Refinement of method of seawater density dynamic measurements using marine autonomous probe-profilers

The study described in the article relates to in situ determination of the vertical profile of seawater density ρ. Traditional density-determining methods by means of CTD parameters (conductivity, temperature, depth) have a multi-stage nature, complicating their use in real-time applications. A promising method is the dynamic measurements using a marine autonomous probe-profiler, which is equipped with only one pressure sensor and could determine the density profile at an order of velocity of 1 m/s. However, the current embodiment of this method requires knowing the seawater density on the surface. The objective of this study is to develop the refinement of the seawater density dynamic measurements method that does not require such information. The method is based on the analysis of the mathematical description of the probe-profiler motion in a field of hydrostatic pressure p during free immersion. It helps to obtain the equation for ρ(p) that does not require a priori information about density. The method involves calculating the first and second derivatives of hydrostatic pressure. To reduce noise amplification because of differentiation, the use of filter-differentiators is proposed, and the frequency suppression band boundary that depends on the profiling velocity is determined. The influence of the probe-profiler immersion velocity on the dynamic error of pressure measurement and the analog-to-digital conversion error is analyzed. The range of profiling velocities has been obtained in which the influence of the indicated errors is reduced. Simulations were conducted to confirm the effectiveness of the proposed method refinement. The obtained results can be applied in determining the seawater density profile in oceanography, fisheries, and underwater work.