Search Documents

By J. J. B. Rutherford

IT is now well known that troublesome intercrystalline corrosion may occur in austenitic stainless steels following exposure of the metal to a temperature within the range 1000° to 1500° F. (540° to 815° C.). The present paper describes a satisfactory method of estimating the extent of such corrosion, based on the measurement of the electrical resistance of a specimen before and after attack. This new method enabled us to study quantitatively the effect of several factors upon intergranular corrosion in the 18 Cr, 8 Ni type of alloy. The results show that susceptibility to this attack increases with increasing carbon content and grain size and appears to pass through a maximum for certain combinations of temperature of sensitization and period during which the alloy was maintained at that temperature; and are in complete accord with our interpretation of the mechanism of this phenomenon. The method is probably applicable to other metals or alloys subject to intercrystalline attack, such as the aluminum alloys of the duralumin type. The methods hitherto in use for estimating the degree of intergranular attack fall into three types: (1) microscopic examination,1 (2) macroscopic examination of fractures, and (3) determination of the time required for complete failure or disintegration .2 None of these lends itself very well to a measurement of the extent of corrosion. The first two are qualitative in nature and the third provides an arbitrary value largely dependent on individual judgment, since failure is a progressive phenomenon and not a sudden event. Our experience is that weight-loss determinations, although valuable in some types of corrosion investigations, bear no simple relation to intercrystalline corrosion in this type of alloy, as is to be expected since the phenomenon probably involves only the thinnest films between adjacent metallic grains.

Jan 1, 1932

By B. M. Larsen

AN increasing amount of attention is being paid to the possible influence of oxygen, in its several modes of occurrence in steel, upon some of the properties of the metal; but clearly investigations along this line are unlikely to yield significant results until reliable analytical methods for the determination of oxygen have been established. This paper presents some results of a study of an improved Ledebur method which aims to determine a definite portion of the oxygen; namely; that which presumably is present in diffusible form in the. solid steel. These more or less preliminary results lead us to believe that the oxygen so determined has some significance; their presentation at this time will, we hope, stimulate discussion of, and focus attention upon, certain rather neglected aspects of the problem. Considerable work has been done recently on methods of estimating inclusions and on the vacuum-fusion method for oxygen; with results which, however, are of somewhat questionable significance. It is commonly presumed that the oxygen in a bath of molten steel in a furnace is present partly as a separate phase consisting of suspended particles of oxides or silicates, partly in solution, the latter being probably the more important. Upon addition of a deoxidizer, it reacts with some portion of the dissolved oxygen and precipitates it as additional suspended particles; the oxygen not eliminated in this way, which remains in solution in the liquid steel, may be regarded indifferently either as dissolved oxygen or as dissolved FeO. When the steel is poured, some of the suspended particles rise towards the surface, others remain trapped in the finished steel, appearing as nonmetallic inclusions in amounts corresponding usually to about 0.01 to 0.03 per cent 02 in the steel. The dissolved oxygen may be partly eliminated as CO during freezing of the ingot, but some remains either in solution or in part precipitated as FeO. Finished steels may thus contain oxygen in several forms: (1) In non-metallic inclusions which are probably quite insoluble in the solid metal at all temperatures; (2) in solid solution which at lower temperatures may be supersaturated; (3) in FeO particles produced by relief of this

Jan 1, 1932

By C Gunther

The purpose of this book is to present the practical side of economic geology concisely and in convenient form; established facts and the applications of accepted views are emphasized; theoretical discussions and questions of genesis are avoided. Coal, iron, and placer deposits are omitted; they are subjects of specialized study that are fully and concisely treated in other works. The reader is assumed to possess a knowledge of mineralogy, petrography, and elementary geology. The arrangement adopted is based on Kemp's theory of magmatic waters, on Lindgren's conclusions on hydrothermal and secondary alterations, and on the theories of secondary enrichment enunciated by Emmons and Weed. No general classification of ore deposits is attempted, nor is any attempt made to fill by hypothesis the wide gaps, nor to explain the apparent contradictions that rank economic geology among the most inexact of sciences. The present knowledge of the subject is too incomplete to warrant such broad generalizations. The demand for good properties generally exceeds the supply. While mines of the first rank will undoubtedly be discovered from time to time, it is probably true that a great proportion of deposits having outcrops of commercial grade or of evident promise have been recognized and explored. A review of mining conditions over long periods shows that the rich discoveries belong to pioneer days, and that as time goes on the more important developments are the result of lower working costs, improved metallurgical processes, and an increasing knowledge of economic geology. As engineers in search of developed mines no longer expect to find properties having positive ore of greater net value than the price asked, so those in search of prospects should not expect to

