) and gravity (RG) anomalies, analytic signal amplitude, Euler deconvolution at varying spectral indices (SI), power spectrum, and source parameter image (SPI), using the submenu of Geosoft Oasis Montaj software. Interpretation of the RM and RG anomalies revealed a primary causative body which perfectly correlates the positive anomalies and iron ore deposit, in form of a horizontal or gently dipping dyke with strike length of 600 m and average width of 110–130 m, within the gneiss complex in the north and trending south of the area. A secondary causative body associated with the negative anomalies and inferred as a vertical/near vertical thin sheet striking northeast–southwest coincided with the granitic and quartzitic intrusion. The NW–SE and E–W lineament trend conformed Kibarian and Liberian orogeny cycles of generally known structural trends in Nigeria, which shows that the iron ore deposit is structurally controlled. Depths to sources were estimated within range ≤ 2–24 m and 37.5–60 m, regarded as shallow and relatively deep depths, respectively. Ten vertical boreholes ranging in depth between 50 and 100 m are recommended, five of which require a priority attention to ascertain the thickness of the primary causative body.

Monitoring the cooling of the 1959 Kīlauea Iki lava lake using surface magnetic measurements

Lava lakes can be considered as proxies for small magma chambers, offering a unique opportunity to investigate magma evolution and solidification. Repeated magnetic ground surveys over more than 50 years each show a large vertical magnetic intensity anomaly associated with Kīlauea Iki Crater, partly filled with a lava lake during the 1959 eruption of Kīlauea Volcano (Island of Hawai’i). The magnetic field values recorded across the Kīlauea Iki crater floor and the cooling lava lake below result from three simple effects: the static remnant magnetization of the rocks forming the steep crater walls, the solidifying lava lake crust, and the hot, but shrinking, paramagnetic non-magnetic lens (>540 °C). We calculate 2D magnetic models to reconstruct the temporal evolution of the geometry of this non-magnetic body, its depth below the surface, and its thickness. Our results are in good agreement with the theoretical increase in thickness of the solidifying crust with time. Using the 2D magnetic models and the theoretical curve for crustal growth over a lava lake, we estimate that the former lava lake will be totally cooled below the Curie temperature in about 20 years. This study shows the potential of magnetic methods for detecting and monitoring magmatic intrusions at various scales.

Fitting Geomagnetic Fields before the Invention of Least Squares: II. William Whiston's Isoclinic Maps of Southern England (1719 and 1721)

The English mathematical practitioner and theologian William Whiston (1667-1752) made the first known maps showing the direction of magnetic inclination (for south-eastern England) in 1719 and 1720. He also anticipated Graham's 1723 measurement of the ratio of horizontal and vertical magnetic intensities, and he understood their relationship to magnetic inclination. Whiston's work is examined to determine how he fitted the planar isoclinic surfaces. This is of interest because his study precedes the publication of 'Meyer's method' by thirty-one years and the Legendre-Gauss 'method of least squares' by eighty-six years. It is suggested that Whiston fitted the surfaces using observations of inclination at a chosen triplet of localities; and that he did this in order to use data from non-included localities as a check on his model. Best fits to his maps are given by inclinations based on Salisbury, Wiltshire, London, and Ainho [Aynho], Northamptonshire, in 1719, and on Wanstead, Essex, Ainho, and Saltfleet, Lincolnshire, in 1720. However, in both cases, the orientation of the isolines appears to be inconsistent with roughly contemporary data from Europe and Scandinavia, as well as a hindcast of the geomagnetic field in 1720, based on the modern Bloxham-Jackson global, time-dependent, spherical harmonic model.

