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2Physical Properties of Marine SedimentsFig. 2.14 Attenuation analysis by the smoothed spectral ratio method. (a) Definition of a moving window ofbandwidth b i=f ui-f li, in which the spectral amplitudes are summed. (b) Seven attenuation curves analyzed from theturbidite layer of gravity core GeoB1510-2 and linear regression to the attenuation curve in 1.82 m depth. (c)Attenuation log of gravity core GeoB1510-2 for 400 kHz frequency. Data from Breitzke et al. (1996).50
2.4 Acoustic and Elastic PropertiesAs the P-wave attenuation coefficientobviously depends on the grain size distributionof the sediment it can be used as a proxy parameterfor the mean grain size, i.e. for a sedimentologicalparameter which is usually onlymeasured at coarse increments due to the timeconsuminggrain size analysis methods. Forinstance, this can be of major importance incurrent controlled sedimentation environmentswhere high-resolution grain size logs mightindicate reduced or enhanced current intensities.If the attenuation coefficients and mean grainsizes analyzed on discrete samples of coreGeoB1510-2 are displayed as a cross plot a secondorder polynomial can be derived from a leastsquare fit (Fig. 2.15a). This regression curve thenallows to predict mean grain sizes using theattenuation coefficient as proxy parameter. Theaccuracy of the predicted mean grain sizes illustratesthe core log in Figure 2.15b. The predictedgray shaded log agrees well with the superimposeddots of the measured data.Similarly, P-wave velocities can also be used asproxy parameters for a mean grain size prediction.However, as they cover only a small range (1450 -1650 m/s) compared to attenuation coefficients (20- 800 dB/m) they reflect grain size variations lesssensitively.Generally, it should be kept in mind, that theregression curve in Figure 2.15 is only an example.Its applicability is restricted to that range ofattenuation coefficients for which the regressioncurve was determined and to similar sedimentationenvironments (calcareous foraminiferal andnannofossil ooze). For other sediment compositionsnew regression curves must be determined,which are again only valid for that specificsedimentological setting.Fig. 2.15 (a) Attenuation coefficients (at 400 kHz) of gravity core GeoB1510-2 versus mean grain sizes. The solidline indicates a second degree polynomial used to predict mean grain sizes from attenuation coefficients. (b)Comparison of the predicted mean grain size log (gray shaded) with the data measured on discrete samples (soliddots). Mean grain sizes are given in Φ = -log 2d, d = grain diameter in mm. Modified after Breitzke et al. (1996).51
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Page 43 and 44: References2.7 ProblemsProblem 1What
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