Sean K. Lehman

Superresolution planar diffraction tomography through evanescent fields

We consider the problem of noninvasively locating objects buried in a layered medium such as land mines in the ground or objects concealed in a wall. In such environments, the transmitter(s) and receiver(s) are frequently within the near‐field region of the illuminating radiation. In these cases, the scattered evanescent field carries useful information on the scattering object. Conventional diffraction tomography techniques neglect, by their design, the evanescent field. Under near‐field conditions, they treat it as noise as opposed to valid data. If correctly incorporated into a reconstruction algorithm, the evanescent field, which carries high spatial frequency information, can be used to achieve resolution beyond the classical limit of λ/2, or “superresolution.” We build on the generalized holography theory presented by Langenberg to develop a planar diffraction tomography algorithm that incorporates evanescent field information to achieve superresolution. Our theory is based on a generalization of the Fourier transform, which allows for complex spatial frequencies in a manner similar to the Laplace transform. We specialize our model to the case of a two‐dimensional multimonostatic, wideband imaging system, and derive an extended resolution reconstruction procedure. We implement and apply our reconstruction to two data sets collected using the Lawrence Livermore National Laboratory (LLNL) Micropower Impulse Radar (MIR).

Development of a multiview time-domain imaging algorithm with a Fermat correction

An imaging algorithm is presented based on the standard assumption that the total scattered field can be separated into an elastic component with monopolelike dependence and an inertial component with dipolelike dependence. The resulting inversion generates two separate image maps corresponding to the monopole and dipole terms of the forward model. The complexity of imaging flaws and defects in layered elastic media is further compounded by the existence of high contrast gradients in either sound speed and/or density from layer to layer. To compensate for these gradients, we have incorporated Fermat’s method of least time into our forward model to determine the appropriate delays between individual source-receiver pairs. Preliminary numerical and experimental results are in good agreement with each other.

Transmission mode acoustic time‐reversal imaging for nondestructive evaluation

In previous ASA meetings and JASA papers, the extended and formalized theory of transmission mode time reversal in which the transceivers are noncoincident was presented. When combined with the subspace concepts of a generalized MUltiple SIgnal Classification (MUSIC) algorithm, this theory is used to form super‐resolution images of scatterers buried in a medium. These techniques are now applied to ultrasonic nondestructive evaluation (NDE) of parts, and shallow subsurface seismic imaging. Results are presented of NDE experiments on metal and epoxy blocks using data collected from an adaptive ultrasonic array, that is, a ‘‘time‐reversal machine,’’ at Lawrence Livermore National Laboratory. Also presented are the results of seismo‐acoustic subsurface probing of buried hazardous waste pits at the Idaho National Engineering and Environmental Laboratory. [Work performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W‐74...

Flaw localization in a structure using model‐based backpropagation.

Damage localization is an important part of structural health monitoring. In this talk we present the results of a method of localizing pointlike damage in a structure using changes in the vibrational response induced by the damage. The vibrational response is measured at discrete locations on a structure both before and after damage is induced. The difference in response is used to drive a numerical model of the undamaged structure (backpropagation). The damage location is marked by a peak in the calculated displacement field. Results are shown using laboratory measurements of a cylinder and two nested spherical half‐shells. Numerical simulation is used to demonstrate performance in more complex structures. [Work supported by the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE‐AC52‐07NA27344.]

On‐line failure detection of a vibrating structure: A model‐based approach

Model‐based failure detection is based on the principle that the MBP for a normal or pristine structure is developed first and tuned during the calibration stage assuring a statistically validated processor. Once developed, the MBP is used as the integral part in a sequential detection scheme to decide whether or not the structure under investigation is operating normally. When an abnormality is detected, a failure alarm is activated and the type of failure is classified based on a library of potential failure modes. Here we use high‐order parametric models to capture the essence of the structures over a limited frequency band known to be sensitive to structural changes. These estimated or identified models for normal operations are then used to develop the MBP which in this instance is a recursive Kalman filter. The filter is known to produce zero‐mean/white residuals when optimally tuned to the data. Failure is declared when these properties are no longer valid. Once the detection is accomplished, the n...

Passive measurements of random wave fields in an instrumented structure

A passive measurement system using fiber Bragg gratings is presented to interrogate the health of an instrumented part. Estimation of the structures Green’s function from diffuse sound fields present during typical operating conditions is the basis for our approach. Experimental studies are conducted using coherent processing techniques of random and generated sound fields to investigate a structure for defects and/or deviations from an initial or pristine state. We are interested in developing a monitoring process that is minimally invasive and robust enough to survive the environment that the structure operates in and provide a quantitative assessment of the structure throughout its lifetime.

Passive acoustic fiber Bragg grating sensor measurements of Green function in an instrumented part

The development of fiber Bragg grating (FBG) sensors for acoustic measurements permits the instrumentation of manufactured composite parts for the purposes of nondestructive evaluation (NDE) and lifetime monitoring. The parts can be designed from conception with an array of FBG acoustic receivers integrated into its internal structure for the purpose of multistatic data collection for NDE and monitoring of internal changes. The FBG array is accessed via a port on the outer surface and requires no additional power demands on the operation of the part. Data collection is accomplished by activating sources on the surface of the part and measuring the fields along the FBG array. We are particularly interested in Green function (GF) estimation of a FBG instrumented composite structure. By passively measuring diffuse acoustic fields throughout the volume, we use a technique developed by Lobkis and Weaver [J. Acoust. Soc. Am. 110(6), 3011–3017 (2001)] to estimate the GF within the part and monitor it over time. ...

Applying geophysical imaging algorithms to nondestructive evaluation problems

The Center for Subsurface Sensing & Imaging Systems (CenSSIS, www.censsis.neu.edu) has been advancing the concept of ‘‘Diverse Problems‐Similar Solutions,’’ whereby imaging, inversion, and tomography (IIT) algorithms developed for one field or physical regime are applied to another. Following this philosophy, algorithms developed for geophysical imaging have been successfully applied to a nondestructive evaluation (NDE) problem in a planar multilayer. The depths and acoustic frequencies of geophysical problems are on the orders of kilometers and sub‐hertz, while those of NDE are millimeters and megahertz. Just the same, the propagation phenomenon is entirely wave‐based in both cases and thus, algorithms developed for one can be applied to the other. [This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under Contract No. W‐7405‐Eng‐48. UCRL‐ABS‐216281.]

Acoustic archeology at the lost city of Helike

In 373 BC an earthquake and tsunami destroyed and submerged the classical Greek city of Helike (HELL‐E‐KEY) on the north Peloponnese shore of the Gulf of Corinth. In June 2004, surface seismo‐acoustic surveys were performed in the area of present Helike in order to collect non‐invasive tomographic data to guide the archeological excavations. Prior to tomographic inversion and imaging, the raw data time series must be preprocessed to estimate ground acoustic velocity, and to remove the ground roll signal. We present a generalized cross‐correlation technique which appears successful in solving both problems. We describe the technique as applied to the data and present tomographic inversions.

Planarly layered diffraction tomography with accurate Green function

Diffraction tomography (DT) imaging techniques require knowledge of the background Green function. Due to its simplicity, it is standard practice to use a homogeneous medium Green function. We have developed a model of a planarly layered Green function that can be used in DT imaging of planarly layered media where the layer acoustical properties and dimensions are known. We present the theory and applications. [Work performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W‐7405‐Eng‐48.]