The distance between one crest of a wave and the next. The frequency is the number of time is a second that a wave advances by one wavelength. Hz is the most useful. Once generated the frequency of a wave cannot vary. This means that a seismic wave has half the amplitude at 200m as it does at 100m.
Absorption is the other reason for loss of energy.
This is the transfer of wave energy to heat. The loss through absorption is the same for each wavelength travelled. Thus longer wavelengths suffer less proportionally. The limitation of seismic is its resolution, which is controlled by the size of the seismic wave. This will vary, but as a wave propagates into the earth, the higher frequencies are absorbed, leaving the lower longer frequencies, and thus reducing resolution.