14. Laboratory Measurements of Velocity and Attenuation in Sediments

Abstract

Laboratory measurements are required to establish relationships between the physical properties of unconsolidated sediments and P-and S-wave propagation through them. Previous work has either focused on measurements of compressional wave properties at depths greater than 500 m for oil industry applications or on measurements of dynamic shear properties at pressures corresponding to depths of less than 50 m for geotechnical applications. Therefore, the effects of lithology, fluid saturation, and compaction on impedance and P-and S-wave velocities of shallow soils are largely unknown. We describe two state-of-the-art laboratory experiments. One setup allows us to measure ultrasonic P-wave velocities at very low pressures in unconsolidated sediments (up to 0.1 MPa). The other experiment allows P-and S-wave velocity measurements at low to medium pressures (up to 20 MPa). We summarize the main velocity and attenuation results on sands and sand-clay mixtures under partially saturated and fully saturated conditions in two ranges of pressures (0-0.1 MPa and 0.1-20 MPa) representative of the top few meters and the top 1 km, respectively. Under hydrostatic pressures of 0.1 to 20 MPa, our measurements demonstrate a P-and S-wave velocity-dependence in dry sands around a fourth root (0.23-0.26) with the pressure dependence for S-waves being slightly lower. The P-velocity-dependence in wet sands lies around 0.4. The Vp-Vs and the Qp-Qs ratios together can be useful tools to distinguish between different lithologies and between pressure and saturation effects. These experimental velocities at the frequency of measurement (200 kHz) are slightly higher that Gassmann's (Gassmann, 1951) static result. For low pressures under uniaxial stress, Vp and Vs were a few hundred meters per second with velocities showing a strong dependence on packing, clay content, and microstructure. We provide a typical shallow soil scenario in a clean sand environment and reconstruct the velocity profile of such a sediment packet.

Characterization of the shallow aquifers by high‐resolution seismic data

“…Assuming the same physical properties for both main aquifers (layers 2 and 4; Table 1), theoretical velocities versus pore pressure has been computed in order to verify the possibility to detect overpressures through seismic data analysis. Laboratory measurements indicate the following relationship between compressional ( V P ) and shear ( V S ) velocities and effective pressure p e, which is the confining pressure reduced by the pore pressure (Prasad et al 2004): where the coefficients a i and b i are obtained by fitting the model parameters to the V P and V S values reported in Table 1 and c 1 was fixed equal to the value proposed by Prasad et al (2004). The values of a 1 and b 1 obtained from the fit are 973.4 and 985.2 respectively, while a 2 and b 2 are equal to 662.6 and 0.349 respectively.…”

“…Assuming the same physical properties for both main aquifers (layers 2 and 4; Table 1), theoretical velocities versus pore pressure has been computed in order to verify the possibility to detect overpressures through seismic data analysis. Laboratory measurements indicate the following relationship between compressional (V P ) and shear (V S ) velocities and effective pressure p e, which is the confining pressure reduced by the pore pressure (Prasad et al 2004):…”

“…where the coefficients a i and b i are obtained by fitting the model parameters to the V P and V S values reported in Table 1 and c 1 was fixed equal to the value proposed by Prasad et al (2004). The values of a 1 and b 1 obtained from the fit are 973.4 and 985.2 respectively, while a 2 and b 2 are equal to 662.6 and 0.349 respectively.…”

P- and S-wave velocity models of shallow dry sand formations from surface wave multimodal inversion

“…1776-1781, doi: 10.1190 surface velocity models, such as P-wave tomography, can perform poorly on loose granular formations, due to their low quality factor, which causes high attenuation (Prasad et al, 2004;Carpentier et al, 2012). The poor quality factor of soft, uncompacted granular media affects indeed the propagation of SWs as well.…”

A semi‐empirical approach to model pressure dependence of elastic moduli in granular media accounting for variations of coordination‐number and Poisson‐ratio

“…Other authors found similar high values of Poisson's ratio for dry glass beads and sands (Prasad and Meissner 1992;Jia and Mills 2001;Kuwano and Jardine 2002). In the figure, we plot upper and lower bounds for Poisson's ratio suggested by Prasad et al (2004) as reference for dry granular systems. The majority of the data presented here lay inside this range.…”