Jan 1, 1932

By W. G. Bradley

THIS PAPER describing the quarrying, mining, and crushing of gypsum at the Blue Diamond mine, 11 miles from Arden, Nev., on the Union Pacific Railway, is one of a series on gypsum mining being prepared for and published by the Bureau of Mines. These papers are designed to disseminate technical information regarding the methods used. The cost tabulations represent operating expenditures only and not total costs. It is recognized that publication of total production costs might in some instances cause embarrassment to individual producers as well as to the industry as a whole. On the other hand, operating costs are essential to the technical discussion and study of the methods employed. The attention of the reader is specifically called to this differentiation in order that no misunderstanding of the scope of the cost tabulations shall ensue.

Jan 1, 1932

By Wheeler P. Davey, Robert T. Phelps

A study of solid solutions has long been a source of interest because of the conditions controlling their formation. X-ray investigations so far have been conducted with the idea that there were two types of solid solutions. In the first type, an atom of the solute was supposed to replace an atom of the solvent in the crystal lattice. In the second type, an atom of the solute was supposed to lie in one of the interstices between the atoms of the solvent. These pictures both require that the sizes of the atoms must play an important part in the process of solution. It is the purpose of the present investigation to study the mechanism of solid solution, using pure silver as the solvent, and pure aluminum as the solute. X-ray measurements show that these elements have the same "shape" of atomic domains1 (they both crystallize as face-centered cubes) and that their atomic diameters differ by less than one per cent.' The results of the present investigation point toward a chemical rather than a physical picture of the nature of solid solution, and make the older pictures merely special cases of a more general picture. The new theory has the advantage, too, that it offers a rational explanation of certain phenomena which hitherto have been only interesting detached facts. The aluminum-silver system which was chosen for this study has been studied by various methods by several workers. By means of thermal analysis Gautier2 and Petrenko3 have determined the phase diagram. Hansen studied the aluminum-rich alloys by means of thermal and microscopic analyses. He showed that at high temperatures silver dissolved in aluminum to the extent of 47 per cent by weight at 560" C. But this solubility decreased to 1 per cent at 200" C. Westgren and Bradley5 made an extensive X-ray study of these alloys. They used the change in the lattice parameter of silver as a criterion of the solution State College, Pa.

Jan 1, 1932

By R. D. Leitch

"INTRODUCTION The character of mine drainage from areas of low pyritic coal3 and from average pyritic coal4 has been investigated, and to complete the study it seemed desirable to select a number of mines situated in high pyritic are as to determine whether or not this variation determines the relative acidity of drainage from mines operating in these characteristic coal fields. Advantage has been taken of some worked on in 1918 by Henry Leighton 5, then Professor of Economic Geology, University of Pittsburgh. He visited and has reported certain areas of high pyritic coal which we also visited during this study. Many of the mines were found to be idle in 1930 and a few were caved shut, apparently for several years. However, samples of the outflow from all mines visited were collected if possible and analyzed, and some general information was gathered for each mine. The map (fig. 1) shows the location of these areas as given by Leighton.ACKNOWLEDGMENTSThe information and assistance obtained from officials and others at the mines visited during this study are gratefully acknowledged, as well as the original suggestion from Dr. Glenn V. Brown of the State Sanitary Water Board who called attention to the desirability of such a study.Thanks are also especially due the coal laboratory of the United States Bureau of Mines, under the direction of H. M. Cooper, for the analyses and determination of sulphur forms in the large number of solid samples necessary to the work reported,This study has been carried on under the direction of R. R. Sayers, chief, health and safety branch, and W. P. Yant, supervising engineer, Pittsburgh Experiment Station, of the U. S. Bureau of Mines."

Jan 1, 1932

By Chas. F. Jackson

During the current business depression the Bureau of Mines has received many inquiries concerning placer mining, the prospects for men of small means to make wages by small-scale placer operations, the locations of placer deposits in the United States, methods of working placers on a small scale, and allied subjects. It has therefore seemed desirable to prepare a circular covering, in nontechnical language, the principal questions which have boon referred to the Bureau of Mines for answer. There are a number of bureau publications dealing with placer mining, most of which are now out of print, but which may be consulted in the larger libraries. Other articles on placer mining nave appeared from time to time in the technical press and several good textbooks have been written on the subject. A partial bibliography of this literature appears at the end of this paper. A recent pamphlet published by the Idaho Bureau of Mines and Geology to assist the prospector and inexperienced small operator has received wide and favorable attention and is recommended as a useful reference. In the present paper no attempt will be made to cover the subject of placer mining in a comprehensive manner, but discussion will be confined to elementary questions of particular interest to persons of limited experience.

