Dispersion Curves of Transverse Waves Propagating in Multi-Layered Soils from Experimental Tests in a 100 m Deep Borehole

Abstract

The estimate of the velocity of shear waves (Vs) is essential in seismic engineering to characterize the dynamic response of soils. There are various direct methods to estimate the Vs. The authors report the results of site characterization in Macerata (Italy), where they measured the Vs using the seismic dilatometer in a 100 m deep borehole. The standard Vs estimation originates from the cross-correlation between the signals acquired by two geophones at increasing depths. This paper focuses on the estimate of the dependence of Vs on the wavenumber. The dispersion curves reveal an unexpected hyperbolic dispersion curve typical of Lamb waves. Interestingly, the contribution of Lamb waves may be notable up to 100 m depth. The amplitude of surface waves decrease rapidly with depth; still, their influence may be essential up to depths considered unusual for standard geotechnical investigations, where their effect is generally neglected. Accordingly, these waves may bias the outcomes of the standard Vs estimations, which ignore frequency-dependent phenomena. The paper proposes an enhancement of the accepted procedure to estimate Vs and addresses the importance of Lamb waves in soil characterization.

Head Waves in Modified Weiskopf Sandy Medium

“…As the presented literature review shows, the problem of bulk wave propagation in a laminated anisotropic halfspace is analyzed apparently for the first time; previous studies are mainly related to the bulk wave propagation in either isotropic halfspaces, e.g., [36], or to the dispersive SAW (surface acoustic waves) in anisotropic layered media [39,[51][52][53][54]. Analyzing the peaks related to the arrival of P and SP waves in both sandy media shows that the peaks arrive at the same time and have almost identical magnitudes.…”

Acoustic Sounding of Hydraulic Fractures in a Low-Permeability Reservoir

“…Here the plus sign corresponds to J = O, G, the minus sign does to J = R . By the second boundary condition (18) and in view of expression (24), we can obtain the expression for the reflection coefficient…”

“…The present work takes into consideration the radial geometry while describing the filtration into the reservoir and a hydraulic fracture perpendicular to the wellbore, and it is a continuation and a generalization of the cited works. In contrast to the known methods in the seismic exploration and acoustic emission [18,19] used in practice for studying the filtration characteristics of the reservoir, the diagnostic wave is generated and propagates in a hydrocarbon liquid with an informative frequency range determined by the characteristics of the channel and the liquid in it, and it is approximately 160-16,000 Hz. The proposed method of the acoustic TV set is new for diagnosing the quality of the made hydraulic fracture in the reservoir in low-permeability reservoirs.…”