GEOPHONE SPRING

Abstract: A spring for use in geophones to suspend coils within a magnetic field assembly. The spring is formed from a disc and comprises an outer ring, an inner ring and three legs connecting the outer and inner rings, which legs are formed by removing portions of the disc in a predetermined pattern. The inner edge of each leg (7) lies on an arc having a radius at least 1.25 times greater than the distance from the center (9) of the spring (4) to the inner edge of the leg (7) at the point (11) where the leg joins the inner ring (6). Preferably each leg (7) is wider at its ends (10, 11) than it is in its middle (12).

Description: This invention relates to a spring which is formed from a disc for use in geophones to suspend coils within a magnetic field assembly, the spring comprising an outer ring, an inner ring and three legs each extending from a juncture point on the outer ring to a juncture point on the inner ring, which legs are formed by removing portions of the disc in a predetermined pattern. Such a geophone spring, of the type which is known as spider spring, is disclosed e.g. in applicant's US-A-4323994. In seismic operations, seismic waves are imparted into the earth's crust at or near the earth's surface, and portions of those seismic waves are reflected or refracted from the boundaries of subsurface layers. Geophones are arranged in arrays or groups on the earth's surface, and when the reflected or refracted waves encounter a geophone, the coil form, which is suspended between two springs, tends to stand still while the geophone housing and its connected magnetic circuit move with the earth's surface. The movement of the coil form through a magnetic field causes a voltage to be generated at the output of the geophone. The outputs of the arrays of geophones are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape of subsurface formations, and the likelihood of finding an accumulation of minerals, such as oil and gas. In present day geophones, spider springs are used extensively. The legs of geophone springs generally have a rectangular cross-section and are curved along their lengths between the junctures with the inner and outer rings of the spring. The spring is "preformed" according to known techniques. When preforming is complete, the inner ring is offset or displaced relative to the outer ring, such that when a mass is suspended between two such springs, the inner ring, legs and outer ring of each spring lie in the same plane. A geophone is intended to sense motion from a direction which is roughly parallel to the axis of movement of the coil form with respect to the geophone housing. Therefore, it is desirable to eliminate or minimize the effects of any lateral motion of the coil form in response to forces which are not parallel to the axis of movement of the suspended coil form within the geophone. In seismic operations, an impulse which is not truly parallel with the geophone axis and which contains a frequency component at or very near to that of a spurious resonant frequency causes the geophone to produce an undesired or false EMF in the coils. Because of its high Q, the movement will continue for some time after the force which caused it has subsided. This resonance is considered to be the main spurious frequency in a geophone and it is highly undesirable as it limits the geophone's upper clean bandwidth. Inasmuch as the geometry and the mass of the coil form for a given model of geophone are constant within the manufacturing limits, the spurious resonance is also constant. The frequency of the spurious resonance can therefore be raised or lowered by changing the geometry of the suspension springs. This characteristic has been used to raise the frequency of these false signals until they are beyond the desirable frequency spectrum of the geophone by increasing the lateral stiffness of the spring. With this approach, these false signals do not interfere with or corrupt signals of interest. A method of increasing the lateral stiffness of the spring, applied in US-A-4 323 994, is to shorten the spring legs. Unfortunately, the signal distortion caused by spring nonlinearity is increased when the legs are relatively short, and spring life is reduced. Furthermore, according to said US patent, each leg was designed to consist of three segments, of which a central segment was straight. According to this arrangement the central straight segments of the legs move in a manner which reduces the lateral bending forces by axial movement of the inner ring relative to the outer ring. In this way the linearity of the spring was improved. The present invention wants to further improve the spring structure by achieving much higher spurious resonant frequencies while still maintaining the linear response of the spring. The geophone spring according to the invention is characterized in that when the spring is flat, the inner edge of each leg lies on a single arc having a radius at least 1.25 times the distance from the center of the spring to the inner edge of the leg at the juncture