Jan 1, 1932

By Charles Will Wright

This paper is one of a series dealing with mining problems and summarizing the data contained in the Information Circulars on individual mines that have been prepared by the managers or superintendents and published by the Bureau of Mines. The purpose of this paper is to present the problems encountered in the development of an efficient mine organization and to show the means used by certain mining companies in promoting efficiency. There is also a discussion of the advantages and disadvantages of the various systems of paying and handling labor and a description of their application at the individual mines.

Jan 1, 1932

By Joseph Pogue

PRORATION in the petroleum industry has come to mean a method for curtailing the production of crude petroleum by artificial effort, and it is in this sense that the term is employed throughout this paper. Although a contrived curb on supply has been invoked at various times in the past in particular fields and to meet specific situations, it is not until the past few years that this procedure has become general and assumed a dominant role in the oil business. In the sense that we now understand it, proration had its inception in the Seminole area of Okla-homa in 1927, from which time and place the practice spread to include practically all the flush oil fields of the country, especially those of California, Kansas, Oklahoma and Texas. At present there is perhaps no phase of the oil business that commands more attention than proration, nor one that engages more time and effort on the part of executives in the business. The practice of proration, indeed, has not only become an integral part of the policy of most companies and operators, but it has also taken on the aspects of a "movement" involving two schools of thought with considerable feeling, and at times some bitterness, on both sides. In this atmosphere it is difficult to find calm, dispassionate consideration of proration, for most discussions are either vigorous exhortations by its proponents, or else sharp condemnations by its opponents; although it appears that the majority of the industry is clearly "in favor of" proration. It is hoped that this analysis will be taken as an attempt to appraise the movement as an economic force, without bias and without regard to the incidence of the conclusions upon any preconceptions that the industry may entertain.

Jan 1, 1932

By Will Coghill

FLOTATION has been so closely allied with the sulfide minerals and their early and associated oily reagents that the term "oil flotation" has erroneously been applied to the entire flotation process. Today, the term "chemical flotation" might be used to advantage. Flotation literature has dealt primarily with the removal of sulfides from non-sulfide gangues, and the separation of one sulfide from another. Now, the application of the process to the separation of nonsulfide minerals is coming to the fore; and from the standpoint of the number of commercial minerals involved, the flotation of nonsulfides offers the larger field. Here, soap or soap-forming substances, such as the fatty acids and their derivatives, are used; hence there may well be a subdivision of the chemical flotation process called "soap flotation." Early experimenters learned that it was easy to form a froth or foam that would carry a part or all of the ore charge over the lip of the flotation machine; but this could not be regarded as flotation because of the lack of selectivity. Doubtless soap was one of the first reagents tried, and doubtless it was rejected on account of its gangue-carrying properties. Now this same gangue-carrying ability makes soap an acceptable reagent, for we are dealing with that, which among sulfides, is considered "gan-gue." As in the flotation of sulfides, so in the flotation of nonsulfides, the selective flocculation of the mineral and the dispersion of the gangue are the first objectives. In fact, these two phenomena epitomize flotation. They depend upon the unlike surface properties of the respective miner-als; and as long as the components of an ore have unlike surface proper-ties, a separation is possible, at least in theory. Often in nonsulfide flotation it makes little difference which mineral floats and which is depressed; generally only a separation is sought.