Practical Application of Theoretical Models to Magnetometer Surveys on Hazardous Waste Disposal Sites — A Case History

AbstractWastes were to be exhumed and removed from a hazardous and chemical waste disposal site. The major disposal practice was shallow burial of wastes in metal drums in parallel trenches. The actual boundaries of the trenches beneath the landscaped cover were not accurately known.Theoretical two‐dimensional models were used to evaluate the application of magnetic methods to define trench boundaries on the disposal site. The models predicted that both the total field and the vertical component anomalies were diagnostic; however, the vertical component anomaly was more focused on the trench boundaries.A survey with a vertical component fluxgate magnetometer was successful in defining trench boundaries. Observations made along traverses that trended perpendicular to the long axis of the parallel trenches indicated vertical magnetic intensity values with a cyclical pattern of large positive values over trenches and large negative values over intertrench corridors. Vertical magnetic intensity values determined with theoretical two‐dimensional models compared favorably to the observed vertical magnetic values. The models were used to constrain the interpretation of the field observations to define trench boundaries; the magnetic survey defined trench boundaries with an accuracy within ± 2 feet.

Dominerende forkastningszoner på Bornholm baseret på anomalierne af den vertikale magnetiske intensitet

In earlier works (1945, 1957) the author has, among other things, demonstrated horizontal displacements of 4-500 m. from the existence of rift valleys in the Bornholm bedrock. By magnetic measurements across the bedrock-sediment boundary both horizontal displacements of 100-200 m. and vertical movements have been demonstrated. The present work discusses further studies of the same problem; the author has used the vertical magnetic intensity (Z) to localize large, dominating fault zones in the Bornholm bedrock, (600-2500 m.).It has been found that the granite varieties and gneiss in southern Bornholm are so different, magnetically speaking, that their limits become visible on the map. The construction of an Z-anomaly chart showing the variations in the vertical magnetic intensity gives the necessary basis for the main section of this work (section B). The chart (supplement plate I) is based on measurements over the whole of Bornholm on a standard quadrat grid with station intervals of 500 m. (more than 3,000 stations). As well as the magnetic measurements which form the basis for the Z-anomaly chart, a great number of magnetic detail profiles have been measured in the boundary zone between the bedrock area and the sediment-covered areas. These detail profiles have been utilized to determine where the boundary is formed by a fault, and where it represents an erosion boundary (section C).

Magnetic interpretation on uneven topography

In most mining prospects, the topography is irregular. Geophysical data collected on such topography cannot be interpreted quantitatively, since all available methods of interpretation assume a flat, horizontal, observational plane. For geologic bodies that can be magnetically represented by infinite line pole, point pole, infinite line dipole and point dipole, this paper gives quantitative interpretation procedures for vertical magnetic intensity components, as measured over an uneven topography, using pairs of observations made on equal elevations. Master curves that can be directly used by the interpreter are provided. The master curves of Fig.2, for infinite line and point poles, can also be used for gravity effects of spherical and cylindrical ore bodies.

Magnetic studies on Kilauea Iki lava lake, Hawaii

Ground surveys made during August, 1961, show large vertical magnetic intensity anomalies associated with the partly lava filled crater of Kilauea Iki. A vertical magnetic variation of 11,600 gammas occurs along a north-south profile across the crater, the maximum being on the north rim of the crater and the minimum on the south edge of the encrusted lava lake below the south rim. An east-west profile shows less vertical magnetic variation, with lake-surface measurements 1500 to 2500 gammas lower than measurements on the east rim of the crater.

QUANTITATIVE INTERPRETATION OF VERTICAL MAGNETIC ANOMALIES OVER VEINS

By using ordinary magnetic induction methods of analysis, Haalck, Heiland, and others have developed formulas which express the magnetic anomaly over a vertical or inclined vein of tabular shape as a function of the susceptibility, dimensions, shape, and disposition of the vein, and of the strength and direction of the earth’s magnetic field. On the basis of these fundamental formulas, other formulas for the vertical component of the magnetic field are derived in the present paper for such veins in intermediate northern magnetic latitudes. Special emphasis is given to the orientation of the veins relative to the magnetic north direction. Several families of vertical magnetic intensity curves for veins with different strikes and dips are given. All theoretical curves for veins striking magnetic north are plotted in terms of a parametric unit so that, once plotted, they can be used repeatedly in different districts, provided a proper multiplying factor is chosen for the observed curve. The importance of the transverse horizontal magnetization effect under certain conditions of orientation is demonstrated. It is shown mathematically that small vertical magnetic anomalies are to be expected for thin veins striking east and dipping south at an angle equal to, or approximately equal to, the complement of the angle of magnetic inclination.

