## 4 Results and Discussion
### Coherent Noises Associated with the Impact of Medium Inhomogeneities during Hologram Recording  
In particle holography, it is assumed that when a hologram of the volume of the medium with particles is recorded, its holographic image is reconstructed layer-by-layer, the best image planes of the particles are determined, and the geometric parameters of the particles (size, shape, position in space) are analyzed. The optical properties of the particles (transmittance, refractive index) differ from the parameters of the medium, and the medium itself is optically homogeneous. In real media, the degree of optical homogeneity may be low, which may occur in the case of sea or lake water and make it difficult to use a submersible DHC to study plankton and other particles [23; 36].  
The optical inhomogeneity of the medium with studied particles may be caused by the inclusions of other phases of a substance (microbubbles in water, surface defects, suspensions of microparticles of a different nature; Figure 14a), as well as variations in the refractive index of the medium caused by disturbances of optical constants (turbulence from propellers, thermohaline currents, upwelling, temperature gradients, etc.; Figure 14b).  
Figure 14: Examples of digital holograms recorded in field conditions aboard AMK. (**a**) Small-scale granularity caused by multiple interference of light waves formed as a result of scattering on microinhomogeneities of an aqueous medium (speckle structure). (**b**) Turbulent flows from AMK thrusters (example of a spotted structure associated with refractive index modulation).  
Figure 13: Processing of a hologram recorded by the DHC aboard Academician [PERSON] (AMK) in the Lapteve Sea in October 2020. (**a**) Camera; (**b**) digital hologram (red color marks the area of interest for further processing); (**c**) reconstructed image without eliminating the twin-image effect; (**d**) result of suppressing the twin-image effect by the spatial-frequency filtering (described in Section 3.2).  
Thus, there are noises in the image of a particle reconstructed from a hologram associated with the influence of medium inhomogeneities (wave aberrations and phase shifts caused by them) during hologram recording by a coherent radiation source. Such noises may be stationary if the cause does not change over time, but they are most often non-stationary.  
There are several situations that require the suppression of such noises:  
* Averaging using a multispectral radiation source at the stage of digital hologram recording [5; 6];
- Averaging using a multispectral radiation source at the stage of particle image reconstruction from a digital hologram [37; 38];
- Averaging over several digital holograms [12];
- Averaging over several reconstructed holographic images [4; 39].
* Non-stationary noises and a moving object: limits on the geometry of the holographic experiment.  
Despite the difference in the physical principles of generating such noises, they are united by the random nature of a phase shift, so the methods of suppression can be the same and based on the generation of random changes in the phases of interfering waves, followed by averaging the resulting intensity distributions. Therefore, this section considers approaches and some practical issues to improve the image using the example of averaging holograms and holographic images, as well as determining a reasonable number of averaging scenarios to achieve the desired image quality. Besides, the paper presents the averaging results for a non-stationary object with a justification of the limits on the geometry of the holographic experiment.