Typical absolute gravimeters and gradiometers operate by tracking the freefall of an object, or “test mass”, within an instrument known as an interferometer. An interferometer contains a laser beam that is split into two paths, each portion of the laser beam reflects off a retro-reflector, contained within a test mass located in a vertical chamber. In one vertical chamber, a test mass containing a retro-reflector is placed in freefall. In the second vertical chamber, if the retro-reflector is held fixed, the signal at the combined output of the two beams can be used to determine the acceleration of gravity (g). If the retro-reflectors in both chambers are placed in freefall, the signal at the combined output of the two beams can be used to determine the gradient of the gravity field between the two objects.
Because forces that act upon a test mass during freefall may alter its acceleration (and hence the gravity measurement), the choice of material used to make the test mass is critical. For example, some forces that might act upon a test mass include frictional drag, micro-accelerations due to out-gassing of the mass itself, and attractive and repulsive electromagnetic forces. Frictional drag on the test mass may be reduced to a negligible effect by evacuation of the vertical chambers, and out-gassing can be avoided by a suitable choice of test mass materials. However, the influence of electric and magnetic fields is difficult to remove because gravity meters often employ electric components such as vacuum pumps, drive motors and computer circuitry.
As any electrically-conductive object falls through an electromagnetic field, it is susceptible to attractive and repulsive forces. The forces can include direct electric attraction or repulsion (positive and negative charges interacting), magnetic attraction or repulsion (north and south poles interacting), or a combination of the two. For example, an interferometer test mass incorporating a magnetic material will be attracted or repelled by external magnetic fields caused by magnetic devices or the earth's magnetic field. Any external magnetic field will induce a flow of free electrons, called an eddy current, in an accelerating conductive object. This eddy current creates its own magnetic field, which attracts or repels the original magnetic field, thus creating an additional force on the freely falling test mass.
All of these electromagnetic forces can alter the freefall course or rate of freefall of the test mass and cause a non-negligible change in a measured gravity value. For this reason, attempts have been made to minimize the influence that electrical and/or magnetic components have on a measurement. For example, vertical chambers have been surrounded with shielding material and electrical components have been placed as far away from a test mass as possible. These shielding and isolation approaches, however, lead to excessively large and heavy instruments due to the weight of the shielding material and the distal placement of the electrical components. Additionally, it is not desirable to manufacture a test mass of entirely non-conducting material, because a non-conducting mass will build-up charge (static electricity). This built-up charge will cause electrostatic attraction or repulsion between the test mass and a conductor or other charged surface, resulting in disturbance of the freefall measurement. In addition, a large charge can damage electrical equipment.
A smaller, lighter system that experiences minimal effects from nearby electric or magnetic fields is needed. Such a system would reduce the cost and burden of transporting an interferometer to a test site; would improve the accuracy with which gravity can be measured; and would allow systems to be used in areas encumbered by large electric or magnetic fields (e.g., near power plants, quarries, pump houses, etc.).