The Gulf Absolute Magnetometer

In response to the need for a more accurate method of measuring the vertical magnetic intensity in the field, voiced to me in 1938 by Captain N. H. Heck, of the United States Coast and Geodetic Survey, the Gulf Research and Development Company built an absolute magnetometer of new design for the determination of declination (D), vertical intensity (Z), and horizontal intensity (H). The declination is measured in the customary manner by means of a fiber‐suspended magnet. The intensity‐measurements are made by comparing the field of a Helmholtz coil with the component of the Earth's field that is to be measured. H is obtained by the sine‐galvanometer method. The zero‐field detecting device for Z is a special vertical field‐balance in which the polarity of the magnet can be reversed by discharging a condenser through magnetizing coils fastened to the case. The effect of residual mechanical unbalance is thus made to disappear from the average of two sets of observations taken with the magnet oppositely polarized, The Helmholtz coil‐constant for the Z‐magnet is determined by suspending the Z‐magnet in the sine‐galvanometer housing, and comparing the value of H so obtained with an observatory base‐line.From preliminary tests at Cheltenham in April, 1940, the declination‐error was 1.′6. The precision of the intensity‐measurements is limited entirely by the stability of electrical standards in the field. This is estimated to be between 0.01 and 0.02 per cent. Six standard cells, two standard resistors, and one Rubicon potentiometer comprise the electrical measuring‐equipment. The determination of Z on the basis of the Cheltenham base‐line for H and the electrical equipment of the CIW sine‐galvanometer No. 1 was 17γ greater than the Cheltenham base‐line for Z. This discrepancy can be accounted for either by pier‐differences or by a magnetic impurity in the construction of the Z‐balance. The tests were interrupted by the war. Since little remains to be done to the instrument, it is hoped that it will be made ready for the postwar observational program at an early date.

Changes in the solar‐diurnal variations in vertical magnetic intensity at the Huancayo Observatory, 1922–1943

The diurnal variations in terrestrial magnetism are dependent upon geographical location. The variations seem more closely dependent upon geomagnetic latitude than upon geographic latitude, which leads to the belief that the configuration of the Earth's general magnetic field is a determining factor in the form of the diurnal variations.

The Kentucky Dam, Power House and Lock

THIS is a project of the Tennessee Valley Authority, and owing to the proximity of the site to the secondary zone of disturbance of the New Madrid earthquake of 1812, all structures forming part of the main dam were proportioned to resist seismic disturbances (Earthquake Notes, 14, Nos. 3 and 4 ; June 1943). The following seismic loadings were adopted: for mass structures of great rigidity an equivalent inertia force equal to 0-05 g. and for less rigid structures an inertia force varying from 0-06 g. to O'lO g., based upon rational calculations of the dynamic action of the structure. The seismic increase in reservoir pressures was calculated by the Westergaard formula, using an earthquake acceleration of 0-05 g. For the two large retaining walls at the end of the earth embankment at each side of the river, special investigations were undertaken to establish the proper seismic loading. An extensive programme of shaking-table tests was conducted at Leland Stanford University by Prof. L. S. Jacobsen. On the basis of these tests an equivalent inertia force of 0-18 g. was applied to the conventional Coulomb wedge, the point of application being about 0-60 the height above the base. For structures sustaining hydraulic loads, the increase in overturning moment due to seismic effect was only 7 per cent, while the corresponding increase for structures such as retaining walls carrying earth loads was of the order of 35 per cent, Maps recently sent out by the State Geologist of the State of Missouri indicate large anomalies in both gravity and vertical magnetic intensity in Stoddard County, the locality of the great New Madrid earthquake of 1811. Whether there is any significance in the coincidence is not established, but the seismic precautions mentioned above do not appear to lack wisdom.

The Base‐Line Drift Of Schmidt Vertical Magnetic Field‐Balances

The creation of a network of magnetic stations at which the vertical magnetic intensity is measured with better precision than can be done with the earth‐inductor would be useful in mineral prospecting and would also be indispensable for unifying the vast amount of observational material gathered by private organizations. This unification would be very valuable to geomagnetic science as well as to the study of large‐scale geological problems. It is estimated that by operating Schmidt balances in groups of three or four instruments and by standardizing them at magnetic observatories every three months, it would be possible to reduce to less than five gammas the contribution of base‐line drifts to the probable error of an absolute determination of the vertical magnetic intensity at field‐stations. The data on which this estimate is based are given in full so as to enable the reader to arrive at his own conclusions.

