Gravity gradiometer

#The present invention relates to a gravity gradiometer, an accelerometer and components for high precision measurement instruments.

#Gravimeters are used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field, because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.

#Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.

#FIG. 1 is a schematic view of a gravity gradiometer according to a specific embodiment of the present invention.

#FIG. 2 is a perspective view of a first mount forming part of a mounting of the gravity gradiometer of according to the specific embodiment of the present invention;

#FIG. 3 is a perspective view of a second mount of the mounting according to a specific embodiment of the present invention;

#FIG. 1 is a schematic view of a gravity gradiometer 1 according to a specific embodiment of the present invention. The gravity gradiometer 1 is arranged for vertical positioning relative to a ground plane. Throughout this specification the ground plane coincides with an x-y plane of an x,y,z-coordination system and consequently the gravity gradiometer is in this embodiment arranged for orientation along the z-axis so that the Γxy and (Γxx-Γyy) components of the gravity gradient tensor can be measured.

#The function of the gravity gradiometer 1 may be briefly summarised as follows. The gravity gradiometer has in this embodiment two substantially identical sensor masses which are pivotally mounted on a mounting so that they can oscillate relative to the mounting. The sensor masses with mounting are rotated about the z-axis and with an angular frequency that approximately equals half the resonance frequency of sensor masses. A gravity gradient will result in a force on the sensor masses which will then oscillate relative to the mounting during that rotation. Components of the gravity gradient tensor can be determined from the oscillating movement of the sensor masses. For further details on the general principal of such measurements are described in the applicants co-pending PCT international patent application number PCT/AU2006/001269.

#The gravity gradiometer shown in FIG. 1 comprises a housing 2 which is connected to mount 3 for connection to an external platform (not shown). The external platform is arranged for rotation of the housing 2 at a suitable angular frequency about the z-axis. Further, the external platform is arranged for adjusting the housing 2 about three orthogonal axes.