Using a New Virtual Survey Line Method to Determine the System Deviation of Offshore Shipborne Gravity Data

Objectives Accurate shipborne gravity measurements are essential for marine geophysical surveys and for enhancing underwater navigation capabilities. However, inconsistencies among nearshore shipborne gravity datasets, collected by various organizations at different times, hinder the development of a comprehensive national database of shipborne gravity measurements.

Methods We addressed the challenge of data distribution in scenarios where no intersecting lines exist but the primary survey directions are aligned. By comparing gravity anomaly differences along the primary survey direction at the interface of adjacent survey areas, we determined the presence of systematic deviations. For adjacent test areas lacking both intersecting lines and overlapping regions, we introduced a virtual point scheme. This scheme interpolates or extrapolates gravity anomalies for virtual points based on data from neighboring test areas, using the differences to assess systematic biases. Additionally, we proposed a virtual line scheme capable of detecting any tilt in the load force measurements of offshore vessels and of constructing a unified gravity ano-maly datasets free from systematic deviations.

Results Among the nine offshore survey areas in China, only R2, R5, and R6 required adjustment. The R2 area data showed an overall decrease of 9.053 1 mGal, while the R5 area data increased by approximately 14.953 6 mGal and the R6 area data decreased by 8.852 7 mGal, all relative to the R3 test area. Post-optimization, the overall deviation and standard deviation between adjacent test areas across all regions were less than 0.5 mGal and 3.3 mGal, respectively.

Conclusions The datasets, after accounting for systematic deviations between adjacent measurement areas, offer valuable insights for the research outcomes of domestic gravity and altimetric satellites. They also serve as a reference for the accuracy verification and grid value calibration of domestic marine gravimeters. It is recommended that in future engineering projects, 2-3 marine standard survey lines can be established across the maritime areas under our country's jurisdiction. These lines should be surveyed by the same vessel and multiple instruments simultaneously to establish reference values. In subsequent maritime gravity surveys, the survey vessel should measure along these standard lines to harmonize the gravity anomaly results of each vo-yage, thereby minimizing systematic deviations that may arise from different periods, vessels, and departments.