Q measurements from compressional seismic waves in unconsolidated sediments

Abstract

Several processing methods in both time and frequency domains have been used to compute the in‐situ specific dissipation function 1/Q, or its inverse, the quality factor Q, in water‐saturated unconsolidated sediments. These methods are based on measurements of spectral amplitude ratio, peak‐to‐peak and first‐peak amplitude ratio, rise time, pulse broadening, and the Futterman causal attenuation operator of an attenuating signal. Compressional seismic waves were generated from explosive sources ranging in size from 1 to 64 mg of silver azide and recorded at a depth of 7.62 m below the surface near Wendover, Utah. The medium consisted of silty sandy clays with mean grain size diameter of 7.48 μm and water saturation of 70 percent. The hydrophone receivers were spirally distributed at distances ranging from 25 to 200 m from the source. An average Q value of 26 was obtained using analysis of spectral amplitude ratios over the frequency range 450–725 Hz for the five different sizes of explosive sources. Measurements of peak amplitude ratio produced an average Q value of 123. The computed Q value from the rise‐time method is found to have a wide range of Q values, from 50 to 207. The Q values determined from the pulse‐broadening technique ranged from 25 to 158 for quarter‐cycle measurements and from 26 to 114 for half‐cycle measurements. The Futterman causal attenuation operator yields high Q values, on the order of 200 to 300. The wide variation of the computed Q values indicates that different computational techniques can result in different Q values for the same type of materials. Results show that the rise‐time and pulse‐broadening methods are probably source‐dependent. This makes the spectral amplitude‐ratio method preferable to these two methods since it is independent of the source. A correct geometrical spreading factor appears to be sufficient to account for the observed amplitude decay with distance, which makes the Q value computed from the Futterman operator questionable. This suggests that the Q value computed from the spectral‐ratio method is probably more reliable than values computed by other techniques.