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Abstract
Three high-sensitivity spinner magnetometers for palaeomagnetic research are described; these demonstrate the versatility of design possible when commercial phase-sensitive detection systems are applied to both air turbine and shaft spinning pick-up coil systems.
Magnetic and non-magnetic mineral analyses were conducted on a lacustrine sequence from Lago Verde in the tropical coast along the Gulf of Mexico that covers the last 2000 years. The site witnessed the transformation of the environment since the early Olmec societies until forest clearance in the last century. Through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes, and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical America. Volcanic activity has played a major influence on sediment magnetic properties, as a purveyor of Ti-magnetites/Ti-maghemites, and as a factor of instability in the environment. Anoxic reductive conditions are evident in most of Lago Verde's sedimentary record.
A spinner type rock remanent magnetometer is described. The instrument measures specimens having moments from 2 × 10−6 emu upward, and measures moments from 10−5 emu with precisions of 1° in direction and 5 per cent in magnitude. The lower limit for measurement corresponds to an intensity of magnetization of 5 × 10−8 emu cm−3 for the largest size of specimen which can be accommodated. A discussion is given of the factors which limit the sensitivity of a spinner magnetometer. Annulment of the geomagnetic field at the rotor is shown to be desirable when rocks having low remanence and high susceptibility are to be measured.
The instrument is primarily designed to measure the magnitude and direction of the magnetization of small rock samples. The samples in the form of cylinders (5 cm3) are rotated at 375 c/s in a freely suspended air turbine. The noise level is equivalent to a remanence of about 3 × 10-7 e.m.u. cm-3 (assuming a standard sample).
The magnetizations of 343 samples of Permian and Triassic sediments from the United States are compared to magnetizations that have been reported for sediments of approximately the corresponding age in England. It is demonstrated that an impressive number of the observations are in essential agreement simply on the basis of assuming that the rocks were magnetized by a geomagnetic field about like the one today, the essential difference being that this field was in a significantly different orientation. This result is discussed in terms of slip of an outer shell of the earth relative to the axis of revolution.
Описявается чувствительный рок-генератор для измерения слабой остаточной магнитной поляризации горных пород, предназначенный
прежде всего для палеомагнитных исследований. Датчиком прибора является система двух плоских катушек, похожая на систему Гельмгольца.
Датчик магнитно экранирован трехкратным пермаллоевым кожухом. Образец горной породы в виде куба с ребром длиной в 20мм врашается внутри датчика со скоростью 85об/сек. Одновременно измеряются косинусоидальная и синусоидальная слагающие индуцированного в датчике напряжения; для выпрямления
применяются два синхронных демодулятора. Измеряемые слагающие напряжения пропорциональны двум слагающим вектора остаточной
магнитной поляризации. Для определения всех трех слагаюших вектора надо провести хотя бы два измерения образца в двух положениях
(но на самом деле образец измеряют в 6 положениях). Чувствительность прибора—5нтг, те. 4. 10−9 единиц системы ЦГСМ. В статье тоже приведена краткая обшая теория чувствительности рок-генераторов.