The optical volume scattering function (VSF) defines the angular distribution of scattered light within an element of water. The VSF is dominated by scattering from particles, and strongly depends on concentration, particle size, and composition. Despite the importance of the VSF, it has seldom been measured in the field. Here we report some of the first field VSF measurements in decades, made in the nearshore bottom nepheloid layer (BNL) off the coast of Martha's Vineyard, Massachusetts. Specifically, we continuously measured the near-forward VSF for several weeks with two co-located laser scattering instruments (LISST-B and LISST-floc, Sequoia Scientific), covering an angular range of roughly 0.01 to 15 degrees, an order of magnitude further near-forward than the venerable observations of Petzold decades ago.

An understanding of near-forward VSF variability in the BNL is needed: for assessment of diver visibility; for effective marine operations such as underwater navigation, object detection, and imaging; and for modeling the light field for shallow water remote sensing studies. Dependence on size proves particularly thorny for prediction of optical properties since size distribution in the BNL is strongly mediated by bottom stress and turbulence. However, given appropriate models, the dependence of the VSF on particle size allows us to describe particle dynamics with high temporal resolution using optical sensors.

In the field, we observed marked variability in the shape and magnitude of the near-forward VSF over the course of resuspension and settling events. Furthermore, we observed correlation between total scattering and VSF shape which are explained well by conceptual models of particle dynamics. In this study, we compare the measured VSF with theoretical optical models, such as Mie theory and Fraunhofer diffraction, as well as examine the link between VSF shape and magnitude as a first attempt to constrain and model scattering in the BNL.