Prerequisite Condition of Diffuse Attenuation Coefficient K<sub>d</sub>(490) for Detecting Seafloor from ICESat-2 Geolocated Photons During Shallow Water Bathymetry

Bathymetry refers to the depth measurement of the topographic seafloor surface and is essential geophysical data for understanding the land-ocean interplay. Recently, researchers have taken advantage of photon penetration of the green laser of NASA ICESat-2 to profile the seafloor as a part of the bathymetric mapping of shallow nearshore coastal waters. Prerequisite conditions for using the ICESat-2 geolocated photons for reconstructing the bathymetric profiles include a preference for using nighttime acquisitions followed by applying refraction correction to the water column returned photons to correct the apparent depths due to the change in the speed of light that occurs at the air-water interface. The success of detecting the seafloor from the bathymetric profiles from ICESat-2 photons will depend on the optical clarity of the water. The diffuse attenuation coefficient for downwelling irradiance, K_d(490), measures how light dissipates with depth in water and indicates how strongly light intensity at 490 nm of wavelength is attenuated in the water column, providing a hint about the water clarity. In this research, we have explored ICESat-2's photon-based bathymetric mapping potential in relation to the K_d(490). ICESat-2 photon data and K_d(490) data from level-2 OLCI of Sentinel-3 A/B mission were acquired with overlapping dates to investigate the possible depth penetration of ICESat-2 photons in the shallow waters during clear water conditions and sediment load periods. Two nearshore study sites were chosen that are located at the head of the Bay of Bengal. This research proves that the ICESat-2 photons can successfully reflect from the seafloor in shallow waters while the optical water condition is clear, during which the K_d(490) is less than 0.12 m^-1. On the contrary, during the periods of sediment load in the water, where the K_d(490) is above 0.15 m^-1, the ray tracing mechanism of ICESat-2 photons has been impacted due to absorption and scattering caused by the sediments load in the water column; thus, seafloor detection by ICESat-2 photons will not be successful in sediment loaded waters. The results from this research suggest the necessity of K_d(490) to be complementary data with ICESat-2 photons for successful bathymetric applications.