Earth's Total Field Research Articles

Responses to small magnetic variations by the trigeminal system of the bobolink

Electrophysiological recordings from the ophthalmic nerve and the trigeminal ganglion of the bobolink ( Dolichonyx oryzivorus) indicate the presence of units (14% of the spontaneously active cells) that are sensitive to small changes in the magnetic field. The most common response was an increase in the rate of spontaneous activity. The most sensitive units responded to changes of 200 nT (<0.5% of the earth's total field). Other responses included reaction to a 0.5 Hz sinusoidal variation of the magnetic field, and to the movement of a hand-held bar magnet. Because of their sensitivities, the responses to small variations in the magnetic field may be involved in detection of the proposed magnetic navigation map. One possible transducer substance that could account for such a sensitivity is magnetite, which has been previously reported in the upper beak area of the bobolink.

Report from IUGG Assembly: IAGA session on lithospheric mapping from local anomaly charts

The objective of this session was to review recent advances in utilizing magnetic survey data to delineate the geological formations that comprise the earth's crust, and to provide information on their genesis.P. J. Hood commenced the session with a review of the national aeromagnetic survey, which has been carried out by the Geological Survey of Canada, since 1947, as an aid to both geological mapping and mineral exploration programs in Canada. Since that time some 8 million line kilometers of aeromagnetic survey data have been obtained and have resulted in the production of, 8100 aeromagnetic maps at various scales. The specifications for aeromagnetic surveys in Canada have been continually improved upon over the years, mainly as a result of developmental work undertaken by the Geological Survey of Canada. This developmental work has also resulted in the fabrication of an inboard aeromagnetic gradiometer survey system, which measures the vertical gradient of the earth's total field in addition to being a high‐resolution total field instrument. The main advantages of the aeromagnetic gradiometer as a geological mapping tool, compared to the single‐sensor (total‐field) technique, are: (a) superior resolution of anomalies produced by closely spaced geological formations; (b) anomalies produced by nearsurface features are emphasized with respect to those resulting from more deeply buried rock formations; (c) regional gradient of the earth's magnetic field and diurnal variation are automatically removed; (d) direct delineation of vertical contacts by the zero‐gradient contour value, i.e., vertical contact mapper. In 1967 the first, colored Magnetic Anomaly Map (MAM) of Canada was produced, at a scale of 1:5,000,000, in which the dominating effects of the main earth's field were removed. Two more editions of the map have been produced in the subsequent 10‐year period. The main uses of such a magnetic anomaly map are threefold: first, as an index map it presents an overview of the aeromagnetic survey coverage of Canada; second, the map presents the major patterns produced by the continental rocks of the Canadian landmass and may be utilized both as an aid in the delineation of structural provinces and in the interpretation of regional features in the basement rocks; third, the map stimulates comparisons of magnetic features with other geological and geophysical compilations on a regional scale. More recently, commencement of colored 1:1,000,000 MAM's has also begun, and these are clearly superior to the 1:5,000,000 MAM in showing the finer details of the regional anomalies, since short wavelength features, especially dykes, tend to be eliminated at the smaller scale.

A Proton Precession Magnetometer

A simple portable proton precession magnetometer designed and constructed for Kodaikanal Observatory is described. Except for the signal amplifier, the instrument is transistorized. The accuracy of measurement of a absolute value of the earth's total field with the magnetometer is ±1 gamma. The precession signal voltage from the sampling coil is amplified and squared by a Schmitt trigger circuit, the output of which drives 10 stages of binary dividers. T4e first pulse in the output of the chain of dividers opens a gate allowing pulses from a 100 Kc crystal oscillator to pass to a pulse counter. The pulse immediately following -the opening pulse closes the gate. By suitable feed back of this closing pulse any following pulse is prevented from opening the gate again. For the next observation, a manual resetting is necessary. The number of 100 Kc Is pulses counted are used to compute the scalar field.

A nuclear magnetometer

The basic principles of the nuclear free precession magnetometer are briefly described, and full details given of the coils, switching circuits and transistorized amplifying equipment of such a magnetometer. Accuracy of absolute measurement of the earth's total field is ±10 μG, and even better when only relative measurements are required. Setting-up of the instrument, which requires no calibration, and taking a reading takes only a few seconds. Suggestions are put forward for possible further applications of the free precession technique.

CANADIAN AERIAL MAGNETIC SURVEYS (M.A.D.)

Tests concluded with an airborne magnetic detector measuring the earth's total field indicate a high degree of accuracy and speed for reconnaissance work over large areas. Cheapness of operation and freedom from small local anomalies make this a valuable tool for geophysical prospecting.