Estimating Oceanic Turbulence Dissipation from Seismic Images

Abstract

Seismic images of oceanic thermohaline finestructure record vertical displacements from internal waves and turbulence over large sections at unprecedented horizontal resolution. Where reflections follow isopycnals, their displacements can be used to estimate levels of turbulence dissipation, by applying the Klymak-Moum slope spectrum method. However, many issues must be considered when using seismic images for estimating turbulence dissipation, especially sources of random and harmonic noise. This study examines the utility of seismic images for estimating turbulence dissipation in the ocean, using synthetic modeling and data from two field surveys, from the South China Sea and the eastern Pacific Ocean, including the first comparison of turbulence estimates from seismic images and from vertical shear. Realistic synthetic models that mimic the spectral characteristics of internal waves and turbulence show that reflector slope spectra accurately reproduce isopycnal slope spectra out to horizontal wavenumbers of ;0.04 cpm, corresponding to horizontal wavelengths of 25 m. Using seismic reflector slope spectra requires recognition and suppression of shot-generated harmonic noise and restriction of data to frequency bands with signal-to-noise ratios greater than about 4. Calculation of slope spectra directly from Fourier transforms of the seismic data is necessary to determine the suitability of a particular dataset to turbulence estimation from reflector slope spectra. Turbulence dissipation estimated from seismic reflector displacements compares well to those from 10-m shear determined by coincident expendable current profiler (XCP) data, demonstrating that seismic images can produce reliable estimates of turbulence dissipation in the ocean, provided that random noise is minimal and harmonic noise is removed.

Recovery of temperature, salinity, and potential density from ocean reflectivity

“…[], Holbrook et al . [], and the interpretation of dynamics of the Sub‐Antarctic Front done by Sheen et al . [].…”

Submesoscale Features and Turbulent Mixing of an Oblique Anticyclonic Eddy in the Gulf of Alaska Investigated by Marine Seismic Survey Data

“…Vertical geostrophic shear ∆u across an isopycnal surface can be estimated using , where g is the gravitational acceleration, ∆ρ is the density change across the isopycnal, ρ is the mean density of two layers, and γ is the isopycnal slope (Margules, ; Sheen et al, ). Here the assumption is made that the seismic horizons approximate isopycnals (Holbrook et al, ; Krahmann et al, ; Sheen et al, ). In the GoA, this assumption is feasible because previous works have shown the consistency between the temperature (primary cause of the seismic reflection) and the density fields (Ladd et al, ; Ladd et al, ), and calculations of the historical CTD data show that the in situ density gradient is consistent with the seismic reflectivity or the synthetic seismogram (not shown).…”

“…Previous works have used the above model to derived mixing rates over the seismic sections (e.g., Holbrook et al, ; Mojica et al, ; Sheen et al, ). To derive the spectra in the turbulence regime properly, these studies suggest a few crucial steps before calculating diffusivities.…”

“…For the 50‐m (i.e., k = 0.02 m ‐1 ) shot spacing of the seismic data, a notch filter centered at the prominent and harmonic spikes ( k = n × 0.02 cpm, where n is any integer) can be applied to suppress the noise spikes. Second, slope spectra are produced from the displacement spectra as (Holbrook et al, ; Klymak & Moum, ). Such a transformation allows us to determine the crossover from the negative to positive gradients for a slope spectrum, corresponding to the transition from an internal wave subrange at lower wavenumbers with an exponent of −1/2 (Garrett & Munk, ) to a turbulence subrange at higher wavenumbers with an exponent of +1/3 (Falder et al, ; Sheen et al, ).…”

“…These parameters are (1) internal wave tracking with an instantaneous phase contour cos(α) = 0.6 and tracking length longer than 2.5 km (400 points), (2) power spectrum calculation with Fourier transform length of 256 points, and (3) dissipation rate fitting within an average turbulence subrange of 0.008–0.04 m −1 (25–125 m). The above calculation is also based on the assumption that the seismic horizons represent isopycnals approximately (Holbrook et al, ; Krahmann et al, ; Sheen et al, ). Besides, following possible error sources in K ρ estimation should come into notice.…”