MODE LOCALISED ACCELEROMETER

1 . An accelerometer comprising:

a frame;

one or more proof masses suspended from the frame by one or more flexures and movable relative to the frame along a sensing axis;

a resonant element assembly, the resonant element assembly comprising a first resonant element and a second resonant element coupled to one another, the first resonant element connected between the one or more proof masses and the frame, the second resonant element connected between the one or more proof masses and the frame, such that movement of the one or more proof masses relative to the frame along the sensing axis results in one of the first and second resonant elements undergoing compression and the other of the first and second resonant elements undergoing tension; and

drive circuitry configured to drive the resonant element assembly into one or more resonant modes and a sensing circuit configured to determine a measure of acceleration based on changes in resonant behaviour of the first and second resonant elements.

2 . An accelerometer according to claim 1 , wherein the first and second resonant elements are substantially identical.

3 . An accelerometer according to claim 1 or 2 , wherein the one or more proof masses comprises a single proof mass and the first and second resonant elements are both coupled to the single proof mass.

4 . An accelerometer according to claim 3 , wherein the first and second resonant elements are surrounded by the single proof mass.

5 . An accelerometer according to any one of the preceding claims wherein one or both of the first and second resonant elements is connected to the one or more proof masses through an force amplifying lever.

6 . An accelerometer according to any one of the preceding claims wherein the first resonant element is coupled to the second resonant element by a mechanical coupling.

7 . An accelerometer according to claim 6 , wherein the first resonant element is connected to the frame at a first anchor and the second resonant element is connected to the frame at a second anchor, and wherein the mechanical coupling is positioned at or close to a nodal point of a mode of vibration of the resonant element assembly.

8 . An accelerometer according to claim 6 , wherein the mechanical coupling is a portion of the frame, the first and second resonant elements being connected to the frame at a common anchor.

9 . An accelerometer according to claim 6 or 7 , wherein the mechanical coupling comprises a serpentine beam.

10 . An accelerometer according to any one of the preceding claims, wherein the first and second resonant elements are electrostatically coupled to one another.

11 . An accelerometer according to any one of the preceding claims, comprising a third resonant element coupled to one or both of the first and second resonant elements.

12 . An accelerometer according to claim 11 , wherein the third resonant element has different mechanical properties to the first resonant element and the second resonant element.

13 . An accelerometer according to claim 12 , wherein the third resonant element has a different stiffness to the first resonant element and the second resonant element.

14 . An accelerometer according to any one of the preceding claims, wherein the drive circuitry is configured to provide a parametric pumping signal to the resonant element assembly.

15 . An accelerometer according to any one of the preceding claims, wherein the sensing circuitry is configured to provide an output based on the amplitudes of vibration of the first resonant element and the second resonant element.

16 . A gravimeter comprising an accelerometer according to any one of the preceding claims.

17 . A borehole tool comprising one or more accelerometers in accordance with claims 1 to 15 .