There is a continuous large demand for gravity and magnetic surveys all over the world for a variety of exploration applications, all of which require the geophysicist to provide some new insight into the geology of an area at scales ranging from very large to very small. To achieve this objective, (a) surveys must be carried out accurately, and (b) their results must be interpreted in sympathy with what is already known of the geology. The methodology for acquiring and compiling data appears to be keeping pace with modern technology. Methods of quantitatively interpreting anomalies in terms of models of causative bodies are adapting rapidly to the burgeoning availability of computing power, from large, powerful machines to inexpensive and field‐portable microcomputers. Geologic interpretation, or the identification of physical property distributions in terms of realistic geologic models and processes, is still relatively neglected—in practice and, regretably, in the geophysical literature. Research into the relationships between physical rock properties—particularly magnetite distribution—and geology is gaining momentum, but research still lags behind the requirements of the conscientious geophysical interpreter.
Theoretical and experimental studies have been made of the manner in which sound waves are propagated in porous granular aggregates. A cylindrical piezo-electric source is used and this simulates the explosion of a charge in a seismic shot-hole.It is found that in general two waves of volume expansion are propagated and that these involve coupled displacements of both constituents of the media. The waves are termed frame-waves, air-waves or liquid-waves depending upon the nature of the pore-filler and the relative displacements of the constituents.Theframe=waveve&city is dependent upon the strength of the frame, the densities of solid and pore-filling materials and the texture of the medium. Air-and liquid-wave velocities are related to the texture of the medium and to the density and viscosity of the pore-filler. Frame-strength is important to a lesser degree. Waves are dispersive only in the case of media of very low permeability.Attenuation is related to viscosity, texture and frequency. Scattering is probably important only at the highest frequencies and largest particle diameters used in the experiments.It is shown that porosity and permeability of a beach sand can be inferred from velocity measurements. These properties provide information regarding grain-size, sorting and the nature of the pore-filler.
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