Laboratory measurements of seismic attenuation and Young’s modulus dispersion in a partially and fully water-saturated porous sample made of sintered borosilicate glass
S. Chapman, B. Quintal, K. Holliger, L. Baumgartner and N. Tisato
Journal name: Geophysical Prospecting
Issue: Vol 66, No 7, September 2018 pp. 1384 - 1401
Info: Article, PDF ( 7.33Mb )
We measured the extensional-mode attenuation and Young’s modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to 99%, and by gas exsolution from an initially fully water-saturated state down to 99%. During forced oscillations of the sample effective stresses up to 10MPa were applied. We observe frequency-dependent attenuation, with a peak at 1–5 Hz, for 99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave-induced gas exsolution–dissolution. At full water saturation, the left-hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave-induced fluid flow associated with mesoscopic compressibility contrasts in the sample’s frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced-oscillation experiments in response to the applied stresses. The agreement of the measured frequency-dependent attenuation and Young’s modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.