Modern multibeam echosounders (MBE) employ frequency-division techniques (FDT) to ensonify multiple sectors within the same ping cycle. This leads to improved performance in coverage rate, and yaw and pitch stabilization. However, it introduces a bias among sectors because MBES systems are frequency dependent. It also reduces the maximum pulse bandwidth compared to a single-sector sonar. In this study, we consider the code-division technique (CDT) as a solution to this problem. A set of orthogonal coded pulses are received within the same frequency band, and each sector is separated with a matched filter. We assess the feasibility of the technique through two stages. 1) First, we formulate an analytical model describing the power and crosstalk budgets of any multisector MBES. The model can then be used to design transmission sequences fitting these budgets. 2) Then, we display the practical usage of the technique for MBES imaging and mapping through simulated case studies. For the same total timebandwidth budget, we compare the performance of FDT, CDT, and multicarrier CDT (MC-CDT), a hybrid method employing CDT and FDT, which is robust to strong dynamic backscatter. This study considers only bottom detection based on signal amplitude. Our results show that it is possible to share a larger frequency bandwidth between multiple sectors while maintaining an acceptable bottom detection performance similar to FDT. Our choice of time-bandwidth product with CDT offers a crosstalk suppression of −25 dB between sectors, but may display low-magnitude residual artefacts in the water-column data. MC-CDT provides a significant gain of pulse bandwidth while it offers interband separation performance comparable to FDT and reduces significantly the water-column artefacts.
Small waveform and directivity variations of marine airgun signatures due to waves interacting with the source float are a source of 4D noise. We are assessing the magnitude of this noise by first measuring the amount of variability from near-source auxiliary data and then modeling synthetic time-lapse ocean bottom seismic data with realistic source variations based on the measured statistics and standard ocean wave models. We quantify the contribution of source variations to 4D noise as a function of sea state by calculating the NRMSD attribute in the image domain. We find that up to 4% NRMSD can be attributed to source variations under realistic scenarios, with two main contributing effects: variations of individual gun signatures due to pressure changes, and array directivity variations due to the wave-induced pitch and roll of the source floats. The latter effect has a larger impact on the 4D noise in our simulations and depends more on the wave steepness rather than the wave height. While waveform variations can be addressed by a nearfield-based shot-by-shot designature, directivity variations are difficult to correct without knowledge of the sea surface shape.
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