point where the leg joins the inner ring, the distance between the inner edge of the leg and the centre of the ring decreasing continuously from the outer juncture point towards the middle of the leg, and in that each leg is wider at its end than it is in its middle. The present invention achieves a highly improved ratio of the spurious resonance of the spring to the natural frequency without deleterious effect to the linearity of the spring's response. This has been accomplished in part by an analysis which isolates the primary non-linear force acting in the spring. Then, structure has been designed specifically to counteract that non-linear force, while at the same time increasing the lateral stiffness of the spring in order to have a high ratio of the spurious resonance of the spring to the natural frequency To counteract the non-linear force, the width of each leg is tapered so that it is narrower at its middle than it is at either of its ends. This taper reduces the signal distortion caused by the non-linear force exerted by the legs. This taper also reduces destructive stress which can appear at the junctures of each leg with the inner and outer rings. The non-linear force has also been reduced by making the legs as long as possible. To improve the lateral stiffness, each leg is given as little curvature as possible and is as straight as possible considering the architecture of the particular geophone to which the invention is applied. Thus for each leg, the radius of the arc forming the inner side of the leg is at least 1.25 times greater than the distance from the center of the spring to the inner side of the leg, at the point where the leg joins the inner ring. A geophone spring in accordance with the present invention, when compared with previously used spring, has the advantage of an increased lateral stiffness of such a degree that the resulting spurious frequency is at last forty times greater than the natural frequency of the spring. This increase in the available clean bandwith at the high frequency end is achieved without compromising the linearity of the spring. US-A-3602409 discloses a geophone spring of the type which is known as the double cantilever spring. In this kind the legs flex differently than in the spider springs to which the present invention relates. In the latter type, the inner and outer ring will, when they move axially one with respect to the other, rotate a little and this rotation does not occur in cantilever or double cantilever springs. When therefore this document discloses (in its fig. 3) tapered legs, this has as purpose to distribute the vertical bending of the legs, in order to prevent bending to be concentrated in the shoulders or juncture points, so that, seen in lateral view, they will bend in an "S" shape. This document does not contain any teaching in connection with spider springs where a certain rotation occurs. The invention will be clarified with reference to the accompanying drawins. Figure 1 is a planar view of a prior art geophone spring; Figures 2a and 2b are schematic planar views of part of a spring Figure 3 is a graph showing the primary linear restoring force in a spring; Figure 4 is a graph showing the primary non-linear restoring force in a spring; Figure 5 is a graph showing the combination of the linear and non-linear restoring forces; Figure 6 is a planar view of a geophone spring according to the present invention, and Figure 7 is a perspective view of the geophone spring of fig. 6 in a preformed state. Referring to Fig. 1, a typical prior art spring is shown although not exactly the same as according to the state of the art (US-A-4 323 994) comprising an outer ring 1 and an inner ring 2, connected by three legs 3. As can be seen, the legs of the prior art spring exhibit an excessive amount of curvature. If the capability for lateral flexing is great, then it can be readily seen that a curved beam will flex more easily than a substantially straight beam. Therefore a curved beam increases lateral compliance and decreases the spurious frequency, which is undesirable. Geophone springs may be formed from spring material such as beryllium copper alloy by any suitable process, for example, by stamping out the spring configuration, by taking thin discs of an appropriate material and etching slots between the legs and rings, and so forth. The spring is then preformed so that the legs are permanently set in an upward bend from the inner ring and a downward bend from the outer ring. That is, the outer ring is displaced above the plane of the inner ring. When a geophone spring is installed in a geophone, the mass suspended by the spring attempts to push the outer ring back down to the plane of the inner ring. When the outer ring is pushed down, the inner and outer rings must rotate slightly with respect to each other to allow the downward displacement of the outer ring. This rotation changes the angular relationship between the rings and each leg. The downward force of the mass is influenced primarily by two restoring forces in the spring. The