Robert W. Haupt

Laser vibrometry from a moving ground vehicle.

We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 μm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz(1/2)) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.

Damage inspection of fiber reinforced polymer-concrete systems using a distant acoustic-laser NDE technique

In this paper, a distant acoustic-laser NDE technique is proposed, utilizing a high powered standoff parametric acoustic array (PAA) and laser Doppler vibrometry (LDV), for the detection of debonding and delamination in multi-layer composite systems. Fiber-reinforced polymer wrapped concrete cylinder specimens with artificial defect were manufactured and used in the validation of the technique. Low-frequency (50 Hz 2 kHz) and highfrequency (2 kHz 7 kHz) focused sound waves were generated by PAA, and surface dynamic signatures of the specimens were remotely measured by LDV. From the results it is found that the proposed technique successfully captures the presence of near-surface debonding/delamination.

A numerical study of shock waves generated through laser ablation of explosives

Shock waves resulting from irradiation of energetic materials with a pulsed ultraviolet laser source have been shown to be an effective indicator for explosives detection. Here, the features of shock wave propagation are explored theoretically. The initial stage of the shock motion is simulated as a one-dimensional process. As the nonlinear wave expands to form a blast wave, a system of conservation equations, simplified to the Euler equations, is employed to model wave propagation. The Euler equations are solved numerically by the 5th order weighted essentially non-oscillatory finite difference scheme with the time integration carried out using the 3rd order total variation diminishing Runge Kutta method. The numerical results for the shock wave evolution are compared with those obtained from experiments with a meltcast 2,6-dinitrotoluene sample. The calculations lay a theoretical foundation for a recently investigated technique for photoacoustically sensing explosives using a vibrometer.

Seismic Barrier Protection of Critical Infrastructure from Earthquakes

Each year, on average, a magnitude-8 earthquake strikes somewhere in the world. In addition, 10,000 earthquake related deaths occur annually, where collapsing buildings claim most lives. Moreover, in recent events, industry activity of wastewater reinjection is suspected to cause earthquake swarms that threaten high-value infrastructure and properties. Earthquake engineering technology has evolved over many years to minimize the destructive effects of seismic waves. However, even under the best practices, significant damage and fatalities can still occur. Here we present a novel concept that redirects and attenuates hazardous seismic waves using an engineered subsurface seismic barrier. The barrier consists of borehole array and trench complexes that inhibit destructive seismic waves from entering a designated ‘protection zone’. The barrier is designed to counter not only surface waves in the aerial-horizontal plane, but employs a vertical ‘V’ shaped muffler structure composed of opposing boreholes or trenches to mitigate seismic waves from diffracting and traveling in the vertical plane. Computational seismic wave propagation models suggest that air or fluid filled subsurface Vshaped muffler structures are critical to the redirection and self-interference of broadband hazardous seismic waves in the vicinity of the structure to protect. The computational models are compared with experimental data obtained from large bench-sized models containing borehole arrays and trenches. The computer models and bench scale measurements indicate that effects of a devastating 7.0 Mw -magnitude earthquake can be attenuated to those of a minor magnitude-4.5 or -5.5 Mw earthquake within a specified protection zone.

Non-contact laser ultrasound concept for biomedical imaging

The potential of a fully noncontact, standoff, laserultrasound system that acquires ultrasonic images within biological tissue is examined. A pulsed laser converts optical energy into ultrasound via photoacoustic mechanisms, while laser Doppler vibrometry measures emerging ultrasonic waves at the tissue surface. Differing from photoacoustic tomography (PAT), which maps spatial variations in tissue-optical absorptivity in the acoustic near field, the laser ultrasound (LUS) approach developed here, is driven by shallow, non-varying optical absorptivity that creates a laterally consistent acoustic source enabling ultrasound propagation well into the far field. LUS acoustic wave generation is explored in tissue and bone including longitudinal, shear, and Rayleigh wave components. Using information from LUS wave types can yield 1) tissue and bone anatomical images and 2) mechanical property distributions that apply to the emerging field of medical elastography. Imaging capabilities using a demonstration LUS system are also presented for complex bio-tissues. Ultrasonic images compare well with ground truth geometries, orientation, and depth of staged samples. 2D cross-sectional echo reflection images are generated for a phantom limb containing muscle and bone materials and use data inversion techniques to yield the elastic moduli distributions in the specimen.

Seismic barrier protection of critical infrastructure

Each year, on average a major magnitude-8 earthquake strikes somewhere in the world. In addition, 10,000 earthquake related deaths occur annually, where collapsing buildings claim by far most lives. Moreover, in recent events, industry activity of oil extraction and wastewater reinjection are suspected to cause earthquake swarms that threaten high-value oil pipeline networks, U.S. oil storage reserves, and civilian homes. Earthquake engineering of building structural designs and materials have evolved over many years to minimize the destructive effects of seismic surface waves. However, even under the best engineering practices, significant damage and numbers of fatalities can still occur.

Standoff acoustic-to-seismic landmine detection

This paper addresses a potential method to advance acoustic landmine detection by increasing operator and equipment standoff range from the minefield and developing a lightweight system that is potentially more practical than many currently researched systems. In this study, a parametric array acoustic source is evaluated to understand its potential for landmine detection. The array can transmit audible signals over 100 meters in air and has a weight of just four pounds. A proof-of-concept system was built at M.I.T. Lincoln Laboratory that uses a commercial parametric source to insonify the ground and excite buried mines. A commercial laser vibrometer was then used to measure the displacement velocity at the ground surface on and off the mine. This system has been demonstrated at an outdoor landmine facility and has measured signatures from buried anti-personnel mines. The overall concept shows promise; however, the parametric source used in this preliminary test was developed for home entertainment and will require substantial modification to be practical for landmine detection.