Recent improvements in gravity precision from CG-6 sensors

Gravity sensors for high-precision monitoring or mapping can be grouped into 1) Relative spring, 2) Absolute free-fall, 3) Absolute cold atom, and 4) Superconducting. While all can provide valuable data, few comparisons of performance or cost have been published. Here we report on performance of CG-6 relative gravimeters and discuss how it relates to other sensors.

The Scintrex CG-6 sensor has weight of 5.5 kg and volume of 10,8 litre, which is less than previous quartz sensors. While the manufacturer specifies 5 µGal repeatability, Francis (2021) reported better performance and improved drift, noise level, tilt susceptibility and temperature influence. Mao et al., (2022) reported uncertainty down to 0.1 µGal in the laboratory. We have analysed more than 2000 survey records from as diverse environments as the desert and the seafloor. Station repeatability is a robust measure of the precision for surveys with multiple station visits and sensors. Data redundancy allows in-situ calibration of scale factors and parameters for tilt and temperature corrections. Up to 10^oC temperature difference between night and day gave no remaining correlation between sensor temperature and gravity residuals, but some diurnal drift periodicity, and repeatabilities <1.5 µGal were achieved. For more stable external temperatures, the scatter of residuals were well below 1 µGal. Both merits are significantly better than for the older CG-5 sensors in similar survey setups.

More instruments and measurements will improve the precision – and increase the cost. Most microgravity surveys have so far been done in R&D settings, and the costs have been baked into a wider project. The relation between cost and precision can be predicted and the optimal choice of survey parameters made in an industrial setting. The absolute free-fall A-10 gravimeter had in our case inferior precision compared to CG-6 for the same acquisition effort. The benefits of absolute measurements for monitoring surveys remain to be demonstrated, and it is too early to judge the performance of cold atom gravimeter developments. Superconducting sensors give time-series of superior resolution, but their limited mobility reduces the spatial resolution – or drive the cost. They can give control points with a lower detection threshold, but arial surveys are required for fair coverage of a subsurface target.

Obtaining <1 µGal precision with relative gravimeters requires good instruments, multiple sensors/repeats, and comprehensive data processing. Recent improvements by CG-6 gravimeters increase technical and economic opportunities for providing valuable gravity monitoring data. Future sensor developments by e.g. cold atom or MEMS should be benchmarked against the CG-6, not the older and less precise CG-5 sensors.

 

References

Francis, O. 2021: Performance assessment of the relative gravimeter Scintrex CG-6. Journal of Geodesy 95: 116.

Mao, Q., Xu, H., Cheng ,Y., Huang, T., Huang, J. And Li, Q. [2022] Apparatuses for verifying the precision of gravimeters with lifting spherical source masses. Recv. Sci. Instrum. 93, 124503.