Full waveform ultrasonic transmission seismograms - a fast new method for the determination of physical and sedimentological parameters of marine sediment cores

M. Breitzke, H. Grobe, G. Kuhn, P. M\"
Fachbereich Geowissenschaften, Universität Bremen, P.O. Box 33 04 40, Bremen, D-28334, Germany

Abstract:

Detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic P-waveforms recorded automatically during full waveform core logging. The physical parameter which predominantly affects the elastic wave propagation in water-saturated sediments is the P-wave attenuation coefficient. The related sedimentological parameter is the grain size distribution. A set of high-resolution ultrasonic transmission seismograms (50-500 kHz), which indicate downcore variations in the grain size by their signal shape and frequency content, are presented. Layers of coarse-grained foraminiferal ooze can be identified by highly attenuated P-waves, whereas almost unattenuated waves are recorded in fine-grained areas of nannofossil ooze. Color-encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation. A modified spectral difference method is introduced to determine the attenuation coefficient and its power law . Applied to synthetic seismograms derived using a 'constant Q' model, even low attenuation coefficients can be quantified. A downcore analysis gives an attenuation log which ranges from 700 dB/m at 400 kHz and a power of n = 1.0 - 2 in coarse-grained sands to few decibels per metre and n < 0.5 in fine-grained clays. A least-squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient. When it is used to predict the mean grain size, an almost perfect coincidence with the values derived from sedimentological measurements is achieved.

AGU Index Terms: 3094 Instruments and techniques; 5144 Wave attenuation; 5102 Acoustic properties; 3099 General or miscellaneous
Keywords/Free Terms: Marine geophysics, physical properties of sediment cores, full waveform core logging, P-wave attenuation and grain sizes.

JGR-Solid Earth 96JB01891