Fig. 4

Cross-sectional view of the results along selected survey lines directing NE–SW: a–h correspond to Lines 1–4, 8, 5–7 from northwest to southeast. Line numbers correspond to those in Fig. 3a. The horizontal coordinate is rotated 39° counterclockwise with respect to the origin point (32°06.2′N, 139°52.0′E) because the survey direction is approx. N51°E: (upper panel) effective self-potential; (upper middle panel) electric field along the dive track; (lower middle panel) ambient temperature (black curve, left axis) and salinity (red curve, right axis); and (lower panel) AUV depth with turbidity (upper curves; color scale is shown at the right), water depth (top of the brown area), and the survey direction (arrow). The water depth is calculated by summing the AUV depth and altitude