Acoustic backscattering properties of manganese nodules: Numerical and laboratory experiments based on Sub-bottom acoustic profile surveys

Abstract

An underwater acoustic sensing system can be used to investigate the characteristics of manganese nodule deposits, such as the nodule size distribution and concentration. We numerically analyzed the relationship between the backscattering intensity obtained using a sub-bottom profiler (SBP) and the manganese nodule distribution by considering higher-order scattering terms among the nodules, in addition to the interaction between wave propagation and the combined effect of the manganese nodules and seafloor. Numerical experiments revealed that complex acoustic reflective waveforms and corresponding spectra increase in complexity with an increase in the sphere size. We categorized this scattering phenomenon with respect to several scattering regimes. As observed, the total reflection response can be attributed to the interference between the scattered waveforms from the nodules and the reflected waveforms from the seabed surface. The laboratory experiments were conducted using actual manganese nodules, and demonstrated a macroscopic decrease in reflectivity with an increase in the area ratio for different nodule sizes. It is, therefore, necessary to eliminate the wave interaction between manganese nodules and the seafloor to quantitatively estimate the density of the nodule distribution.

Keywords:

• Manganese nodule
• acoustic sensing
• scattering phenomenon
• sub-bottom profiler
• reflection response

Acknowledgements

This study was conducted as part of a project entrusted by the Ministry of Economy, Trade and Industry (METI), Japan, and Japan Oil, Gas and Metals National Corporation (JOGMEC). We greatly acknowledge the thorough reviews and constructive comments of the four anonymous reviewers, which helped increase the quality of the manuscript. We are also grateful for insightful discussions with Akihiko Chiba and Atsushi Shirota.

Disclosure statement

No potential competing interest.

Data availability

The data can be provided upon reasonable request to the corresponding author.