Magnetic Contour Map Research Articles

Magnetic mapping of the western Alps: compilation and geological implications

Extracted from large surveys of France, Italy and Switzerland, airborne magnetic data covering the western Alpine Arc have been compiled into a single homogeneous map of magnetic anomalies at the constant altitude of 3000 m. For this purpose, each data set has been revised thoroughly and accurately to give a single coherent large-scale pattern. The magnetic contour map reflects the anomaly pattern over the entire length of the Western Alpine collision suture. The distribution of polarities exhibits a large anomalous low located by reduction to the pole over the whole external part of the belt. The observed anomaly suggests a large gap of magnetization between the Adriatic microplate and the European crust. The analysis of the waveband shows that the broadest wavelengths are produced in the lower crust close to the transition zone, in the granulite facies. This highly magnetic layer is used as a marker to describe the geometry of the European and Adriatic deep seated crust. The main results are presented on a composite synthetic profile showing the sloping side of the European slab and an important crustal thinning to the southeast of the Adriatic slab. This feature is emphasized on the magnetic contour map by a linear magnetic low attributed to major transcurrent fault. This trend is called the Sestri-Voghera trend and extends from the Ligurian basin by the Sestri-Voltaggio Zone to the Judicarian system. Sinistral movements can be recognized along the whole axis as well as possible uplift of rift shoulders. The magnetic anomaly pattern over the complete length of the anomalous body of Ivrea as well as the Insubric-Canavese Line limit the extension of the Adriatic microplate by a well defined linear trend. The symmetrical shears deduced from consecutive anomalies are used to propose a structural scheme.

Application of 2-D Hilbert transform in geophysical imaging with potential field data

The relationship between two-dimensional (2-D) Hilbert transforms and three-dimensional (3-D) potential field data (gravity and magnetic) is reviewed and applied to quantitative interpretation of magnetic and gravity anomalies as well as their derivatives. The vertical and horizontal derivatives of potential field anomalies due to multiprism models were used to test the 2-D Hilbert transform algorithms and their application. The gravity and magnetic contour maps of the Sudbury area, Canada, were digitized and Hilbert-transformed to be correlated with satellite digital images and with the previously known geology. The results indicate that the 2-D Hilbert transform technique can, in a robust fashion, be utilized for the extrapolation of geological boundaries through overburdened areas and subsequently provide a mathematical tool for a step-by-step integration of potential field data and geological remote sensing. >

Extension of the Narmada — Son lineament on the continental margin off Saurashtra, Western India as obtained from magnetic measurements

Magnetic total intensity values and bathymetric data collected on the continental margin off Saurashtra were, used to prepare magnetic anomalies and bathymetric contour maps. The magnetic anomalies are considered to have been caused by the Deccan Trap flood basalts which underlie the Tertiary sediments. Interpretation of the magnetic data using two-dimensional modelling method suggests that the magnetic basement is block faulted and deepens in steps from less than 1.0 km in the north to about 8.0 km towards the southern portion of the study area. The WNW-ESE trending faults identified in the present study extend across the Saurashtra continental margin between Porbandar and Veraval and appear to represent a major linear tectonic feature. The relationship of these fault lineaments with the regional tectonic framework have been discussed to indicate that they conform better as the northern boundary faults of the Narmada rift graben on the continental margin off Saurashtra.

A Magnetic Anomaly Map of the Japan Sea

A magnetic anomaly contour map (contour interval is 50nT) is compiled with all the available data of the total intensity of geomagnetic field. Data are very dense in the coastal areas of the Japanese islands so that contour control is good while in the coastal areas of U. S. S. R. and the Korean Peninsula, data are not so dense. The amplitudes of anomalies are generally small compared with that in the northwestern Pacific Ocean. The magnetic anomaly lineations exist in the southern half of the Japan Basin. From identification of the lineations, the Japan Basin stopped opening 15Ma BP on the assumption of two-limbed opening and the spreading rate of 3.4cm/yr. There are many seamounts and uplifts which have anomaly patterns suggesting their reversed magnetization. This seems to be a crucial point towards understanding the tectonics of the Japan Sea.

