Fine‐scale thermohaline ocean structure retrieved with 2‐D prestack full‐waveform inversion of multichannel seismic data: Application to the Gulf of Cadiz (SW Iberia)

Abstract

This work demonstrates the feasibility of 2‐D time‐domain, adjoint‐state acoustic full‐waveform inversion (FWI) to retrieve high‐resolution models of ocean physical parameters such as sound speed, temperature and salinity. The proposed method is first described and then applied to prestack multichannel seismic (MCS) data acquired in the Gulf of Cadiz (SW Iberia) in 2007 in the framework of the Geophysical Oceanography project. The inversion strategy flow includes specifically designed data preconditioning for acoustic noise reduction, followed by the inversion of sound speed in the shotgather domain. We show that the final sound speed model has a horizontal resolution of ∼ 70 m, which is two orders of magnitude better than that of the initial model constructed with coincident eXpendable Bathy Thermograph (XBT) data, and close to the theoretical resolution of O(λ). Temperature (T) and salinity (S) are retrieved with the same lateral resolution as sound speed by combining the inverted sound speed model with the thermodynamic equation of seawater and a local, depth‐dependent T‐S relation derived from regional conductivity‐temperature‐depth (CTD) measurements of the National Oceanic and Atmospheric Administration (NOAA) database. The comparison of the inverted T and S models with XBT and CTD casts deployed simultaneously to the MCS acquisition shows that the thermohaline contrasts are resolved with an accuracy of 0.18oC for temperature and 0.08 PSU for salinity. The combination of oceanographic and MCS data into a common, pseudo‐automatic inversion scheme allows to quantitatively resolve submeso‐scale features that ought to be incorporated into larger‐scale ocean models of oceans structure and circulation.

Seismic Oceanography in the Tyrrhenian Sea: Thermohaline Staircases, Eddies, and Internal Waves

“…In the last decade, a new technique called seismic oceanography (SO) was developed to image and investigate the properties of water masses using low-frequency acoustic methods. This technique is based on multichannel seismic (MCS) reflection profiling, and it has been shown to be a powerful tool to study temperature (and less so, salinity) contrasts associated with oceanic fine structure (Sallarè s et al, 2009) in unprecedented horizontal resolution (e.g., Biescas et al, 2008;Buffett et al, 2010;Dagnino et al, 2016;Gonella & Michon, 1988;Holbrook et al, 2003).…”

A Locally Generated High‐Mode Nonlinear Internal Wave Detected on the Shelf of the Northern South China Sea From Marine Seismic Observations

“…The black line in the box shows the analyzed subsection of the whole seismic line SO1N (gray) conducted on 27 July 2014. Therefore, many inversion techniques have been proposed to recover the water T/S structure (e.g., Biescas et al, 2014;Dagnino et al, 2016;Papenberg et al, 2010;Wood et al, 2008). Whatever the strategy is, using starting models smoothed from concurrent hydrographic observations during the inversion can improve the results significantly with a nominal temperature accuracy of less than 0.18C (Biescas et al, 2014;Kormann et al, 2011;Padhi et al, 2015).…”

Appraisal of Instantaneous Phase-Based Functions in Adjoint Waveform Inversion

“…It is suitable for all types of acquisition geometries and it is appropriate for marine and land data. It has so far been applied to a broad range of frequencies; long wavelengths (up to 0.1 Hz) for regional studies with global earthquake records [3], [9], for active seismic source experiments for subsurface data at medium scale (10 Hz), and for the water column at small scale (50 Hz) [8] and even at ultrahigh frequency marine seismic reflection data (1 kHz) [21]. Also, it allows multiparameter inversion; e.g., p and s wavevelocities, anisotropy, density, and attenuation.…”