All underwater acoustic devices use Time Variable Gain (TVG). Put simply, TVG is the gain in the receive cycle. TVG increases as the time increases in the receive cycle.
The first acoustic returns, usually from the nadir region, are the strongest and need the least amount of gain.
Acoustic returns that arrive later in the receive cycle require more gain.
Therefore the gain increases during the receive period.
Absorption is frequency, temperature and salinity dependent, and increases if any of these factors increase. There is an inbuilt calculator for absorption, or the user can refer to the table in the Operation Manual.
Spreading loss is only dependent on the range from the sonar head; the deeper the water, the more the spreading loss.