GB791928A - Improvements relating to geophones - Google Patents

Abstract

791,928. Vibration detectors. BRITISH PETROLEUM CO., Ltd., and DENNISON, A. T. Oct. 2,1956 [Oct. 19, 1955], No. 29821/55. Class 40 (4). In a moving-coil geophone in which the two moving coils 23 are suspended by a pretensioned helical spring 27, each in one of two annular magnetic air gaps, the concentric position of the coils being maintained by flat leaf-springs 32, conically shaped or V-shaped stops 36, 37, 39 are provided for limiting the upward movement of the coils, so that when the geophone is turned into a position in which the helical spring pulls the coils up to the stops they are locked in position by the spring action. The outer cylindrical casing 10 tapers to a point at its lower end 11 to facilitate penetration of the ground, the instrument being specially designed for use in swampy areas. The head member 12 screws into casing 10, an 0-ring 13 making the joint waterproof. The magnetic system provides annular air gaps 22 for the two coils 23 which are attached to inertia masses 25 secured together by a rod 26. The supporting helical spring 27 is attached at its upper end to an adjusting screw 28 screwed into a suspension bracket 30 secured to the chassis 15 which carries the magnetic system, and at its lower end to a fine adjustment nut 31 screwed to the upper end of rod 26. Two flat leaf-springs 32 are provided for each inertia mass, each being clamped at one end to suspension platforms 33 secured to the chassis 15 and at the other end to suspension blocks 34 secured to the inertia masses 25. The upper suspension platform 33 has adjustable conical studs 35 and the upper inertia mass 25 has a conical hole 36 and a V-shaped slot 37 running radially inwards from the circumference. The lower inertia mass 25 has a conical stop 38 and a corresponding conical hole 39 is provided in the magnet. When the geophone is tilted beyond 45 degrees from the vertical, the helical spring 27 pulls the suspended system up to the stops so that when the geophone is in the normal carrying position (horizontal) the suspended system is locked in position by spring action. Under very severe mechanical shocks the moving system may jump out of the stops but the clearances are so arranged that the centre spindle 26 strikes the polepiece 17 before the coils 23 can touch it. The geophone may be planted under water or mud by means of a rod fitting over the neck 40. Specification 740,505 is referred to.