US3212057A - Long period seismometer - Google Patents

Description

1955 F. E. ROMBERG LONG PERIQD SEISMOMETER 5 Sheets-Sheet l Filed March 27, 1961 INVENTOR. Frederick E. Romberg BY Oct. 12, 1965 F. E. ROMBERG LONG PERIOD SEISMOMETER 5 Sheets-Sheet 2 Filed March 2'7, 1961 331 1.53" EFE INVENTOR. Frederick E. Romberg BY A 6 EN T Oct. 12, 1965 F. E. ROMBERG 3,212,057

LONG PERIOD SEISMOMETER Filed March 27, 1961 5 Sheets-Sheet 3 INVENTOR. Frederick E. Romberg BY AGENT United States Patent 3,212,057 LONG PERIOD SEISMOMETER Frederick E. Romberg, Dallas, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Mar. 27, 1961, Ser. No. 98,564 11 Claims. (Cl. 340-17) This invention relates generally to vibration detecting seismometers, and more particularly to that group of seismometers that convert a physical vibratory motion into an electrical output signal.

A primary object of this invention is to provide an extremely accurate long period horizontal motion detecting seismometer.

Another object is to provide a long period horizontal motion detecting seismometer which is accurate in operation and yet extremely durable and rugged in use.

An additional object of this invention is to provide a horizontal motion detecting seismometer which operates on the pendulum principle, and yet which operates at a controlled period that is longer and selectively adjustable relative to the period that a corresponding conventional gravity-type pendulum would have.

A further object of this invention is to provide a relatively short length pendulum-type seismometer which will have a period of operation that would correspond to an extremely long length conventional type pendulum.

A further object of this invention is to provide a simple, compact, unitary seismometer which operates to detect long period horizontal motion, and yet to provide such a seismometer that is quickly and easily installed in the field for operation, and one that is easily adjusted for misalignment.

A still further object of this invention is to provide a long period horizontal motion detecting seismometer in which the geometry of the construction of this seismometer is such that it will tolerate misalignment at installation which would prevent conventional long period horizontal motion seismometers from functioning accurately.

Certain prior art seismometers are known which are specifically intended to measure long period horizontal component vibrations, but these earlier devices were very sensitive to tilt so that it was necessary to install them permanently in their properly adjusted position before they were suited for use. This virtually eliminated their value as portable instruments. Many of these prior art horizontal seismometers were disturbed in their operation if subjected to a tilt of as small as 2 seconds of arc; the present invention can operate satisfactorily with as great a tilt as A2 of a degree.

Horizontal seismometers have been used in the past to detect earthquakes throughout the world. Detecting earthquakes is a continuous endeavor and most of the stations that are intended for this purpose are of a permanent nature; however, there has arisen at the present time a need for a long period seismometer that is extremely rugged and durable and relatively insensitive to slight tilt so that it may be used as a portable instrument. This need is in establishing a world wide nuclear explosion detection system in which seismometers are located in arrays to not only accurately determine the location of a large vibration but also to distinguish between the vibration resulting from an explosion and from an earthquake. This is done by an examination of certain characteristics of the long period sections of a visual record seismogram of an earthqutke and a corresponding seismogram which was triggered by nuclear explosion.

The present invention utilizes the principle of a vertically extending, horizontally moving pendulum but incorporates sufiicient structure and geometry to actually 3,212,057 Patented Oct. 12, 1965 increase the period of the pendulum in this seismometer beyond the period it would experience as a solely gravity operated pendulum. This advantage is gained by the use of a relatively short length pendulum which is pivoted at a point in vertical alignment with the null position of the mass of the pendulum, but which includes a zero length spring which attaches to the mass at the lower end of the pendulum and also attaches to a point a certain distance above the pivot so that in effect the spring is considerably longer than the pendulum itself. This structure in combination with the characteristics of the spring itself and the use of frictionless bearings will permit this seismometer to achieve the results desired in this invention.

The lateral dimensions of a seismometer constructed according to this invention are small and of such shape that the seismometer is adaptable for use in a bore hole beneath the surface of the earth and may be raised and lowered from its installed position by means similar to those used in oil field drilling and exploration operations.

