An algorithm for quantitatively modeling reflected ultrasonic bounded pulses and beams.

The study of the reflected acoustic waves plays an important role in our understanding of media. We provide an algorithm to propagate the ultrasonic bounded beam source and study its reflection from any horizontal and homogenous water-solid boundary. This algorithm implements a hybrid combination of the phase-advance wavefield continuation in the frequency domain and the complex analytic solution for the acoustic reflectivity. The peak amplitude of the specularly reflected beam is in agreement with the laboratory measured acoustic reflection from water-Aluminum and water-Copper alloy boundaries. The algorithm is able to model the observed critical reflection as well as the null in the reflected amplitude at the Rayleigh critical angle from the acoustic wave. This algorithm is a crucial tool to understand the full reflected wave from material immersed in water in any azimuthal or incidental angles. The software of this algorithm and acoustic reflectivity from both solid materials are provided.

The formation of peak rings in large impact craters.

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

A versatile facility for laboratory studies of viscoelastic and poroelastic behaviour of rocks.

Novel laboratory equipment has been modified to allow both torsional and flexural oscillation measurements at sub-microstrain amplitudes, thereby providing seismic-frequency constraints on both the shear and compressional wave properties of cylindrical rock specimens within the linear regime. The new flexural mode capability has been tested on experimental assemblies containing fused silica control specimens. Close consistency between the experimental data and the results of numerical modelling with both finite-difference and finite-element methods demonstrates the viability of the new technique. The capability to perform such measurements under conditions of independently controlled confining and pore-fluid pressure, with emerging strategies for distinguishing between local (squirt) and global (specimen-wide) fluid flow, will have particular application to the study of frequency-dependent seismic properties expected of cracked and fluid-saturated rocks of the Earth's upper crust.

Quantitative modeling of reflected ultrasonic bounded beams and a new estimate of the Schoch shift.

The wavefields of bounded acoustic beams and pulses reflected from water-loaded plates are fully modeled with the phase advance technique. The wavefield produced at the source is propagated at any incidence angle using phase shift modeling that incorporates the full analytic solution for the acoustic reflectivity at the interface. This approach provides for the ready visualization of both the stationary monofrequency beam wavefield and animation of the temporally bounded pulse. The model images are reminiscent of the classic Schlieren photographs that first illustrated the nonspecular behavior of the reflected beams incident near critical angles. Various phenomena such as the lateral displacement and the null zone at the Rayleigh critical angle are recreated. A new approximation for this shift agrees well with that of the peak energy of the reflected beam. Similar effects are observed during the reflection of a bounded pulse. Although more computationally costly than existing analytic approximations, the phase advance technique can facilitate the interpretation of reflectivity measurements obtained in laboratory experiments. In particular, the full visualization allows for a better understanding of the behavior of reflected waves at any angle of incidence.

A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration.

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.