In this paper, we report a novel approach based on a lensless Synthetic Optical Holography (SOH) that is aimed to recover the complex scattered field from buried surfaces at different wavelengths with sub-nanometric spectral resolution, without affecting the phase retrieval in depth. The proposed technique is applied to characterize and image the field scattered from a rough embedded surface of a microfluidic channel. The real and imaginary part of the random complex field revealed the presence of 2D optical vortices at each location in which a phase singularity is located. A statistical study of optical vortices is presented and the high spectral resolution is exploited to study the behavior of topological charges with the frequency shift.