## 1 Introduction
### Methods for Estimating the Sound Speed in the Seafloor
For instance, the uncertainty of the SAMS was from \(\pm 8.7\) up to \(\pm 24.8\) m s\({}^{-1}\) depending on the position and frequency used [20], which is higher than the uncertainty of laboratory measurements reported by [PERSON] [10] and [PERSON] et al. [12]. In addition, direct acoustic methods, both in the laboratory and in situ, require immense efforts and become expensive when large areas are investigated, especially in spatially and temporally heterogeneous regions [7; 22]. Acoustic and seismic remote-sensing methods are non-invasive, cost-effective and have commonly been used for the rapid scanning and mapping of the sedimentary parameters of the seafloor over large shelf/coastal areas [23]. Thus, they present a good practical alternative to direct methods. Geoacoustic inversion is an effective approach to investigate the remotely sensed data and constrain the seafloor sediment acoustic properties by matching the experimental data with the predictions from modeling. It is based on the quantification of the influence of the bottom parameters on the sound propagation and makes possible the estimation of sediment parameters such as sound speed, density, and porosity. A variety of geoacoustic inversion methods have been developed in recent years using both active and passive sound sources [3; 24; 25; 26]. Active geoacoustic inversion methods are based on data acquisition using powerful, controlled sound sources [27], whereas passive geoacoustic inversion methods are based on using the ubiquitous ambient noise as a replacement for the active sound sources [28]. For instance, [PERSON] et al. [29] and [PERSON] et al. [30] presented a passive geoacoustic inversion method using two hydrophones, which combined noise interferometry and time reversal mirror techniques. They used the noise data in the frequency band of 10-70 Hz, which was recorded by two hydrophones at the distance of 8 km in the Shallow Water 2006 (SW06) experiment, in order to estimate the sound speed and density at the bottom. For gas-rich bottoms, the sound speed can be estimated using geoacoustic inversion by matching experimentally recorded pulse responses of sound signals and sound fields simulated by ray theory [3; 7]. If a gassy layer with low sound speed is thin, its thickness can be estimated using the resonance effects of the frequency-dependent reflection coefficient [4; 5]. Furthermore, [PERSON] and [PERSON] [6] suggested a method to estimate the sound speed in gassy sediments from the modal attenuation at \(<\)100 Hz frequencies. Mode-attenuation coefficients were estimated from the discrete components of the research vessel noise that was recorded by a vertical line array of hydrophones in shallow water (Lake Kinneret, \(\sim\)40 m maximum bottom depth). Seismic methods are routinely used for estimations of the sediment sound speed at various settings and water depths but are rarely used to investigate the geoacoustic parameters of the seafloor itself. For instance, [PERSON] et al. [31] performed a Migration Velocity Analysis on pre-stack time-migrated seismic data in order to estimate the compressional sound speed in shallow, muddy sediments of the Bornholm Basin, Baltic Sea. Where the seabed sound speed is higher than in the water, a critically refracted head wave is formed. A downward-traveling acoustic ray that is incident on the bottom boundary at the critical angle \(\theta_{c}\) is refracted at the seabed to propagate horizontally through the sediment at the seabed compressional sound speed \(c_{0}\). As it progresses, it re-radiates acoustic energy back into the water column, also at the critical angle \(\theta_{c}\). For many years, acoustic head wave data from seismic refraction experiments have been widely used in sound-speed estimations of the soil on land and the seafloor sediments in relatively shallow aquatic environments [32].