## 1 Introduction
### Methods for Estimating the Sound Speed in the Seafloor  
The sound speed in the seafloor marine sediments can be estimated through several approaches: (1) direct methods [10; 11; 12], (2) empirical relations [10; 13] and/or physical models [14; 15; 16; 17], (3) in situ acoustic measurements [2; 18; 19; 20] and (4) remote sensing using acoustic and seismic methods. Direct laboratory methods involve the extraction of the sediment cores and their subsequent analysis in the laboratory. For instance, [PERSON] and [PERSON] [10] used a pulse transmission technique at a frequency of about 200 kHz to measure the sound speed in the cores; the estimated margins of error were \(\pm 3\) m s\({}^{-1}\) for clays and \(\pm 5\) m s\({}^{-1}\) for sands. [PERSON] et al. [12] conducted laboratory measurements of compressional sound speeds of shelf sediments (-50 m bottom depth) in the South Sea of Korea for six 1-3.5 m-long cores using an automated sound-speed-measurement technique with a 1 MHz PZT transducer [21]. It was modified from the pulse transmission technique [11] and provided sound-speed accuracy of approximately 1%. However, the estimation of in situ sound speed from laboratory measurements is challenging due to: (1) the disturbance of the sediment cores during their extraction from the sediment, which introduces bias into the laboratory measurements; and (2) different scales and environmental conditions between the laboratory and the seafloor (i.e., pressure, temperature, measurement frequency) [12]. Compilations of direct measurements have been used to derive empirical relations between seismic velocities and seafloor sediment properties. [PERSON] [13] analyzed the data obtained from 456 cores (8287 samples) taken from various physiographic regions (mainly in deep water) of the Pacific and Atlantic Oceans, and the Norwegian, Mediterranean and Black Seas in order to derive a statistical relationship between the sound speed, density and porosity. The analysis of the cores showed that the sound speed and porosity varied from 2000 to 1450 m s\({}^{-1}\) and from 25% to 90%, respectively. The length of the cores analyzed by [PERSON] sometimes exceeded 10 m, so the relationship obtained from the samples was valid for at least 10 m into the sea bottom. Also, several empirical relationships were developed by [PERSON] and [PERSON] [10] in order to link the sound speed in the sediment to various sediment types (by mean grain size or porosity). [PERSON] and [PERSON] [10] suggested separate equations for three general environments: a second-order polynomial for continental margins (shelf and slope) and linear equations for pelagic and turbiditic abyssal hills and plains. The limiting porosity \(P\) for their models was 35% to 85% for continental margins, and 70% to 90% for abyssal hills and plains. Most of the samples in their report came from the upper 30 cm of the seafloor, limiting the sub-bottom-depth extent of their models [10]. Such analytical expressions can be convenient for rough sound-speed estimations if the type and/or density of the sediment are known, for instance, from sediment cores. In situ acoustic methods involve the use of either manually buried acoustic systems [18] or specially designed underwater mechanical systems, which can penetrate into the sediment through gravitational or external forces. The penetration depth of such devices can vary greatly. The in situ Sediment Geoacoustic Measurement System (ISSAMS) [19] is designed to measure the sound speed within the topmost 30 cm, whereas the Sediment Acoustic-Speed Measurement System (SAMS) [20] can reach a maximal sediment depth of 3 m, and the Acoustic Lance [2] has a maximum penetration depth of 5 m. The use of such methods helps to preserve in situ conditions but may still inflict sediment disturbance by the installation of the measurement device. The uncertainty of such measurements may be high relative to the laboratory measurements.