These and other objects of this invention will be apparent from an examination of the following specification and drawings in which:

FIGURE 1 represents a side elevational view of the long period horizontal motion seismometer of this invention;

FIGURE 2 is a fragmentary sectional view of the pendulum of this invention showing the manner in which the electrical output signal is generated;

FIGURE 3 is a schematic diagram explaining the principle of operation of the pendulum-spring combination employed in this invention;

FIGURE 4 is a schematic illustration similar to that shown in FIGURE 3 but intended for a theoretical explanation of the effect of vertical tilt on the seismometer of this invention;

FIGURE 5 is a graphical illustration of a comparison between a seismogram or pictorial record of an earthquake versus a nuclear explosion;

FIGURE 6 is an elevational view of a single portable seismometer installed for field use at a pre-selected location;

FIGURE 7 is an outline plan view showing how an array of these seismometers may be used to completely examine the vibration of the earth due to one of the mentioned causes.

Referring now more particularly to the characters of reference on the drawing, it Will be observed in FIG- URE 1 that the seismometer assembly of this invention identified generally at 2 is seen to include .a frame 3 having an open area 4 in which is supported a spring loaded short pendulum 5 and having a flat generally horizontal base 6 which provides support for the electrical signal generating means 7 which permits a replica of the move ment of the pendulum to be electrically transmitted and remotely recorded. Vertical sides 8 and a top cross member 9 of the frame 3 outline and define the open area 4.

The frame 3 also includes a pair of inwardly projecting bearing support arms 10, each containing a removable block 11 secured on the arms 10 by means of capscrews 12. The pendulum 5 includes corresponding blocks 11:: and screws 12a located on outwardly projecting arms 13. Each of the blocks 11 and 11a are installed initially so as to be in vertical alignment with each other. This construction permits the combination of one block and its screws to act as a clamp to bind a short length of tungsten wire 14 between the block and the corresponding area of arm 13 (or 10). The tungsten wires 14 are formed in a generally V-shape to provide support for the free ends of the small bearing springs 15. This construction will provide a substantially frictionless bearing in which pendulum 5 may oscillate when an external force is applied to the frame containing the pendulum. An upward force is supplied to the pendulum to keep springs in tension during operation by a Zero length" spring 16 which is itself suspended and supported by V-shaped tungsten wires 17 which are held in a similar manner to the frame and pendulum respectively by their mounting blocks 11b and 11c.

The lower end of the pendulum 5 is enlarged at 20 to provide an increased mass at the lower extremity of the pendulum. Small adjustments in the effective weight of the pendulum may be gained by repositioning the small nuts 21 on the threaded upright 22. At each of three locations on the base 6, there is installed an adjustment screw 23 having a knurled head 24 to permit hand rotation of the screw 23 in either direction in its threaded hole (not shown) to provide a micrometer-type adjustment for the base 6 and consequently a leveling action for the entire seismometer assembly 2.

In FIGURE 2 the construction of the electrical signal generating unit 7 may be observed in more detail. This unit includes two spaced apart generator housings 25 made of soft iron. Each housing includes a permanent magnet 26 rigidly attached to each inner end of the housing in such a manner as to be suspended centrally of the path of movement of the circular armature 27 which is integrally attached to the lower end of the pendulum 5 just below its enlarged mass portion 20. Electrical leads 30a and 30b connect opposite ends of pick-up coil 31 and transmit the signal to a remote recorder or the like. Since the movement of the lower end of the pendulum is very slight (from O to .1" during earthquakes), the small slack in the leads and their cable housing 30 will permit their use in the manner shown. As an alternate signal read-out a photo-electric cell 40 is installed on base 6 in alignment with a light source (not shown) at the null position of the pendulum,

In FIGURE 5 a comparison may be had between the seismogram of an earthquake, or natural phenomenon and a nuclear explosion or man-made vibration. FIGURE 5a is a typical visual record of earthquake vibration wave, and it will be observed that the frequency of certain modes of the principal shock wave amplitudes is such that the wave spacings, or period is relatively large, for example approximately 15 seconds. Whereas in FIGURE 5b it will be seen that the period or spacing S of the peak vibration waves of the same modes following a nuclear explosion is substantially smaller, for example approximately 8 seconds. The seismometer assembly of this invention is very capable of detecting both of these long period vibrations in a horizontal direction.

