Buried Inclusions Research Articles

Impulse backscattering in granular beds: Introducing a toy model

Impulses efficiently propagate into nominally dry granular beds and backscatter from buried inclusions in such beds may be potentially exploited to image shallow buried objects (SBOs). However, reliable imaging of SBOs requires "cleaning up" of surface vibrations, and, in addition to three-dimensional (3D) particle dynamics simulations, a phenomenological model to parametrize the bed surface may be useful for field applications. We introduce a 1D mean-field-like toy model with two parameters, which allows one to model surface vibrations, is consistent with experiments in a granular bed, and can help estimate the approximate signal transmission properties of the bed.

Amplification of elastic waves due to a point source in the presence of complex surface topography

This paper first presents the amplification or de-amplification in the three-dimensional wave scattering generated by the half-space when subjected to a dilatational point load placed at some point in the medium, when the free surface is changed to simulate a two-dimensional ridge or a valley deformation. This model is then extended to include the presence of a long, circular, buried cavity. The solution is formulated using the boundary elements method for a wide range of frequencies and spatially harmonic line loads, which are subsequently used to obtain time series by means of (fast) inverse Fourier transforms into space–time. Our simulation results indicate larger wave amplification and de-amplification in the time and frequency domains when the topographic surface includes a smooth ridge and canyon deformations. The presence of a buried inclusion further complicates the wave field displacement pattern, including additional reflections.

Nonlinear acoustics in granular assemblies

We present a brief review of the problem of acoustic propagation in granular media and discuss recent progress on nonlinear acoustics in granular media. The presentation emphasizes the solitary wave like properties of impulse propagation in granular media at vanishingly small loading conditions and discusses the possible spectroscopic applications of nonlinear impulse acoustics in the detection of buried inclusions.

Backscattering of Nonlinear Acoustic Impulses from Buried Inclusions in Granular Beds

ABSTRACTNonlinear acoustic impulses travel as weakly dispersive bundles of energy through 3-D, dry, granular assemblies. Such impulses can therefore be exploited to probe for buried objects in 3-D granular assemblies. In this article, we shall discuss the details of propagation of an impulse generated at the surface of a granular bed. The role of polydispersity will be briefly addressed. The calculations suggest that backscattered data, as received at predetermined locations at the surface, may be helpful in reconstructing an approximate image of the buried inclusion, as viewed from above.

40Ar- 39Ar laser probe studies of clinopyroxene inclusions in eclogitic diamonds

40Ar- 39Ar laser probe ages are reported for individual clinopyroxene inclusions in thirty-two eclogitic diamonds from four localities: Jwaneng and Orapa (both in Botswana), Udachnaya (Siberia), and Argyle (Australia). Ages obtained from cleaved diamonds from Jwaneng (244 Ma), Orapa (102 Ma), and Udachnaya (425 Ma) are similar to those of their host kimberlites. However, there is considerable variation in age between individual laser extractions obtained from the same inclusion, which we suspect is mainly due to partial retention of radiogenic argon that diffused to the inclusion/diamond interface while the diamond was in the mantle. Some of this argon was lost when the diamonds were cleaved as part of the experimental procedure. The presence of an ambient mantle 40Ar component trapped in inclusions may be an additional cause of scatter in ages; however, this cannot be validated with the present data. In support of the diffusive loss process, the results from an Udachnaya inclusion completely enclosed within host diamond, with the argon being released by laser drilling, yielded an age of 1149 ± 37 Ma, which is considerably older than the host kimberlite (about 340–400 Ma). Likewise, the scatter in 40Ar K ratios for eclogitic diamonds from Jwaneng and Orapa indicates that they too are probably xenocrysts. Therefore, a combination of sample preparation technique and laser drilling can be used to offer a useful extension of the original method, enabling both diamond formation (from buried inclusions) and host rock emplacement ages (from cleaved inclusions) to be determined. Argyle clinopyroxenes, similarly cleaved to the diamond surface, also show a considerable range of inclusion ages (1159–1540 Ma); only in this case, there is a relationship between age and K Ca ratio of the inclusions. Such a relationship is difficult to reconcile in terms of partial loss of 40Ar and, instead, indicates that there was a protracted period of diamond growth.

Slowness-time and tau-slowness spectra for signals scattered for boundary reverberation from the seafloor

The spatial properties of reverberation from the seafloor is often characterized both theoretically and experimentally in terms of a directional, or slowness, spectrum. These slowness spectra of signals scattered from the seafloor determine the power that propagates to become long-range reverberation. Slowness-time spectra are essentially short-time-duration directional spectrum estimates and are used when the excitation signals are transient such as a shot or a short pulse. Slowness-time spectra and their transformation to τ − p (intercept time-slowness) spectra are very powerful analysis methods for a array data since they incorporate both slowness and travel time estimates of the scattered energy and identify scattering sources. To illustrate the power of τ − p spectra in the interpretation of boundary reverberation, two different numerical approaches are applied. A finite-difference approach is applied to generate the time-domain array response, which is then subsequently transformed into the τ − p domain and compared to direct simulations of the τ − p response using a hybrid boundary element-wave-number integration algorithm. This comparison clearly illustrates the signal-processing issues involved in estimating the τ − p representation of real reverberation data. The τ − p spectra for signals scattered from (i) a rough seafloor and (ii) a buried inclusion in a stratified seabed have been estimated and used to analyze the energy partitioning from the various propagation modes and the locus of the dominant scattering centers.

Acoustic Imaging of Subsurface Features

The characterization of sites containing buried hazardous wastes and the selection of new waste disposal sites have become important problems for environmental engineers. Because of the potential risks associated with uncertainty of the subsurface environment, increasing effort has been dedicated to application of available remote sensing techniques. One such technique which is quantitative and offers high resolution is geophysical tomography (GT). Two approaches to GT are examined: backprojection, which traces straight ray paths, and backpropagation, which inverts the wave equation. Results of field studies demonstrate that buried inclusions having length scales on the order of 0.5 m can be resolved using GT and that identification of differing buried waste forms is possible. This method has been successfully demonstrated for a number of applications and shows great promise for many more.

Rayleigh wave interaction with inhomogeneities—Part II: The buried cavity and inclusion

The elastodynamic interaction between an explosively generated Rayleigh pulse and a buried imperfection such as a cavity or an inclusion in a half plane is investigated. Dynamic photoelasticity was employed to obtain full field information for data analysis. Measurements of the stress distribution along the free boundary of the half plane and the cavity and along the cavity/inclusion interface have been made. Results show that fractures occur for shallow as well as for deeper burial depths initiating from the cavity boundary.