## 2 Materials and Methods  
### Data Processing  
Figure 1 shows the proposed workflow, investigating the impact of sound absorption of the water column while excluding other parameters, in order to better understand its contribution to backscatter variations. The legacy MBES data used in this study were collected from three coastal sites in the East Sea of Korea near survey Lines 102, 208, and 209, as depicted in Figure 2. Legacy MBES data acquisition was conducted using the Kongsberg EM3000 and EM2040C MBES systems(Kongsberg, Norway) installed aboard the Ocean 2000 vessel operated by the Korea Hydrographic and Oceanographic Agency. The raw MBES data were recorded using Kongsberg's Seafloor Information System software (SIS5, Kongsberg, Norway) [1,2]. The legacy MBES backscatter data, originally collected for non-research purposes, were obtained without adequately considering site-specific water column variability. However, the Korean seas exhibit significant temporal and spatial variability in oceanographic conditions due to dynamic factors such as seasonal currents and regional characteristics. To analyze the extent of these variations, historical CTD datasets from the Korea Oceanographic Data Center (KODC) were utilized to visualize and assess the spatiotemporal changes in absorption parameters across the East Sea. At locations where legacy MBES data were acquired, site-specific SVP measurements during the surveys were used and compared with the 10-year averaged KODC data to evaluate the deviations in absorption parameters. Using the mean salinity values derived from the KODC CTD data, water temperature profiles were estimated using the SVP measurements obtained from the legacy MBES surveys. These updated parameters were then applied to re-process the legacy MBES backscatter data. The calibration workflow for legacy MBES datasets began with decoding of the raw MBES data to extract key acoustic parameters, including beam angle, position, orientation, pulse length, power level, and backscatter intensity [28]. In the decoding stage, Time-Variable Gain (TVG)--which was previously applied without considering site-specific acoustic conditions--was re-assessed to account for signal loss due to water column absorption. Following the decoding phase, georeferencing was performed to align the MBES data with the Earth-fixed coordinates. This step involved correcting vessel motion parameters, such as pitch, roll, and heave, and synchronizing sensor data with precise positional information. The integration of these corrections ensured accurate mapping of the backscatter data to their corresponding seafloor locations. Figure 1: Workflow for calibrating legacy MBES data using historical KODC CTD and SVP data to improve backscatter measurements, including parameter explanations for each stage. In the next phase, source level correction was applied. The transmitted signal strength was compared with system log values to correct for power variations during data acquisition. This step standardized the backscatter intensity measurements, making them suitable for comparative analysis across different regions and conditions. The effects of sound absorption through the water column on the MBES backscatter data were investigated. Geometric spreading loss and absorption loss parameters were re-calculated using site-specific water column information derived from the KODC datasets. Absorption loss--represented by the absorption coefficient (\(\alpha\))--was determined based on salinity, temperature, and frequency, enabling more precise compensation for signal attenuation and scattering effects. The final step involved applying Time-Variable Gain (TVG) correction using the re-calculated absorption coefficients. This step mitigated the signal loss caused by distance and water column variability by performing line-by-line adjustments of the backscatter intensity data. By focusing specifically on the absorption parameter of the water column, this workflow isolated its contribution to backscatter variations while excluding other factors.