The form of dip chart here presented for the use of the profession was devised by the author when he was confronted with a particularly heavy job of geological section making and did not have any of the usual tables or charts at hand. This was at the property of the Teziutlan Copper Co. in Puebla, Mexico, where the orebodies are much faulted and lie conformably with a slightly inclined and much folded schist. Because of the low dip and the folding of the orebodies, the engineering and geological data respecting them were best represented on vertical cross-sections quite closely spaced.
Calculated Table of Apparent Dips in Vertical Section Geological Dip Chart
A less finished form of the chart was published in the Mining & Scientific Press of Aug. 6, 1921, with an explanation of its use. I have checked the new drawing for accuracy against the calculated table given, and constructed an explanatory diagram beneath. As some geologists are accustomed to using the table, I have included it, since no advantage is obtained by mere interpolations from the chart to get the required angle, except as it gives a mental picture of this angle. The chart gives a more rapid and much easier solution when cases multiply so that it becomes desirable to eliminate the tedious manipulations of a separate protractor. In fact, using the parallel ruler as described, it is not even necessary to make note of the angle sought. It is brought out graphically, leaving the mind free to note the correlations. The chart itself may be made more convenient by reproduction on celluloid with a slot running on the zero line of the protractor.
When I approached the problem of constructing a dip chart, it was with this idea of eliminating all possible mechanical labor. In the form discovered, two sets of curved lines, each tangent to the 90° line at the center of the protractor, constitute a system of coordinates by means of which one may pick a point on the protractor that will determine the angle of apparent dip, of a vein or other geological structure, upon the vertical section. A line from this point to the center of the protractor establishes the angle. In finding the point, it is usually best to follow the coordinate of the strike angle to its intersection with the coordinate of the real dip angle. The apparent dip angle sought, of course, may face to the left or the right. If to the left, as happens in the diagram (vertical section), the upper side of the chart is used; if to the right, the lower.
The chart is constructed on the center or zero line of a protractor, as representing the line or course of a vertical section. This line is divided into eighteen equal parts, representing from outside to center each five degrees of real dip. The radiating lines of the protractor then represent every five-degree position of the strike of veins. The illustration given will show the graphical construction, which, carried out for all even cases, gives the series of points that determine the coordinate lines for any odd case. The solution ordinarily employed to find an apparent dip is used here in a series of cases condensed to conform with the conditions of the problem as set forth above. It is also conditional that all the perpendiculars (P) erected on the strikes pass through the line of section at a common point, that representing the particular dip being used. The angle of apparent dip sought for each is then resolved so that it also falls into the protractor. The succession of base points of the strike perpendiculars generates the arc of a circle and forms the coordinate for the dip being used (30 in the illustration). The intersections with this arc of each line representing an apparent dip angle give points at the crossing of the strike-angle coordinates. The latter coordinate lines are then drawn connecting points for an identical strike angle in all the arcs of the dip angles. The chart thus constructed then permits a rapid solution of any combination of strike and dip. To make more clear that portion of the chart adjacent to the center of the protractor, where the lines converge, a portion has been doubled in size.
Most mining geological sections are prepared on transparent material, so that by squaring them over the chart and orienting the notes or plan alongside, all the drafting detail can be executed with pencils and a parallel ruler. The calculated table is a useful supplement, and a job of section making that lends itself to this method will require far less time and labor. Not the least advantage is its freedom from annoying mechanical detail. An outline of the method is given at the bottom of the chart.
New Life-saving Aid
As the result of experiments made by the U. S. Bureau of Mines, the geophone, an instrument used during the war for the detection of earth and rock sounds in the construction of military mines and tunnels, promises to become an active factor in the saving of the lives of miners entombed as the result of fires and explosions. Blows with a sledge on a coal face were heard by the Bureau’s investigators, by means of the geophone, at a distance of 650 ft., with various rooms and entries intervening. On a suspended pipe line, light hammering with the knuckles was detected at a distance of 1500 ft. Ordinary talking and singing could be detected through 150 ft. of solid coal. By the use of two geophones, one instrument to each ear, it is possible to determine the direction from which a sound is coming through the earth, and thus to locate the approximate position of the entombed miner. The Bureau gives recommendations for a signal code to establish communication between the rescuing party and those entombed.
By means of the instrument, it is possible to hear water circulating in the pipes of ordinary city mains situated 10 to 15 ft. below the surface. Tests conducted at a busy corner in the downtown district of Pittsburgh located a leak in a water main which the water company had for two weeks vainly sought to detect.
The mine geophone is sensitive, yet extremely simple and easily portable. The instrument was invented by the French during the war. It was developed by the United States Engineers, and the instruments now used by the Bureau of Mines were made according to plans drawn by them, except for the introduction of different diaphragms. The geophone is essentially a small seismograph, as it embodies the same principles as the ponderous apparatus that records earthquake tremors. It consists of a lead weight suspended between two elastic diaphragms cutting across a small airtight box. If the instrument is placed on the ground and anyone is pounding or digging in the vicinity, energy is transmitted as wave motion to the earth, and the earth waves shake the geophone case. The geophone, therefore, transforms the earth wave into an air wave, which is heard by the ear as sound, and at the same time magnifies the wave so that the sound is louder than if the ears were placed in direct contact with the earth.
The observations of the Bureau of Mines indicate that the geophone may be useful in the location of mine fires. Usually a mine fire makes enough noise, either by drawing air or by breaking off slate and coal, to be heard for a considerable distance through the coal and even through the strata above. The distance that these sounds can be heard depends a great deal on the nature of the strata above the mine, yet the sounds originating at a mine fire should be detected through 100 to 300 ft. of cover and through 500 to 800 ft. of coal.