restoring force A is a force which the legs exert against the lowering of the outer ring. As shown in Fig. 3, restoring force A is linear, within a limited distance about the flat position of the spring, indicated by 0, with respect to the downward displacement D of the outer ring. Fig. 2a schematically shows the planar relationship of the outer ring, leg and inner ring of a spring in the preformed state, viewed from above, before a mass exerts downward pressure on the outer ring. Fig. 2b shows the changes that occur when a mass exerts a downward force on the outer ring. The angle α before the mass is suspended is less than the angle α' after the mass is suspended. In contrast, angle β before suspension is greater than angle β' after suspension. Restoring force B is a force which the legs exert in an attempt to return to the same angular relationship with the inner and outer rings that existed before the mass caused a downward displacement D of the outer ring. Restoring force B is non-linear with respect to the downward displacement D of the outer ring. Fig. 4 shows the non-linear restoring force B. Fig. 5 shows a resultant force C which is the combination of the restoring forces A and B. The resultant force C which is the combination of forces A and B is substantially linear for a short distance X of displacement above and below the flat position 0 of the spring. The strength, or amplitude, of force B determines the distance X and therefore the linearity of the resultant force C. The longer the distance X, the greater the amplitude of movement of the earth's surface the geophone can transduce, and thus the greater the dynamic range of the geophone. According to the invention, to limit the lateral flexing, the legs have been designed to be as straight as possible considering the architecture of the geophone. In Fig. 6 the preferred spring configuration is shown in planar view. A spring 4 includes an outer ring 5 connected to an inner ring 6 by three legs 7. These parts of the spring 4 are formed by removing the areas designated 8 from the disc by suitable means, e.g. by etching. The legs 7 are each curved about the center 9 of the spring 10, extending from a juncture point 10 on the outer ring 5 to a juncture point 11 on the inner ring 6. The three legs are substantially equal in length and are disposed substantially equidistant from each other. To reduce the effect of force B, each leg 7 tapers somewhat in its width from each juncture point 10, 11 to the center 12 of each leg, so that the center 12 is narrower than the juncture points 10, 11. Depending on the amount of the taper, the leg 7 can flex in a lateral direction. The lateral flexing of the leg 7 absorbs some of the non-linear restoring force B, thus reducing the non-linearity of the geophone's output signal. Referring to Fig. 6 the spring has been designed so that before the spring is preformed, that is, when it is flat, the inner edge of each leg 7 lies on an arc having a radius at least 1,25 times greater than the distance from the spring center 9 to the inner edge of the leg at the juncture point 11. The non-linear restoring force B has also been reduced in another way. Referring to the preformed spring shown in Fig. 7, a longer leg 7 will subtend a smaller angle from the plane of the rings for an equal amount of preform. The smaller the angular relationship between the rings and the leg 7, the less the amount of force B. Therefore, given a particular size of disc from which the spring is formed, the legs have been made as long as possible in order to reduce force B. The foregoing description of the invention has been directed in primary part to a particular preferred embodiment in accordance with the requirements of the patent statute and for purposes of explanation and illustration. It will be apparent, however, to those skilled in this art that many modifications and changes in this specific device may be made without departing from the scope of the invention.

Claims: A spring (4) which is formed from a disc for use in geophones to suspend coils within a magnetic field assembly, the spring comprising an outer ring (5), an inner ring (6) and three legs (7), each extending from a juncture point (10) on the outer ring (5) to a juncture point (11) on the inner ring (6), which legs are formed by removing portions of the disc in a predetermined pattern, characterized in that when the spring is flat, the inner edge of each leg (7) lies on a single arc having a radius at least 1.25 times the distance from the center (9) of the spring (4) to the inner edge of the leg (7) at the juncture point (11) where the leg joins the inner ring (6), the distance between the inner edge of the leg (7) and the centre (9) of the spring decreasing continuously from the outer juncture point (10) towards the middle (12) of the leg, and in that each leg (7) is wider at its end (10, 11) than it is in its middle (12).

Inventors: VITRINGA, FREDERICK ANTON
Applicants: HALLIBURTON GEOPHYSICAL SERVICES, INC.
Application year: 1985
Main classification: G01V1
IPC codes: G01V1