Jan 1, 1932

By J. E. Moran

A groupd of French capitalists, La Compagnie Française des Mines d'Or du Canada, has undertaken the considerable task of developing Otter creek, in the Atlin district, B.C., by hydraulic mining methods. It has long been known that the gravels of this stream carry sufficiently high values in gold to make it attractive as a mining venture, but it was impossible to ignore the existence of formidable natural obstacles to hydraulicing them, and these needed to be taken into serious account before embarking upon any costly programme of development. The tide to the Otter Creek leases was originally held by Mr. H. Maluin, a French engineer well known throughout the north, to whom was to fall the task of financing whatever programme might be decided upon. It was he, too, who would have to assume the responsibility of finally passing judgment upon the practicability of the various devises for overcoming the adverse natural conditions. With an infinite attention to detail, which I believe to be a characteristic of the French people generally, he studied his problem and was finally able to submit to his financial backers a programme which left the very minimum possible to the hazard of accident or chance. Dr. Mandy, on his visit to Otter creek this season, expressed the opinion that our method of opening this difficult creek presented certain features which might be successfully applied in placer operations elsewhere in British Columbia, and it is largely for this reason that the present paper has been prepared. I shall set forth, as well as I can, the practices which have enabled us to overcome rather easily some of the obstacles which nature had unkindly placed in the path which we must take. The natural flow of water in Otter creek is far too little to deal success-fully with the rather excessive amount of glacial drift overburden which lies directly over the gold-bearing gravels. For this natural defect, the only remedy was to procure more water. This we have clone through a system of flumes and ditches, totalling between ten and eleven miles in length, which bring to the mine a supply of water adequate for our needs.

Jan 1, 1932

By H. B. Freeman, E. J. Gleim

During the 12 months ended July 1, 1931, engineers of the Bureau of Mines examined electrical equipment in 80 bituminous-coal mines. The purpose of these examinations was to ascertain the condition of such equipment with respect to possible hazards of gas and dust ignitions, also of shock. Although only 3 of the 80 were being worked with open lights and therefore did not use explosion-proof equipment, there were others where electric cap lamps were used by the men but where it was apparently assumed that conditions as to gas and dust did not require equipment designed to give protection against ignitions. Early in its history the United States Bureau of Mines originated certain standards covering the construction and testing of electrical equipment in order to minimize the dangers of ignitions from exposed electric sparks and flashes ; accordingly, it is comparatively easy to determine wherein a machine built to these standards falls short of continuing to meet them if damaged or neglected. Many so-called flame-proof and dust-proof machines, however, have not been built to any standard comparable with that developed by the Bureau of Mines. In addition, certain machines have parts which pass Bureau of Mines tests, but do not satisfy the full requirements in other respects, and others make no pretense of inclosing the spark-producing parts. It is not uncommon to find two or more of these various types in the same mine with " permissible " machines, the type that has been built to conform to the Bureau of Mines standard. Consequently, inspection of nonpermissible machines becomes somewhat difficult without a standard of comparison. Because of its unprotected condition obviously open-type electrical equipment must be used with great care, if at all, in a mine where precautionary measures are adopted in guarding against gas and dust ignition. The unsafe nature of such equipment is widely recognized. For that reason it is to be installed only where there is no likelihood of gas being present. It is not apparent to some, however, why flame-proof and dust-proof equipment and equipment having tested parts should not be regarded in the same light as permissible equipment.

Jan 1, 1932

In 1776 two Franciscan friars, Dominguez and Escalante, started to find a direct route from Santa Fé to Monterey, and in their misguided wanderings northward they reached Timpanogos, now known as Utah lake, where they heard of a much larger body of water to the northwest, of which they were told that it was extremely salt.* They never saw this great lake, because they passed south of it on their way to the Pacific coast. The Dead Sea of North America was discovered in 1835 by James Bridger, a trapper. He and other trappers had ascended the Missouri and had reached the Bear river. A discussion arose as to the course of this river, and a bet was made, to settle which Bridger followed the Bear river to its source, in the Uintah mountains, and from the crest of the watershed he caught sight of the lake, to which he then descended.† He tasted the briny water of the inland sea and wondered if it were an arm of the Pacific Ocean. The lake was first seen by the Mormons on July 24, 1847. A band of them, numbering 145 and led by Brigham Young, had migrated from Missouri in search of a new home in the western wilderness. It is recorded that when Brigham Young stood on the summit of the pass over the Wasatch range and saw the valley of the Jordan outspread before him, he exclaimed: "It is enough. This is the right place". He had seen it in a vision, so he claimed, and foresaw the future glory of the new Zion that was to be planted in that happy

