Recent Advances in the FG5 Absolute Gravity Meter, Dynamic Gravity Meters, the Gphone, and Borehole Gravity

English Abstract:

There have been large advances in the gravity meters that are being developed at Micro-g LaCoste, Inc. and Scintrex Ltd. These instruments span applications of absolute gravity, static spring gravity instruments that are useful for looking at low frequency gravity signals, dynamic gravity meters that can be used to measure gravity on an airplane or ship and even drone aircraft. Very small gravity meters are being developed to deploy in very deep vertical or horizontal boreholes for reservoir characterization.

The FG5 absolute gravity meter has recently been upgraded to have a longer dropping length. The additional length of freefall allows the extraction of the gravity and gravity gradient during a single drop. A counterweight has been added to the dropping chambers to greatly reduce the recoil of the dropping chambers. A novel method has been developed to balance the free-fall mass to reduce rotational errors.

Two other instruments derived from the FG5 are being developed that have multiple simultaneous freefall masses in the same vacuum system. These instruments directly measure vertical gravity gradients and higher order gradients can be measure simultaneously.

The gPhone noise has recently been reduced to very low noise level (~0.2microGal/sqrt(Hz)) consistent noise measurements measured by superconducting gravity meters. An interesting new method has been developed that is useful for characterizing instrument noise levels even in the presence of ambient seismic noise without the need to difference gravity meters.

An important advance in dynamic gravity meters that allow the much heavier damped beams to be used with a true force feedback loop to greatly reduce the crosscouplings that limited the performance of these gravity meters in high turbulence. The sensors are also being reduced in size so that they can be used in drone aircraft.

Finally, a new line of ultra-small gravity meter sensors have been developed to deploy in boreholes many miles deep in the earth. The sensors can be leveled in vertical or horizontal boreholes. These instruments will have great utility in reservoir monitoring applications in oil, gas, or water.