US2375433A - Seismograph - Google Patents

Description

May 8, 1945- l J. P. MINTN 2,375,433

SEISMOG'RAPH Filed March 29, 194s Patented May s, 1945 UNITED STATES PATENT orifice SEISMOGRAIH John P. Minton, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation o! New York Application Maren 29,1943, serial No. 481,021

a claims. (o1. aai-36.5)

This invention relates to geophysical prospecting and more particularly to means for placing indications of elapsed time on a seismograph record, and is a continuation-impart of my copending application, Serial No. 174,974, filed No- J vember 17, 1937.

' In subsurface surveying, by thev use of seismographic equipment, accuracy in time measurement is of greatimportance. Anerror of as much as a few thousandths of a second in measuring the over-al1 period of elapsed time required for seismic waves to travel from their point of origin toaJ substratum and be reiiected or refracted to the recording point will frequently give rise to substantial errors in depth calculations of strata.

Considerable difficulty has been encountered in- I one-,or both times manually have been used.

Electrically driven forks have had theobjection that more or less complicated additional circuits must be provided. The type of fork which depends for its energy on an initial impulse being delivered to it by hand has proven unsatisfactory, for the reason that it must be mounted in a position Where it will intercept a beam of light in the optical system, thereby making it inconvenient for the operator to deliver a physical impulse to it in order to start itvibrating.

The primary object of this invention is the provision of means for recording photographioally denite intervals of time, simply and accurately.

' The invention provides a vibrating reed having a v predetermined natural frequency and having. a

substantially negligible thermal coefficient of eX- .pansion Means are provided for electrically delivering an impulse 'to the vibrating reed by the same means and at the same time that the photovgraphic iilm is set into motion, since the timing device does not serve any useful function until the photographic lm has been set into motion. The reed, energized by the electrically delivered impulse, then vibrates at its natural frequency.

The reed carries, v4mounted on the end thereof, shutter means for intercepting a beam of light.

Figure 1 is a diagrammatic illustration of .al

complete seismograph unit, showing the application of the .instant invention thereto;

Figure 2 is a plan view of the vibrating element and its base;

Figure 3 is a sectional view taken along the lines 3--3 of Figure 2, showing the relative dimensions of a section of'the reed;

Figure 4 is a side elevation of the vibrating element showing a modified form of the impulse delivering means; and

Figure 5 is a diagrammatic illustration of a further modication of the invention showing common control means for both the starting means for the reed and for the camera that is used forrecording the time indications as well as the seismic waves.

Referring to thevdrawing in detail, and particularly to Figure 1, there` is shown diagrammatically a conventional seismograph comprisy ing a geophone lli adapted to lbe impinged by seismic waves to generate electrical signals corresponding-to them. These electrical signals are conducted to the ampliiier il by means of conductors i2. The amplified signals are conducted from the ampliiier by means of conductors it to a recorder ld where they are recorded on a moving sensitized paper or photographic film l5. The recorder used in practice is usually that of a vibrating string or coil type galvanometer. As-

suming a coil type recording galvanometer is,

used, alight ray from'the source i6 traverses the path il to a mirror on the vibrating element, not shown in detail, thence over the path It through a lens D-to the photographic lm or sensitized paper l5. Simultaneously with therecording ci the signals which correspond to the seismic waves, there is also placed on the moving photographic film or sensitized paper l5 indications of elapsed time in the form of transverse lines. These lines are generated by the interception of a beam of light from the source i9 which follows the path 2liA through a shutter 2| carried by avibrating -reed 22, which in turn is secured to the base 23.

This beamy of light is reected from a fixed mirror 24 on the recording galvanometer. I4 and follows the path 25 through the lens D to the moving sensitized paper or photographic illm I5. 'Ihe reed 22 is caused to start lvibrating through the medum of an electromagnet 26 and associated circuit which comprises the core 21, the winding 28,

the battery 29 and the switch 30. The battery 29, on closing the switch 30, energizes the core 21 to draw the free end of the 4reed downwardly against it where it is held until it is desired toI start it vibrating, then the switch 30 is opened, thus de-energizing the core 21 and permitting the reed to swing into vibration.