Jan 1, 1932

By Howard Eavenson

PRIOR to 1907 nearly all coke was made in beehive ovens, and most of the gas produced was made in the old-style gas retorts, and while there were a few coke plants in southern West Virginia, southwest Virginia and Kentucky operating on high-volatile coals the proportion of the total product made by them was small and was usually used locally. The greater part of the coke made, and the coal gas produced, came from the Pittsburgh seam of western Pennsylvania and northern West Virginia-which was then regarded as the standard coking and gas coal of the coun, try, and the best coal available anywhere for these purposes. With the advent of the by-product oven and the building of coke plants at the furnaces, the restriction of the use of one grade of coal for making coke was removed and the use of mixtures of low-volatile and high-volatile coals began. The change in the locations of the coke plants removed the necessity of using Connellsville coal and also made sources of gas available to the public utilities other than of their own manufacture. Most of the first oven plants were built at points distant from the Connellsville region and this afforded an opportunity for the use of coals from the southern Appalachian fields, which were at approximately equal distances from the ovens. Since that time development has been made on a number of seams of coal of great purity in southern West Virginia, southwestern Virginia and eastern Kentucky, which have been largely used for cokemaking, gas and other metallurgical purposes where low amounts of ash, sulfur and phosphorus are determining factors. These coals are much lower in the objectionable elements than the best of the coal from the Pittsburgh seam, or than any northern coal, and in by-product ovens produce coke that is not excelled for any purpose by any coal mined in this country, and is equaled very seldom. Coal from several of these seams is shipped in great quantities without beneficiation and is lower in ash and sulfur than the best washed product from any seam mined in the north.

Jan 1, 1932

By H. C. Rickaby

The Bannockburn gold area covers the four townships of Bannockburn, Argyle, Hincks, and Montrose, lying in the western part of the Matachewan district of Ontario (see Figure 1). The geology of these four town-ships was studied by H. C. Cooke of the Geological Survey, in 1917-18, and is shown in his map of the district. In the fall of 1930, a rich gold-bearing quartz vein was discovered in the northwest part of Bannockburn by B. S. Ashley. A prospecting rush followed the discovery; many claims were staked in the Bannockburn area, and, during the summer of 1931, a considerable amount of prospecting was done.

Jan 1, 1932

By H. R. MacMichael

THE rate, at which zinc ore can be sintered varies widely, depending on the characteristics- of the ore. A coarse table concentrate already roasted to a very, low sulfur content may sinter at the rate of-8.6 tons per hour per 100 sq.ft., of wind-box area. At the other extreme is a finely ground flotation, concentrate, with considerable sulfur remaining in the ore as delivered to the sinter plant, for which corresponding rate may be only 3.8 tons per hour, or 44 per cent of the first rate. A usual, size-of sinter machine in zinc plants is one with wind-box 3.5 by 33 ft., making 115.5 sq. ft. of area, and using a 100-hp. fan. Such a machine with the coarse concentrate described would produce 10 tons of sinter per hour; and with the fine, flotation concentrate would produce only 4.4 tons per hour. Ore of the latter class is -sintered at the plant of the Mexican Zinc Co. (subsidiary of the American Smelting & Refining Co.) at Rosita, Mexico. ; Consequently, an unusually large sintering capacity per ton of ore to be treated is required, and provision was made for treatment of 9000 tons concentrates per month, preroasted to. 5 to 9 per. cent sulfur. For this duty three of the usual 33-ft., 100-hp. sintering machines would have been needed.. Considering the auxiliary equipment. necessary for proportioning, handling, and mixing charge, as well as means for disposition of the sinter, a three-unit plant becomes-very complicated, especially if the capacity should be doubled later. After much study it was decided to build a single sintering unit of ample capacity for the work, the plant thus being much simpler and cheaper in construction as well as in operation (Figs. 1 and 2). The sintering machine was built with a wind-box 5.25 by 60 ft. (315 sq. ft.) and provided with a 300-hp. fan.

Jan 1, 1932

Abbey, Robert Graham, District Mgr.. The W. W. Sly Mfg. Co., 50 Church St.. New York, N. Y. '21 Abbott, A. N., Mines Supt., Mazapil Copper Co., Ltd Concepcion del Oro, Zac., Mexico. '28 Abbott, Argyle Campbell, Dev. Dept., E. I. du Pont de Nemours & Co., Wilmington, Del. '29 Abbott, Clarence E., V.P., Charge of Raw Materials, Tenn. Coal, Iron & R. R. Co., 1242 Brown-Marx Bldg.. Birmingham, Ala. '04 Abbott, Louis Benjamin, Chief Engr., Elkhorn Div., Consolidation Coal Co., Jenkins, Ky. '16 Abeel, George H., Jr., Cons. Min. Engr Box 427, Santa Monica, Cal. '18 Abel, Albert E., Asst. to Pres., Valley Mould & Iron Corp Hubbard, Ohio. '19 Abel, Raymond L., Asst. Prof., Dept. of Pet. Refin., Univ. of Pittsburgh Pittsburgh. Pa. '28 Abeel, Leo M., Research Engr., Sullivan Concentrator Chapman Camp, B. C., Canada. '31

