Measurement of Wave Properties with a Custom Built Sensor System as Part of the One Ocean Expedition (2021-2023): Technical Aspects and Post Processing of Field Data

Abstract

In-situ measurement data of ocean waves is important for the validation and tuning of several types of numerical models. A recent inexpensive alternative to the widely used buoy measurements is given by a wave sensor system based on an altimeter and an inertial measurement unit (IMU). This project’s goal is to determine the applicability of such a system on a ship, not designed for research purposes, in order to investigate if such a system is suited for wide usage. The aim is to prove that such a system combines low cost in production with practicality and reliability, while delivering quality wave data. Therefore, a custom wave sensor system is designed, built and installed on board the Statsraad Lehmkuhl as part of the One Ocean Expedition. The measured raw data is post processed to receive the water surface elevation as a time series. Resulting wave properties are compared to model and satellite data. Further, the influence of the Doppler Effect on the measured data is investigated. The results show, that an altimeter and IMU based wave sensor system is capable of reliably measuring wave data. Imprecise motion measurement leads to bias for higher waves. The ship’s hull may falsify the measurements, depending on the probes location and the angle of the incoming waves. The Doppler Effect has a significant effect on the measured wave spectra, but a correction can be applied easily, if the ship’s velocity and the angle of the incoming waves to midships are known. These results suggest, that wave sensor systems based on altimeters and IMUs pose a widely applicable possibility for open ocean wave measurements. Challenges, which are faced in further development of the system, are the concrete positioning of the altimeters and IMUs, as well as the availability of the ship’s speed and the angle of the incoming waves for the Doppler Effect correction.