Page 283 - Acquisition and Processing of Marine Seismic Data

274 5. PREPROCESSING

FIG. 5.33 Schematic illustration of the attenuation effect on a sine wave. (A) Input monofrequency sinusoid, after
(B) spherical divergence and (C) absorption effects. T and A represent period and amplitude of the sinusoid, respectively.
Only amplitude decay occurs in the case of the spherical divergence effect, while both amplitude decay and frequency
decrease occur due to the absorption effect. Here, T 1 ¼ T 2 < T 3 and A 1 > A 2 > A 3 .

e αr absorb the signal quickly, and this causes serious
ArðÞ ¼ A 0ðÞ (5.6)
r issues in land seismic surveys because the surfi-
cial weathered soil layer does not only absorb the
Eq. (5.6) suggests several implications: first, signal, but also creates lateral velocity variations.
higher frequency components are absorbed To overcome this issue, dynamite is located into
more quickly with respect to the lower fre- the shallow bore holes penetrating the base of the
quency components of the signal. Therefore, surficial weathered layer.
higher frequency amplitudes of the propagating Although fluid saturation and free gas con-
signal die rapidly, the period (and hence the tent of the pore spaces contribute to absorption,
wavelength) of the signal increases and the dom- the primary reason is the transformation of the
inant frequency decreases. In addition, the kinetic energy into heat energy due to the fric-
absorption of the signal becomes higher as the tion of the rock particles during the wave pas-
absorption coefficient increases. For a constant sage. The heat emerging during the
velocity subsurface, the absorption coefficient compression when the acoustic wave travels
of the medium is mainly controlled by the rock through a medium is consumed during the dila-
quality factor, Q, which has a relatively small tation and cannot be recovered. The longer the
value for weathered layers while it is larger for signal travels, the higher the effect of the absorp-
massive and compact media. Weathered layers tion is, and therefore reflections from deeper