The present invention describes a monoblock for a plurality of lithium ion cells. The method includes a step of performing a chemical mechanical planarization processing on a poly-silicon layer before fabricating a poly-silicon gate such that the poly-silicon gates obtained in subsequent fabrication process are kept at the same height, which thus avoids the silicon nitride residues issue that occurs in the prior art. The echoes are resampled at a sampling frequency which takes into account the depth of the water, sediment conditions generally in the area and the duration of the ping, resulting in an improved classification of the seabed according to the selected attribute. An outlet passage extends through the housing into one of the first and second chambers.