In actual use as shown in FIGURE 6, the small portable seismometer assembly 2 will be installed in a thermally insulated housing 32 after the assembly has been placed on the top flat surface of a concrete pedestal 33 and leveled by means of adjustment screws 23. The output signal from the assembly 2 is carried by the electric cable 30 to a remotely located recorder 34, which may be a portable instrument or may be truck mounted if desired.

In FIGURE 7 it is shown that this seismometer assembly 2 may be used in combination with a conventional vertical motion detecting seismometer indicated at 35 to provide an array to detect earth movements in any direction. These arrays may be located throughout the world and their combined outputs will permit an accurate location of the desired event, be it an earthquake or explosion.

The various elements and the geometry of the construction of this invention are so combined that the following advantages of a long period horizontal motion detecting seismometer are obtained:

(a) A short length, short movement vertical pendulum having a long period,

4 (b) A pendulum type horizontal motion detecting seismometer whose period is relatively insensitive to nominal tilt.

The long period of this seismometer is attained by the specified relationship between the mass of the short length pendulum and by the force of the restraining spring. This relationship is determined by the geometry of the pendulum-spring combination, the spring constant of the main spring, and the mass of the pendulum, and this relationship is such, that, once the pendulum mass is put into motion, the torque due to gravity which would tend to return the pendulum to its Zero or null position is almost, but not quite balanced by the torque due to the spring tension of the restraining spring which would tend to rotate the pendulum in an upward direction. The following mathematical analysis will illustrate this relationship:

The torque due to gravity '1'.; may be seen by reference to FIGURE 3 and is obtained from the following equation;

Where M is the mass, g the acceleration due to gravity, d the effective pendulum length.

The torque due to the spring tension T (this being a zero length spring) is obtained from the following formula:

T =Fa sin 5 Where a is the height of the upper spring suspension above the hinge or bearing line and B is the angle between the vertical and the line of action of the spring.

The spring force in tension, F, is obtained from the equation:

Where k is the spring constant, r is the extended length of the spring at force F, and c is effective or virtual length of the unstressed spring. In a zero length spring, 0 equals Zero, and in a negative length spring, 0 is a negative nuumber; in an ordinary or positive lengt spring 0 equals a positive number. In this invention a zero length spring is preferred, but operative units may be made employed slightly positive or slightly negative springs.

From trigonometry the following values of FIGURE 3 may be equated:

sin fi=(b/r) sin T =F(ab/r) sin (I: T =kmb(1c/r) sin Thus the total torque T=T +T is given by T=[kab(lc/r) Mgd] sin 5 Then if c=0, and the system is adjusted so that kab=Mgd, T becomes zero and the period of the system becomes infinite. If Mgd kab, the system becomes oscillatory when subjected to a small shock or vibration. The academic equation for undamped motion is:

Md =(kabMgd) sin and the period t for small oscillations is:

The sensitivity of the period of this seismometer to nominal tilt in the vertical plane perpendicular to the plane of motion, may be examined by reference to FIG- URE 4 in which 0 is the angle of tilt. The component of gravity is thus reduced by a factor of the cosine of the angle of tilt, while the spring torque T remains unchanged. This reduces the original period to then the rate of change of the period with tilt (cit/d0) becomes:

dt/d0=1rg sin 0(d) (g cos 0 kab/Md) which is small if 0 is small.

In a practical illustration, if the seismometer has a period of 30 seconds, and a pendulum length of 20 cm., then a tilt of as much as 36 minutes of arc will increase the period of time by only one second. In field installations however, even with this portable seismometer it is relatively easy to quickly set up the instrument to an accuracy of one-fourth of a degree (15 minutes of arc).

The sensitivity of the displacement of this seismometer from its pendulum null position to tilt may be examined by reference to FIGURE 4 in Which 0 is the angle of tilt in the plane of motion of the pendulum in a manner similar to the analysis above relative to the sensitivity of the period. Here the gravitational torque T is unchanged, while the spring torque T will be changed.

In actual practice, if there is a tilt from zero to one second of arc, in the seismometer of the previous example, then the zero position will be shifted by 19 minutes of arc.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof are possible. The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the scope of the appended claims interpreted in the light of the spirit of this invention.

