Showing papers in "Geophysical Supplements to the Monthly Notices of the Royal Astronomical Society in 1949"

Compressibility-Pressure Hypothesis and the Earth's Interior

Abstract: Summary The hypothesis has been set down that at pressures ranging from 0.4 × 1012 to at least 3.6 × 1012 dynes/cm.2 (the latter being the pressure at the Earth's centre) the compressibility K−1 of a substance at temperatures of the order of those in the Earth's deep interior is essentially independent of its chemical composition. A simple form of the hypothesis to the effect that K and dk/dp (where p denotes pressure) change continuously and smoothly throughout the Earth below a depth of 1000 km. has been discussed in relation to seismic data. The hypothesis entails an abnormally large density gradient in the lowest 200 km. of the Earth's mantle, and in the Earth's inner core, and that these two regions are not chemically homogeneous. It entails that the density near the Earth's centre lies between about 16 g./cm.3 and 20 g./cm.3 (apart from a possible accumulation of even denser material very close to the centre). The hypothesis in its simplest form would require the Earth's density to be a little higher in the region above the 1000-km. level than the figures given in a previous model. The hypothesis would entail that the inner core is solid, capable of transmitting S waves at a speed not less than about 4.9 km./sec. and probably less than 6.0 km./sec.

The Seismological Implications of Aeolotropy in Continental Structure

Abstract: Summary The seismological consequences of a departure from isotropy in the surface layers of the Earth are discussed. It is noted that erroneous values of the focal depth and the thickness of the layers might be found in near earthquakes. For a transversely isotropic continent it is shown that SH waves would travel with a different velocity from that of SV, and that the law of variation of velocity with direction is different. With body waves the sharp distinction into compressional and distortional waves does not hold, and an explosion would generate an S wave as well as a P wave; an apparent difference in the instant of generation of P and S might arise. It is proved that waves of the Rayleigh type can be propagated over the surface of a transversely isotropic body in which the axis of circular symmetry is normal to the free surface, supposed plane; in such waves the diminution of amplitude with depth is different from that in an isotropic body, so that the amplitudes of the surface waves generated by a source at a given depth will be different from what they would be in an isotropic medium. Love waves can be propagated as in isotropic media.

The Electrical and Magnetic Effects of Tidal Streams

Abstract: Summary The tidal movement of sea water relative to the Earth's magnetic field induces electromotive forces of a few millivolts per kilometre. Recent measurements off Plymouth show that the potential gradient is at right angles to the streams in that part of the English Channel. Observations on cross-channel telephone cables indicate that a considerable flow of electric current takes place, which can be accounted for b assuming the mean conductivity of the sea-bed to be of the order of 6 × 10-5 (ohms-cm.)-1. Tidally generated earth-currents spread into the land on either side of the Channel and have been measured near Lulworth. It may be possible to use them for the measurement of tidal stream velocities. In the second part of the paper the potential gradients generated by water flow in shallow channels of rectangular or elliptical section are examined. The horizontal gradient in the water is almost independent of vertical variations in water velocity but is affected critically by the depth of the channel and the conductivity of the channel-bed. The induced electric currents can be expected to extend to depths comparable with the width of the channel.

The Magnetic Properties of the Tholeiite Dykes of North England

Abstract: Summary The magnetic profiles taken over various members of the tholeiite dykes of the North of England show that they possess abnormal magnetic polarization. Direct measurement of the residual magnetism gives a series of magnetic vectors which could arise from a magnetizing field in opposition to the present field. Minor deviations in direction can be explained by flow of the molten material below the Curie point of the magnetic content.

On the absolute measurement of gravity

Abstract: Summary A simplified method is given for finding the corrections for bending and stretching of a pendulum used for making an absolute determination of gravity. It appears that the correction given by Clark is incorrect and that the value of g for Teddington consequently needs an increase of about 1.7 mgal. Examination of several minor corrections mentioned by Heyl and Cook for the Washington determination shows that they mount up and that the value given by these authors also needs an increase of about 1.5 mgal. When taken in conjunction with the relative determination by Bullard and Browne, the revised values give a discrepancy of 4.7 ± 1.6 mgal., which is large enough to need attention. An independent determination is desirable.

Observational Aspects of the Sunspot-Geomagnetic Storm Relationships

Abstract: Summary With the use of international magnetic character figures, the statistical rise of geomagnetic activity is investigated for the solar disk passage of four area groupings of sunspots, 1914-1944. This is the reverse process of earlier Greenwich work in which the comparison was made from geomagnetic storms to sunspots. For sunspots exceeding 1500 millionths of the Sun's hemisphere there is a very definite increase of geomagnetic activity centred at about two days after the date of central meridian passages of the sunspots. But this relationship rapidly degenerates, and from the area criterion alone ceases to be significant below the area group 1000-750 millionths. Other sunspot groupings based on solar flare incidence greatly improve the correlation. and for the minor naked-eye sunspots, around 500 millionths, a peak of geomagnetic activity now appears. Twenty-seven-day recurrence tendencies of the geomagnetic activity associated with the various sunspot groupings are examined, and the pseudocharacter of what is found is contrasted with the strong and more precise twenty-seven-day recurrence tendency of the smaller storms, especially those without SC onset and not apparently dependent upon sunspot or solar flare incidence. The statistical non-recurrence of great magnetic storms is confirmed from 1907-1944 data.

A Gravity Survey of Southern Andros Island and the Adjoining Marine Area in the Bahamas

Abstract: Summary The authors present gravity data covering a small part of the Bahama platform in the hope that they will assist in solving the problems concerning the formation and folding of the interesting area. The results suggest that the Bahamas are essentially part of the Florida foreland, but that Cuban diastrophism may have affected the S. W. marginal zone.