GeoEurasia’21. Results of Russia’s first airborne gravity survey with a strapdown gravimeter

RESULTS OF RUSSIA’S FIRST AIRBORNE GRAVITY SURVEY WITH A STRAPDOWN GRAVIMETER
German Igorevich Brovkin
JSC GNPP Aerogeofizika

In recent decades, airborne gravimetry has established itself as a reliable and relatively inexpensive geophysical method for studying the gravity field [Mogilevsky et al., 2015]. Its specific features arise from the need to conduct surveys aboard aircraft in flight and impose certain requirements on the equipment used. Airborne gravimeters with gyrostabilised platforms have become widespread: various technical solutions arrange their measurement systems to minimise the effects of disturbing accelerations and other interference on the vertical accelerometer, the system’s principal measuring element. Strapdown systems use a fundamentally different measurement approach: three mutually orthogonal accelerometers record acceleration components (the superposition of the geological signal and platform acceleration), while a gyroscopic system records angles of deviation from the local vertical at each observation point. Processing these data allows the acceleration components to be transformed and their vertical component extracted [Vyazmin, 2019]. Eliminating the need for stabilisation systems in strapdown airborne gravimeters reduces their mass and dimensions, which in turn opens up opportunities for use on a broader range of aircraft. Instruments of this type have been used to measure gravity-field increments for more than 20 years [Glennie, 2000]; this presentation describes the first Russian experience with a new-generation strapdown airborne gravimeter.

In October–November 2020, Aerogeofizika conducted concurrent field observations using two airborne gravity systems: the GT-3 (Gravtechnology LLC, Russia) [GT-1A (GT-2A) Gravimeter, 2011] and a strapdown system. This paper considers the processing, comparison and quality assessment of the resulting data. Data processing. Preliminary processing of measurements from each instrument system took account of all the particular features of each observation dataset and was performed in several variants. For the GT-3 gravimeter data, gravity increments were calculated using a Kalman filter with 80 and 100 s windows. For the strapdown system data, filtering was performed with processing parameter β equal to 5 and 10 km.

Subsequent gravity anomaly calculations followed the
processing workflow below:

• • Calculation of free-air gravity anomalies;
  • Calculation of the gravity field of terrain masses with a density of 2.67 g/cm3,
  followed by low-pass filtering.
  • Calculation of Bouguer gravity anomalies;
• • Statistical adjustment of observations using the tie-line network (levelling), and assessment of accuracy
  at intersections of survey lines and tie lines;
• • Correction for residual disturbing accelerations using the AirGrid procedure [Mogilevsky et al.,
  2010].
  

With rare exceptions arising from measurement characteristics, the same processing parameters were applied to all datasets. The final stage assessed the quality of the resulting materials [Mogilevsky et al., 2018]. For each processing variant, two quantities characterising accuracy were calculated: the root-mean-square deviation (RMSD) of anomaly differences based on internal consistency [Sander et al., 2002], and the RMSD of Bouguer anomalies at survey-line/tie-line intersections.

Plotting the gravity fields measured by different gravimeters along survey lines allows a qualitative comparison (Fig. 1). The maximum RMSD of the differences between instruments was ±0.91 mGal. Overall, the resulting anomalies are similar, with the principal differences confined to the highest-frequency components. The main outputs of airborne gravity data processing are gravity-anomaly models in which residual disturbing accelerations are minimised or fully accounted for. Figure 2 shows images of Bouguer-anomaly grids that have undergone the full processing sequence. As in the one-dimensional case, the overall similarity of data from different types of airborne gravimeter is visible to the unaided eye, but additional patterns also emerge: the visible detail in the gravity models increases successively from left to right (from А to Г). These features are especially clear in transforms emphasising the high-frequency field components. The maximum RMSD between field models from different instrument systems is ±0.81 mGal. In the spectral domain, some anomalies with spatial periods of 3–5 km are better resolved in the strapdown system data, particularly in the processing variant with β=5 km.

Conclusion

Concurrent field trials of the strapdown airborne gravity system yielded measurements of gravity-field increments meeting quality requirements within the survey area. Final processing was performed in the office. Comparison of the data from different gravimeters shows their overall similarity. The final stage assessed the accuracy of the resulting anomalous gravity-field models. During the study, staff of the Laboratory of Control and Navigation at Moscow State University developed and successfully tested dedicated software for processing data from the new strapdown airborne gravimeter.

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