Ronald Green

Gravity Gradients And The Interpretation Of The Truncated Plate

The truncated plate and geologic contact are commercially important structures which can be located by the gravity method. The interpretation can be improved if both the horizontal and vertical gradients are known. Vertical gradients are difficult to measure precisely, but with modern gravimeters the horizontal gradient can be measured conveniently and accurately. This paper shows how the vertical gradient can be obtained from the horizontal gradient by the use of a Hilbert transform. A procedure is then presented which easily enables the position, dip angle, depth, thickness, and density contrast of a postulated plate to be precisely and unambiguously derived from a plot of the horizontal gradient against the vertical gradient at each point measured. The procedure is demonstrated using theoretical data.

The seismic refraction method—a review

Abstract The seismic refraction method had its beginning in the war of 1914–1918, but it became established as a prospecting method in the 1920s and 30s as a result of successes in the Gulf area of the United States. The success was an outcome of the parallel development of improved instrumentation and improved methods of interpretation. Instruments generally become smaller, lighter, more portable and reliable. Field technique has steadily improved the signal-to-noise ratio. The interpretation methods have tended to be simpler to apply and applicable to more realistic geological field situations. The seismic refraction method has found new applications in crustal geophysics, reconnaisance surveying in sedimentary basins, structural engineering, and mining geophysics. This review brings together the history of the development of the method, a discussion of the basic theory, field procedures and instrumentational developments and a discussion of methods of interpretation. Some examples are given as an illustration. It is suggested that students will find the review of value in having an account of the history, the theory and applications in the one paper, and professional geophysicists will find it of interest and of value in indicating the continued developments that have taken place over the years. It would seem to indicate that future developments will be towards lighter equipment with improved information gathering capabilities coupled with automatic and portable data processors which will interpret the data directly in terms of realistic geological structures.

Ultra-rapid magnetic surveying in archaeology

Abstract The cause of magnetic anomalies arising from archaeological sources together with established overseas field surveying and interpretation techniques are reviewed. Faster and less complicated methods are required for investigations of Australian aboriginal sites. This is because anomalies associated with aboriginal habitation are generally much weaker than those found over European archaeological sites and larger areas usually have to be covered. A very simple system is described which enables interpretation to be carried out on the site without the need for computing. A method of data presentation is given which enables up to 5:1 signal enhancement over temporal noise. As an example, the magnetic field originating from a camp fire-site is mapped, illustrating the advantages of the use of the equipment and the technique.

Bouguer gravity map of Indonesia

Abstract A Bouguer gravity map of Indonesia on Mercator projection at a scale of 1: 5,000,000 and with a contour interval 20 mGal has been prepared over the past few years as part of a joint research program of the Geological Survey of Indonesia and the University of New England, Armidale. A new base station network was set up throughout Indonesia and tied to the IGSN stations at Sydney and Singapore. A discussion of the gravity features and the tectonic implications are given. The map is obtainable, in folded form only, from the Publications Department, University of New England, Armidale, N.S.W., Australia 2351 for

The spectrum of a set of measurements along a profile

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Correction of pole positions for palaeomagnetic data from the Mesozoic of Australia

Abstract There is a growing practice of measuring various quantities (e.g., elevation) along a profile and then using a digital computer to obtain the spectrum. The objective is often to obtain the amplitude of the longer wavelengths, and a density of sampling points along the profile is taken which seems adequate to secure this. However, if there is in addition to the gentle longer wavelengths a superimposed roughness (i.e., high frequencies), then there results a curious (to many people) effect, but one that has long been known to communication engineers, whereby the high frequencies contribute to the amplitudes of the low frequencies. In other words the amplitudes obtained for the low frequencies are incorrect, and consequently any deduction based on the amplitudes (which presumably was the reason for obtaining the spectrum) will be wrong. The effect is called “aliasing” and in this paper the circumstances which give rise to this phenomenon are discussed both in terms of the Fourier transform method and in terms of picture diagrams. A recent paper on landforms is criticised but the main function of this paper is: (1) to bring the problem of “aliasing” to the notice of engineering geologists, and (2) to point out a method of detecting the degree of “aliasing” in a given situation by recording the amount of change in low frequency amplitude with change in sampling density.

New evidence on the form of the granitic intrusions in new England, N.S.W.

Abstract Embleton and Schmidt (1977) have published an account of the detailed movement of the palaeomagnetic pole, basing their discussion on recent palaeomagnetic data of Schmidt (1976). The poles used by Embleton and Schmidt (1977) do not accord with the palaeomagnetic data. Schmidts data appear to need drastic modification, and even if they were correct, the deduced polar wandering trail of Embleton and Schmidt (1977) is of doubtful validity.

The harmonic method of inverting a magnetic profile over a contact

Abstract Four hundred and thirty‐two gravity stations have been established at intervals of about 1 km in the Uralla‐Kingstown district. The Bouguer gravity anomaly map shows close correlation with the surface geology. There are gravity lows over the granitic intrusions; this is to be expected, because the surrounding sedimentary rock is 0.06 g/cm3 denser than the granite. The gravity work shows that the granitic intrusion in the Uralla‐Kingstown district, which forms part of the ‘New England Batholith’, does not continue to any great depth, as had been postulated previously from geological field mapping, but is a lenticular body with a thickness of 4 km. From the regional gravity anomaly throughout the area (—60 mgal) it can be seen that the 1 km high New England Plateau is isostatically compensated in terms of an Airy crustal model.

A METHOD FOR THE DIRECT INTERPRETATION OF ELECTRICAL SOUNDINGS MADE OVER A FAULT OR DIKE

Abstract The profile of the magnetic gradient over a buried geological contact has four significant points: two turning points and two inflexion points. The four points are harmonic. Using this invariant property of the position along the profile of any three of the four points, a simple graphical method is given which determines: 1. (1) the position of the contact. 2. (2) the depth to the contact. 3. (3) the angle of dip of the interface.

The depth location by induced polarization of a target equivalent to a simple induced dipole in a semi-infinite homogeneous medium

The potential function for a point electrode in the vicinity of a vertical fault or dike may be expressed as an infinite integral involving Bessel functions. Beginning with such an expression, two methods are presented for the direct analysis of resistivity data measured both normal and parallel to dikes or faults. The first method is based on the asymptotic expansion of the Hankel transform of the field data and is suitable for surveys done parallel to the strike of the dike or fault. The second method is based on a successive approximation technique which starts from an initial approximate solution and iterates until a solution with prescribed accuracy is found. Both methods are suitable for programming on a digital computer and some illustrative numerical results are presented. These examples show the limitations of the methods. In addition, the application of resistivity data to the interpretation of induced‐polarization data is pointed out.