As shown in the planview of Figure 2, the base 23 to which the reed 22 is secured by means of screws 3| or other conventional means, is made relatively large and is formed of solid metal, such as iron, brass, etc., having a relatively large mass. The base 23 is rmly anchored to the housing of the recording instrument, not shown. Forming .the base 23 of relatively large mass -eliminates the possibility of introducing into the vibrating reed 22, that is secured thereto, harmonics which would tend to distort or render inaccurate the timing lines formed thereby. Another advantage obtained by having the base of a large mass relative to the reed is that there is no tendency of the vibrating reed Vto set the mass into vibration tothe reed. Since substantially all of the starting energy remains in the reed, it will vibrate over a considerable period of time before it comes to rest. Preferably the .mass of the base is sev- 'eral hundred times that of the reedund 36% nickel. This material is lsold on ther market under the trade. name of "Elinvar.

to draw it downwardly toward themagnet core 21 and hold it in the down position until it is desired to record waves, at whichy time the current supplied by the source 29 is cut off. The removal of this current from the winding 28 on the core 21 causes an immediate de-energization of the core 21 and a resultant release of the soft iron element which will start the element vibrating at its constant natural frequency. The reed 22 is of graduated length. The mass secured to the end thereof in the form of element 2| can- .not as a result of changes in temperature affect the natural period of vibration of the reed.

As soon as the force exerted by the electromagnet 26 on the soft iron member 2| carried by the reed 22 is removed, the reed will vibrate at its natural frequency for a period of time that is sustained for as vlong as ve or six seconds.

. T0 those skilled in the art, it will be apparent that this period of time is ample to record seismic waves from any conventional spread-distance. It is at* this point admitted that the instant ini vention would have no application in refraction surveying where as much as or 40 seconds are required for recording dependent upon the distance traveled by the seismic waves.

Where a timing reed is usedfor marking seismograph records, in which extreme accuracy is required, the ordinary ironv or steel reed does not have suiiicient accuracy. These instruments conditions, involving a range ont temperatures of as much as 150 F. or greater. By making the reed of a metal which has a zero thermal coemcient of expansion, or nearly so, the length of the reed and hence the frequency of vibration is practicallyl constant. I have found that a reed made of Elinvar is accurate over a temperature range of from 430 F. to 120 F. within one part in five thousand, whereas the ordinary iron or steel reeds have a frequency variation of from five to ten times as great. y

As shown in Figure 3, the reed is wide in comf parison to its thickness. The ratio oi.' width to are used in the open under all typesof climatic thickness should be within the range of from is secured to the rtree end of the reed, by welding leg portion 22 of the element to the reed.- Leg portion 32 extends vertically upwards from the end of the reed, and is provided with a window 34 in the form of a slit through which light raysare adapted to pass. The-'vibration of the reed carrying the element will intercept rays from the source of light I9 to expose the moving sensi-4 tized paper or photographic' nlm Ii in the manner described above.

The soft iron element 2| in addition to deilning a window through which light rays are projected, also provides 1by means of the portion' Il .an armature whereby electromagnetic forces can-be exerted on the assembled reed 22 and element 2| carried by the free end thereof. described above, the purpose of placing these forces upon the assembled reed and element is On the other hand, when the conventional method of reflection seismic-surveying is being carried out, or refracted waves over a surveyed distance are being recorded, this deyice not only has application but is more practical than any of the other means known inthe art, such as electrically driven forks.

This novel timing device `also affords the ad-` vantage that it is free from variations of frequency due to changes in ,temperature A1- vthough the instant invention has been described in connection with chromium-nickel-steel, known by the trade name Elinvar, it is obvious to those skilled in the art that other materials having a substantially zero coefficient of expansion would serve equally as well.

In Figure 4 there is shown a modified form of electromagnet for use in imparting the starting impulse to the vibrating element.A This form of the invention differs from the impulse deliver ing means described infconnection with Figure 1 in that the core 35 is substantially U-shaped. One arm 36 of the U-shaped core cooperates with the soft iron -portion of element 2| to exert an attracting force thereon in the manner described above. The other arm-31 ont the core 3B is provided with an elongated soft iron pole tip Il so shaped to enable it to cover a substantial area of the vibrating reed 22, and which ouenergization of the electromagnet will exert an additional force on the. reed 22 to attract it'toward the electromagnet. This form of the invention has y the advantage that with `a relatively small source of D. C. supply to the winding 2B of the electromagnet, a stronger force will be exerted on the bottom of the vibrating element to draw it downwardly against the polesxof the core `and hold it in'the down position until it is desired to release it' for vibration to intercept the beam of light.

, In Figure 5 there is shown stillJ another modiiication of the present invention, in that common means are shown for setting vibrating element 22 into 'vibration and i'or starting the camera 3l to cause the photographic film 0r sensitized paper I5 to move past the lens D.

