Comparison of Laser Particle Sizers and Sediment Settling Instruments in Marine Sediment Grain Size Analysis

Coastal marine environments are characterized by different sediment types, representing the final outcome of hydrodynamic, meteorological, climatic, and morphological processes. Once sediment particles enter the marine environment, they undergo resuspension, transport, and deposition in different sedimentary settings based on their specific sizes, ultimately settling. Understanding the structural characteristics of sediments in various environments is crucial, as it serves multiple purposes, such as environmental assessment, beach nourishment, and benthic community studies.

Figure 1:Laser Granulometer (LG) and X-ray Sedigraph (XS).

In this context, sediment grain size determination should be conducted with the highest accuracy. In the study by M. Celia Magnoa et al., 24 sediment samples with a wide range of sizes, mineralogy, and composition were collected from five coastal marine sites in Italy. Fine particles smaller than 63 μm were analyzed using a Laser Granulometer (LG) and an X-ray Sedigraph (XS) to compare the data and highlight potential differences. The LG is based on the principle of laser diffraction, where particles diffract light at a given angle. The diffraction angle is inversely proportional to particle size, and the intensity of the diffracted light at any given angle represents the number of particles with a specific cross-sectional area within the laser beam path. A parallel monochromatic light beam passes through the suspension, and the diffracted light is focused onto a detector.

To calculate grain size from the light intensity detected by the sensor, two diffraction theories—Fraunhofer and Mie—were applied. Both theories assume that all particles are spherical, meaning that particle size is represented by the diameter of a sphere with a cross-sectional area equal to that measured by laser diffraction. The grain size distribution is calculated based on volume percentage.

Analysis of the fine fraction obtained through wet sieving using LG and XS highlighted the variable percentage of sand. The higher the recorded sand content by both instruments, the greater the percentage of sand in the total sample. The highest values were recorded at the shallower stations of Bagnoli (up to 48.4% by XS and 24.2% by LG), Montalto (up to 3.0% by XS and 4.9% by LG), Torvajanica, and Gaeta. A comparison of the sand content detected by the two instruments showed that XS recorded higher sand content in Bagnoli and San Benedetto samples, while LG recorded higher sand content in Montalto, Gaeta, and Torvajanica samples.

A key difference was that the silt percentage determined by XS was consistently lower than that measured by LG, while the opposite was true for clay content (Figure 2). The unexpected variability in sand content across nearly all samples was attributed to clay mineral aggregates. The highest sand content detected by XS was linked to the local compositional characteristics of the sediments. In the samples analyzed by XS, clay content increased by 15–25%, which was attributed to the different working principles of the two instruments. However, the final results successfully characterized the structure and distribution of sediments as a function of water depth.

Figure 2:Percentages of sand, silt, and clay detected by XS and LG in sediment fractions < 63 μm.

In conclusion, it is essential to select the appropriate analytical instrument based on sediment characteristics, adjust the medium solution density for XS analysis, and take precautions to prevent the formation of aggregates in both XS and LG measurements.

[1] M. Celia Magno, F. Venti, L. Bergamin, G. Gaglianone, G. Pierfranceschi, E. Romano, A comparison between Laser Granulometer and Sedigraph in grain size analysis of marine sediments, Measurement, Volume 128, 2018, Pages 231-236,