A novel contactless technique to measure water waves using a single photon avalanche diode detector array.

R Zhang,I Underwood,D M Ingram,S Draycott,I Gyongy

doi:10.1098/rspa.2020.0457

Abstract

Commonly deployed measurement systems for water waves are intrusive and measure a limited number of parameters. This results in difficulties in inferring detailed sea state information while additionally subjecting the system to environmental loading. Optical techniques offer a non-intrusive alternative, yet documented systems suffer a range of problems related to usability and performance. Here, we present experimental data obtained from a 256 × 256 Single Photon Avalanche Diode (SPAD) detector array used to measure water waves in a laboratory facility. 12 regular wave conditions are used to assess performance. Picosecond resolution time-of-flight measurements are obtained, without the use of dye, over an area of the water surface and processed to provide surface elevation data. The SPAD detector array is installed 0.487 m above the water surface and synchronized with a pulsed laser source with a wavelength of 532 nm and mean power <1 mW. Through analysis of the experimental results, and with the aid of an optical model, we demonstrate good performance up to a limiting steepness value, ka, of 0.11. Through this preliminary proof-of-concept study, we highlight the capability for SPAD-based systems to measure water waves within a given field-of-view simultaneously, while raising potential solutions for improving performance.

Highlights

We assess the ability of a Single Photon Avalanche Diode (SPAD) detector array—synchronized with a laser source—to measure surface gravity waves
A range of wave conditions are tested with various frequencies and amplitudes—resulting in a range of wave steepness values
The preliminary results are promising and demonstrate that good quality data can be collected up to a limiting value of ka. This limit is well predicted by the presented optical model which shows that the data collection capability is highly macropixel dependent: a function of the angles of incidence of light rays for different wave phases

Summary

Background and Introduction

Optical techniques are an attractive approach for measuring water surface waves as they offer a non-intrusive approach which limits surface disturbances and the subsequent effect on both the wave measurement itself and other equipment/instruments. Gain calibration was performed empirically and post-experiment by reconciling the peak-to-peak amplitude of the SPAD measurements with the corresponding wave gauge data one of the test cases (Test 6, a low steepness condition was well captured by the SPAD detector array). Based on this approach, the empirical gain value was calculated to be 1.67, and was subsequently applied to all macropixels for all test cases. Steepness values shown are those calculated from the wavenumbers associated with the known (actual) frequencies along with the target amplitudes

Results

Discussion

Findings

Conclusion