Although forpurpose of illustration. the form of the device illustrated in Flture 4 is shown in connection with the novel starting means ot Pig- A vthen releases the reed 22 which springs free and mic record to be obtained. c

ure 5, it is obvious that the form of the invention shown in Figure l can=be used equally as well. In Figure 5, the camera 3 8 is shown diagrammatically. and the take-up roll 40 which may be driven by electric motor o r conventional spring motor, -is provided with a brake shoe14| secured to one arm 42 of a lever 43 that ispivotally mounted at 44. The second ann 45 of the lever 43 has secured to it, at the end oppositel pivot 44. a contact 48, for engaging the stationary contacts 41, 48 andv 48. Contact 48, carriedby the e lever ,arm 45, andthe stationary contacts 41, 48

and 49, serve to energize and de-energize the cir- 4cuit of the electromagnet 26 in the` manner hereinafter described. Normally, spring 50, holds brake shoe 4I, iirmly -against nlm take-,up roll 40, contact 48, resting on dead contact, 41. To place the apparatus in operation, manual. switch key 5I is pressed down, thereby causing current to ilow in the circuit vmade up of Abattery 52, switch" 58, and coil 54.

The closing of, this circuit energizes coil 54, and soft iron magnet 55. Magnetic flux from pole piece 58 attracts soft iron lever arm 42, causing it to move from its original position, indicated in Figure@ by the solid lines, to a position indicated by the dotted lines. where it is in firm contact with `electromagnet polepiece 58. This movement of lever 42 causes a simultaneous movement of lever arm 45 from its original position indicated in -the drawing by the solid/lines, to a position f shown by the dotted lines/The motion of` arm 45 causes contact 48 topa'ss over live contact 48, which is connected to one side of coil 28,'.on core 35, as shown in Figure 5. Current then 'flows fromv battery 28, to'contact 48, and across to va reed having one end secured to lsaid base, said 1 reed being of a predetermined length determinable by the interval of time desired between the of said. core to exert a pull on the reed, the other arm of said core being adapted to exert a force contact 46. Lever arm 45, is provided with suit-I I able electrical conduction means (not shown).

timing. lines, electromagnetic meansfor vdelivering an impulse to said reed to start the" reed vibrating,n common means secured to the free endl of said reed forming an armature for the elec. tromagnet and defining an aperture through which-light will pass at regulated intervals when the reed is vibrating to expose timing lines on the seismogram, saidy electromagnetic means comprising a substantially Ushaped core, a pole piece on one arm of said core adapted on energizaton on the armaturecarried by the free end of said reed,a winding on said core, and an energizing circuit to energize saidwinding and core to Aattract the reed and armature to the core, and

means for de-engergizing said core to release the reed to start it vibrating.l

2. In a timing system for electric seismographs l having a record sheet and driving means there- A for, the combination of a timing device for placing transverse parallel lines as indications of elapsed time on said record sheet comprising a base, a thin elastic plate having one end secured to said basel and the opposite end free to vibrate,

a shutter carried by said free 'end of said elastic plate, an electromagnet, an energizing circuit for said -electromagnet whereby lupon energization thereof it will move said free end or Said plate to an angular position to store energy therein for extending from contact 48, to the pivoted end of arm 45, from whence a connection is provided to the other side of coil 28. Current thus flows from contact 46, through these conductors tor ccil- 28 and thence back to the battery 29. The closing of this circuitienergizes coil 28' and core 85, drawing the f ree end of reed 22 down against the arm 38 ci.' the U-shaped-magnet. As soon as the contact 48 has moved away from the contact 48 to the second dead contact 48, the circuit is broken de-energizing coil 28 and core 35, This starts vibrating. 'I'hus a single operation, namely'. pressing switch key 5|, simultaneously places the cameraroll 48 in motion, and starts'the reed 22 vibrating. Key 5l isheld in the down po-i sition for sufcienttime to allow a complete seis- Inl practicing the invention as disclosed* in Figure 5 one is able to coordinate the starting` of the camera and the timing udevice to the extent -that a minimum )of illm will be-wasted in recording a seismogra'm andy such that the en-Av tire useful period' ofv vibration of the vibrating element isutilized. l

' driving means. s 3. In a timing system for electric seismographs subsequent vibration during a substantial period of time, and common means for starting said driving means and for controlling said circuitto de-energize said electromagnet to initiate vibration of said elastic plate upon starting of said having a record sheet and driving means. therefor, the combination of a timing device for plac ing transverse parallel lines as indications of l elapsed time on said record sheet comprising a base, a thin elastic plate having one end secured to said base and the opposite end free to vibrate a shutter carried by said freev end of said reed,

- an elctromagnet for moving saidjree end of '60 ing means.

said reed to an angular position to store-energy therein, an energizing circuit4 for said electromagnet, and common means for starting said v driving means and in quick succession closing and opening said energizing circuit to initiatevibration of saidlreed upon starting or! said drive JonN P. more.'

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