Atomic Gravimeter Cuts Measurement Drift across 24 Square Kilometre Tropical Survey

Scientists are tackling the challenge of obtaining high-precision gravity measurements in remote and difficult terrains with a novel hybrid approach. Nathan Shettell, Kai Sheng Lee and Fong En Oon, from the Centre for Quantum Technologies at the National University of Singapore, alongside colleagues including Rainer Dumke working with the School of Physical and Mathematical Sciences at Nanyang Technological University, present a field-deployable gravimetry system that combines the advantages of both absolute and relative gravimeters. Their research details a successful 24km² survey of dense tropical terrain, utilising an on-site atomic gravimeter to calibrate mobile spring gravimeters and suppress instrumental drift to unprecedented levels. This innovative technique extends laboratory-grade gravimetry to real-world geophysical surveys, offering a scalable calibration backbone for large-area, high-precision data acquisition in previously inaccessible environments.

Researchers typically encounter limitations with both absolute and relative gravimeters; absolute gravimeters are typically bulky and difficult to deploy, whereas relative gravimeters are lightweight and mobile but intrinsically limited by time-dependent drift. They demonstrate a hybrid quantum-enabled gravimetry approach in which an on-site atomic gravimeter provides routine, μGal-level calibration of two mobile spring gravimeters during a field survey spanning 24 km2 of dense tropical terrain. The atomic reference enables high-precision, asynchronous cross-comparison of relative measurements acquired over seven days, effectively suppressing instrumental drift to a level required for demanding geophysical applications. This deployment captures regional gravity gradients with high fidelity under challenging field conditions. Throughout the seven-day field survey, two CG6 Autograv spring gravimeters were routinely cross-calibrated against the atomic gravimeter, achieving microGal-level drift correction and enabling high-precision asynchronous measurements. Measurements from the spring gravimeters were acquired at stations spaced at least 50 metres apart along predefined survey axes, with each station occupied for approximately 30 seconds to verify mechanical stability. Data processing incorporated post-processed kinematic (PPK) analysis of GPS measurements, achieving centimeter-level vertical precision for a substantial portion of the measurement stations; specifically, solutions either reached centimeter-level vertical precision, provided reduced accuracy, or failed to converge, with only the latter being excluded from further analysis. This precise positioning allowed for accurate terrain corrections and validation of the atomic gravimeter’s stability as a temporal reference. Measurements at each station consisted of five gravity readings recorded at one-minute intervals over a five-minute period, providing robust data for analysis. Careful correction for elevation and terrain dependencies was applied to the gravity data, ensuring that residual gravity signals could be confidently attributed to subsurface mass distributions. The resulting data captured regional gradients with high fidelity under challenging environmental conditions, demonstrating the viability of field-deployed quantum gravimeters as calibration backbones for large-area, high-precision gravity surveys. Scientists are increasingly deploying quantum sensors in challenging environmental conditions, illustrating how field-operable quantum sensors can extend quantum-grade gravimetry beyond laboratory settings and serve as scalable calibration backbones for large-area, high-precision geophysical surveys in remote or logistically constrained environments. Gravimetry is a cornerstone of subsurface investigation, providing a passive and non-invasive probe of density variations beneath the Earth’s surface. Because gravitational fields cannot be shielded or attenuated, gravity measurements can reveal geological structures inaccessible to electromagnetic or acoustic methods. Field gravity surveys are traditionally performed using either relative or absolute gravimeters. Relative instruments, including spring-based gravimeters, are compact and well suited to dense spatial sampling, but rely on internal references that introduce time-dependent drift and sensitivity to environmental conditions. Absolute gravimeters, by contrast, measure gravitational acceleration directly in SI units and provide intrinsically drift-free references. Atomic gravimeters exploit quantum effects of ultracold atoms such as matter-wave interference to achieve high precision, long-term stability, representing a major advance in gravimetry. Hybrid gravimetry has recently emerged as a promising strategy to bridge the gap between absolute and relative gravimeters by combining mobile relative gravimeters with an absolute reference. In this approach, an absolute gravimeter provides calibration that suppresses drift in relative instruments while preserving their operational flexibility. Researchers extended this concept by deploying a compact atomic gravimeter as a continuously operating quantum reference during a large-area field survey, reporting results from a hybrid gravity survey conducted over a 24 km2 region of dense tropical terrain where environmental variability and limited infrastructure posed significant challenges. A site reconnaissance identified a suitable location for 00 and 17:00, weather permitting. Two CG6 spring gravimeters were used for mobile measurements and returned each evening to a fixed location adjacent to the atomic gravimeter base station. During overnight periods (approximately 18:00 to 09:00), both the atomic and spring gravimeters remained stationary and recorded continuous gravity data at one-minute intervals, providing a common reference for monitoring and correcting time-dependent drift in the relative gravimeters. This calibration scheme enabled high-precision cross-comparison between mobile measurements acquired on different days. At each station, the gravimeter was leveled by adjusting its tripod until the internal tilt readings were within ±10 arcseconds, after which the instrument was observed for approximately 30s to verify mechanical stability. If significant drift was detected, the instrument was repositioned to a more stable surface and re-leveled. Once a stable setup was achieved, gravity data were acquired over a 5 min interval, with five measurements recorded at one-minute cadence. GPS measurements provided precise latitude, longitude, and elevation information to resolve spatial variations in the gravity field and to compute terrain-related corrections, including Bouguer anomalies. Dense canopy cover limited satellite visibility, necessitating GPS measurements at locations with sufficient sky exposure. Each spring gravimeter was fitted with a rigid metal bracket supporting a GPS antenna, enabling position data to be recorded concurrently with each gravity measurement, while a continuously operating GPS receiver on a nearby building served as a fixed reference station for post-processed kinematic (PPK) analysis. PPK processing combined reference-station and mobile-antenna data to refine the position estimates for each measurement. Stations with centimeter-level precision provided high-quality elevation control for validating the atomic gravimeter as a stable temporal reference, while stations with reduced accuracy were retained to preserve spatial coverage and support analysis of longer-wavelength gravity variations across the survey area. Understanding gravity variations across the survey area requires careful correction for both temporal and spatial effects, ensuring that the residual gravity signal can be confidently attributed to subsurface mass distributions. Spring gravimeters measure relative gravity rather than absolute values, necessitating calibration against an absolute reference to ensure consistency across instruments and survey periods. The spring gravimeter measurements at horizontal position x (latitude and longitude) and time t can be expressed as gsg(x, t) = g0(x) + δtg(x, t) + δzg(x) + ∆sg(t), where g0(x) is the absolute gravity at position x, δtg(x, t) captures time-dependent variations, δzg(x) accounts for elevation-dependent spatial corrections, and ∆sg(t) represents instrument-specific, slowly varying temporal effects. In this analysis, all gravity measurements are referenced to mean sea level, as determined from GPS-derived elevations. Data correction is divided into two components: temporal corrections, addressing instrumental drift, tidal effects, and inter-instrument calibration; and elevation corrections, accounting for vertical variations in gravitational acceleration due to changes in elevation. Earth tides, which can reach amplitudes up to 150 μGal, were removed from both absolute and relative gravity data using external tidal models. The continuously recording atomic gravimeter was used as an on-site absolute reference. Each evening, the spring gravimeters were co-located with the atomic instrument and left recording overnight under stable conditions (18:00, 07:00), enabling direct comparison and quantification of instrument-specific drift. Over short timescales, the drift was well-approximated by a linear trend ∆sg(t0 + ∆t) = ksg∆t. Both the drift rate and the offset of each spring gravimeter were determined by fitting the calibration data to simultaneous atomic measurements. This nightly calibration effectively removed short-term drift and stabilised the instruments’ local reference points. After tidal correction, the atomic gravimeter measurements exhibit a gradual increase of approximately 50 μGal over the survey period, likely reflecting environmental and regional gravity variations beyond solid Earth tides, predominantly due to ocean loading. Because the nightly calibration is tied directly to the atomic reference, these long-term variations are naturally incorporated into the spring gravimeter data. The persistent challenge of accurately mapping gravity variations across large and difficult terrains has long hampered detailed geophysical surveys. This work elegantly sidesteps that trade-off, demonstrating a hybrid approach that marries the portability of relative gravimeters with the unwavering accuracy of a compact atomic gravimeter used for continuous, in-field calibration. The ability to routinely calibrate mobile sensors during a survey, rather than relying on sparse absolute measurements taken before and after, represents a substantial leap forward, particularly crucial in areas like dense tropical forests where access is limited and environmental noise is high, as demonstrated by the successful 24km survey. The suppression of instrumental drift to levels suitable for demanding applications opens possibilities for improved resource exploration, volcano monitoring, and detailed studies of Earth’s internal structure. While the scalability of this approach remains to be fully tested, future work might focus on miniaturising the atomic gravimeter further, or exploring the potential of satellite-based calibration techniques to complement ground-based efforts. Ultimately, this hybrid approach isn’t just about better gravity maps; it’s about democratising access to high-precision geophysical data, enabling a more detailed understanding of our planet’s hidden forces.