5:15 PM - 7:15 PM
[HGM04-P17] Relationship between salt mapping and tafone-like depressions using spectral reflectance
Keywords:salt weathering, tafoni-like depression, sandstone, spectral reflectance
Wave erosion mediated by salt weathering due to seawater supply and solar radiation has been suggested to be the main factor in the growth of tafoni-like depressions in the seawater splash zone. However, there are no quantitative measurements of salt deposition or deposition distribution. In this study, In this study, we mapped salt deposition and adhesion on rock surfaces to analyze their distribution and examined the relationship between salt deposition patterns and depression growth.
The study area is the Yayoi Bridge over Aoshima, Miyazaki Prefecture, Japan. The bridge was built in 1951 and the piers are made of blocks of the same sandstone as the bedrock of Aoshima (Neogene Miyazaki Group). There are 144 sandstone blocks forming the wall, set in 11 layers. The layers are numbered from the bottom to the top, with the lowest being the 1st layer and the highest being the 11th layer. The sandstone blocks have been weathered and eroded over a period of 73 years, forming depressions on their surfaces. The depth of these depressions depends on the orientation and elevation of the sandstone block surface, and the location of the piers. Continuous studies on the formation of tafoni-like depressions and salt weathering processes have been conducted here since 1970s.
In this study, the spectral reflectance of rock surfaces were measured using a portable spectrometer in order to establish a method for mapping salt deposition and adhesion on rock surfaces. The results of the measurements showed that the spectral characteristics of the salts were significantly detectable. The salt map of the piers shows that the salt concentration is higher in layers 4 to 6 just above the mean high water level (MHWL) at the piers, while it is lower in layers 8 to 11. There was a positive correlation of about 0.5 between salt concentration and depression volume, indicating that the higher the salt deposition concentration, the larger the depression. Therefore, it is suggesting that salt accretion distribution, or salt weathering, is the main cause of weathering of the sandstone blocks.
In addition, a comparison of depression depths at 20, 38, 50, and 73 years after completion showed a decreasing trend in the overall rate of increase, but a high rate of increase was still observed in certain layers. The main reason for this is that the potential for salt weathering is still high in the deeper part of the depression, which continues to expand. Further studies are needed to investigate the mechanisms of salt precipitation and temporal changes in salt distribution in order to understand the processes of depression formation. Additionally, the analysis method using spectral reflectance characteristics is effective for visualizing salt distribution and is expected to be applicable to other regions and structures. Furthermore, it is desirable to develop a measurement method that can acquire salt distribution more simply and efficiently by utilizing multi-hyperspectral cameras.
The study area is the Yayoi Bridge over Aoshima, Miyazaki Prefecture, Japan. The bridge was built in 1951 and the piers are made of blocks of the same sandstone as the bedrock of Aoshima (Neogene Miyazaki Group). There are 144 sandstone blocks forming the wall, set in 11 layers. The layers are numbered from the bottom to the top, with the lowest being the 1st layer and the highest being the 11th layer. The sandstone blocks have been weathered and eroded over a period of 73 years, forming depressions on their surfaces. The depth of these depressions depends on the orientation and elevation of the sandstone block surface, and the location of the piers. Continuous studies on the formation of tafoni-like depressions and salt weathering processes have been conducted here since 1970s.
In this study, the spectral reflectance of rock surfaces were measured using a portable spectrometer in order to establish a method for mapping salt deposition and adhesion on rock surfaces. The results of the measurements showed that the spectral characteristics of the salts were significantly detectable. The salt map of the piers shows that the salt concentration is higher in layers 4 to 6 just above the mean high water level (MHWL) at the piers, while it is lower in layers 8 to 11. There was a positive correlation of about 0.5 between salt concentration and depression volume, indicating that the higher the salt deposition concentration, the larger the depression. Therefore, it is suggesting that salt accretion distribution, or salt weathering, is the main cause of weathering of the sandstone blocks.
In addition, a comparison of depression depths at 20, 38, 50, and 73 years after completion showed a decreasing trend in the overall rate of increase, but a high rate of increase was still observed in certain layers. The main reason for this is that the potential for salt weathering is still high in the deeper part of the depression, which continues to expand. Further studies are needed to investigate the mechanisms of salt precipitation and temporal changes in salt distribution in order to understand the processes of depression formation. Additionally, the analysis method using spectral reflectance characteristics is effective for visualizing salt distribution and is expected to be applicable to other regions and structures. Furthermore, it is desirable to develop a measurement method that can acquire salt distribution more simply and efficiently by utilizing multi-hyperspectral cameras.