Volume 2, Issue 3, 01 March 1932
Index of content:
2(1932); http://dx.doi.org/10.1063/1.1745037View Description Hide Description
Electrical prospecting is defined as the science and the art of determining the variations of the electrical constants (resistivity, magnetic permeability and the dielectric constant) of the earth's crust and of interpreting these variations in terms of geological structure. The most successful systems of prospecting are based on the study of resistivity variations. The basic assumption made is that in general changes of resistivity follow the bedding planes. Electrical methods of exploration may be divided into two classes, direct current methods and alternating current methods. In part II the fundamental theory of direct current method is discussed and a typical survey is described. Part III deals with the theory of alternating current methods, with particular reference to the optimum frequency to be used. It is shown that in general very low frequencies are desirable. Two alternating current surveys made by the Swedish American Prospecting Corporation are briefly described. In the conclusion (part IV) some of the difficulties of electrical prospecting are discussed. The depth to which investigations may be carried is limited. In the present stage of the art, it would take exceptionally favorable conditions to obtain reliable information much in excess of 2,000 feet. However, it is stated that improvements in methods of interpretation and in field technique should give electrical methods a definite field of usefulness in prospecting for oil.
2(1932); http://dx.doi.org/10.1063/1.1745038View Description Hide Description
Description of a new portable instrument for measuring relative values of gravity to within two or three parts in ten million, specially designed for geophysical exploration. The principles of the design are analysed and the methods for eliminating effects of elastic hysteresis, temperature changes, variations in barometric pressure, etc., are discussed. Also effects of initial stresses in materials, defects in alignment of locking mechanism, inaccurate leveling, etc. These difficulties are serious but seem to have been overcome. Results of preliminary field measurements near Houston, Texas, are presented and some comparison between the type of information given by this instrument and by the torsion balance.
2(1932); http://dx.doi.org/10.1063/1.1745040View Description Hide Description
When a suspended system is supported by a fine wire the equilibrium position usually changes slowly for a long time after the load is applied. The equilibrium position also changes with temperature. It is found that both of these disturbing factors can be eliminated by a suitable heat treatment of the wire. Observations have been made on tungsten and platinum‐iridium wires of sizes suitable for use in the Eötvos torsion balance.
2(1932); http://dx.doi.org/10.1063/1.1745041View Description Hide Description
An instance of regional variation in Oklahoma and two cases of local variation, one at Long Beach, California, and the other at Salt Creek, Wyoming, have been selected for consideration from a large number of geothermal surveys conducted during the past few years by the U. S. Geological Survey and the American Petroleum Institute. The causes of local and regional variations are unknown. Possible explanations such as radioactivity, proximity to crystalline rocks, and transfer of heat along the strata are given careful consideration in attempting to explain the observed relations between the strata and the isogeothermal surfaces.
2(1932); http://dx.doi.org/10.1063/1.1745042View Description Hide Description
The Michigan College of Mining and Technology, in cooperation with the Calumet and Hecla Copper Company and the author, is carrying out a program of temperature measurements in the deep coppermines of Northern Michigan, extending the previous work of Agassiz and others. Temperatures are measured with mercurythermometers mounted in bakelite tubes, placed in drill holes in mine workings where the rock has been freshly exposed, special attention being given the effects of drilling, blasting, and other heat conduction considerations. Present results give as the average gradient from the surface to 5679 feet below (temp. 95.3°F), 1°F in 108.5 feet (0.0168°C/meter). The gradient is more nearly uniform than has sometimes been supposed. A preliminary attempt has been made at calculating the previous ``thermal history'' of this region. Diffusivity of specimens of the rock measures 0.0075 c.g.s., and on this basis calculations of theoretical temperature‐depth curves have been made for 25 different assumptions of previous temperature conditions, and compared with the actual curve. Results as yet are inconclusive but indicate that at least 30,000 years have elapsed since the last glacial epoch, a longer period than usually assumed.
2(1932); http://dx.doi.org/10.1063/1.1745043View Description Hide Description
The velocity of elastic waves in granite was determined at Quincy and Rockport, Massachusetts, and Westerly, Rhode Island. The waves measured were generated by dynamite explosions. They were recorded by portable seismographs at distances ranging from fifty feet to four thousand six hundred feet. The observed velocities for longitudinal waves were:A three‐component seismograph, used only at Quincy, recorded transverse waves, the velocity of which was 8150±90 ft/sec., or 2.48±0.03 km/sec. From the two velocities determined at Quincy and the density of specimens taken from the shooting location, 2.65 grams/cm3, values for the bulk modulus,k, compressibility, β, rigidity,μ, Poisson's Ratio, σ and Young's Modulus E, were obtained as follows: k=44±1×1010 dynes/cm2; β=2.28±0.05×10−12 cm2/dynes; μ=16.3±0.4×1010 dynes/cm2; σ=0.333±0.005; E=43±1×1010 dynes/cm2. The form of the time‐distance curves, straight lines through the origin, indicated that the waves did not penetrate deeply. Accordingly, the values obtained are for pressures of only a few atmospheres. The bearings of these results upon earlier investigations of the elastic constants of granite are discussed. Although direct comparisons between laboratory and field results are not conclusive, they indicate that the Adams and Williamson curve is incorrect for pressures below 2,000 megabars, and that there is no marked difference between dynamically and statically determined compressibilities of granite.
2(1932); http://dx.doi.org/10.1063/1.1745044View Description Hide Description
In the course of explorations of subsurface geology by the seismograph the authors have frequently observed the pronounced effect of stratification on the velocity of seismic waves in shales, and this effect has often been utilized in practical seismography. Recently an opportunity was afforded for securing additional quantitative data on the velocity normal to and parallel to the bedding planes. The paper points out that the velocity parallel to the planes of stratification is, in some instances, as much as fifty percent higher than the velocity in a direction normal to the bedding planes. It is shown also that inclined stratified beds exhibit a higher apparent point‐to‐point velocity when sound travels in an up‐dip direction than when traveling down‐dip. The paper describes a procedure whereby this effect may be utilized for determining the direction and approximate magnitude of the dip in such stratified deposits. The method has proved to be of considerable practical importance where the stratified formations are obscured by overlying deposits.
2(1932); http://dx.doi.org/10.1063/1.1745045View Description Hide Description
The equation of motion of a mechanical seismograph is −ẍ=ÿ+2kẏ+p 2 y where x is the ground displacement and y the seismograph deflection. This equation may be solved for y when x is supposed known or for x when y has been observed as a function of the time. In this paper both of these ways of solving the equation are considered. The motion of the seismograph due to a train of waves starting at t=0 is considered and also the motion due to the arrival of a single wave. In each case seismographs with several periodic times and either undamped or critically damped are considered. Curves are given showing the motion of the ground and the calculated motion of the seismograph. The motion of the ground corresponding to several simple assumed seismograms is also worked out and shown by means of curves. The motion corresponding to a given seismogram depends greatly on the periodic time and damping of the seismograph. Finally the ground motion is deduced from two actual seismograms due to dynamite explosions. An integraph is described which enables the calculations to be done more quickly.