## 1 Introduction

[2] Seismic oceanography is an acoustic method used to explore the thermohaline structure of the ocean's interior using multichannel seismic (MCS) systems [*Holbrook et al.*, 2003]. The amplitudes of the reflections at the horizons of the ocean's thermohaline fine structure depend on the acoustic impedance contrasts across these horizons, which are in turn a function of sound speed (*c*) and density (*ρ*) contrasts. Therefore, MCS data contains implicit information on the temperature (*T*) and salinity (*S*) structure. According to the Rayleigh principle, the potential vertical resolution of seismic data is ∼*λ*/4 (where *λ* is the wavelength that corresponds to the peak frequency of the seismic source), being between 1 and 10 m for conventional MCS systems. The theoretical lateral resolution, defined by the size of the first Fresnel zone, is between 10 and 100 m, depending on the frequency content of the seismic source used and the depth of the target, greatly improving the horizontal resolutions compared with more traditional oceanographic techniques.

[3] Until now, applications of inversion methodologies to oceanographic seismic data have centered on retrieving 1-D sound speed models from stacked MCS traces, using either local optimization methods such as full waveform inversion [*Wood et al.*, 2008; *Kormann et al.*, 2011] or processing-based approaches [*Papenberg et al.*, 2010], since it is the parameter with the strongest influence on the acoustic reflectivity [*Ruddick et al.*, 2009; *Sallarès et al.*, 2009]. In order to obtain the *T* and *S* models from a single *c* model, two additional equations are required: (i) an equation relating *c* with *T*, *S*, and depth (*z*), e.g., *Millero et al.* [1980]; and (ii) an empirical *T*-*S* relationship, which can be obtained from local or regional oceanographic data. Since a single *T*-*S* relationship should be used for all the seismic section, covering different mesoscale structures, this step can be a source of significant errors in this inversion approach.

[4] In this work, we propose and present a new full waveform inversion (FWI) approach, which resolves directly both *T* and *S* from a 1-D synthetic seismic trace without the need of a *T*-*S* relationship. FWI was originally proposed to extract information on the ground elastic properties from the complete seismic wavefield [*Lailly*, 1983; *Tarantola*, 1984] and is commonly formulated as a local optimization problem where the gradient of the inverted parameters is calculated based on adjoint techniques. We show that it is possible to implement sensitivity kernels allowing for the direct inversion of *T* and *S*, as they appear implicitly in the acoustic wave equation. In a second step, *c* and *ρ* can be retrieved through the equations of state, so the main oceanographic scalar physical parameters can be determined without additional approximations.