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A Subsoil Inspection in a Mangrove Habitat Using Electrical Resistivity Tomography73Marine Research of the National Academy of Marine ResearchDOI: 10.29677/MR.202406_4(1).0005 An ERT raster image could give investigators the spatial information associated with the horizontal extension of resistivity features. Based on a raster image, one can preliminarily form an intuitive prediction of possible change trends and the three-dimensional (3D) underground spatial distribution for in-situ soil layers. Then, the subsoil depth of organic carbon storage in the mangroves could be rapidly determined. An ERT raster image was formed by using joint inspection results from survey lines BL-S-R3 and BL-S-R5 as shown in Figure 15. The west portion beyond Point A in the ERT raster image belongs to the mangroves. Focusing on the shallow subsurface, the apparent resistivity pattern in the mangroves has constantly an identical image feature, but it is significantly different from that in the sand flat, i.e., the east portion beyond Point A. Therefore, the subsoil depth could be positively identified as the scope affected by the mangrove root system, i.e., the depth for organic carbon storage. The representative subsoil depth is around 4-5 meters in the southern area of the mangrove reserve.5.3 Subsoil volume estimateFigure 14. The electrical resistivity tomography image along survey line BL-S-R2. Another survey line BL-S-R2 was fully located within the mangroves, parallel to survey line BL-S-R3, as shown in Figure 14. Using a shorter electrode spacing of 1 meter, the DD-type ERT image indicates obvious apparent resistivity clusters totally scattering along the survey line within 5 meters deep. Such an ERT image formation is usually identified as the main distribution layer of mangrove roots. In brief, the subsoil depth of mangroves, i.e., organic carbon storage, at the southern area of the mangroves can be determined as being approximately 4-5 meters deep from the surface.