Fig. 4
Top panel shows the projected travel-time (P-TT) residuals in the nadir direction by the cross symbols with the color indicating the seafloor transponder number. The black curve represents the estimated NTD time series expressed by the 3d B-spline functions. Bottom panel shows the final travel-time residuals by the cross symbols with the color indicating the seafloor transponder number, which are obtained by removing the black curve from the cross symbols in the top panel