A gravity gradiometer measures gradients in a gravitational field, that is, spatial changes in the gravitational field. Such changes may be caused by inhomogeneous subterranean distribution of mass, such as oil, mineral deposits, tunnels or other structures.
Schweitzer et al. describe utility of a gravity gradiometer in the context of secondary oil recovery in U.S. Pat. Nos. 6,125,698 and 6,212,952. The gradiometer includes eight accelerometers mounted about the periphery of a rotor assembly that is rotated about a spin axis. For a gradiometer having its spin axis aligned with the field lines of an ideally uniform gravity field, each accelerometer experiences the same acceleration force as it proceeds along its orbital path. However, where the local gravity field is perturbed by the presence of one or more masses, or the spin axis is tilted relative to the local field lines, each accelerometer may experience different accelerations throughout its orbit. The gradiometer is operated with different orientations of its axis to eliminate undesired effects of horizontal gradients.
Kohel et al. propose a gravity gradiometer for space in “Quantum Gravity Gradiometer Development for Space,” Jet Propulsion Laboratory, California Institute of Technology. Specifically, Kohel et al. proposes deploying two vertically-oriented interferometric accelerometers. Each interferometric accelerometer would utilize a magneto-optic trap (“MOT”) to hold and trap cold atom clouds using magnetic fields and circularly polarized beams of laser light. A Raman laser beam is directed along the vertical path of the atoms. Each interferometric accelerometer provides a phase reading based on three interactions between the Raman laser beam and the atoms. The gradiometer calculates a gravity gradient from phases determined by the two interferometric accelerometers.
The proposed gravity gradiometer of Kohel et al. has several drawbacks. For example, to provide a reasonable level of accuracy, the height of Kohel et al.'s two interferometric accelerometers would have to be on the order of several meters. A gravity gradiometer of this size presents numerous obstacles in its deployment. Due to its height, the unit could not reasonably be deployed in standard unmanned aerial reconnaissance vehicles. Another drawback is that Kohel et al.'s gravity gradiometer is highly sensitive to acceleration within a horizontal plane. The travel path of the cold atom clouds must remain within the beam width of the Raman laser, which requires that the Raman laser and the atoms remain at substantially the same transverse velocity. Once a cold atom cloud is launched, however, it may move out of the Raman beam if the structure accelerates transversely, destroying the measurement.