摘要:

This paper is an expository review of the theory of guided waves that occur in the earth's atmosphere. We introduce the subject by treating the problem of radio propagation around the surface of an airless spherical earth. This leads readily to the classical solutions of van der Pol and Bremmer and the more recent work of Fock in the USSR. The influence of a troposphere with a smooth profile of refractive index is then considered. This analysis, which follows the recent work of Armand, confirms that a monotonically varying or smooth profile will not change the basic structure of the diffraction field. The modifications resulting from the presence of a tropospheric duct are developed by using a parabolic profile of refractive index in the manner suggested by Fock, Weinstein, and Belkina. It is shown that the dominant modes in this system have low attenuation. The second and major part of the paper is devoted to the theory of the mode propagation of VLF radio waves. Here the effective waveguide is the space formed by the earth's surface and the lower edge of the ionosphere. The mode solution is developed as a natural generalization of the classical airless‐earth theory. It is shown, for frequencies less than about 10 kc/s, that the field may be described in terms of flat‐earth modes analogous to those in a straight rectangular microwave guide. At higher frequencies, however, the earth curvature plays a major role and the character and excitation of the modes are changed drastically. Complications resulting from the anisotropy of the ionosphere are also considered. A critical discussion of the recent work on the subject is gi

ISSN:8755-1209    

DOI:10.1029/RG001i004p00481

年代:1963

数据来源: WILEY

摘要:

In all three principal methods of measuring the earth's gravitational field—gravimetry, satellite orbit perturbations, and astrogeodetic networks—both mathematical theory and observational techniques have been developed in recent years to more than sufficient accuracy to define and determine the irregularities of the field significant on a planetary scale. The common defect of all three methods is inadequate distribution of observations. At present, the different methods agree only for major features such as the Indian Ocean minimum, and the normalized spherical harmonic coefficients of the potential are known to be of an order of magnitude of about ±1.3 × 10−6/(n‐ 1) with an uncertainty of about ±0.6 × 10−6for the tesseral terms. Gradual improvement is anticipated with additional observations and the application of better statistical techniques made possible by mod

ISSN:8755-1209    

DOI:10.1029/RG001i004p00507

年代:1963

数据来源: WILEY

摘要:

Electronic surveying—its feasibility, portability, and accuracy in geophysical mensurations—is analyzed. Geometries of various systems are presented, and circular, hyperbolic, and direct approaches are compared. Short derivations are made, and several phenomena of wave propagation are surveyed: shape of the ray path, velocity anomalies due to atmosphere and variable soil conductivity, reflections of long waves from theElayer and microwaves from the ground. The meaning of the use of various wavelengths in geophysical surveying is explained. An encyclopedic review is given of the different methods used in air to ground, ground to sea, and ground to ground surveying missi

ISSN:8755-1209    

DOI:10.1029/RG001i004p00553

年代:1963

数据来源: WILEY

摘要:

Gravity and heat flow observations demonstrate that, on the average, mass and radioactivity per unit area are equal under continents and oceans. A global representation of the anomalies in the heat flow and gravity fields shows many similarities, and horizontal gradients in both fields are correlated with earthquake zones. The regional variations of Rayleigh and Love wave velocities provide evidence additional to that of gravity and heat flow that the mantle under continents differs from that under oceans to a depth of 400 to 700 km. Taken together the observations of the planetary fields imply that vertical segregation has been the dominant feature of the process of continent formation. The significance of deep structure of continents and oceans is explored by a number of numerical experiments. The difference in radioactivity between continents and oceans results in a concentration of thermal stresses at the continent‐ocean boundary. The ensuing fault zones provide a preferred location for the release of strain energy accumulated by dynamic processes such as those associated with changes in the earth's rotation. The decay of radioactivity implies a past rate of heat production in excess of the heat loss. During the first two to three aeons, the earth expanded at an average rate of about 10 km aeon−1. The resulting fracturing of the outer surface with accompanying upward flow of matter led to the localization of the early continents. Continental growth proceeded by secular differentiation of the subjacent man

ISSN:8755-1209    

DOI:10.1029/RG001i004p00587

年代:1963

数据来源: WILEY