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Modelling of the vibro-impact drilling system is undertaken in this study, and the results of the numerical analysis and comparison between two selected models are presented. The first one is a newly developed model of an existing experimental rig (three masses model) and the second one is a simplified low dimensional model (Pavlovskaia et al., 2001 [22]) created to describe the dynamic interaction between the drill-bit and the drilled formation. The optimal loading parameters are identified in this work based on the analysis of the system responses, and the influence of the additional degrees-offreedom on the loading optimisation strategy is investigated. Three main control parameters are considered, and they are the applied static force, and the amplitude and the frequency of the applied dynamic force.Our investigations confirm that the best progression rates are achieved when the system response is periodic and the frequency of the response is the same as the frequency of the applied dynamic force, and the value of the static force is smaller than the amplitude of the applied dynamic excitation. This result is valid for both models considered. Also in both cases, zero progression rates are obtained for lower values of the excitation amplitudes and the average progression increases with the increase in the dynamic amplitudes. Both models also predict zero progression rates at the higher excitation frequencies.Based on the analysis undertaken, it can be concluded that the low dimensional model provides good estimates of the optimal static force and the amplitude of the dynamic force, and it could be used for the operational control of the drilling system to adjust the loading parameters while drilling through different formations. The choice of the optimal frequency, however, should be made based on the predictions of the more detailed model of the drilling system as additional degrees of freedom significantly influence the structure of the internal resonances and they should be taken into account.

Conventional and digital holographies are proving to be increasingly important for studies of marine zooplankton and other underwater biological applications. This paper reports on the use of a subsea digital holographic camera (eHoloCam) for the analysis and identification of marine organisms and other subsea particles. Unlike recording on a photographic film, a digital hologram (e-hologram) is recorded on an electronic sensor and reconstructed numerically in a computer by simulating the propagation of the optical field in space. By comparison with other imaging techniques, an e-hologram has several advantages such as three-dimensional spatial reconstruction, non-intrusive and nondestructive interrogation of the recording sampling volume and the ability to record holographic videos. The basis of much work in optics lies in Maxwell's electromagnetic theory and holography is no exception: we report here on two of the numerical reconstruction algorithms we have used to reconstruct holograms obtained using eHoloCam and how their starting point lies in Maxwell's equations. Derivation of the angular spectrum algorithm for plane waves is provided as an exact method for the in-line numerical reconstruction of digital holograms. The Fresnel numerical reconstruction algorithm is derived from the angular spectrum method. In-line holograms are numerically processed before and after reconstruction to remove periodic noise from captured images and to increase image contrast. The ability of the Fresnel integration reconstruction algorithm to extend the reconstructed volume beyond the recording sensor dimensions is also shown with a 50% extension of the reconstruction area. Finally, we present some images obtained from recent deployments of eHoloCam in the North Sea and Faeroes Channel.