## 2 The Experimental Data
### Methodology for Seafloor Sound Speed Estimation Based on Travel Times Matching  
The critical distance (the minimum distance at which the head wave can be registered on the HLA records) increases with the bottom depth, and with a decrease in the acoustic impedance contrast at the seafloor [36]. For a constant sound speed in the water layer, the minimum distance at which the seafloor interface head wave can be registered with a Horizontal Line Array (e.g., the black solid line in Figure 5) is  
\[R_{c}=2H_{1}\tan(\theta_{c}), \tag{2}\]  
Figure 4: (**a**) A depth-migrated section of TGS line ISEM-2073-108 and ISEM-2073-109, showing the seafloor to salt layer structure from the continental shelf to the studied area, and the geometrical configuration of the experiment. A yellow dashed line depicts the Horizontal Line Array (HLA) of 576 hydrophones used in the experiment. The position of sound source #2290 is depicted by a red triangle. The stretch of the bottom, which was insonified through the recording of source #2290 by the HLA (their footprint), is highlighted with a red dashed line. (**b**) A zoomed section of the area of the studied source #2290. Red dashed and dash-dot lines approximate the reflection footprint stretch of this source for the seafloor and the top salt, respectively. (**c**) A reduced time vs. offset display for source #2290, overlaid with the chosen seafloor reflection (red), top salt reflection (blue), and sub-bottom layer refraction (orange). The reduction velocity is 2000 m s\({}^{-1}\). where \(H_{1}\) is the bottom depth, \(\theta_{c}=\text{asin}\frac{c_{w}}{c_{0}}\) is the critical angle of incidence, \(c_{w}\) is the sound speed in the water near the seafloor, and \(c_{0}\) is the sound speed at the seafloor. An additional offset coverage is needed to properly identify the seafloor interface head wave refracted arrival in the HLA's records (e.g., as depicted by a dashed line in Figure 5). For high-sound-speed seafloors, such as coarse sand, gravel, and basalt, \(R_{c}\) is significantly smaller than the length, \(L\), of the HLAs, which are commonly used for 2D marine seismic data acquisition and are 7200 m in our case. However, for the common abyssal seabed with a relatively low sound speed (e.g., with a predominantly silt to clay composition with \(c_{0}\)\(\sim\)1500-1600 m s\({}^{-1}\)) [10]\(R_{c}\) becomes bigger than \(L\), and an alternative approach is required for the estimation of the seafloor sound speed based on data recorded by such HLAs. In the following section, we exploit acoustic rays that bend and reach the critical angle below the seafloor, where the sound speed increases with the depth. The essence of the methodology introduced in this study is a geoacoustic inversion, which is based on the recursive matching of experimentally recorded and simulated acoustic-seismic arrival times. Reduced-time (Equation (1)) displays were applied to facilitate the comparison of experimental and simulated arrival times. For horizontal interfaces, if the assumed sound speed matches the refractor sound speed, then the refraction will display as a horizontal line on the reduced time vs. offset plot. Arrivals with an assumed sound speed lower than the reduction velocity will have a positive slope, whereas those with a sound speed greater than the reduction velocity will have a negative slope. Three alternative methodologies for calculating the arrival times of head-wave refractions correspond to three cases of water and bottom stratification:  
Figure 5: The critical distance \(R_{c}\) as a function of bottom depth \(H_{1}\) and sound speed at the seafloor \(c_{0}\) calculated with Equation (2) for the sound speed in water \(c_{w}\)= 1527 m s\({}^{-1}\) (color scale). This critical distance determines the minimum lengths of an HLA that would be required to record the upwardly traveling seafloor interface head wave at the sea surface.