Fig. 3: Field survey.
a, Gravity gradient data with standard error (black scatter) and total inferred uncertainty when including model uncertainty (blue scatter), 1σ (dark blue shading) and 2σ (light blue shading) credible interval bands, and a site model (dashed) (see Methods). b, Scale schematic of the site showing the dimensions and position of the tunnel and leading contributions to the gravity signal. The origin for coordinates (red dot) is defined in the vertical direction by the lowest point on the survey line, and in the horizontal direction by the expected location of the centre of the tunnel. c, POE (blue contours) inferred from the gravity gradiometer data, and expected tunnel position (dashed). d, Estimation of soil density obtained by shifting the focus of the inference process to use assumed knowledge of the tunnel geometry and inference of the gradiometer data, showing the 68% highest density interval (HDI).
