Development of a grain-size determination method based on digital images of muddy sediments in coastal areas

• Published in: Marine geology 2023-07, Vol.461, p.107066, Article 107066
• Main Authors: Umehara, Akira, Takashima, Hikaru, Yoshimori, Yusuke, Sekito, Satoshi, Ohno, Masaki, Nakai, Satoshi, Nishijima, Wataru
• Format: Article
• Language: English
• Subjects: Autocorrelation; Digital camera; MATLAB; Mud content; Seto Inland Sea
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/j.margeo.2023.107066

The grain-size distribution is a fundamental parameter for assessment of coastal environments. In coastal areas, grain-size analysis of sediments is hampered by arduous collection of sediments from the seafloor at each sampling site and long analysis time. Although rapid determination methods using digital imaging have been developed over the last two decades, these methods cannot be applied to muddy sediments in coastal areas. In the present study, a digital image analysis method applicable to muddy sediments was developed based on sediment color analysis combined with an autocorrelation algorithm introduced by Rubin (2004). For digital image collection in the laboratory, a waterproof digital camera (16 megapixels) was used; it was placed 9 cm from the sediment surface. The imaging equipment consisted of the digital camera, an acrylic plate, and a housing filled with tap water, which was used to avoid effects of suspended sediment in the seawater. Model sediments were prepared and images of the sediment surfaces were taken. The mud and coarse fraction of the sediment were successfully estimated through binarization of the image based on brightness values and the grain-size distribution of the coarse fraction was determined using the autocorrelation algorithm introduced by Rubin (2004). In the field, the imaging apparatus consisting of a stainless-steel cage, lighting equipment, a housing filled with tap water, and a wide acrylic plate attached to the bottom of the housing provided a sturdy structure for obtaining clear digital images from uneven field sediments with suspended particles in the bottom water. The ship was moved several hundred meters at a time; the imaging apparatus was allowed to settle on the sediments and images were then taken continuously at set intervals. From these images, grain-size distributions of sediments containing >15% mud were successfully estimated. Thus, the digital image analysis method developed here will be suitable for measuring the fraction of surface area covered by mud and the size of the visible coarse fraction, and will be useful for monitoring of various sediment types, including muddy coastal sediments, demonstrating great promise for elucidating benthic ecosystem structures and functions in coastal areas. •Grain-size is a fundamental parameter for assessment of coastal environments.•Rapid determination methods using digital imaging have been developed previously.•Previous methods cannot be applied to muddy sedime