What is claimed is:

1. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a bearing in said support means, a vertically extending pendulum supported by said bearing and adapted to oscillate therein, and a vertically extending spring means connecting said pendulum to said frame, to apply to said pendulum a torque opposite and substantially equal to the gravitational restoring torque on said pendulum.

2. A seismometer according to claim 1 wherein said spring means includes a spring that has substantially a zero length.

3. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a bearing in said support means, a vertically extending pendulum supported by said bearing and adapted to oscillate therein in a vertical plane, and a vertically extending spring means connecting said pendulum to said frame above said bearing to apply to said pendulum a torque opposite and substantially equal to the gravational restoring torque on said pendulum.

4. A seismometer according to claim 3 wherein said spring means includes a spring that has substantially a zero length.

5. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a base in said frame, a substantially frictionless bearing in said support means, a vertically extending pendulum of a length substantially shorter than the height of said frame supported by said bearing and adapted to oscillate therein in a vertical plane, a vertically extending spring means connecting said pendulum to said frame and opposing the gravitational restoring torque of said pendulum, and electrical generating means to detect said movement of said pendulum supported on said base means and adjacent the lower end of said pendulum.

6. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a substantially frictionless bearing in said support means, a vertically extending pendulum supported by said bearing and adapted to oscillate therein in a vertical plane, a vertically extending spring means connecting said pendulum to said frame and opposing the gravational restoring torque of said pendulum, and electrical generating means to detect said movement of said pendulum; said spring means extending a substantial distance above said bearing support means to said frame.

7. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a base in said frame, a substantially frictionless bearing in said support means, a vertically extending pendulum of a length substantially shorter than the height of said frame supported by said bearing and adapted to oscillate therein in a vertical plane, a vertically extending spring means connecting said pendulum to said frame and opposing the gravitational restoring torque of said pendulum and electrical generating means to detect said movement of said pendulum, said electrical generating means comprising a generator housing attached to said base adjacent the lower end of said pendulum and containing a permanent magnet, and a pickup coil attached to the free end of said pendulum for interacting with the magnetic field of said permanent magnet.

8. A long period horizontal motion detecting seismometer comprising a frame, bearing support means on said frame, a substantially frictionless bearing in said support means, a vertically extending pendulum supported by said bearing and adapted to oscillate therein in a vertical plane, a vertically extending spring means connecting said pendulum to said frame and opposing the gravitational restoring torque of said pendulum, electrical generating means to detect said movement of said pendulum, and adjusting means on said frame to level said seismometer in all directions.

9. A long period horizontal motion detecting seismometer, comprising a frame, a vertically extending pendulum supported by said frame and adapted to oscillate in a vertical plane, and a vertically extending spring means connecting said pendulum to said frame and opposing the gravitational restoring torque of said pendulum.

10. A long period horizontal motion detecting seismometer, comprising a frame, a vertically extending pendulum, bearing means connecting said pendulum with said frame and adapted to enable said pendulum to oscillate in a vertical plane when said pendulum is biased upward, and a vertically extending spring means connected to said frame and biasing said pendulum upward so as to oppose the gravitational restoring torque of said pendulum.

11. A seismometer according to claim 10 wherein the spring constant of said spring means has a magnitude sufiicient to cause said gravitational restoring torque to be substantially equalized.

References Cited by the Examiner UNITED STATES PATENTS 1,774,379 8/30 Jones 340-17 2,390,328 12/45 Roberts 340-17 2,636,160 4/53 Loper et al. 340-17 3,026,428 3/62 French 340-17 SAMUEL FEINBERG, Primary Examiner. KATHLEEN H. CLAFFY, NEIL C. READ, Examiners.

Claims (1)

1. A LONG PERIOD HORIZONTAL MOTION DETECTION SEISMOMETER COMPRISING A FRAME, BEARING SUPPORT MEANS ON SAID FRAME, A BEARING IN SAID SUPPORT MEANS, A VERTICALLY EXTENDING PENDULUM SUPPORTED BY SAID BEARING AD ADAPTED TO OSCILLATE THEREIN, AND A VERTICALLY EXTENDING SPRING MEANS CONNECTING SAID PENDULUM TO SAID FRAME, TO APPLY TO SAID PENDULUM A TORQUE OPPOSITE AND SUBSTANTIALLY EQUAL TO THE GRAVITATIONAL RESTORING TORQUE ON SAID PENDULUM.

Images