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The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.

Edit: this can be done even in air: have a time with same amplitude be played to system before and after deployment keeping same geometry.

The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.

Edit: this can be done even in air: have a time with same amplitude be played to system before and after deployment keeping same geometry.

Edit2: this applies also to hydrophones, even if sensitivity is not as high as in water, but for the OP (changes in sensitivity) only the difference matter.

Edit3: Obviously, using a single tone only implies that changes in sensitivity are not spectral but effect whole spectrum in the same way, Otherwise multiple tones with different frequencies are required.

The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.

The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.

Edit: this can be done even in air: have a time with same amplitude be played to system before and after deployment keeping same geometry.

Not knowing why sensitivity changes (as discussed in comments to OP) and one cannot develop a model describing the temporal functionality of this degradation, one can only assume a linear degradation as function of time.

So, if calibration before deployment differs from calibration after deployment, one must assume that calibration changed linearly with time.

The only way to know if calibration has changes is to calibrate the full system before and after deployment. This can be done for a single frequency, e.g. 2 kHz and has not to be done for the whole frequency range.