Robert A. Guyer

Determination of elastic moduli of rock samples using resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy (RUS) is a method whereby the elastic tensor of a sample is extracted from a set of measured resonance frequencies. RUS has been used successfully to determine the elastic properties of single crystals and homogeneous samples. In this paper, we study the application of RUS to macroscopic samples of mesoscopically inhomogeneous materials, specifically rock. Particular attention is paid to five issues: the scale of mesoscopic inhomogeneity, imprecision in the figure of the sample, the effects of low Q, optimizing the data sets to extract the elastic tensor reliably, and sensitivity to anisotropy. Using modeling and empirical testing, we find that many of the difficulties associated with using RUS on mesoscopically inhomogeneous materials can be mitigated through the judicious choice of sample size and sample aspect ratio.

Three component time reversal: Focusing vector components using a scalar source

In acoustics, it is known that, for a given response signal at an arbitrary location, reciprocity and time reversal (TR) can be used to focus high levels of acoustic energy at that position. In solid media, elastic waves generally induce different disturbances in three directions. In this paper, both experimental and numerical wave propagation results for solid materials demonstrate the ability to use a scalar source, a three component detector and the reciprocal TR process to selectively focus each of the different vector components, either individually or collectively. The principle is explained from an analytical point of view. The numerical and experimental study demonstrates excellent temporal and spatial focalization. Applications of the selective vector component focusing can be found in damage imaging techniques using both linear or nonlinear ultrasonic waves.

Modelling the dynamic response of bistable composite plates for piezoelectric energy harvesting

This paper presents analytical modelling and experimental characterisation of an arrangement of bistable composite plates with bonded piezoelectric elements to perform broadband vibration-based energy harvesting from ambient mechanical vibrations. These bistable devices have the potential to exhibit improved power generation compared to conventional resonant systems by exploiting nonlinear modes of oscillation driven by a ‘snap-through’ mechanism. Snap-through behaviour is shown to lead to higher average power outputs over a much broader frequency bandwidth than a resonant device. The conditions which yield these snap-through modes are investigated in terms of drive frequency and amplitude of vibration, revealing the emergence of intermittent and continuous snap-through modes for higher amplitude oscillations. These modes are found to widen the half-power bandwidth from 7Hz for linear oscillations (106mW for 4g peak acceleration) to 22Hz for high amplitude snap-through behaviour (244mW for 10g peak acceleration).

Role of hydrogen bonding in hysteresis observed in sorption-induced swelling of soft nanoporous polymers

Hysteresis is observed in sorption-induced swelling in various soft nanoporous polymers. The associated coupling mechanism responsible for the observed sorption-induced swelling and associated hysteresis needs to be unraveled. Here we report a microscopic scenario for the molecular mechanism responsible for hysteresis in sorption-induced swelling in natural polymers such as cellulose using atom-scale simulation; moisture content and swelling exhibit hysteresis upon ad- and desorption but not swelling versus moisture content. Different hydrogen bond networks are examined; cellulose swells to form water–cellulose bonds upon adsorption but these bonds do not break upon desorption at the same chemical potential. These findings, which are supported by mechanical testing and cellulose textural assessment upon sorption, shed light on experimental observations for wood and other related materials.Water uptake of natural polymers is accompanied by swelling and changes in the internal structure of the polymeric system but the exact mechanism of water-uptake and swelling remained unknown. Here the authors use atom-scale simulations to identify a molecular mechanism which is responsible for hysteresis in sorption-induced swelling in natural polymers.

Using time-reversal to locate non-volcanic tremor and to fulfill the monitoring objectives of the nuclear-test ban treaty

In this paper are presented the latest results of our group effort to apply Time Reversal (TR) to different seismology problems. The first problem considered is source location of non-volcanic tremor (NVT). NVT episodes involve quasi-continuous emissions of seismic energy making the identification of distinct events and phase arrival times very difficult. We locate 2 NVT episodes that were recorded near Hemet: one triggered by the 2002 Denali earthquake and one by the 2009 Mexicali earthquake. Locations indicate sources slightly off the known scarp of tectonic faults. In both cases, determination of the source mechanism is impossible. The second problem is determination of the depth of seismic events. There is currently no robust method to estimate the depth of small events (below Mw5.0), when depth determination is crucial in discriminating small earthquakes from man-made blast, since the blast will have to be within about 2 km of the surface where few earthquakes occur. We propose to use stacked autocor...

Chapter 12. Field Observations

Impact of biomass burning on surface water quality in Southeast Asia through atmospheric deposition: field observations P. Sundarambal, R. Balasubramanian, P. Tkalich, and J. He Tropical Marine Science Institute 14 Kent Ridge Road, National University of Singapore, 119223, Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, Engineering Drive 1, 117576, Singapore Division of Environmental Science and Engineering, National University of Singapore, Engineering Drive 1, 117576, Singapore Singapore Delft Water Alliance, National University of Singapore, Engineering Drive 1, 117576, Singapore Received: 27 February 2010 – Accepted: 12 March 2010 – Published: 25 March 2010 Correspondence to: R. Balasubramanian (eserbala@nus.edu.sg) Published by Copernicus Publications on behalf of the European Geosciences Union.

Slow nonlinear dynamics in rock

Recent experiments on the elastic response of Berea sandstone show that nonlinear excitation of a resonant bar is accompanied by a slow dynamics, a dynamics involving time scales many orders of magnitude longer than the excitation period, 2π/ω. A lumped‐element description of a resonant bar is developed. A complementary phenomenological theory of slow nonlinear dynamics is postulated. This phenomenological theory is used in the lumped‐element description to explain the experiments. The results illustrate the possibility of experimental exploration of slow nonlinear dynamics using the acoustic analogue of a NMR experiment. [Work partially supported by OBES, Engineering and Geosciences Grant No. W7405‐ENG‐36, the National Science Foundation Grant No. EAR‐9528965, and the Institute of Geophysics and Planetary Physics at Los Alamos National Laboratory.]