Jan 1, 1932

By J. W. Furness

Before attempting any discussion of the importance of tariff enactments as applied to exhaustible resources, certain basic factors should be emphasized. First, it must be remembered that minerals differ from organic substances in that they are irreplaceable, and when once taken from the earth's crust enter into uses which ultimately dissipate them, so . that they cannot be recovered for future use. A second factor entering into the problem is the unequal distribution of the mineral deposits of the world. No industrial nation has within its political confines a sufficient quantity of all the minerals necessary for the maintenance and advancement of present civilization; hence international exchange of these commodities is inevitable. As minerals are essential to modern industry, the supply available to any given nation constitutes a factor which limits its ambition to advance industrially. A nation which has this ambition endeavors to secure not only a supply of minerals sufficient for its current needs, but also sources which will assure it expansion. Should mineral raw materials be withheld from such a nation by enactment, it seems inevitable that every effort will be made to obtain them, and that, should the nation deem itself sufficiently powerful, its ultimate recourse will be force. Thus the importance of tariff enactments, not only from the standpoint of industrial prosperity but from the standpoint of international peace, can be appreciated.

Jan 1, 1932

By A. B. Kinzel

ALTHOUGH vanadium is an important constituent of almost every brand of high-speed steel manufactured today, little is known as to its role in this series of alloys. The now standard 18 per cent tungsten and 4 per cent chromium constituents were the result of developments and improvements before 1903. In that year, vanadium was added to the steel by Dr. J. A. Mathews. This proved to be an outstanding con-tribution, so that the standard high-speed steel of today is generally referred to as the 18-4-1 type-18 per cent W, 4 per cent Cr and 1 per cent V. Various modifications have been attempted, since then, but with the exception of the introduction of cobalt, no striking improvements have occurred. Vanadium has been increased to 2 per cent in many of the common brands, and at least one brand contains 5 per cent V in addition to the usual tungsten and chromium. The carbon has been gradually increased so that it is not uncommon to find 0.80 per cent in the standard grade, although the carbon in the 5 per cent vanadium steel mentioned above is approximately 1 per cent, and even 1.2 per cent has been made experimentally. The role of vanadium has never been well understood, and although vanadium has been increased to 2.5 and even 5 per cent in certain high speed steels, it has not been generally established that the higher vanadium contents 'result in superior cutting properties. However, in all of the investigations, of which the writers are aware, the interrelation of the vanadium and carbon contents was not considered as a prime factor. The carbon content was held at 1.2 per cent or less-generally at the usual 0.65 to 0.75 per cent.

Jan 1, 1932

By C. Schlumberger, M. Schlumberger

The object of this paper is to clarify the idea, so important when exploring by potential methods, of the depth of investigation attainable by electrical measurements. After defining, with some precision, .the term "depth of investigation'' in geophysics, the authors show how it is possible to regulate this depth in potential methods and to obtain an 'electrical drilling" from surface observations. They consider also the difficulties encountered in deep measurements and indicate how it is possible to reach greater depths than those having an industrial interest. Depth of Investigation The term, depth of investigation, in its general sense, is easily understood. For instance, a driller who can pull cores from a depth of 2000 m. is capable of working, with accuracy, to that depth. By means of a properly placed drill, he may locate an orebody at a depth of 1999 m., however small this orebody may be. On the contrary, the term as used in geophysics requires precise elaboration. As a matter of fact, no geophysical method, irrespective of the physical property studied (density, magnetic permeability, conductivity), can discover a very small body at a great depth because, in practice, the physical parameter upon which such a geophysical method is based is never regular enough, throughout the ground, to distinguish the feeble anomaly caused by the small, deep-lying body from the numerous anomalies due to irregularities of the encasing terrains. Aside from this question of the ratio between the size of the body being hunted for and its depth of burial, a second factor plays its role in the possible depth of investigation; that is the difference, for the parameter under consideration, between the body and its surrounding material. The greater this difference, and the more homogeneous the encasing terrains, the greater is the possible depth of investigation. This being generally established for all geophysical methods, it is possible to distinguish two main classes of these methods, from this same point of view

Jan 1, 1932