THE RESOLUTION OF COMBINED EFFECTS, WITH APPLICATIONS TO GRAVITATIONAL AND MAGNETIC DATA

A simple but rigorous and quite general mathematical method is given for finding the minimum separation of two nearby bodies at which their observed combined effect indicates the presence of two separate bodies. Geophysical applications of the method are illustrated by investigating the resolution of gravity and torsion balance data for the two limiting cases of spheres and infinite horizontal cylinders, the resolution of the vertical magnetic intensity for infinite rectangular plugs, and the direct interpretation of the infinite horizontal rectangular block. The possible usefulness of such analysis in the selection of a geophysical field method and in the choice of station spacing is discussed.

Short‐time magnetic fluctuations of local character

The comparison of the short‐time fluctuations of the Earth's vertical magnetic intensity afforded by data taken during the daylight hours by the exploration troops of the Gulf Research and Development Company indicates that at points separated by 800 miles, 58 per cent of the fluctuations are of local origin. It is found that on a few magnetically quiet days, the time‐variations of the vertical magnetic intensity are locally so different in magnitude and in phase, as to condemn the theories of magnetic variations which are based solely upon the difference between night‐ and day‐ionization of the upper atmosphere. The observed irregularities are independent of magnetic activity. They can be accounted for only by the dynamo‐theory, and indicate the presence of local air‐circulations in the ionosphere. It is suggested that simultaneous registration of the magnetic elements at points spaced about five times the ionospheric height might delineate the circulation of the upper air, and thus provide perhaps information useful to meteorology.

Secular change in the magnetic solar‐diurnal variations at the Huancayo Magnetic Observatory

A complete change of the character of the solar‐diurnal variations of vertical magnetic intensity observed at the Huancayo Magnetic Observatory during the past sunspot‐cycle is described. Characteristics of the southern type of variation, present during the northern summer solstice during 1922, give place to a form of variation characteristic of the Northern Hemisphere in 1932. This change is associated with the secular change of the Earth's general magnetic field which consists partly of a southern shift of the magnetic equator in the region around Huancayo.Detailed features of this change are discussed and found to be qualitatively in agreement with expected changes. The large magnitude of the changes indicates that the solar‐diurnal variation is a complex function of the Earth's general field such as is called for by the atmospheric‐dynamo theory of Balfour Stewart. Statistical considerations lead to the conclusion that the reality of the change is established to an extraordinarily high degree of probability.

Differences in diurnal variation of vertical magnetic intensity in Southern California

This paper sets forth the results of an experimental investigation of the diurnal variation in the intensity of the vertical component of the Earth's magnetic field in the northern half of Southern California. It also presents a discussion of the effect which the diurnal variation may have on the results of magnetic investigations of geologic features in this region. Attention was directed primarily to determining to what degree the diurnal variation is or is not constant over the various areas of the region regardless of the local weather prevailing at the different localities.Southern California is divided into local topographic areas characterized by diverse atmospheric conditions. Areas open to oceanic breezes are frequently covered with fog and are consequently cool in summer; other districts closed to the sea by ridges may receive simultaneously extreme amounts of solar energy directly from the Sun, or may become heated by foehn or valley winds from adjacent regions. These variable local conditions of energy‐absorption and radiation may influence earth‐currents if they are considered as induced currents circulating in conductive paths in the Earth's crust. Any change in earth‐currents would influence magnetic intensities by the amount of change in magnetic flux produced by the disturbed earth‐currents.

A magnetometer for the measurement of the Earth's vertical magnetic intensity in C. G. S. measure

The measurement of the vertical component of the earth’s magnetic field is a less simple operation than that of the horizontal component. The horizontal field measurements are on a satisfactory basis, whether made by the swinging magnet method, or by the more recently developed electric magnetometers, in which known magnetic fields may be provided by means of known currents flowing through coils of known dimensions.