The term shallow refraction, as used in this paper, refers to investigations confined to the superficial layer of rocks, composed primarily of unconsolidated material. To define the scope of the discussion it will be assumed that the term shallow refraction applies to work of which the lower limit is approximately 300 feet and the upper of the order of a few feet. The consideration of the small magnitude of quantities measured (distances and times) determines the perspective of the problems involved, since difficulties encountered in the interpretation, although equally disturbing whether in deep or shallow work, will cause a greater percentage of error in the latter case. The purpose of this paper is to discuss and illustrate these problems. The factors considered include the location of the geophone spread in relation to the topography of the site, influence of the ground conditions in the vicinity of geophones on recorded times, consideration of the shallow uphole shots and problems arising from the repeated use of the same shothole. The rapid variation in the vertical velocity of the overburden and errors due to it are discussed together with the effects of a non homogeneous unconsolidated material and velocity reversal. The effects of the ill-defined solid rock surface are also considered. It appears that as the depth of investigations becomes shallower, the limits of the practical capabilities of the method are approached, because the differences between the theoretical assumptions and the actual conditions become more pronounced.
The term “structure”, as used here, includes synclinal and anticlinal folds and folding in general, faults, cross fractures and various conditions associated with intrusions. Structure, both regional and local, has a very important role in emplacement of mineralization. Under certain circumstances which occur fairly commonly, structural conditions are reflected significantly in the trends and intensities apparent on aeromagnetic maps. A number of documented illustrations based on Canadian Shield conditions are discussed. Illustrations are drawn from areas of known geology and from proven mining camps, as well as from loci of recent discoveries. Canadian examples are chosen because of existence and availability of extensive aeromagnetic cover, although it is logical to extend the argument to other shields and indeed to regions of other geological, but similar magnetic character. The importance of aeromagnetics in structural approach to exploration and the correctness of such approach seem to be fully substantiated by results discussed.
The gravity difference between two stations, one at the surface and the other underground vertically below the former and at a given distance from it, depends on the mean density of the earth, Us as well as on the density of the layer of rock contained between the two stations. When one of these densities is known, the other can be computed from this gravity difference. The reliability of this determination depends on the relative accuracies with which (I~ and (r can be obtained.These accuracies are discussed in the paper. The mean density of, the earth has been determined with an accuracy of approximately 0.01 grJcm3. The determination of the density of a layer of rock depends on density determinations of rock samples which are not representative of the layer as a whole. Thus the accuracy of the value of u based on a number of sample determinations will depend on many factors, including the method of averaging the density values obtained from the samples and the degree of uniformity in the geology.To investigate the problem discussed above, three sets of gravity measurements were made under differing conditions. In each instance a number of pairs of stations vertically above each other were occupied on the surface and underground. The results computed from the data on each pair of stations in a set of measurements were considered as repeated measurements of the same quantity, and the most probable value was calculated.The results demonstrated that the accuracy varied with the conditions prevailing in the area where the observations were made. In Godstone Quarries the dip of the strata was negligible, the rocks fairly uniform and structural conditions undisturbed. Consequently, although the rock layer between the surface and underground stations was only of the order of a hundred feet, the mean density of the easrth computed from the average density of the rock samples, was very close to the accepted standard value of 5.52 gr./cms. This agreement, however, was easily upset when only one random sample density was assumed as representative of a given formation.In a different locality in Cumberland the observations were made in a mine and on the surface. The rock layer between the surface and the underground stations was approximately a thousand feet thick. One set of measurements fol!owed a line parallel to a fault, the other a line crossing this fault. The results differed appreciably from the standard value of (I llz, particularly in the latter case.It is concluded that the gravity difference between a surface and an underground station can be used satisfactorily to determine the average density of a rock layer in situ and en bloc, using the standard value for the mean Earth density.
The paper describes and discusses the results of an experimental gravity survey which was carried out underground on different levels of a mine, in the mine shafts, and on the surface above the mine workings. The paper is composed of three complementing sections. The part dealing with gravity measurements in the shafts gives also attention to the particular problem of the terrain corrections underground, due to the surface topography. The interval densities from gravity measurements in the shafts are computed and adjusted in accordance with known geology and compared with the stratigraphical columns of the shafts. The effect of the ore body on the stations in the shaft is derived theoretically and compared with the observed one. The gravity contours are constructed on different levels in the mine workings and discussed in relation to the known extent of the ore body. The gravity profile across a fault underground is presented and discussed. Another gravity profile was run underground in the same plan position as a surface traverse 1000 feet above it. The line of boreholes along this traverse gives good account of geology which includes step‐faulting. This known geology is compared with the deductions based on the gravity results. This is also done in the case of another gravity profile run over a known geological section. A number of gravity measurements were also taken in the same plan position, separated by the vertical distance of 800–1,100 feet. These points were placed by the boreholes previously drilled in the area. Attempt in correlation of these and gravity results is made. The densities computed from the gravity measurements are compared with the laboratory determinations of the densities, carried out on samples from different parts of the mine. The contours on the top of the base formation are constructed from the information obtained from the boreholes, and are compared with the gravity contours on the surface above. A simple method of computation of the effects of slabs and blocks is presented as applied to the calculation of the corrections for underground drifts and faults. A table is appended for use with this method.
A brief description of the procedure usually adopted for ascertaining the accuracy o aeromagnetic data is given as a background to the discussion of factors which affect this accuracy. These factors fall mainly into two groups: spatial positioning, i.e. relation to the ground surface, and effects contributing to the observed relative magnetic values. The evaluation of the observed anomalies in terms of the most probable causes and the anomalies themselves are directly influenced by the amount of the available control of factors mentioned above. The effects of various factors, such as plan positioning, height keeping, diurnal drift, are discussed, the arguments being based on theoretical and practical premises.
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