The acoustic wave equation describes wave propagation directly from basic physical laws, even in heterogeneous acoustic media. When numerically simulating waves with the wave equation, contrasts in the medium parameters automatically generate all scattering effects. For some applications - such as propagation analysis or certain wave-equation based imaging techniques - it is desirable to suppress these reflections, as we are only interested in the transmitted wave-field. To achieve this, a modification to the constitutive relations is proposed, yielding an extra term that suppresses waves with reference to a preferred direction. The scale-factor α of this extra term can either be interpreted as a penetration depth or as a typical decay time. This modified theory is implemented using a staggered-grid, time-domain finite difference scheme, where the acoustic Poynting-vector is used to estimate the local propagation direction of the wave-field. The method was successfully used to suppress reflections in media with bone tissue (medical ultrasound) and geophysical subsurface structures, while introducing only minor perturbations to the transmitted wave-field and a small increase in computation time.

Conventional Full Wavefield Migration (FWM) is a full-wavefield inversion method based on recursively applying one-way convolutional propagation and reflection operators in the space-frequency domain at every depth level. Therefore, it struggles to model diving waves and image steep reflectors accurately. In this paper, the Interface Contrast imaging technique, an imaging technique based on the scattering integral developed in the context of medical ultrasound, is presented and used to provide a natural omni-directional extension to the conventional FWM method. The resulting algorithm is applied to a synthetic 2D model featuring a steep reflector. The results of these simulations are given and show that the technique can successfully image steep reflectors. This result yields a proof-of-concept for further research into this algorithm, where including internal scattering is a top priority.

Full Wavefield Migration (FWM) is a full-wavefield inversion method based on the so-called WRW model in the context of seismic imaging. This WRW model describes seismic data in terms of convolutional propagation and reflection operators in the space-frequency domain. By recursively applying these operators, multi-scattering data can be described and the FWM algorithm can find the underlying reflection properties (i.e. the ‘image’). However, the current FWM algorithm only allows for acoustic imaging. This approach disregards elastic phenomena such as wave conversions. In this report, an extension to the FWM method is given to incorporate these effects. The resulting algorithm is then applied to a simple 1.5D model, to allow for proof-of-concept numerical simulations. The results of these simulations are given and show that the proposed model can model wave conversions and other elastic effects, yielding a good proof-of-concept for further research into this extended algorithm.

Sperm that swim collectively to the fertilization site have been observed across several vertebrate and invertebrate species, with groups ranging in size from sperm pairs to massive aggregates containing hundreds of cells. Although the molecular mechanisms that regulate sperm-sperm adhesion are still unclear, aggregation can enhance sperm motility and thus offer a fertilization advantage. Here, we report a thorough computational investigation on the role of cellular geometry in the performance of sperm aggregates. The sperm head is modelled as a persistent random walker characterized by a non-trivial three-dimensional shape and equipped with an adhesive region where cell-cell binding occurs. By considering both, a simple parametric head shape and a computer reconstruction of a real head shape based on morphometric data, we demonstrate that the geometry of the head and the structure of the adhesive region crucially affects both the stability and motility of the aggregates. Our analysis further suggests that the apical hook commonly found in the sperm of muroid rodents might serve to shield portions of the adhesive region and promote efficient alignment of the velocities of the interacting cells.