Aeromagnetic survey of Kansas

An aeromagnetic survey of Kansas, conducted by the Kansas Geological Survey during the past 5 years, has been completed. The total intensity magnetic field contour map, along with spectrally filtered versions, reveals largescale magnetic patterns, which we correlate with basement composition and paleotectonics.A second vertical derivative map clearly demonstrates that the southern part of the Proterozoic Central North American Rift System (CNARS) does not terminate in central Kansas but continues along a southwestern trend to at least the Oklahoma border. Some of the current seismicity within the state appears to be correlated with reactivated faults within the CNARS.Apparent in northeastern Kansas is the presence of numerous highly magnetic shallow granitic plutons, known from drilling to be approximately 1350 m.y. old, embedded within the older, 1600—1700‐m.y.‐old mesozonal granitic crust.The total intensity magnetic field map reveals a series of magnetic lows trending approximately east‐west across the state. These may correspond to a paleoplate boundary between the older granitic terrane to the north and the younger, 1400‐m.y. rhyolitic and epizonal granitic terrane to the south.

An interpretation of some magnetic data off the west coast of Ireland

AbstractA total field magnetic anomaly contour map of the continental margin west of Ireland is described by reference to geological structure previously established by seismic reflection profiling. The western Irish Mainland Shelf, Porcupine Seabight Trough and Porcupine Ridge were previously defined as major north‐south elements in the structure of the margin. Each is readily distinguished on the magnetic anomaly contour map. To the north these features terminate against, or merge with, the east‐west Slyne Ridge. A pronounced change in the magnetic anomaly pattern at about 53°N suggests that the southern limit of this Ridge coincides with a major geological discontinuity extending from near the coast of Ireland to the continental slope, some 320 km to the west.The magnetic anomaly contour map gives general support to the idea that the Slyne Ridge and Porcupine Ridge represent submerged blocks of marginal continental crust, and that the Porcupine Seabight Trough is founded upon crust significantly different in character.

An aeromagnetic survey of the afar triangle of Ethiopia

In 1968, an aeromagnetic survey was flown over the Afar triangle of Ethiopia. The survey extended from 9.5° N to 15° N and was bounded in the west by the Ethiopian scarp (about 40°h E) and in the east extended over the Gulf of Tadjoura and the western part of the southern Red Sea. The flight height was 1.83 km (6000 ft). 34 profiles were flown in a direction N 100° and 42 profiles in a direction N 150°, the spacing between profiles being about 10 km. In addition, there were 14 tie lines. The total flight path was approximately 24000 km. The magnetic and radioaltimeter records were digitised and profiles prepared showing both total intensity magnetic field (with I.G.R.F. epoch 1965.0 removed) and topography. Several analyses have been carried out: correlation studies were made in an effort to find trends and locate transform faults. Spectral analyses have yielded information on the shape, size and depth of the causative bodies and correlations between the magnetic anomalies and the geology are being studied. In addition, a coloured magnetic anomaly contour map has been prepared with contour interval 100 nT, and scale 1:1,000,000 at 33° N. The contour map reveals several interesting features. First, the magnetic lineations previously mapped in the Gulf of Aden are found to extend westwards over parts of southern Afar. Secondly, there are regions of small amplitude, short wavelength anomalies which are thought to be associated with continental (sialic) material. Thirdly, the anomalies north of about 12° N have a NW-SE trend fanning towards the south. The bearing of the survey on the tectonic evolution of the area is discussed. A full account is in preparation with large format diagrams.

On methods of graphical displaying of polar magnetic disturbances

Some simple and useful methods of mapping the field of polar magnetic disturbances are discussed. Contour maps showing the storminess in a local time-universal time plane are used to trace the variation with time of the longitudinal extent and intensity of the auroral electrojets. In effect this kind of map is a way of ascribing to each longitude an index analogous to the AE index. For the study of north-south motions of the auroral electrojets during polar magnetic substorms contour maps are used showing the disturbances of the three field components in a latitude-universal time plane.

A MAGNETIC SURVEY IN THE WESTERN ENGLISH CHANNEL

Since 1956, when a nuclear spin magnetometer was first developed for towing behind a ship, measurements of the earth’s total field in the western English Channel have been made on six expeditions of R.R.S. Discovery II and R.V. Sarsia. There are now enough profiles to allow a magnetic contour map of the area to be drawn. The most striking feature of the map is an anomaly with its peak value approximately 15 miles south of the Eddystone and with a total area of 30 × 12 square miles. The discovery of this anomaly has necessitated a reappraisal of the underlying geology which had been previously postulated from coring and seismic results. Sharp magnetic anomalies to the east of the Lizard show good correlation with the basic intrusive rocks on land but the limited extent of the anomalies to the east appears to disprove the idea of a continuous Lizard-Dodman-Start thrust. The boundary of New Red Sandstone outcrops on the sea-floor, postulated from previous coring results, shows a remarkable coincidence with the extent of the sharp anomalies off the Lizard and off the Start peninsula.