Seismometer

P. T. ELSKAMP ETAL 3,239,804

SEISMOMETER

March 8, 1966

Filed Aug. 10, 1962

Sheets-Sheet 1

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P T. ELSKAMP

J. G. HAGEDOORN ^BY:

THEIR ATTORNEY

P. T. ELSKAMP ETAL

SEISMOMETER

March 8, 1966

Filed Aug. 10, 1962

3,239,804

Sheets-Sheet 2

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INVENTORS:

R T. ELSKAMP

J. G. HAGEDOORN ^BY: λ/. z&gt; //...

THEIR ATTORNEY

3,239,804

Patented Mar. 8, 1966

United States Patent Office

3,239,804 SEISMOMETER

Petrus T. Elskamp, Voorburg, and Johan G. Hagedoorn, Scheveningen, Netherlands, assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware

Filed Aug. 10, 1962, Ser. No. 216,221

Claims priority, application Netherlands, Aug. 15, 1961, 268,232

Claims. (Cl. 340—17)

The invention relates to an apparatus for measuring earth vibrations or seismometer provided with a coil system capable of moving in a magnetic field and connected to the stationary casing by means of one or more springs. It is known to use for this purpose disc-shaped flat leaf springs whose resilience has been increased, if necessary, by means of incisions. In the known seismometers the circular outer edge of these springs is riveted or screwed to the casing, the central part being secured to the body of the coil.

The object of the invention is to provide a construction for a simple, inexpensive seismometer of small dimensions having other advantages which will be described below. The construction according to the invention has at least one flat disc-shaped leaf spring, but its circular outer edge fits in the body of the coil, i.e. in a groove provided in the inside thereof at a very short distance from the end; in addition, the outer edge of the leaf spring is provided with a small projecting resilient lug fitting in a narrow radial slot of the coil body and capable of being held in the said slot.

This slot is arranged at the end of the coil body and extends at least as far as the groove, the lug is preferably integral with the leaf spring; the latter is usually made from a single thin sheet of resilient material such as phosphor bronze or beryllium copper by etching or stamping. The spring thickness is of the order of 0.1 mm. The central part of the leaf spring is secured to the casing (for example, the magnet of the pole pieces). The slot is preferably as deep as the distance between the bottom side of the slot and the end (about 0.5 mm.). The slot is as thick as or slightly thicker than the leaf spring.

In this way, a cheap construction is obtained permitting a very compact structure as well as a great sensitivity of the seismometer.

The construction according to the invention also has a number of advantages relating to the electrical part of the seismometer. The coil may be electrically connected to the connecting lines in a simpler manner than before by means of one or both springs. It should be noted that it is known per se to use the suspension springs as current conductors.

It is also easy to determine whether the seismometer is properly connected, i.e. with the correct polarity. The relevant method is disclosed in the U.S. Patent 2,423,970. In order to carry out this method, namely, supplying a direct-current voltage from the measuring station as a result of which the seismometer acts as an interrupter in one poling but not in the other, an extra contact is required; with the present seismometer, however, one spring can produce the desired effect through direct contact with the casing (viz. the magnet or a pole piece).

These additional advantages result from the fact that the new construction permits a more efficient use of the insulation which is already present for other reasons (the coil body is insulated; the earth is not used as a return conductor).

In the accompanying drawing, FIG. 1 is a cross-section in elevation of the coil body with two suspension springs.

FIG. 2 is a plan view of FIG. 1.

FIG. 3 is a diagrammatic elevation partially in crosssection of the assembled seismometer.

Referring to FIGS. 1 and 2 of the drawing, the coil body 1 consists of a housing of a non-magnetic material, for example, aluminum, the surface of which is provided with an insulating layer; it is preferred to use a coil body 5 of aluminum which has been anodized afterwards. Two coils 2 and 3 connected by a wire 4 are arranged in the coil body. Each of the ends is provided with a circular groove which, measured radially, makes an incision of about 1 mm. in the coil body and of which the bottom side (the side turned away from the corresponding end of the coil body) is at about 0.5 mm. from the end. A disc-shaped leaf spring 7', 7 fits in each of these grooves. The resilience of this spring is increased by incisions 8. The spring has a central opening 9 enabling it to be se15 cured to the casing and two openings 10 and 11 for the passage of the terminals for the electrical connection. Each leaf spring also has a projecting resilient lug 12 fitting in a narrow radial slot 13 of the coil body. At one side or, as shown in the drawing, at both sides of the point 20 of projection of the lug is a small incision in the spring which has the effect of making it diverge slightly from the circular shape at this point and enabling it to be fitted in the coil body. FIGS. 1 and 2 show the spring in the position it occupies after being fitted. In order to dis25 assemble the spring it is only necessary to lift the lug 12 until it can be pushed over the edge of the coil body; the spring may then be removed from the coil body by turning it. To mount the spring it is placed on the end of the coil body in such a way that the lug 12 remains on 30 the outer edge, for example, to the left of the slot 13 but close thereto. The edge of the spring 14 is then pressed into the groove and when the spring is rotated ? to the left it automatically screws into the groove until the lug 12 finally falls into the slot 13 after a complete 35 rotation; thus blocking further movement. Thus, the lug 12, measured in a radial direction, projects into the slot 13 slightly further than the groove 5.

The structure of the seismometer is shown diagrammatically in FIG. 3. The casing (stationary part) of the ⁴⁰ seismometer consists of a hollow iron cylinder 15 closed at the top and bottom by cover plates 16 and 17, of nonmagnetic material, preferably anodized aluminum, which likewise serve as fitting pieces. This cylinder contains a permanent magnet 18 and iron pole pieces 19 and 28. 45 The coil body 1 is suspended from the two springs 7', 7 between the cylinder 15 and the inside parts 18, 19 and 28. The electrical connections are indicated at 21 and 22. The entire assembly may also be enclosed in a shell, not shown (for example, of aluminum) to ensure water50 tight sealing.

The electrical circuit is formed by connection 21, spring 7', coil 2, line 4, coil 3, spring 7, the inside parts 20, 18, 19 and connection 22. It should be noted that the inside part is electrically insulated from the outside 15 55 of the seismometer. Hence, the lines 21 and 22 remain insulated from the earth.

The seismometer according to the invention may be mounted in a simple manner by stacking the various parts. The inside parts may be secured to each other and to the θθ iron cylinder by flanging.

In order to determine whether the seismometer is properly connected to the measuring cable a sufficiently high direct-current voltage (from the measuring center) may be provided between the points 21 and 22. In one meth05 od of poling there is a strong downward movement of the coil; the upper spring touches the pole piece 19 and short-circuits the direct current through the coil. As a result the coil body springs back, the current again flows through the coil, etc. This intermittently interrupts the 7q direct current. This interrupted direct current may be made detectable, in the measuring center, for example, audible. In the other method of poling the coil moves

3,239,804 upwards but no interrupted direct current is generated. Hence this enables the poling of the seismometer to be checked.

It will be understood that the invention is not limited to seismometers with two coils and/or one magnet; these 5 numbers may vary.