Here is a simplified schematic of the green laser pulse waveform from lidar. Both surface return and bottom return times are picked at the half-peak amplitude. The volume backscatter is caused by the light reflecting off particles and colored dissolved organic material (CDOM) in the water column.
In reality, these curves would not be smooth but would actually be quite noisy. As I start to work with some of the lidar data, I will post more information about the waveforms.