Relationships among compressional wave attenuation, porosity, clay content, and permeability in sandstones

TLDR

Anelastic attenuation converts compressional waves into heat, altering their amplitude and phase, and understanding its causes is essential for seismic data acquisition, processing, interpretation, and for inferring rock physical properties. The study measured compressional wave attenuation in 42 sandstones at 40 MPa and 0.5–1.5 MHz using a pulse‑echo method with perspex buffer rods, estimating attenuation from the logarithmic spectral ratio of reflected signals corrected for beam spreading. The results show that at 1 MHz and 40 MPa, attenuation correlates strongly (R = 0.88) with porosity and clay content (α = 0.0315ϕ + 0.241C − 0.132), and that samples with permeability below 50 md exhibit high attenuation (>1 dB/cm) while those above 50 md show low attenuation (<1 dB/cm).

Abstract

Anelastic attenuation is the process by which rocks convert compressional waves into heat and thereby modify the amplitude and phase of the waves. Understanding the causes of compressional wave attenuation is important in the acquisition, processing, and interpretation of high‐resolution seismic data, and in deducing the physical properties of rocks from seismic data. We have measured the attenuation coefficients of compressional waves in 42 sandstones at a confining pressure of 40 MPa (equivalent to a depth of burial of about 1.5 km) in a frequency range from 0.5 to 1.5 MHz. The compressional wave measurements were made using a pulse‐echo method in which the sample (5 cm diameter, 1.8 cm to 3.5 cm long) was sandwiched between perspex (lucite) buffer rods inside the high‐pressure rig. The attenuation of the sample was estimated from the logarithmic spectral ratio of the signals (corrected for beam spreading) reflected from the top and base of the sample. The results show that for these samples, compressional wave attenuation (α, dB/cm) at 1 MHz and 40 MPa is related to clay content (C, percent) and porosity (ϕ, percent) by α=0.0315ϕ+0.241C−0.132 with a correlation coefficient of 0.88. The relationship between attenuation and permeability is less well defined: Those samples with permeabilities less than 50 md have high attenuation coefficients (generally greater than 1 dB/cm) while those with permeabilities greater than 50 md have low attenuation coefficients (generally less than 1 dB/cm) at 1 MHz at 40 MPa. These experimental data can be accounted for by modifications of the Biot theory and by consideration of the Sewell/Urick theory of compressional wave attenuation in porous, fluid‐saturated media.