In this paper the correction for the gravitational attraction of the topography on a gravity station is considered as consisting of two parts; (1) the restricted but conventional “Bouguer correction” which postulates as a convenient approximation that the topography consists of an infinite horizontal plain, and (2) the “Terrain correction” which is a supplementary correction taking into account the gravitational effect of the undulations of the terrain about the plane through the gravity station. The paper illustrates the necessity of making terrain corrections if precise gravity surveys are desired in hilly country and presents terrain correction tables with which this quantity may be determined to a relative accuracy of one‐tenth milligal. This accuracy is required to fully utilize the high instrumental precision of modern gravimeters.
Gravimeter observations, in a vertical shaft, 2,247 feet deep, of the Pittsburgh Plate Glass Company’s limestone mine at Barberton, Ohio, for the purpose of determining the densities of the subsurface rock strata, are reported. The survey was made with a standard‐type gravimeter to simulate the data which would be obtained by a borehole gravimeter to aid in the anticipation and formulation of problems in the development and application of a borehole gravimeter for gravity prospecting. Density measurements on many selected core drill rock samples are compared with the densities determined from the gravimeter data. The individual sample measurements show large scatter and systematically low values. Attempts to restore the samples to initial conditions underground were unsuccessful. It appears that density determinations of finite intervals of underground rock strata can be done better with the gravimeter than by laboratory measurements of rock samples.
“Old gravity data never die.” Review interpretations of gravity surveys can be made whenever warranted by new geological concepts, development information, or improved techniques. Important additional uses can be gained by extending the interpretation to deeper horizons by calculating and subtracting the gravity effects of overlying strata whose structure becomes known in detail from shallower development. This constitutes the new technique reported in this paper. An essential new factor which makes gravity “stripping” practicable is the advent of the gamma‐gamma density log which determines subsurface density relations in strata penetrated by development drilling. Combined stratigraphic and density information defines the mass anomalies in the upper strata. Subtracting the calculated shallower gravity influences improves the definition of the deeper gravity prospects. Applications of the method are illustrated by selected examples.
The definitive article “Airborne gravity is here!,” as well as my responses to published “Discussions” of it, were consciously directed to all readers of Geophysics. After 55 years of associations with geologists, geophysicists, exploration managers, university students, and editors (with more than 50 publications), I believe that I may have some competence to judge this.
The interpretation of the results of gravitational prospecting surveys is considered, from a theoretical point of view, in terms of the magnitude of the causative mass as distinct from the conventional interpretation in terms of the mass distribution (size, shape and depth). A general proof is given, based on Gauss’ Theorem in potential theory, that the former problem is unique and the uniqueness is illustrated by an analytical example which also serves to demonstrate the well‐known lack of uniqueness of the latter problem. Practical formulae are presented for estimating the total mass directly from the gravity data and the precision of the mass estimate is considered. The method is applied to a practical gravimeter survey over a known chromite ore body and the estimated mass is found to be in excellent agreement with estimates from core drilling.
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